2021-04-23 12:33:29
𝗣𝗥𝗜𝗡𝗖𝗜𝗣𝗟𝗘 𝗢𝗙 𝗜𝗡𝗛𝗘𝗥𝗜𝗧𝗔𝗡𝗖𝗘
Heredity is a process of transmission of heritable traits from parents to their offsprings.
Genetics is the branch of biology dealing with the principles and mechanism of inheritance and variation.
Inheritance is the basis of heredity and by this process, traits are passed on from the parents to the offsprings. Continuity of the gene pool is maintained by the process of inheritance.
Genes are the basic unit of inheritance and located on chromosomes.
Variation exists among individuals of one species. Variation is due to crossing over, recombination, mutation and environmental effects on the expression of genes present on chromosomes.
𒊹Mendel’s Laws of Inheritance
Gregor Johann Mendel is called “Father of genetics”.
Mendel performed experiments on Garden pea. He took 14 true-breeding plants of pea having seven distinguishable characters, which have two opposite traits.
He called genes as “factors”, which are passed from parents to offsprings.
Genes, that code for a pair of opposite traits are called “alleles”.
He gave three laws of inheritance based on his observation:
𒊹Law of Dominance
: One of the alleles is dominant and gets expressed in the phenotype in case of the heterozygote, e.g. When we cross homozygous tall (TT) and dwarf (tt) plants, in the offsprings we get all the tall plants having the genotype Tt, so tallness is a dominant trait over dwarfness.
➪Law of Segregation of genes:
Each allele separates during meiosis at the time of gamete formation. There is no blending and characters are passed to different gametes. Homozygotes produce only one kind of gametes and heterozygotes produce different kinds of gametes.
♲Law of Independent assortment:
It states that alleles for different traits are inherited independently. He showed that using a dihybrid cross.
Test Cross: It is to find out the genotype of the plant showing dominant trait, the given plant is crossed with the recessive homozygote. The two observations are:
If the phenotype of offsprings shows only the dominant trait, then the parent plant was homozygote to the dominant trait
If the offsprings produced are of both phenotypes, then the parent plant was heterozygote to the dominant trait
𒊹Incomplete Dominance
When neither of the two alleles is dominant and the phenotype of the heterozygote does not resemble any of the parents. The heterozygote expresses intermediate or a mixture of two parents’ traits
Example: The flower colour inheritance of snapdragon (dog flower). On crossing true breeding red (RR) and whiteflower (rr), we get all pink colour flowers in the F1 generation, which on self-pollination give red: pink: white flowers in the ratio 1:2:1 in the F2 generation
☞Co-dominance
When both the alleles express themselves together in an individual, they are said to be co-dominant
Example: The inheritance of the ABO blood group in humans is controlled by the gene I. The gene I has three allelic forms, IA, IB and i. In a human being, any two out of three alleles are presentIA and IB code for different kinds of sugar polymers present on the surface of RBC and ‘i’ does not produce any sugarIA and IB are dominant over ‘i’, but IA and IB are co-dominant and express themselves together.
➪Chromosomal Theory of Inheritance
Sutton and Boveri supported Mendel’s observations and stated that chromosomes are the carrier of genes
Chromosomes occur as a homologous pair and the two alleles of a gene are located on the homologous pair of chromosomes at the same site
Homologous chromosomes separate during meiosis in the process of gamete formation
Chromosomes segregate and assort independently
During fertilization, gametes combine and produce the offsprings with the diploid no. of chromosomes, that is similar to the parent
Morgan extensively worked on fruit flies, Drosophila melanogaster and provided experimental evidence to support the chromosomal theory of inheritance
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