Question

Who discovered ‘Y’ chromosomes?
A. McCarthy
B. McClung
C. Gregor Mendel
D. Nettie Stevens

Answer 1

One of two sex chromosomes in therian mammals which includes humans, and many other animals is the Y chromosome. X chromosome is the other one. In many species, Y is generally the sex determining chromosome since the presence or absence of Y determines the male or female sex of offspring that is produced during sexual reproduction.

Complete answer:
Option A) McCarthy: John McCarthy, who was born in the year $1927$ was an American computer scientist and cognitive scientist. One of the founders of the discipline of artificial intelligence was McCarthy. He did not discover Y chromosomes.
Hence option A is not correct.
Option B) McClung: One of the eminent American zoologist and prairie pioneer cytologist who discovered the role of chromosomes in sex-determination was Clarence Erwin McClung. He graduated from pharmacy at the University of Kansas. He did not discover the Y chromosomes.
Hence option B is not correct.
Option C) Gregor Mendel: Gregor Johann Mendel was one of the famous biologists who was born in the year $1822$. He was well known for his contributions in genetics and was the first person to lay the mathematical foundation in the science of genetics, which came to be known as Mentalism. Gregor Mendel did not discover the Y chromosomes.
Hence option C is also not correct.
Option D) Nettie Stevens: An American geneticist who discovered sex chromosomes is Nettie Maria Stevens. Soon after the rediscovery of Mendel's paper on genetics in $1900$, she observed that male mealworms produced two kinds of sperm, in which one was with a large chromosome and the other with a small chromosome in the year $1905$. She discovered the Y chromosome; hence option D is the correct answer.

Hence, Option D Nettie Stevens is the correct answer.

Note:
Nettie Stevens proposed that the chromosomes always existed in pairs and that the Y chromosome was the pair of the X chromosome, which was discovered in $1890$ by Hermann Henking. She came to the conclusion that the previous idea of Clarence Erwin McClung, that the X chromosome determines sex, was wrong and that sex determination was in fact due to the Y chromosome's existence or absence.