1. Explain how each of the following phenomena can disrupt Mendelian phenotypic ratios.

a. lethal alleles

Answer – Lethal alleles can cause death of the organism which has them. They can be recessive or dominant. When two heterozygotes are crossed, in Mendelian genetics, one would get the ratio 3:1 (dominant: recessive). However, when lethal alleles/genes are involved, on crossing two heterozygotes, 2:1 ratio for dominant: recessive is obtained.
e.g. Yellow coat color in mice. Yellow allele AY is dominant over agouti allele A. On crossing two heterozygous yellow mice (AYA), we obtain a phenotypic ratio of 2: 1 with genotype AYA: AA. The mice with AYAY dies as embryo because the AY allele is lethal as homozygous.

b. multiple alleles

Answer – Multiple alleles involve more than two alleles to determine the phenotype, unlike traits studied by Mendel, which had a dominant and recessive allele to give a 3:1 ratio (dominant: recessive) when heterozygotes were crossed. Human ABO blood group (3 alleles), coat colour in rabbits are example of multiple alleles. The types of genotype in case of multiple alleles depends on the number of alleles involved and is calculated by the formula n(n+1)/2, where n is number of alleles.
e.g. In ABO blood group, three alleles IA, IB and i are involved resulting in different blood groups. Of these IA and IB are codominant and i is recessive. These alleles result in the following genotype and phenotype: ii gives blood type O; IAIA and IAi give A blood type; IBIB and IBi give B blood type and IAIB gives AB blood type.

c. incomplete dominance

Answer – In this case, the dominant allele does not have complete effect in heterozygous condition. A phenotype which is intermediate between the homozygous dominant and recessive allele is found in heterozygotes. Thus, the Mendelian ratio (3:1) obtained on crossing two heterozygotes alters in case of incomplete dominance and we get a 1: 2: 1 ratio for homozygous dominant: heterozygotes: homozygous recessive.
e.g. In snapdragon plant (Antirrhinum majus), when homozygous white flower (CWCW) is crossed with a homozygous red flower (CRCR), the offspring have pink flower (CWCR), showing incomplete dominance. When these pink flowers are self-crossed, we get 1 red flower (CRCR): 2 pink flowers (CWCR): 1 white flower (CWCW).

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