Question

In an organism, pink spot is a sex- linked recessive trait and black hair is dominant to white. If a pink spotted black heterozygous female is mated to a white male which is not spotted, then phenotypic ratio of the offspring would be:
A. ¼ spotted black; ¼ pink spotted white; ¼ unspotted black; ¼ unspotted white
B. ½ pink spotted black; ½ pink spotted white
C. ¾ pink spotted black; ¼ pink spotted white
D. ¾ unspotted black; ¼ pink spotted white

Answer 1

Hint: Genetic linkage is described as the way in which two genes that are located close to each other on a chromosome are often inherited together while recombination is a process by which pieces of DNA are broken down and recombined to produce new combinations of alleles. This process creates greater genetic diversity at the level of genes.

Complete answer: Phenotype: It is referred to as the observable physical characteristics of the organisms.
This includes:
1. Organism appearance
2. Development
3. Behavior
The phenotype of an organism is determined by its genotypic ratio. The genotype is the set of genes the organism carries. The pink spot is a sex-linked recessive trait, which means that the trait is expressed only if it is present in homozygous condition.
Let, allele B denote Black color, and R denote Pink spot.
-Pink spotted black hair heterozygous female is Bbrr.
-Not spotted white hair male is bbRr.
Parents are Bbrr and bbRr.
Gametes are Br, Br, and bR, br.

	bR	Br
Br	BbRr	BBrr
Br	BbRr	BBrr

This implies that the phenotypic ratios of the offspring produced are: ¼ Black Unspotted; ¼ Black-spotted pink; ¼ White unspotted; ¼ White pink spotted.

So, the correct option is option A. ¼ Black Unspotted; ¼ Black-spotted pink; ¼ White unspotted; ¼ White pink spotted

Note: Morgan and his group knew that the genes were located on the X chromosome and saw quickly that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combination was much higher than the non-parental type.