Mendelian Inheritance And Neo-Mendelian Inheritance Prof. Madane B.B.

Mendel’s Postulates or Principles Principle of unit character Principle of dominance and recessiveness Principle of independent assortment Principle of segregation

Mendel’s Laws Of Inheritance 1st – Law of Dominance. 2nd – Law of Segregation. 3rd – Law of Independent Assortment.

Experimental Design Step 1 - Selection of parents and obtaining pure lines. Step 2 - Artificial cross to raise F1 generation. Step 3 - Selfing of F1 to raise F2 generation.

Graphic Representation of Monohybrid Cross Uniform expression in F1 generation lead to Law of Dominance. Reappearance of recessive trait in F2 generation lead to Law of Segregation. Punnet square shows that only 1 out of 4 receives recessive trait from both the parents and expresses the recessive trait. The ratio of dominants and recessives is 3:1.

Graphic Representation of Dihybrid Cross Dihybrid forms four types of gametes due to segregation and independent assortment. Appearance of new combinations in F2 proves the law. Probability of two independent events occurring simultaneously is product of their individual probabilities. Dihybrid ratio is product of two monohybrid ratios. (3:1)x (3:1) = 9:3:3:1

Trihybrid Cross and Trihybrid Ratio Presuming that there is an Independent Assortment, trihybrid (YyRrCc) will form eight different types of gametes : YRC,YRc,yRC,YrC,Yrc,yRc,yrC,yrc F2 generation will show eight different types of phenotypes in 27:9:9:9:3:3:3:1 ratio.

Polyhybrid No. of traits No. of types of gametes No. of phenotypes No. of genotypes 1 2 3 4 9 8 27 16 81 N 2N 3N

Deviations From Mendelian Ratios Many gene interactions were discovered on the basis of different ratios. 1:2:1 - Incomplete or co-dominance. 2:1 - Pleiotropic genes. 9:7 - Complementary genes. 9:3:4 - Supplementary genes. 1:4:6:4:1 - Polygenes 15:1 - Duplicate genes.

Neo Mendelism – Gene Interactions Gene interactions are of two types. Intrageneic or interallelic- these occur between alleles of same gene. e.g. Incomplete dominance, co-dominance and multiple alleles. Intergenic – these occur between the alleles of different genes on the same or different chromosomes. e.g. Pleiotropy and polygenes.

Graphic Representation of Incomplete Dominance. In incomplete dominance one gene cannot suppress the expression of the other completely. There is an intermediate expression in F1 generation. Both the parental traits reappear in F2. Therefore this does not favour the blending theory of inheritance. Phenotypic and genotypic ratio is same.

Graphic Representation of Co-dominance. In co-dominance, both the traits express themselves equally in F1 generation. Both the parental traits reappear in F2 generation. In co- dominance also phenotypic and genotypic ratio is same.

Multiple Alleles More than two alternative forms (alleles) of a gene in a population, are known as multiple alleles. (They occupy the same locus on homologous chromosomes.) They arise by mutations of the wild type and a series of alternative expressions is observed. They show dominant recessive relation or may show incomplete/co-dominance among themselves. Wild type is dominant over all other mutant alleles.

Multiple Alleles in Drosophila Phenotype Genotype Normal wings Vg+ Nicked wings vgni Notched wings vgno Strap wings vgst Vestigial wings vg

ABO Blood Groups in Human Beings The gene ‘I’ controls the ABO blood groups. There are three alleles: IA, IB and i. IA and IB are co-dominants and are completely dominant over i. There are 6 different genotypes and only 4 phenotypes, i.e. Blood groups as shown in next slide.

Blood Groups in Human Beings. Genotype Phenotype- blood group IA IA or IA i A IB IB or IB i B IA IB AB i i O

Pleiotropy When a single gene controls two or more traits it is called pleiotropic gene and this phenomenon is called pleiotropy. Single gene produces two related or unrelated expressions. The ratio is 2:1 instead of 3:1 due to lethal effect.

Sickle Cell Anaemia In human, the disease sickle cell anaemia is caused by a gene HbS. Normal or healthy gene HbA is dominant. The carriers (heterozygotes- HbA HbS ) show signs of mild anaemia as their RBCs become sickle- shaped (half moon shaped) in oxygen deficiency. RBCs of normal person RBCs of person with sickle cell trait

Graphic Representation of Sickle Cell Anaemia The homozygotes with recessive trait HbS die of fatal anaemia. There are two expressions by the single gene HbS.

Polygenic or Quantitative Inheritance In human population, characters such as height, skin colour and intelligence show gradations ( continuous variations ) in expressions. These characters are determined by two or more gene pairs and they have additive or cumulative effect. These genes are called polygenes or multiple factors and the inheritance pattern is called quantitative inheritance. The ratio is 1:4:6:4:1, if there are two genes having additive effect. The ratio is 1:6:15:20:15:6:1, if there are three genes having additive effect.

Polygenic Inheritance of Wheat Kernel F2 Generation

Polygenic inheritance of Human skin colour

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