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Question

What were the exceptions for the the law of dominance , segregation and independent assortment respectively?


Solution

Exception of law of dominance:

Incomplete dominance

In incomplete dominance when a red snapdragon flowered plant is crossed with a white flowered plant an intermediate phenotype appears in the F1 hybrid instead of a parental phenotype.

Codominance

Unlike dominance in codominance when a A (IAIA) blood group individual mates with B (IBIB) blood group individual the offsprings have blood group AB (IAIB) instead of A or B.

Exception of law of segregation:

Nondysjunction

During meiosis homologous chromosomes/sister chromatids and hence genes may move to a common gamete violating law of segregation.

Exception of law of independent assortment:

Linkage

When genes are present on the same chromosome they tend to remain together and enter into the same gamete. This is the reason behind deviation of dihybrid test cross ratio from 1:1:1:1 and occurrence of parental combination in high frequencies.



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Exception of law of dominance:

epistasis

Allelic effect at a given locus depend on the variants at other loci

environment dependence and specific case of frequency dependence

The concept of dominance can be used for any quantitative trait. Typically, when the quantitative trait of interest is fitness, then the trait often depend on the environment. A typical and interesting case is when the fitness depends on the frequency of alleles in the population. Depending of the frequency of the allele at a given locus, the relationship might be a relationship of dominance, additivity or dominance for the other allele.

Exception of law of segregation:

meiotic drive

from wiki: "Meiotic drive is a type of intragenomic conflict, whereby one or more loci within a genome will affect a manipulation of the meiotic process in such a way as to favor the transmission of one or more alleles over another, regardless of its phenotypic expression. More simply, meiotic drive is when one copy of a gene is passed on to offspring more than the expected 50% of the time."

Exception of law of Independent assortment:

epistasis for fitness

Imagine a case where a given combination of allele is lethal at very young age, then you would never see these two alleles together in an individual. It is per se, not a case of exception to independent assortment but it gives the feeling of such exception. A case more extreme when a given combination of alleles prevents fecundation could however be considered as an exception to independent assortment.

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