29. If a colour-blind woman marries a normal man, the chance that their boy child will be colour-blind is %.

Color Blindness Inheritance: Probability of a Colour-Blind Boy Child

Introduction

Colour blindness is one of the most common examples of an X-linked recessive genetic disorder in humans. Because the gene responsible for red-green colour blindness is located on the X chromosome, its inheritance pattern differs significantly from that of autosomal traits. Males possess one X chromosome and one Y chromosome (XY), whereas females possess two X chromosomes (XX). Consequently, males express the disorder whenever they inherit the mutant allele, while females must inherit two mutant alleles to become colour blind.

Understanding X-linked inheritance is essential for predicting the transmission of several human genetic disorders, including colour blindness, haemophilia, and Duchenne muscular dystrophy. These inheritance patterns are frequently tested in competitive examinations because they require students to combine knowledge of chromosome inheritance with Mendelian genetics.

Correct Answer

Correct Answer: 100%

Detailed Explanation

Colour blindness is inherited as an X-linked recessive trait. Let the normal allele be represented by X^C and the colour-blind allele by X^c.

A colour-blind woman must possess two recessive alleles because females require two mutant copies to express this disorder.

Therefore, her genotype is:

X^cX^c

A normal man possesses one normal X chromosome and one Y chromosome.

Therefore, his genotype is:

X^CY

During reproduction, the mother can contribute only X^c gametes because both of her X chromosomes carry the mutant allele. The father contributes either X^C or Y with equal probability.

Punnett Square Analysis

Interpretation of the Offspring

Every son receives the Y chromosome from the father and the X^c chromosome from the mother. Since males possess only one X chromosome, they express the colour-blind phenotype immediately.

Therefore, every boy child born to this couple will be colour blind.

Step-by-Step Calculation

Step 1: Identify the Parents

Mother = X^cX^c

Father = X^CY

Step 2: Determine Possible Sons

All sons inherit:

• Y chromosome from father
• X^c chromosome from mother

Thus, every son has the genotype:

X^cY

Step 3: Determine the Probability

Probability that a boy child is colour blind = 100%

Calculation Summary

Understanding X-Linked Recessive Inheritance

In X-linked recessive disorders, males are affected much more frequently than females because males possess only one X chromosome. A single recessive allele on the X chromosome is sufficient to produce the disorder in males. Females, however, require two recessive alleles to express the condition, making affected females relatively rare.

When the mother is colour blind, every son necessarily inherits her mutant X chromosome. Since the father contributes only a Y chromosome to sons, there is no normal allele available to mask the recessive mutation. Consequently, every male offspring becomes colour blind.

Characteristics of Colour Blindness

Biological Significance

X-linked inheritance illustrates how chromosome composition influences the transmission of genetic disorders. Because males inherit their single X chromosome exclusively from their mothers, maternal genotype plays a decisive role in determining the occurrence of X-linked diseases in sons. Understanding these inheritance patterns is essential in medical genetics, genetic counseling, prenatal diagnosis, and the study of hereditary disorders.

Final Answer

Mother’s genotype = X^cX^c

Father’s genotype = X^CY

Every son receives X^c from the mother and Y from the father.

Therefore, every boy child will be colour blind.

Correct Answer: 100%