Human females have two X chromosomes, each of which can be passed on to their son or daughter with equal probability. Human males have one X chromosome which is passed on to their daughters and one Y chromosome which is passed on to their sons. Assuming equal numbers of males and females in a population, if an X chromosome is randomly sampled from the population, what is the probability that it was inherited from a female of the previous generation?

Correct option is C

Given:

• Human females have two X chromosomes, each of which can be passed to their offspring with equal probability.
• Human males have one X chromosome (passed to daughters) and one Y chromosome (passed to sons).
• The population has an equal number of males and females.
• An X chromosome is randomly selected from the population, and we need to determine the probability that it came from a female in the previous generation.

Formula and Concept:

1. Key observation:
   • Half the population is male, and half is female.
   • Each female has two X chromosomes.
   • Each male has one X chromosome.
2. The total X chromosomes in the population are distributed as:
   • Two-thirds of X chromosomes come from females (since each female contributes 2 X chromosomes).
   • One-third of X chromosomes come from males (since each male contributes 1 X chromosome).

Solution:

1. Total X chromosomes in the population:

   • Let there be $N$ males and $N$ females in the population.
   • Total X chromosomes from females = $2N$.
   • Total X chromosomes from males = $N$.
   • Total X chromosomes in the population = $2N+N=3N$.

2. Probability that a randomly chosen X chromosome came from a female:

   ​$$P(\text{X from female}) = \frac{\text{X chromosomes from females}}{\text{Total X chromosomes}} = \frac{2N}{3N} = \frac{2}{3}.$$

   Final Answer:

   ​(c) 2/3.

   ​