Explain the inheritance of ABO blood groups: phenotypes are A, B, AB and O blood groups and alleles are IA, IB and Io
Explain the inheritance of ABO blood groups: phenotypes are A, B, AB and O blood groups and alleles are IA, IB and Io
Answer
The inheritance of ABO blood groups is a classic example of multiple alleles and codominance in genetics. The ABO blood group system is determined by the presence of specific antigens on the surface of red blood cells, which are controlled by three alleles: IA, IB, and i.
1. Alleles and Their Functions
- IA: This allele codes for the A antigen on red blood cells.
- IB: This allele codes for the B antigen on red blood cells.
- i: This allele does not produce any antigen (recessive).
2. Phenotypes and Genotypes
The combination of these alleles results in four possible phenotypes (blood types) and their corresponding genotypes:
| Phenotype | Genotype | Description |
|---|---|---|
| A | IAIA or IAi | A antigens present; can be homozygous or heterozygous. |
| B | IBIB or IBi | B antigens present; can be homozygous or heterozygous. |
| AB | IAIB | Both A and B antigens present; codominance. |
| O | ii | No A or B antigens present; recessive. |
3. Codominance in ABO Blood Groups
In individuals with the genotype IAIB, both A and B antigens are expressed simultaneously on the surface of red blood cells. This is a clear example of codominance, where neither allele masks the other, resulting in the AB blood type.
4. Inheritance Patterns
When considering inheritance, each parent contributes one allele to the offspring. The possible combinations of alleles can be illustrated using a Punnett square. For example, if a parent with blood type A (IAi) has offspring with a parent with blood type B (IBi), the potential genotypes of their children would be:
Punnett Square Example
Results:
- Offspring Genotypes:
- IAIB (AB)
- IAi (A)
- IBi (B)
- ii (O)
5. Phenotypic Ratios
From this cross, the phenotypic ratio can be summarized as follows:
- 1 AB : 1 A : 1 B : 1 O
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