150. Color blindness in human beings is an X-linked trait. A color-blind man has a 45, X daughter who is also
color-blind. The nondisjunction that leads to the 45, X daughter occurred in which parent and in which
meiotic division?
1. Father; First meiotic division
2. Brother, father and mother; first meiotic division
3. Mother; first meiotic division
4. Father; second meiotic division

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Question

Color blindness in human beings is an X-linked trait. A color-blind man has a 45, X daughter who is also color-blind. The nondisjunction that leads to the 45, X daughter occurred in which parent and in which meiotic division?

1. Father; First meiotic division

2. Brother, father and mother; first meiotic division

3. Mother; First meiotic division

4. Father; Second meiotic division

Detailed Explanation

In order to solve this problem, we need to understand the genetic basis of color blindness and the concept of nondisjunction during meiosis.

Color blindness is an X-linked recessive trait, meaning that the gene responsible for color blindness is located on the X chromosome. A color-blind man has one X chromosome with the mutation and one Y chromosome (X⁻Y), and he will pass on his X chromosome to all his daughters (but not his sons, as he will pass on the Y chromosome to them).

Step 1: Understanding the Daughter’s Genotype

• Daughter’s Genotype: 45, X

  • A 45, X karyotype means that the daughter has only one X chromosome and no second sex chromosome (monosomy of the X chromosome).

  • The daughter is color-blind because she has inherited the color-blind X from her father.

Step 2: Nondisjunction in Meiosis

Nondisjunction refers to the failure of chromosomes to properly separate during cell division. In this case, nondisjunction of the X chromosome occurred during meiosis, resulting in the daughter having only one X chromosome. Since this daughter is color-blind, it means she must have inherited the color-blind allele from her father.

Step 3: Determining the Source of the Nondisjunction

• The father is color-blind, so he has the genotype X⁻Y.

• The mother has two normal X chromosomes (assuming she is not color-blind).

• For a 45, X daughter to arise, the nondisjunction must have occurred in the father’s second meiotic division (during spermatogenesis). In the second meiotic division, the X chromosome from the father should have failed to separate, leaving the daughter with only one X chromosome. The Y chromosome from the father will remain the same.

Step 4: Explanation of the Options

1. Father; First meiotic division:

   • This is incorrect, as nondisjunction in the first meiotic division would result in a XX or XY karyotype, not a 45, X karyotype.

2. Brother, father, and mother; first meiotic division:

   • This is incorrect, as it suggests multiple individuals and divisions are involved. Nondisjunction only happened in the father’s second meiotic division.

3. Mother; First meiotic division:

   • This is incorrect because nondisjunction occurred in the father, not the mother.

4. Father; Second meiotic division:

   • This is correct. The nondisjunction happened in the father’s second meiotic division, which resulted in the 45, X daughter.

Answer

The correct answer is:

4. Father; Second meiotic division

Conclusion

This problem highlights how X-linked recessive traits such as color blindness are inherited, especially when nondisjunction occurs during meiosis. In this case, the color-blind father passed on the color-blind allele, and nondisjunction in his second meiotic division led to the daughter having a 45, X karyotype, making her color-blind despite inheriting only one X chromosome. Understanding these meiotic processes is crucial in genetics and inheritance patterns.