Question

Chromosome theory of XY sex determination was proposed by.
A. Henking
B. Wilson and Stevens
C. Johannsen
D. Punnet

Answer 1

Hint: Two scientist Stevens and Edmund wilson proposed chromosomal theory of XY sex determination at the same time. But, Stevens is generally considered to have made the larger contribution as her theory was ultimately proven correct.

Complete answer:
Stevens was studying the mealworm. She found the male produced two types of cells with both X and Y chromosomes and the females produce only those with X. So, she found that sex is determined chromosomal factor and male parent is mainly responsible for the gender of the offspring. Chromosomal theory of inheritance was not accepted at that time, and people believed that gender was determined by the mother and the environment.
Another scientist, Edmund Wilson gave similar theory of sex determination at about the same time. So, they both get credit for this discovery.

They proposed a theory for sex determination: A zygote forms when male and female gametes fuse. These gametes contain chromosomes, gametes are haploid and the gametes contain 23 chromosomes. The female have two sex chromosomes, which are two X chromosomes. The gamete is formed by meiosis, so the female gamete will have 22+X chromosomes and in male 2 sex chromosomes are found: X and Y. So, the male gamete will have either 22+X or 23+Y. When the Y chromosome is present in the male gamete and it fuses with the X chromosome of the female, a male zygote is formed with 44+XY chromosome. When the X chromosome is present in the male gamete and it fuses with the X chromosome of the female, a female zygote is formed with 44+XX chromosome.
As the chromosomal theory of sex determination is given by both Steven and Wilson.

Hence, the correct answer is option (B).

Note: In the XY sex-determination system, the female always provide X chromosome and the male provides either an X chromosome or a Y chromosome and they fuse to form female (XX) or male (XY) offspring, respectively. The ratio of chromosomes X or Y are determined by the hormone levels in the male parent.