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ICSE BIOLOGY GENETICS NOTES

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Tanay Nayak
Avalon Heights International School, Mumbai
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~1~ 2. Genetics Genetics Scope of Syllabus: Monohybrid cross, dihybrid cross. The following terms are to be covered: gene, allele, heterozygous, homozygous, dominant, recessive, mutation, variation, phenotype, genotype. Sex determination in human beings. Sex-linked inheritance of diseases to include haemophilia and colour blindness (only criss cross inheritance). ~ Part A ~ HEREDITY AND VARIATIONS I. Definitions and Explanations 1. Genetics 2. Heredity 3. Variation 4. Characters Genetics is the study of transmission of body features (both similarities and differences) from parents to offspring and the laws relating to such transmission. The transmission of genetically based characteristics from parents to offspring is known as heredity. The small differences between organisms are known as variations. Any heritable feature is known as a character. Example: height of organism II. Questions-Answers 1. Explain the term like begets like . begets like means that young ones of organisms always look like their parents. Example: The young one of an elephant looks like its parent. A mango seed germinates into a mango plant. Multiplying lactobacilli in curd look exactly like each other. ANSWER Like (i) (ii) (iii) Explain the concept of heredity and variations . ANSWER In nature, there is transmission of genetically based characteristics from parents to offspring. This is known as heredity. But at the same time, there are variations between individuals of the same species. Even two siblings born of the same parents will be unlike in many ways. This is known as variation. The study of genetics is the study of both heredity and variations. 3. What are the applications of genetics in recent times? (Extra) ANSWER In recent times, genetics is applied in:(i) Genetic Engineering: It is a technique in which the genetic constitution of an organism is altered by introducing new genes into its chromosomes. This brings about quick multiplication of modified organisms. (ii) Genetic Counselling: It is a practice in which newly married couples are advised by a specialist regarding the possibilities of any undesirable trait which their children might inherit from them. This can be used to prevent diseases such as thalassemia, sickle cell anaemia and haemophilia. 2. ~2~ Biology for Class 10 ICSE ~ Part B ~ CHROMOSOMES I. Definitions and Explanations 1. Chromosome 2. Karyotype 3. Chromosome Number 4. Homologous Chromosomes 5. Autosomes 6. Sex Chromosomes 7. X Chromosome 8. Y Chromosome II. 1. Chromosome is a threadlike structure of nucleic acids and proteins, found in the nuclei of most cells, which carries hereditary characters from one generation to another. A karyotype is a set of chromosomes of an individual or species. It is the number and visual appearance of the chromosomes in the cell nuclei of an organism or species. Every individual of a species has a constant number of chromosomes, known as its chromosome number. The chromosome number of humans is 46. A pair of corresponding chromosomes of the same shape and size, one from each parent, are called homologous chromosomes. Humans have 23 identical pairs of homologous chromosomes. The chromosomes that determine all the hereditary characters of an organism except their sex, i.e. express their somatic characters, are known as autosomes. Humans have 22 pairs of autosomes. The chromosome pair that determines the sex of the organism is known as sex chromosome. Humans have 1 pair of sex chromosomes. This pair is XX in females and XY in males. The X chromosome is the longer chromosome that determines the female characteristic in organisms. The Y chromosome is the shorter chromosome that determines the male characteristic in organisms. Questions-Answers What is a karyotype? What are its uses? ANSWER A karyotype is a set of chromosomes of an individual or species. It is the number and visual appearance of the chromosomes in the cell nuclei of an organism or species. It is an arrangement of photographs of chromosomes taken during the division of the nucleus. The chromosomes are arranged according to their shapes and sizes. Use: A karyotype is useful because it gives a visual representation of the chromosomes of the organism. 2. What is the chromosome number in humans? How are the chromosomes divided? ANSWER The chromosome number in humans is 46. These 46 chromosomes are divided into 23 homologous pairs. Of these, 22 pairs are known as autosomes. The remaining pair determines the sex of the humans, which is known as sex chromosomes. Sex chromosomes are of two types the longer female X chromosome and the shorter male Y chromosome. 3. How is sex determination done in humans? ANSWER Human beings have 46 chromosomes, out of which 44 are autosomes and 2 are sex chromosomes. The sex chromosomes are of two types: longer X chromosome and shorter Y chromosome. Males have the XY combination of sex chromosomes and females have XX combination. Males = 44 A + XY ~3~ Genetics Females = 44 A + XX When gamete formation takes place, these divide into half. Hence, the male gamete (sperm) can have either X or Y chromosome, but the female gamete (egg cell) can have only X chromosome. Male gamete = 22 A + X or 22 A + Y Female gamete = 22 A + X ova P1 generation XX X Gametes sperm X XY X Y During fertilisation, if the egg cell (X) unites with a sperm containing X chromosome, then XX combination is produced, i.e. the offspring is a daughter. This can be represented in a Punnett s Square as follows:- F1 generation Sperm Ova X Y XX XY XX XY X X Hence, the onus of whether the offspring is a son or a daughter lies on the human father and not the mother. There is a 50% chance that the offspring would be a son and 50% chance that it would be a daughter, but that would depend on the chromosome donated by the father. III. 1. Reasons and Justifications The father is responsible for the gender of the offspring and not the mother. humans, females have XX sex chromosomes and males have XY sex chromosomes. The gamete, i.e. egg cell produced by the mother always contains the X chromosome; but the gamete produced by the father, i.e. the sperm, could contain either X or Y chromosome. When the egg cell (X) unites with an X chromosome containing sperm, then XX combination is produced, i.e. daughter. But when it unites with a Y chromosome containing sperm, then XY combination is produced, i.e. son. Thus, it is the father s sperm that decides the gender of the offspring, since the mother can only produce one type of gametes. ANSWER In IV. Distinctions 1. Autosomes and Sex Chromosomes Autosomes (i) Autosomes determine all the characters of an organism except sex. (ii) Humans have 22 pairs of autosomes in each nucleus. 2. X Chromosome and Y Chromosome Sex Chromosomes Sex chromosomes determine the sex of the organism. Humans have 1 pair of sex chromosomes in each nucleus. Biology for Class 10 ICSE ~4~ X Chromosome (i) X chromosome is the longer sex chromosome. (ii) It determines female character. (iii) It is present in egg cell and may be present in sperm. (iv) It is present in both males and females. (v) It contains X-linked genes. (vi) Due to its longer size, it contains more genetic information. Y Chromosome Y chromosome is the shorter sex chromosome. It determines male character. It is never present in egg cell and may be only present in sperm. It is present in only males. It contains Y-linked genes. Due to its shorter size, it contains less genetic information. ~ Part C ~ GENES I. Definitions and Explanations 1. Gene 2. Genome 3. Alleles or Traits 4. Genotype 5. Phenotype 6. Homozygous Pair 7. Heterozygous Pair 8. Dominant Character 9. Recessive Character 10. Pure Breed 11. Hybrid II. Gene is a specific DNA segment of a chromosome that determines hereditary characteristics. A genome is the full complement of DNA (including all genes and intergenic regions) of an organism. The alternative forms of the same character, which produce different effects, are known as alleles. They occupy same position on homologous chromosomes. Example: tall and dwarf plants are contrasting alleles of the same character height. The genetic constitution of an organism is called its genotype. It is the set of genes present in the cells of an organism. Example: TT, tt, Tt represent the genotype of an organism. The external visible features of an organism are called its phenotype. It is the observable characteristic which can be genetically controlled. Example: tallness and dwarfness represent the phenotype of an organism. When both alleles of a gene pair are identical, they are said to be homozygous. Example: TT is a homozygous tall plant where T and T both represent tall characters. tt is a homozygous dwarf plant where t and t both represent dwarf characters. When both alleles of a gene pair are different, they are said to be heterozygous. Example: Tt is a heterozygous tall plant where T and t represent tall and dwarf characters respectively. The allele which expresses itself in the phenotype is the dominant character. Example: In the heterozygous organism Tt, the T character is dominant, and hence the plant appears tall. The allele which masks itself in the phenotype is the recessive character. Example: The heterozygous organism Tt has the dwarf character t, but the organism still appears tall. Hence, t is masked and is the recessive character. Homozygous individuals formed by the parents with identical characters and bred true for at least the previous three generations are called as pure breed. Examples: TT (pure tall) and tt (pure dwarf). Heterozygous individuals formed by parents having contrasting characters are called hybrids. Examples: Tt (hybrid tall) Questions-Answers ~5~ Genetics 1. What are genes? What is their role in genetics? ANSWER Genes are the specific DNA segments of a chromosome that determine hereditary characteristics. Genes are different from one individual to another, even within the same species. When a male gamete unites with the female gametes, chromosomal matter is exchanged and different genes are produced. Hence, genes are responsible in carrying hereditary information. They transfer information from both parents to the offspring. 2. Where are genes present? ANSWER Genes are present on the chromosomes. They are specific segments containing DNA that are present on chromosomes. 3. What are characters and alleles? Explain with an example. feature that can be inherited is called as a character. The alternative forms of the same character, which produce different effects, are known as alleles. Example: The shape of a person s hair is a heritable character. Character: shape of hair Now, the shape of the hair could be either curly or straight. These are the alternatives of the character (shape) and are known as alleles. Allele: curly hair / straight hair ANSWER Any 4. Explain phenotype and genotype of an organism. external visible features of an organism are called its phenotype. It is the observable characteristic which can be genetically controlled. ANSWER Phenotype: The Genotype: The genetic constitution of an organism is called its genotype. It is the set of genes present in the cells of an organism. A pea plant might be either tall or dwarf. This external characteristic is its phenotype. If we consider the character governing the height of the pea plant, it could be either TT, Tt or tt. This actual genetic constitution of the plant is called its genotype. 5. Explain dominant and recessive alleles with an example. ANSWER An allele has two alternative versions of the same character. They can be either dominant or recessive. Dominant Character: The allele which expresses itself in the phenotype is the dominant character. Recessive Character: The allele which masks itself in the phenotype is the dominant character. Example: Consider a pea plant with the genetic constitution Tt, where T represents tallness and t represents dwarfness. Though this plant has both variations of the height character, it will be a tall plant. This is because tallness is the dominant character and dwarfness is the recessive character. 6. What are homozygous and heterozygous organisms? Considering T as the tallness character and t as the dwarfness character in the pea plant, write the different genotypes possible and their corresponding phenotypes. ANSWER A character is made up of two pairs of genes, each representing one allele for the character. Homozygous gene pair: When both alleles of a gene pair are identical, it is said to be homozygous. Heterozygous gene pair: When both alleles of a gene pair are different, it is said to be heterozygous. Biology for Class 10 ICSE ~6~ The following table shows different possibilities considering tall and dwarf character for pea plant. Genotype Homozygous TT Homozygous tt Heterozygous Tt TT: pure homozygous dominant tt: pure homozygous recessive Tt: heterozygous dominant III. 1. Phenotype tall dwarf tall Reasons and Justifications Heterozygous gene pairs are always dominant in their phenotype. gene pairs contain both variations of the alleles of a character. One of these alleles represents the dominant character and the other represents the recessive character. However, the dominant character is always visible in the phenotype and the recessive character is always masked. Hence, due to the presence of the masking of their recessive character, heterozygous gene pairs are always dominant in their phenotype. ANSWER Heterozygous IV. Distinctions 1. Genotype and Phenotype Genotype The genetic constitution of an organism is called its genotype. (ii) It is the set of genes present in the cells of an organism. (iii) It is influenced only by hereditary factors. (i) (iv) It will contain all hereditary factors even if those genes are not expressed. (v) Example: DNA, susceptibility to diseases 2. Phenotype The external visible features of an organism are called its phenotype. It is the observable characteristic which can be genetically controlled. It is influenced by hereditary factors as well as environmental factors. It contains only factors that are expressed. Example: tallness of pea plant, eye colour in humans Dominant and Recessive Characters Dominant Characters (i) It is the character that shows itself in the phenotype. (ii) It is expressed even if another allele of the same gene pair is present. (iii) In the phenotype, dominant characters are expressed in both homozygous and heterozygous gene pairs. (iv) For colour of human eyes, brown eyes is Recessive Characters It is the character that remains masked in the phenotype. It is expressed only if the other allele of the gene pair is identical. In the phenotype, recessive characters are expressed only in homozygous gene pairs. For colour of human eyes, blue eyes is ~7~ dominant. 3. Genetics recessive. Homozygous and Heterozygous Gene Pairs Homozygous Gene Pairs When a pair of alleles present on homologous chromosomes are similar, they are known as a homozygous gene pair. (ii) They can produce only one type of gamete. (iii) Both dominant and recessive characters can show themselves in their phenotype. (iv) For height of plant, where T represents tallness and t represents dwarfness, the homozygous gene pair is TT and tt. (i) Heterozygous Gene Pairs When a pair of alleles present on homologous chromosomes are different, they are known as a heterozygous gene pair. They can produce two types of gametes. Only dominant characters show themselves in their phenotype. For height of plant, where T represents tallness and t represents dwarfness, the heterozygous gene pair is Tt. V. Tables and Charts 1. Dominant and Recessive Forms of Common Human Traits Character Eye colour Tongue Hand Earlobe Lips Rh Blood Group Colour Vision (red/green) Cheek Dominant Trait brown rolling right-handedness free thick positive normal dimpled Recessive Trait blue non-rolling left-handedness attached thin negative colour-blind normal ~ Part D ~ SEX-LINKED INHERITANCE I. Definitions and Explanations 1. Punnett Square 2. Sex-linked Inheritance 3. X-linked Inheritance 4. Y-linked Inheritance A Punnett Square is a simple diagram in which the different types of gametes of one parent are placed alongside one side of the square and those of the other parent are placed alongside the other side. Sex-linked inheritance is the appearance of a trait in an organism which is due to the presence of an allele exclusively on either the X-chromosome or on the Ychromosome. X-linked inheritance is the appearance of a trait in an organism due to the presence of an allele on the Xchromosome. Conditions like colour-blindness and haemophilia are related to X-linked inheritance. Y-linked inheritance is the appearance of a trait in an organism due to the presence of an allele on the Y chromosome. Biology for Class 10 ICSE ~8~ Conditions like hypertrichosis and pattern baldness in men are related to Y-linked inheritance. II. Questions-Answers 1. What is sex-linked inheritance? What are its two types? ANSWER Sex-linked inheritance is the appearance of a trait in an organism which is due to the presence of an allele exclusively on either the X-chromosome or on the Y-chromosome. Based on the chromosome through which the hereditary trait is transmitted, there are two types of sex-linked inheritance:- (i) X-linked inheritance: The hereditary trait is transmitted on the X chromosome. (ii) Y-linked inheritance: The hereditary trait is transmitted on the Y chromosome. 2. Explain the sex-linked inheritance of colour blindness. ANSWER Colour blindness is caused due to sex-linked inheritance transmitted through the X chromosome. Thus, it is caused due to X-linked inheritance. The following chart explains the transmission of colour blindness. Case 1: Mother is normal (not colour-blind) and father is colour-blind. The normal mother is represented by XX and the colour-blind father is represented by XoY. 3. Explain the inheritance of a rolling tongue with the help of a pedigree chart. ANSWER The following chart is a pedigree chart of a family consisting of two parents and three children, illustrating the trait of rolling tongue. The main points of the pedigree chart are as follows:- Males are shown by squares and females by circles. Ability to roll tongue is shown by a solid circle and inability to do the same is shown by blank circle. The dominant trait here is the ability to roll tongue (genotype R) and the recessive trait is its inability (genotype r). Both parents can roll their tongue. Of the three children, two have the ability to roll their tongues while the third cannot. The child who cannot roll the tongue has only recessive character (rr). One of these r characters must have come from either parent. Hence, the parents must be heterozygous, because each of them have donated one recessive character to the child. The other character in the parents must be dominant because they are themselves able to roll their tongues. Thus, the genotype of each parent is Rr. This can be shown in a Punnett Square as follows:- ~9~ III. Distinctions 1. Genetics X-linked Inheritance and Y-linked Inheritance X-linked Inheritance (i) It is the appearance of a trait in an organism due to the presence of an allele on the X chromosome. (ii) It can occur in both males and females. (iii) Conditions such as colour blindness and haemophilia are caused due to X-linked inheritance. Y-linked Inheritance It is the appearance of a trait in an organism due to the presence of an allele on the Ychromosome. It occurs in only males. Conditions such as hypertrichosis and male pattern baldness are caused due to Y-linked inheritance. ~ Part E ~ MENDEL S EXPERIMENTS ON INHERITANCE I. 1. 2. 3. 4. 5. 6. Definitions and Explanations Monohybrid Cross When only a single pair of contrasting characters is considered in a cross, then it is known as a monohybrid cross. Example: The cross between pure tall (TT) and pure dwarf (tt) pea plants. Monohybrid The ratios of genotype and phenotype that are obtained by conducting a Ratio monohybrid cross is known as monohybrid ratio. Monohybrid genotypic ratio = 1:2:1 Monohybrid phenotypic ratio = 3:1 Dihybrid When two pairs of contrasting characters is considered in a cross, then it is Cross known as a dihybrid cross. Example: The cross between pure yellow round (YYRR) and pure green dwarf (yyrr) pea plants. Dihybrid The ratio of phenotype that are obtained by conducting a dihybrid cross is Ratio known as dihybrid ratio. Dihybrid phenotypic ratio = 9:3:3:1 P Generation The first generation of organisms considered in a hybrid cross is referred to as the P generation, which stands for parent generation. F1 Generation The first generation of offspring obtained in a hybrid cross is referred to as the F1 generation, which stands for first filial generation. 7. F2 Generation The second generation of offspring obtained by the hybrid cross of the F1 generation is referred to as the F2 generation, which stands for second filial generation. II. Statements and Laws 1. Mendel s Laws of Inheritance put forth three laws governing inheritance. (i) Law of Dominance - Out of a pair of contrasting characters present together, only one is able to express itself while the other remains suppressed. (ii) Law of Segregation of Characters (a.k.a. Law of Purity of Gametes) The two members of a pair of factors separate during the formation of gametes. ANSWER Mendel Biology for Class 10 ICSE (iii) ~ 10 ~ Law of Independent Assortment When there are two pairs of contrasting characters, the distribution of the members of one pair into the gametes is independent of the distribution of the other pair. III. Questions Answers 1. Which plant did Mendel select for his experiments on heredity? selected the garden pea plant for his experiments on heredity. Its binomial nomenclature is Pisumsativum. ANSWER Mendel 2. How many pairs of contrasting alleles does the pea plant have? Enlist them. ANSWER The garden pea plant chosen by Mendel for his experiments has seven pairs of alleles. The following is a list of them:Character Colour of flower Colour of seed Shape of seed Colour of pod Shape of pod Position of flower Height of plant 2. Dominant purple yellow round green inflated axillary tall Recessive white green wrinkled yellow constricted terminal dwarf State and explain Mendel s first law of inheritance. law of inheritance is the Law of Dominance. It states that: Out of a pair of contrasting characters present together, only one is able to express itself while the other remains suppressed. The pair of contrasting characters are known as alleles. The gene that expresses itself is the dominant character. The gene that suppresses itself is the recessive character. In the heterozygous genotype Tt, where T represents tall character and t represents dwarf character, it is the tallness character that expresses itself. Hence, the gene for tallness is the dominant character while that for dwarfness is the recessive character. ANSWER Mendel s first 3. ***Show the monohybrid cross between a homozygous tall pea plant (TT) and a homozygous dwarf (tt) of the same species.*** ANSWER In a monohybrid cross, only one pair of contrasting alleles is studied. The cross between a homozygous tall pea plant (TT) and a homozygous dwarf pea plant (tt) is an example of a monohybrid test cross. The monohybrid cross is depicted as follows:- The offspring is heterozygous with the genotype Tt. However, the offspring will still be tall physically, because of the presence of the dominant T character. ~ 11 ~ Genetics State and explain Mendel s second law of inheritance. ANSWER Mendel s second law of inheritance is the Law of Segregation of Characters, which is also known as the Law of Purity of Gametes. It states that: The two members of a pair of factors separate during the formation of gametes. Every character is governed by a pair of genes. Each of them bear contrasting characters known as alleles. For example: pisumsativum may show Tt in its genotype, where T stands for tallness and t stands for dwarfness. However, during the process of formation of gametes, these two factors separate from each other. Hence, the gamete will contain any one of the contrasting characters, but not both. The gametes formed are always pure and hence this law is also known as the Law of Purity of Gametes. 5. Explain the Law of Purity of Gametes using the example of crossing between a homozygous tall pea plant and a homozygous dwarf pea plant. ANSWER Let us consider a pure homozygous tall pea plant (TT) and a homozygous dwarf pea plant (tt). Their crossing yields the following results:- It can be seen that during the gamete formation, the individual characters separate themselves form each other. This brings about a separation of the gametes. Thus, gametes contain only one character and hence they are said to be pure. 6. Explain the Law of Purity of Gametes using the example of crossing between two heterozygous pea plants of the genotype Rr where R stands for red flowers and r stands for white flowers. ANSWER In the garden pea plant, red flowers are dominant (R) and white flowers are recessive (r). When two heterozygous pea plants of the genotype Rr are crossed, there are four possible combinations in the second filial (F2) generation. These are RR, Rr, rR and rr. This can be shown as follows:- 4. 7. Of these, RR and rr are homozygous; Rr and rR are heterozygous. The genotypic ratio is 1:2:1. The parents were both phenotypically red, but in the offspring, RR, Rr and rR are red while rr is white. Thus, the phenotypic ratio is 3 red: 1 white. This happened because the characters of each gene pair separated and then recombined. State and explain Mendel s third law of inheritance. ANSWER Mendel s third law of inheritance is the Law of Independent Assortment. ~ 12 ~ Biology for Class 10 ICSE 8. It states that: When there are two pairs of contrasting characters, the distribution of the members of one pair into the gametes is independent of the distribution of the other pair. Thus, each one of a pair of contrasting characters may be combined with either of the other paired type. In other words, when parents differing in two or more characters are crossed, then the inheritance of one character is independent of the other character. Explain the Law of Independent Assortment with the help of the dihybrid cross between a yellow round seed and a green wrinkled seed of the garden pea plant. ANSWER In seed colour of the garden pea plant, yellow (Y) is dominant while green (y) is recessive. Also, round seeds (R) are dominant and wrinkled seeds (r) are recessive. The dominant homozygous YYRR seed is crossed with the recessive homozygous yyrr variety. When the gametes are formed, the individual characters separate from each other. Four different types of gametes are formed such as YR, Yr, yR and yr. Now, when these gametes unite with each other, they can unite in any combination. They also produce yellow wrinkled and green round seeds which were not present in the original parental generation. This has happened because the gametes have assorted independently in all the combinations possible. 9. Explain Mendel s monohybrid cross with the example of a crossing between a pure tall and pure dwarf pea plant. OR: A pure tall plant (TT) is crossed with a pure dwarf plant (tt). Draw PunnettSquares to show F1 generation and F2 generation. ANSWER When only one pair of contrasting characters is studied in a hybrid cross, it is known as monohybrid cross. Mendel performed a monohybrid cross by taking a homozygous tall plant (TT) and a homozygous dwarf plant (tt) as the parental P1 generation. Tall character is dominant and dwarf character is recessive. The hybrid cross between these parents is represented as follows:- P1 generation TT Gametes tt T t Tt F1 generation Punnett s Square for F1 generation is as follows. Male gametes T T Tt Tt Tt Tt t gametes F1 generation Female t Hence, all offspring obtained in the F1 generation are genotypically and phenotypically similar. When they are further crossed with each other (selfing), different combinations are obtained. ~ 13 ~ Genetics Punnett s Square for F2 generation is as follows. F1 generation F2 generation Tt Tt Female gametes Male gametes 10. T t TT Tt Tt tt T t In the F2 generation, there are the following types of plants: TT, 2 Tt and tt. Thus, the monohybrid genotypic ratio is 1:2:1. Of these, TT and 2 Tt are tall plants while tt is dwarf. Thus, the monohybrid phenotypic ratio is 3 tall : 1 dwarf, i.e. 3:1. Explain Mendel s dihybrid cross with the example of a crossing between a yellow round and a green wrinkled pea plant. ANSWER When two pairs of contrasting characters is studied in a hybrid cross, it is known as dihybrid cross. Mendel performed a dihybrid cross by taking a homozygous yellow round plant (YYRR) and a homozygous green wrinkled plant (yyrr) as the parental P1 generation. Yellow and round character is dominant and green and wrinkled character is recessive. The hybrid cross between these parents is represented as follows:- P1 generation Gametes YYRR yyrr YR yr YyRr F1 generation Punnett s Square for F1 generation is as follows. Male gametes Female YR YyRr YyRr F1 generation gametes YR YyRr YyRr yr yr Hence, all offspring obtained in the F1 generation are genotypically and phenotypically similar. When they are further crossed with each other (selfing), different combinations are obtained. The F1 generation gametes are obtained from YyRr. They are of four types YR, Yr, yR and yr. Punnett s Square for F2 generation is as follows. F generation Notes Prepared1by Neil Valentine D Silva YyRr YyRr ~ 14 ~ Biology for Class 10 ICSE Male gametes F2 generation YR Yr yR yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr Female gametes YR Yr yR yr 11. In the F2 generation, there are the following types of plants: 9 yellow round, 3 green round, 3 yellow wrinkled and 1 green wrinkled. Thus, the dihybrid phenotypic ratio is 9:3:3:1. A pea plant with pure red flowers is crossed with another with pure white flowers. Is this an example of monohybrid cross or dihybrid cross? Explain. Also, draw the Punnett Squares to show their F1 and F2 generations. ANSWER This is an example of a monohybrid cross, since only one pair of contrasting characters (colour of flowers) is considered. A homozygous red flowered plant (RR) is crossed with a homozygous white flowered plant (rr).This is the parental P1 generation. Red flowers are dominant and white flowers are recessive. The hybrid cross between these parents is represented as follows:- P1 generation Gametes RR rr R r Rr F1 generation Punnett s Square for F1 generation is as follows. Male gametes R R Rr Rr Rr Rr r gametes F1 generation Female r Hence, all offspring obtained in the F1 generation are genotypically and phenotypically similar. When they are further crossed with each other (selfing), different combinations are obtained. Punnett s Square for F2 generation is as follows. ~ 15 ~ Genetics F1 generation F2 generation Rr Rr Female gametes Male gametes 12. R r RR Rr Rr rr R r In the F2 generation, there are the following types of plants: RR, 2 Rr and rr. Thus, the monohybrid genotypic ratio is 1:2:1. Of these, RR and 2 Rr are tall plants while rr is dwarf. Thus, the monohybrid phenotypic ratio is 3 red : 1 white, i.e. 3:1. Give the dihybrid ratio. Name and state the law which explains the same. 2008 dihybrid ratio (genotypic) is 9:3:3:1. It is governed by the Law of Independent Assortment. It states that: When there are two pairs of contrasting characters, the distribution of the members of one pair into the gametes is independent of the distribution of the other pair. ANSWER The 13. Mention some applications of Mendel s laws. ANSWER Mendel s laws have the following applications:- IV. 1. Using the Mendel s laws, we can determine new combinations in the progeny of hybrids and can predict their frequency. This information is vastly used by plant and animal breeders to produce better breeds. New types of plants with new combinations of useful characters can be produced by hybridisation. Reasons and Justifications Gametes have a haploid number of chromosomes. 2007 Law of Segregation states that: The two members of a pair of factors separate out during the formation of gametes. This is also known as the Law of Purity of Gametes. As the alleles separate out during the formation of gametes, gametes have only a haploid number of chromosomes. ANSWER Mendel s V. Distinctions ~ 16 ~ Biology for Class 10 ICSE 1. Monohybrid Cross and Dihybrid Cross Monohybrid Cross (i) Only one pair of contrasting characters is considered. (ii) The F2 generation produces 4 variations. (iii) The monohybrid genotypic ratio is 3:1. 2. Dihybrid Cross Two pairs of contrasting characters are considered. The F2 generation produces 16 variations. The dihybrid genotypic ratio is 9:3:3:1. F1 Generation and F2 Generation F1 Generation (i) It is the first filial generation. (ii) All offspring of the F1 generation in Mendel s monohybrid and dihybrid crosses are genotypically and phenotypically identical. F2 Generation It is the second filial generation. All offspring of the F2 generation in Mendel s monohybrid and dihybrid crosses are genotypically and phenotypically different. ~ Part F ~ MUTATION I. Definitions and Explanations 1. Mutation Mutation is a sudden change in one or more genes, or in the number of structure of chromosomes. II. Questions-Answers 1. Mention some harmful effects of mutations. can be harmful in the following ways:- ANSWER Mutations Sickle Cell Anaemia This blood disease is caused due to a gene mutation. The mutation causes changes in the DNA structure that brings about the production of sickle shaped RBCs. Radioactive Radiations These can alter the genetic structure, and their effects can continue for many generations. The atomic bombing of Nagasaki and Hiroshima in 1945 still cause mutative effects in people living in these cities. BOARD EXAMINATION QUESTIONS 2002 1. Name the structure responsible for the functional activity given below:Acts as bearer of heredity units. ANSWER Chromosome (i) State the Mendel s Law of Dominance. (ii) What is dihybrid ratio? (iii) Define phenotype. (iv) What are autosomes? ANSWER (i) Mendel s Law of Dominance states that: Out of a pair of contrasting characters present together, only one is able to express itself while the other remains suppressed. 2. ~ 17 ~ (iii) (iv) (v) Genetics Dihybrid ratio is 9:3:3:1. The external visible features of an organism are called its phenotype. The chromosomes that determine all the hereditary characters of an organism except their sex, i.e. express their somatic characters, are known as autosomes. 2003 3. Name the following:The genetic constitution of an organism. ANSWER Genotype 4. Define the term allele. alternative forms of the same character, which produce different effects, are known as alleles. ANSWER The 2004 5. (i) Name two animals which have 19 pairs of chromosomes. (ii) Differentiate between genotype and phenotype. (iii) In a certain species of animals, black fur (B) is dominant over brown fur (b). Predict the genotype and phenotype of the offspring when both parents have Bb or have heterozygous black fur. ANSWER (i) Two animals with 19 pairs of chromosomes tiger and cat (ii) The differences between genotype and phenotype are:Genotype Phenotype (i) The genetic constitution of an organism is The external visible features of an organism are called its genotype. called its phenotype. (ii) It is the set of genes present in the cells of an It is the observable characteristic which can be organism. genetically controlled. (iii) It is influenced only by hereditary factors. It is influenced by hereditary factors as well as environmental factors. (iv) It will contain all hereditary factors even if It contains only factors that are expressed. those genes are not expressed. (v) Example: DNA, susceptibility to diseases Example: tallness of pea plant, eye colour in humans The crossing will be as follows:P generation F1 generation Bb Bb Male gametes Female gametes (iii) B b BB Bb Bb bb B b Thus, the phenotypic ratio of the offspring will be 3:1; i.e. there are 3 possibilities that the offspring may have black fur and 1 possibility that the offspring may have brown fur. The genotypic ratio is 1:2:1. There is 1 possibility of BB, 2 possibilities of Bb and 1 possibility of bb. ~ 18 ~ Biology for Class 10 ICSE 2007 6. Name the following:Pair of genes responsible for a particular characteristic in an individual. ANSWER Alleles 7. Name the organ or structure that transmits characteristics from parent to offspring. ANSWER Genes 8. Explain the term mutation. is a sudden change in one or more genes, or in the number of structure of chromosomes. ANSWER Mutation 9. Give the difference between pure and hybrid strains (definitions). ANSWER Pure breed: Homozygous individuals formed by the parents with identical characters and bred true for at least the previous three generations are called as pure breed. Examples: TT (pure tall) and tt (pure dwarf). Hybrid: Heterozygous individuals formed by parents having contrasting characters are called hybrids. Examples: Tt (hybrid tall) 10. Expand DNA. Nucleic Acid ANSWER Deoxyribo (i) State Mendel s Law of Independent Assortment. (ii) A homozygous tall plant (T) bearing red coloured (R) flowers is crossed with a homozygous dwarf plant (t) bearing white flowers (r). a. Give the genotype and phenotype of the F1 generation. b. Give the possible combinations of the gametes that can be obtained from the F1 hybrid. c. Give the dihybrid ratio and the phenotype of the offspring of the F2 generations when two plants of the F1 generation above are crossed. ANSWER (i) Mendel s Law of Independent Assortment states that: When there are two pairs of contrasting characters, the distribution of the members of one pair into the gametes is independent of the distribution of the other pair. 11. (ii) The crossing of homozygous tall plant with red flowers and homozygous dwarf plant with white flowers is as follows:P1 generation TTRR Gametes TR F1 generation ttrr tr TtRr a. The genotype of the F1 generation is TtRr The phenotype of the F1 generation is tall plant with red flowers. b. There are 4 possible combinations of gametes: TR, Tr, tR, tr c. The dihybrid ratio is 9:3:3:1 The phenotype of the offspring is:- ~ 19 ~ Genetics 9 tall red 3 tall white 3 dwarf red 1 dwarf white. 12. Give reasons for the following: Gametes have a haploid number of chromosomes. ANSWER Gametes are formed by the meiotic division of somatic cells. According to the Law of Segregation, the alleles present in a gene pair separate from each other when gametes are formed. Thus, gametes have only one haploid factor, i.e. a single chromosome. 2008 13. Name a specific part of a chromosome that determines hereditary characters. ANSWER Genes 14. State whether the following statement is true or false:The number of pairs of autosomes in man is 22. ANSWER True 15. Briefly explain the term phenotype. external visible features of an organism are called its phenotype. Consider the crossing of a pure tall (TT) and pure dwarf (tt) pea plant. In the F1 generation, these will produce a hybrid offspring (Tt). By the Law of Dominance, the offspring will appear to be tall even though it also has an allele for dwarfness. Thus, irrespective of its genetic composition, it will appear tall. This is called phenotype. The following is the phenotype for different combinations considering the tallness character:TT phenotype: tall Tt phenotype: tall tt phenotype: dwarf ANSWER The 16. Explain the following terms: (i) mutation (ii) homologous chromosomes (iii) alleles ANSWER (i) Mutation:Mutation is a sudden change in one or more genes, or in the number of structure of chromosomes. (ii) Homologous chromosomes:A pair of corresponding chromosomes of the same shape and size, one from each parent, are called homologous chromosomes. Humans have totally 46 chromosomes in a nucleus. These are arranged in 23 homologous pairs. In a homologous pair, both chromosomes are identical. (iii) Alleles:The alternative forms of the same character, which produce different effects, are known as alleles. Example: For tallness, the two alleles are T (which represents tall character) and t (which represents dwarf character). 17. Give the dihybrid ratio. Name and state the law which explains the same. dihybrid ratio is 9:3:3:1. The law that explains the dihybrid ratio is the Law of Independent Assortment. It states: When there are two pairs of contrasting characters, the distribution of the members of one pair into the gametes is independent of the distribution of the other pair. ANSWER The 2009 ~ 20 ~ Biology for Class 10 ICSE 18. Identify and name the following process/term from the given statement: A pair of chromosomes carrying dissimilar alleles for a given character. ANSWER Heterozygous 19. Explain the following terms: (i) monohybrid cross (ii) gene (iii) phenotype. ANSWER (i) Monohybrid cross:When only a single pair of contrasting characters is considered in a cross, then it is known as a monohybrid cross. Example: The cross between pure tall (TT) and pure dwarf (tt) pea plants. (ii) Gene:Gene is a specific DNA segment of a chromosome that determines hereditary characteristics. (iii) Phenotype:The external visible features of an organism are called its phenotype. The phenotype is dictated by the dominant allele present in a gene pair. Example: If we consider the characteristic of tallness, the phenotype of TT and Tt will be tall while the phenotype for tt will be dwarf. 20. Name two sex-linked diseases in males. and baldness ANSWER Hypertrichosis State Mendel s Law of Segregation. ANSWER Mendel s Law of Segregation states that: The two members of a pair of factors separate during the formation of gametes. 21. 2010 22. Give one point of difference between the following on the basis of what is given in brackets:Genotype and Phenotype (definition). ANSWER Genotype:The genetic constitution of an organism is called its genotype. It is the set of genes present in the cells of an organism. Phenotype:The external visible features of an organism are called its phenotype. 23. (i) A pure tall plant (TT) is crossed with a pure dwarf plant (tt). Draw Punnett Squares to show (1) F1 generation and (2) F2 generation. (ii) Give the phenotype of the F2 generation. (iii) Give the phenotypic and genotypic ratio of the F1 and F2 generations. (iv) Name any one X-linked disease found in humans. Considering the crossing between homozygous tall plant (TT) and homozygous dwarf ANSWER (i) plant (tt), the F1 and F2 generations can be shown as follows:P1 generation Gametes TT tt T t Tt F1 generation Punnett s Square for F1 generation is as follows. Male gametes F1 generation T T Tt Tt Tt Tt t Female gametes t ~ 21 ~ Genetics If these offspring are selfed, we get F2 generation.Punnett s Square for F2 generation is as follows. F1 generation F2 generation Tt Tt Female gametes Male gametes T t TT Tt Tt tt T t (ii) The phenotype of F2 generation is 3:1. There are 3 tall and 1 dwarf plants. (iii) F1 Generation: Genotypic ratio = 1:1 (all are Tt) Phenotypic ratio = 1:1 (all are tall) F2 Generation: Genotypic ratio = 1:2:1 (1 TT, 2 Tt and 1 tt) Phenotypic ratio = 3: 1 (3 tall, 1 dwarf) (iv) Colour blindness 2011 24. Rewrite and complete the following sentence by inserting the correct word in the space indicated:- Phenotype is the observable characteristic which is __________ controlled. ANSWER genotypically 25. Write down the difference between the following pair as indicated within brackets:Monohybrid and Dihybrid Cross (phenotypic ratio) ANSWER Phenotypic ratio of monohybrid cross = 3:1 Phenotypic ratio of dihybrid cross = 9:3:3:1 2012 26. Match the item in column A with the correct item in column B:Column A Column B Mutation Sudden change in genes Alleles ANSWER Mutation sudden change in genes 27. Briefly explain the following: Allele ANSWER The alternative forms of the same character, which produce different effects, are known as alleles. Example: For tallness, the two alleles are T (which represents tall character) and t (which represents dwarf character). Biology for Class 10 ICSE ~ 22 ~ 28. A homozygous plant having round (R) and yellow (Y) seed is crossed with another homozygous plant having wrinkled (r) and green (y) seeds. (i) Give the scientific name of the plant on which Mendel conducted his hybridisation experiments. (ii) Give the genotype of the F1 generation. (iii) Give the dihybrid phenotypic ratio and the phenotype of the offspring of the F2 generation when two plants of the F1 generation are crossed. (iv) Name and state the law which explains the dihybrid ratio. (v) Give the possible combinations of gametes that can be obtained from F1 hybrid. ANSWER (i) Pisumsativum (ii) Genotype of F1 generation: all RrYy (iii) The dihybrid phenotypic ratio = 9:3:3:1 (iv) The law is Mendel s Law of Independent Assortment. It states: When there are two pairs of contrasting characters, the distribution of the members of one pair into the gametes is independent of the distribution of the other pair. (v) The possible combinations of gametes are: RY, Ry, rY and ry. 2013 29. Given below is a schematic diagram showing Mendel s Experiment on sweet pea plants having axial flowers with round seeds (AARR) and terminal flowers with wrinkled seeds (aarr). Study the same and answer the questions that follow:- (i) (ii) (iii) (iv) ANSWER (i) (ii) (iii) (iv) phenotype of F1 Give the progeny. Give the phenotype of F2 progeny produced upon by self-pollination of F1 progeny. Give the phenotypic ratio of F2 progeny. Name and explain the law introduced by Mendel on the basis of the above observation. Phenotype of F1 progeny = 1:1 (all are axial round) Phenotype of F2 progeny include:a) Axial round b) Axial wrinkled c) Terminal round d) Terminal wrinkled Phenotype of F2 progeny = 9:3:3:1 The law introduced by Mendel after this observation was Law of Independent Assortment. It states: When there are two pairs of contrasting characters, the distribution of the members of one pair into the gametes is independent of the distribution of the other pair. ~ 23 ~ Genetics

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