Question

How can different genotypes result in the same phenotype? Give two examples using genotypes. Type all genotypes in the proper notation, e.g., X^w+, by using the Microsoft superscript function

Answer 1

A phenotype is characterised by a gene which has 2 alleles present on the 2 homologous chromosomes. Generally, alleles can be of 2 types: dominant and recessive. An individual inherits one allele from each parent. Thus, depending on the genotype of the parents, an individual can have 2 dominant alleles, 2 recessive alleles or 1 dominant and 1 recessive allele for any particular gene. The dominant allele usually determines the phenotype because it expresses itself when present, both in homozygous and heterozygous condition. The recessive allele is expressed only when present in homozygous condition.

If an individual is homozygous for the dominant allele i.e has 2 dominant alleles, then the phenotype will be that of the dominant allele.

If the individual is heterozygous, i.e has 1 dominant allele and 1 recessive allele, then the phenotype will be that of the dominant allele.

If an individual is homozygous for the recessive allele i.e has 2 recessive alleles, only then the phenotype will be that of the recessive allele.

For example, let us assume that hair colour (phenotype) is determined by a single gene known as H. The alleles representing this gene are H and h. Here H allele causing black colour is dominant over h allele causing white hair.

Individuals with the genotype HH will have black hair phenotype.

Individuals with the genotype Hh will have black hair phenotype.

Individuals with the genotype hh will have white hair phenotype.

Thus, both HH (homozygous dominant) and Hh (heterozygous) will have the same phenotype (black hair).

Example 1

In Drosophila melanogaster, red eye is dominant to white eye.

Red eye allele is represented by w⁺ and white eye allele is represented by w.

w⁺ is dominant over w.

hence,

individuals with w⁺w⁺ have red eyes

individuals with w⁺w have red eyes

individuals with ww have white eyes

Thus 2 genotypes (w⁺w⁺ and w⁺w) have the same phenotype (red eyes)

Example 2

In humans, there are 3 alleles involved in determining the blood type of a person. These are I^A, I^B and i. I^A and I^B are dominant over i. combination of any of the 2 alleles will determine the blood group (phenotype) of the individual.

S. No.	Genotype	Phenotype (blood group)
1	IA IA	A
2	IA i	A
3	IA IB	AB
4	IB, IB	B
5	IB i	B
6	i i	O

Here, individuals with both I^A I^A and I^A I genotype have the A blood group i.e. same phenotype.

individuals with both I^B I^B and I^B I genotype have the B blood group i.e. same phenotype.