Principles of Inheritance and Variation — BIOLOGY STD 12 Science — Question

• The alleles which are able to express themselves independently, even when present together are called co–dominant alleles and this biological phenomenon is called co–dominance. For example, ABO blood grouping in humans.
• ABO blood groups are controlled by gene I. Gene I has three alleles I^A, I^B and I^O/i.
• I^A and I^B produce RBC surface antigens sugar polymer A and B, respectively, whereas i does not produce any antigen.
• IA and IB are dominant over i hence I^A and I^B are dominant alleles and i is recessive allele as in I^Ai and I^Bi. • When IA and IB are present together, both express equally and produce the surface antigens A and B, hence show co–dominance.
• Since humans are diploid, each person possesses any two of the three ‘I’ gene alleles, resulting into six different genotypic combinations and four phenotypic expressions.

Table showing the genetic basis of blood groups in human population.

Polygenic Inheritance:

• It is a type of inheritance, in which traits are controlled by three or more genes. Such traits are called polygenic traits.
• The phenotype reflects contribution of each allele and is also influenced by the environment.
• For example, human skin colour. Suppose three genes A, B and C control skin colour with A, B, C being the dominant alleles and a, b, c being the recessive alleles. Then,

• The F₂ generation will have varied skin tones, with each type of allele in the genotype determining the darkness or lightness of the skin.