Final answer:
The natural percentage of female offspring in genetic sex determination with XX female and XY male genotypes is C. 50%, because fathers can equally pass on X or Y chromosomes, while mothers always contribute an X chromosome.
Step-by-step explanation:
The natural percentage of female offspring in genetic sex determination where the female genotype is XX and the male genotype is XY is 50%. Since mothers can only provide an X chromosome and fathers have a 50% chance of passing either an X or Y chromosome, the probability that any given child will be female (inherit XX) is 50%. Simply put, there's an equal chance for male and female offspring each time conception occurs.
In mammals, sex is determined by the presence of XX chromosomes in females and XY chromosomes in males. During sexual reproduction, male gametes (sperm) can carry either an X or Y chromosome, while female gametes (eggs) only carry an X chromosome. Therefore, it is the sperm from the father that determines the sex of the offspring.
It is important to note that while the basic percentage chance remains 50:50 for each pregnancy, actual birth ratios might slightly vary due to biological, environmental, or selective factors.