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ICSE Class X Notes 2021 : Biology; Cell Cycle and Genetics

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WGENETICSV Definitions 1. Gene: a heritable factor that controls a specific characteristic, or a section of DNA that codes for the formation of a polypeptide. 2. Allele: a specific form of a gene occupying the same gene locus or position as other alleles of that gene, but differing from other alleles by small variations in its base sequence. 3. Genome: the whole of the genetic information of an organism. OR It is a complete set of chromosomes where every gene and chromosome is represented singly as in a gamete. 4. Gene mutation: a change in the sequence of bases in a gene. 5. Diploid: a diploid nucleus contains two copies of each chromosome, in homologous pairs. 6. Haploid: a haploid nucleus contains one chromosome of each homologous pair. 7. Homologous chromosomes: a pair of chromosomes with the same genes but not necessarily the same alleles of those genes. 8. Somatic cell: a body cell that is not a gamete. 9. Karyogram: a diagram or photograph of the chromosomes from an organism. 10.Karyotype: the number and type of chromosomes present in a nucleus. 11.Recombination: In each homologous pair, one chromosome is a maternal chromosome and the other a paternal chromosome. After crossing over, the chromatids recombine to produce new and unique combinations of alleles, different from both the maternal and the paternal arrangements. This is called recombination. 12.Genotype: the alleles possessed by an organism each allele is represented by a letter; chromosomes come in pairs and so alleles come in pairs a genotype is therefore represented by a pair of letters, such as TT or Tt. PAGE 1| 13.Phenotype: the characteristics of an organism; a characteristic may be an external feature, such as the colour of flower petals, or internal, such as sickle-cell anemia. 14.Dominant allele: an allele that has the same effect on the phenotype when in either the homozygous or heterozygous state; the dominant allele is always given a capital letter for example, T. 15.Recessive allele: an allele that only has an effect on the phenotype when in the homozygous state; a recessive allele is always given the lower case of the same letter given to the dominant allele for example, t. 16.Codominant alleles: pairs of alleles that both affect the phenotype when present in the heterozygous state; these alleles are represented in a different way in genetics a capital letter is chosen to represent the gene and then other (superscript) letters represent the alleles (for example, in human blood grouping, A and B are codominant alleles and are represented as IA and IB). 17.Locus: the specific position of a gene on a homologous chromosome; a gene locus is fixed for a species for example, the insulin gene is always found at the same position on chromosome 11 in humans. 18.Homozygous: having two identical alleles at a gene locus; the alleles may both be dominant or both recessive for example, TT or tt. 19.Heterozygous: having two different alleles at a gene locus for example, Tt. 20.Test cross: testing a dominant phenotype to determine if it is heterozygous or homozygous for example, crossing either TT or Tt with tt; if there are any offspring with the recessive phenotype, then the parent with the dominant phenotype must be heterozygous (Tt). 21.Carrier: an individual with one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele. 22.Pure-breeding: individuals of the same phenotype that, when crossed with each other, produce offspring which also all have that same phenotype. 23.Parental generation: the original parent individuals in a series of experimental crosses. 24.F1 generation: the offspring of the parental generation. 25.F2 generation: the offspring of a cross between F1 individuals. 26.Sex linkage: the pattern of inheritance that is characteristic for genes located on the X chromosome. PAGE 2| 27.Pure line: It is a strain of true -+breeding individuals which have homozygous traits due to continued self - breeding over the generations. Or A result of inbreeding where animals or plants have certain characteristics that are the same through generations. 28.Gene pool: The aggregate of all the genes and their alleles present in a interbreeding population is known as gene pool. 29.Backcross: It is a cross between hybrid and one of its parents in order to increase the number of traits of that parent. Or Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, in order to achieve offspring with a genetic identity which is closer to that of the parent. It is used in horticulture, animal breeding and in production of gene knockout organisms. 30.Homogametic: Individuals having homomorphic sex chromosomes produce similar gametes. E.g. human female or Denoting the sex which has sex chromosomes that do not differ in morphology, resulting in only one kind of gamete, e.g. (in mammals) the female and (in birds) the male. 31.Heterogametic: Individuals with heteromorphic sex chromosome produce two types of gametes. E.g. Human male. Or Heterogametic sex refers to the sex of a species in which the sex chromosomes are not the same. For example, in humans, males, with an X and a Y sex chromosome, would be referred to as the heterogametic sex, and females having two X sex chromosomes would be referred to as the homogametic sex. 32.Satellite / Trabant: A satellite chromosome or SAT chromosome has a chromosome segment that is separated from the main body of the chromosome by a secondary constriction. 33.Genome size: Genome size is the total number of nucleotide base pairs within one copy of a single genome. Measurements are often made in numbers of base pairs. 34.DNA Profiling: Matching the DNA from a sample to a known individual is called DNA profiling. PAGE 3| 35.Amitosis: Cell divides without spindle formation. 36.Disjunction: It is the separation of homologous chromosomes during cell division. 37.Non-disjunction: It is the non-separation of homologous chromosomes during anaphase I of meiosis I. or Non-disjunction is a failure of homologous pairs of chromosomes to separate properly during meiosis. It results in gametes that contain either one too few or one too many chromosomes. Those with too few seldom survive, but in some cases a gamete with an extra chromosome does survive and after fertilization produces a zygote with three chromosomes of one type, as shown in the following figure. This is called a trisomy. Trisomy in chromosome 21 results in the human condition known as Down syndrome. A gamete, usually the female one, receives 24 chromosomes instead of 23 and a baby with 47 instead of the usual 46 chromosomes in each cell is born PAGE 4| 38.Congression: Chromosomes fibres contract and bring the chromosome over the equator. Or The movement of chromosomes to the spindle equator during mitosis is called as congression. 39.Mitogens: The agents which stimulate the cell division are called mitogens. E.g. Cytokinins and some steroids. 40.Mitotic poisons: There are some chemicals which inhibit cell division e.g. azides, cyanides, colchicine. 41.Intranuclear mitosis and pre-mitosis: In protists, fungi and algae the nuclear envelop does not degenerate during mitosis. Instead spindle is formed inside the nucleus. 42.Centromeric Index: It is the ratio of lengths of the two arms of chromosome. Centromere index (Cl) is traditionally defined as the length of the short arm divided by the total length of the chromo some x 100. 43.Clone: a group of genetically identical organisms or a group of cells derived from a single parent cell 44.Autosomal linkage: When two or more gene loci are on the same chromosome, they do not assort independently in meiosis as they would if they were on different chromosomes. The genes are said to be linked. Linkage is the presence of two genes on the same chromosome, so that they tend to be inherited together and do not assort independently. 45.Mutation: A change in the structure of a DNA molecule, producing a different allele of a gene, is a gene mutation. Mutations may also cause changes in the structure or number of whole chromosomes in a cell, in which case they are known as chromosome mutations (or chromosome aberrations). Base substitutions, on the other hand, often have no effect at all. A mutation that has no apparent effect on an organism is said to be a silent mutation. Interphase chromatin During interphase (the period of the cell cycle where the cell is not dividing), two types of chromatin can be distinguished: Euchromatin, which consists of DNA that is active, e.g., being expressed as protein. Heterochromatin, which consists of mostly inactive DNA. It seems to serve structural purposes during the chromosomal stages. Heterochromatin can be further distinguished into two types: PAGE 5| o o Constitutive heterochromatin, which is never expressed. It is located around the centromere and usually contains repetitive sequences. Facultative heterochromatin, which is sometimes expressed. Types of Chromosomes based on the position of the Centromere Depending on the location of centromere, chromosomes are classified into four types (i) Telocentric Centromere at one tip. (ii) Acrocentric Centromere is just below the tip. (iii) Sub-metacentric Centromere is in between the center and tip of the chromosome. (iv) Metacentric Centromere is in the middle. Each chromosome consists of 2 units or arms, called chromatids attached at a point called primary constriction or centromere. Pedigree chart for genetic diseases (Note how carriers are represented) PAGE 6| VProblems On GeneticsW 1. Suppose that fur colour in mice is determined by a single gene. Brown fur is dominant to white. A mouse homozygous for brown fur was crossed with a white mouse. Determine the possible genotypes and phenotypes of the offspring. 2. Seed shape in the pea plant is controlled by a single gene. Smooth shape is dominant to wrinkled shape. A plant that was heterozygous for smooth seeds was crossed with a plant that had wrinkled seeds. Determine the possible genotypes of the offspring and the phenotype ratio. 3. Celia is blood group A and her husband Sanjeev is blood group B. Their daughter Sally is blood group O. Determine the genotypes of Celia and Sanjeev. (based on co-dominance) 4. Hair shape in humans is a codominant characteristic. Straight hair and curly hair are codominant alleles and the heterozygote has wavy hair. Daryll and Shaniqua both have wavy hair. Deduce the probabilities that their children will have straight hair, curly hair or wavy hair. (Straight + curly = wavy) 5. A woman who is homozygous for normal vision married a man who is red green colour blind. Determine the possible types of vision inherited by their two children, one girl and one boy. 6. Cystic fibrosis (CF) is a genetic disorder that causes the excessive production of thick sticky mucus. It is due to a recessive allele that is not sex linked. The pedigree chart shows two generations of a family. A filled-in symbol represents an individual who has cystic fibrosis. Deduce the genotypes of the parents I1 and I2. Deduce the probability that II1 is heterozygous. 7. The pedigree chart shows the family history of a recessive human condition called woolly hair. A filled-in symbol indicates that the person has woolly hair. Deduce whether this condition is sex linked or not. PAGE 7| 8. The pedigree chart below shows the inheritance of a particular genetic condition in a family. A filled in circle or square means that the individual is affected that is, shows the genetic condition. The condition is not sex linked. Deduce whether the characteristic is dominant or recessive. 9. Ludovica is blood group AB and is expecting a baby with her husband Mikhail who is blood group A. Mikhail s mother was group O. Deduce the possible genotypes and phenotypes of their baby using a Punnett grid. 10.Red Poll cattle are homozygous for an allele that gives red coat colour. White Shorthorn are homozygous for an allele that gives white coat colour. When crossed, the offspring all have a mixture of red and white hairs in their coats, producing a colour called roan. a. Suggest suitable symbols for the two alleles of the coat colour gene. b. List the three possible genotypes for the coat colour gene and their phenotypes. c. Draw genetic diagrams to show the offspring expected from the following matings: i. a Red Poll with a roan ii. two roans. 11.In mice, the gene for eye colour has two alleles. The allele for black eyes is dominant, whereas the allele for red eyes is recessive. Choose suitable symbols for these alleles, and then draw a genetic diagram to show the probable results of a cross between a heterozygous black-eyed mouse and a red-eyed mouse. PAGE 8| 12.A species of poppy may have plain petals or petals with a large black spot near the base. If two plants with spotted petals are crossed, the offspring always have spotted petals. A cross between unspotted and spotted plants sometimes produces offspring that all have unspotted petals, and sometimes produces half spotted and half unspotted offspring. Explain these results. 13.In Dalmatian dogs, the colour of the spots is determined by a gene that has two alleles. The allele for black spots is dominant, and the allele for brown spots is recessive. A breeder wanted to know the genotype of a blackspotted female dog. She crossed her with a brownspotted male dog, and a litter of three puppies was produced, all of which were black. The breeder concluded that the female was homozygous for the allele for black spots. Was she right? Explain your answer. 14.A man of blood group B and a woman of blood group A have three children. One is group A, one group B and one group O. What are the genotypes of these five people? 15.Draw a genetic diagram to explain why there is always an equal chance that a child will be male or female. (You can do this in just the same way as the other genetic diagrams you have drawn, but using symbols to represent whole chromosomes, not genes.) 16.Can a man with haemophilia pass on the disease to: a. his son? b. his grandson? 17.One of the genes for colour vision in humans is found on the X chromosome but not on the Y chromosome. The dominant allele of this gene gives normal colour vision, whereas a recessive allele produces red green colour blindness. a. Choose suitable symbols for these alleles, and then write down all of the possible genotypes for a man and for a woman. b. A couple who both have normal colour vision have a child with colour blindness. Explain how this may happen, and state what the sex of the colour-blind child must be. c. Is it possible for a colour-blind girl to be born? Explain your answer. 18.One of the genes for coat colour in cats is sex linked. The allele CO gives orange fur, whereas CB gives black fur. The two alleles are codominant, and when both are present the cat has patches of orange and black, which is known as tortoiseshell. PAGE 9| a. Explain why male cats cannot be tortoiseshell. b. Draw a genetic diagram to show the expected genotypes and phenotypes of the offspring from a cross between an orange male and a tortoiseshell female cat. (Remember to show the X and Y chromosomes, as well as the symbols for the alleles.) 19.The allele for grey fur in a species of animal is dominant to white, and the allele for long tail is dominant to short. a. Using the symbols G and g for coat colour, and T and t for tail length, draw a genetic diagram to show the genotypes and phenotypes of the offspring you would expect from a cross between a pure-breeding grey animal with a long tail and a purebreeding white animal with a short tail. b. If this first generation of offspring were bred together, what would be the expected phenotypes in the second generation of offspring, and in what ratios would they occur? 20.In a species of plant, the allele for tall stem is dominant to short. The two alleles for leaf colour, giving green or white in the homozygous condition, are codominant, producing variegated leaves in the heterozygote. A plant with tall stems and green leaves was crossed with a plant with short stems and variegated leaves. The offspring from this cross consisted of plants with tall stems and green leaves and plants with tall stems and variegated leaves in the ratio of 1:1. Construct a genetic diagram to explain this cross. 21.In a species of animal, it is known that the allele for black eyes is dominant to the allele for red eyes, and that the allele for long fur is dominant to the allele for short fur. a. What are the possible genotypes for an animal with black eyes and long fur? b. How could you find out which genotype this animal had? PAGE 10|

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