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Amogh Shrivastava: Genetics Class X ICSE Notes

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Amogh Shrivastava
South City International School (SCIS), Kolkata

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GENETICS SCOPE OF SYLLABUS The three laws of Mendel Monohybrid cross Phenotype and Genotype Dihybrid cross only phenotype Terms to be covered: gene, allele, heterozygous, homozygous, dominant, recessive, mutation, variation, phenotype and genotype Sex determination in human beings Sex-linked inheritance of diseases to include only X-linked haemophilia and colour blindness INTRODUCTION Genetics is the study of heredity i.e. transmission of body features from parents to offspring and the law relating to such transmission. Heredity is the transmission of genetically based characteristics from parents to offspring or the genetic constitution of an individual. Heredity not only gives rise to variations or small differences among the individuals of the same species but also ensures that like begets like. Any inherited feature of an organism is a character and the different alternative forms of a character are called traits. Characters Traits Colour of Eyes Brown or blue Hair Shape Straight or curly Eyebrows Heavy, bushy, or thin Colour vision Normal or red-green colour blindness Hair on middle of finger joints Growth or no-growth Tongue rolling Rolling tongue or no-rolling Hand Use Right-handedness or left-handedness Skin Colour Albinism or normal Ear Lobe Free or attached Lips Thick or thin Rh Blood Group Rh positive or Rh negative CHROMOSOMES IN GENETICS It is through chromosomes that the different traits of various characters are inherited by an offspring from its parents a haploid number of chromosomes from each parent is inherited by the offspring. The number of chromosomes is constant for individuals of a species. They always occur in pairs. Homologous chromosomes are a pair of corresponding chromosomes of the same shape and size, one obtained from each parent. Humans have 46 chromosomes in total, which implies that they have 23 pairs of chromosomes. Out of these pairs 1-22 are completely identical or homologous chromosomes. They are called autosomes or somatic chromosomes. However, the 23rd pair is different as it may contain sex chromosomes as X and Y which determine the sex of the offspring. The XX pair causes the offspring to be female and the XY pair causes the offspring to be male. Thus, in females, the sex chromosomes are homologous (causing all chromosome pairs to be homologous pairs) but in males the sex chromosomes are heterologous (causing autosomes to be homologous but the 23rd pair to be heterologous). Autosomes (pairs 1-22) are homologous chromosomes which carry genes that determine general body features like complexion, height, seed colour etc. Sex chromosomes (23rd pair), also called allosomes, are the kind of chromosomes which determine the sex of the offspring. Image showing the karyotype of human chromosomes. Karyotype is a chart in which chromosomes are arranged according to their size and shape in pairs. SEX DETERMINATION The offspring gets one of each pair of chromosomes from each parent. Thus, the sperm and egg both contain a haploid number of chromosomes. Sex of the offspring depends upon the allosomes or sex chromosomes. The egg or female gamete always contains X chromosomes. However, half the sperms released into the genital tract of a female are X-bearing and the other half are Y-bearing. If an X-bearing sperm fuses with an egg, the offspring will be female If a Y-bearing sperm fuses with an egg, the offspring will be male GENES AND ALLELES Genes are DNA segments of a chromosome which determine the hereditary characteristics. A genome is the full complement of a DNA including all genes and the intergenic regions of an organism. Alleles or allelomorphs are alternative forms of a gene, occupying the same locus in homologous chromosomes and affecting the same characteristics but in different ways. Alleles are of two types: Dominant allele is the kind of allele that expresses itself regardless of the presence of another allele for a given gene by supressing its contrasting gene. Recessive allele is the kind of allele whose expression is suppressed in the presence of a dominant allele for a given gene. It can only express itself in the presence of the same recessive allele. Chromosomes as a whole are neither dominant nor recessive. GENOTYPE AND PHENOTYPE Genotype is the set of genes present in the cells of an organism i.e. the full genetic makeup of the organism. Phenotype is the externally observable characteristics which are genetically controlled. Types of genotypes: Homozygous dominant diploid individual carrying two identical dominant alleles. (RR) Heterozygous dominant diploid individual carrying two different alleles. (Rr) Homozygous recessive diploid individual carrying two identical recessive alleles. (rr) Heterozygous recessive not possible as in case of a heterozygous pair, the dominant allele will express itself. MENDEL S EXPERIMENTS John Mendel, the Father of Genetics, had conducted breeding experiments on garden pea to understand genetics. Reasons for choosing Pisum sativum: Many varieties were available in alternate forms of a character. Varieties were available in pure forms that bred true i.e. produced the same type generation after generation. They self-pollinated but could also be cross-pollinated artificially. The life-span is short which allowed Mendel to study many generations in less amount of time TERMS RELATED TO CROSSES A monohybrid cross is when one feature is studied whereas dihybrid cross is when two features are studied. A hybrid is the product of a cross between two genetically different individuals of the same genus of species between two different genera. Hybridization is a phenomenon of mixing two unlike genetic constitutions. The organisms used as parents are called the parental generation (represented by P). A filial generation is the progeny obtained as a result of cross (represented by letters F1, F2 etc.). MONOHYBRID CROSS The following diagram shows a self-explanatory diagram of a monohybrid cross: DIHYBRID CROSS The following diagram shows a self-explanatory diagram of a monohybrid cross: MENDEL S LAWS OF INHERITANCE Mendel s Laws of Inheritance are: Law of Dominance: Out of a pair of contrasting characters present together, only one is able to express itself while the other remains suppressed. Law of Segregation/Purity of Gametes: The two members of a pair of factors separate during the formation of gametes. Law of Independent Assortment: When there are two pairs of characters, the distribution of the alleles of one character into the gametes is independent of the distribution of the alleles of the other character. SEX LINKED INHERITANCE Sex linked inheritance is the appearance of a trait which is due to the presence of an allele exclusively either on the X chromosome or on the Y chromosome. Certain disorders like haemophilia and colour blindness are related with the X chromosome. Hence, they are called X linked diseases. They are more common in males as the recessive allele expresses itself in males due to lack of a dominant X chromosome. In females, the recessive allele only expresses itself in case of a homozygous recessive genotype. Certain females, are called carriers of the disease as their genotype contains heterozygous dominant alleles causing them to carry the disease which does not express itself. It is also called criss-cross inheritance because if the mother is a carrier, the son may get the disease from the mother and the daughter may get the disease from the father if the father is also colour blind. MUTATION Mutation is a sudden change in one or more genes, or in the number or structure of chromosomes causing a change in the hereditary material of an organism resulting in change in certain characters or traits. Sickle cell anaemia is caused due to gene mutation. Change in DNA causes erythrocytes to become sickle shaped. Mutation can be caused by radioactive radiations too. _____________________________________________

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