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

Gujarat BoardEnglish MediumSTD 12 ScienceBIOLOGYPrinciples of Inheritance and Variation5 Marks
Question
List the three different allelic forms of gene “I” in humans. Explain the different phenotypic expressions, controlled by these three forms.
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Answer

The three different allelic forms are: $I^A, I^B, I^O/i.$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 $I^A, I^B$ 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.
  • $I^A$ and $I^B$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 $I^A$ and $I^B$ 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 from Parent 1
Allele from Parent 2
Genotype of offspring
Blood groups of offspring
$I^A$
 $I^A$
$I^AI^A$
A
$I^A$
 $I^B$
$I^A​​​​​​​I^B$
AB
$I^A$​​​​​​​
i
$I^A​​​​​​​i$
A
$I^B$ $I^B$
$I^BI^B$
B
$I^B$
i
$I^Bi$
B
i
i
ii
O

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