laws of inheritance by Mendels theory.pptx

PPT for laws of inheritance

1. Laws of Inheritance by Thamizhvani TR, VISTAS

2. LAWS OF INHERITANCE On the basis of the results of his experiments Mendel recognized the phenomenon of dominance and formulated following two laws : 1. Law of segregation ; 2. Law of independent assortment. Actually Mendel himself did not postulated any genetical principle or laws, he simply gave conclusive theoretical and statistical explanations for his hybridization experiments in his research paper. However, it was Correns, the discoverer of Mendel’s work, who thought that Mendel’s discovery could be represented by these fundamental laws of heredity.

3. LAWS OF INHERITANCE Law of Segregation The law states that the hybrids or heterozygotes of F1 generation have two contrasting characters or allelomorphs of dominant and recessive nature. These alleles though remain together for long time but do not contaminate or mix with each other and separate or segregate at the time of gametogenesis,so that each gamete receives only one allele of a character either dominant or recessive.

4. LAWS OF INHERITANCE Law of Independent Assortment Also known as Mendel’s second law of inheritance, the law of independent assortment states that a pair of traits segregates independently of another pair during gamete formation. As the individual heredity factors assort independently, different traits get equal opportunity to occur together.

5. LAWS OF INHERITANCE The law of inheritance was proposed by Gregor Mendel after conducting experiments on pea plants for seven years. Mendel’s laws of inheritance include law of dominance, law of segregation and law of independent assortment. The law of segregation states that every individual possesses two alleles and only one allele is passed on to the offspring. The law of independent assortment states that the inheritance of one pair of genes is independent of inheritance of another pair.

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7. V ariations in Mendel’s Theme

8. Mendel reported full dominance and recessiveness for all the seven gene pairs (or allele pairs) He may have been selective in his choice of pea characters to study, becausevariations on the basic theme appear quite often in analysis. For instance, in some cases the phenotypes of heterozygotes are found to be different from either of the homozygotes leading to 1. Incomplete Dominance 2. Codominance 3. Multiple Alleles V ariations in Mendel’s Theme

9. Sometimes in a heterozygote dominant allele does not completely mask the phenotypic expression of the recessive allele and there occurs an intermediate phenotype in the heterozygote. This is called incomplete dominance. EXAMPLE When a red flowered pea plant (RR) is crossed with whiteflowered pea plant (rr) then the F1 hybrid pea plants are found to have pink flowers. It shows that gene for red colour could not completely dominate the gene for white colour as shown in the figure In such a case, F2 phenotypic ratio and genotypic ratio are the same, as follows : F2 phenotypic ratio = 1 Red : 2 Pink : 1 White F2 genotypic ratio = 1 RR : 2 Rr : 1 rr INCOMPLETE DOMINANCE

10. INCOMPLETE DOMINANCE

11. EXAMPLE When a homozygous Andalusian fowl with black feathers is crossed with a homozygous fowl with splashed white feathers the F1 hybrids are found to contain blue feathers INCOMPLETE DOMINANCE

12. EXAMPLE: INCOMPLETE DOMINANCE

13. CODOMINANCE Sometimes both alleles of a gene in a heterozygote lack the dominant and recessive relationship, i.e., each allele is capable of some degree of phenotypic expression. In a sense, codominance is no dominance at all, the heterozygote showing the phenotypes of both homozygotes Hence, heterozygote genotype gives rise to a phenotype distinctly different from either of the homozygous genotypes.

14. Symbolism for codominant alleles . For codominant alleles, all upper case base symbols with different superscripts are used . The upper case letters indicate that each allele can express itself to some degree even when in the presence of its alternative allele (heterozygous). EXAMPLE The coat colour of the Shorthorn breed of cattle represent a classical example of codominance. when a cattle of red coat (CRCR) is crossed with the cattle of white coat (CwCw), the F1 heterozygote or hybrid is found to possess roan coat (CRCW) . In roan coat the red and white hairs occur in definite patches but no hair has intermediate colour of red and white. CODOMINANCE

15. EXAMPLE The alleles governing the M-N blood group system in humans are codominants and may be represented by the symbols L M and L N , base letter L being assigned in honour of its discoverers (Landsteiner and Levine). Here, three blood groups are possible–M, N and MN–and these are determined by the genotypes L M L M , L N L N , and L M L N respectively. Blood groups actually represent the presence of an immunological antigen on the surface of red blood cells. People of L M L N genotype have both antigens CODOMINANCE

16. CODOMINANCE

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