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

Bihar BoardEnglish MediumSTD 12 ScienceBIOLOGYPrinciples of Inheritance and Variation5 Marks
Question
Explain co–dominance taking an example of human blood groups in the population.
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Answer

Co–dominance:
  • 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 IA, IB and IO/i.
  • IA and IB 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 IA and IB are dominant alleles and i is recessive allele as in IAiand IBi.
  • 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.
Allele fromParent 1 Allele fromParent 2 Genotype of offspring Blood groups of offspring
IA IA IAIA A
IA IB IAIB AB
IA i IAi A
IB IB IBIB B
IB i IBi B
i i ii O

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