Question

Gregor Mendel described allelic relationships as either dominant or recessive, and assumed that alleles and genes pairs are segregating independently during meiosis. What are the exceptions to those two laws of Mendelian genetics

Answer 1

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 (IAIAIAIA) blood group individual mates with B (IBIBIBIB) blood group individual the offsprings have blood group AB (IAIBIAIB) 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.