A woman having only daughters is a matter of chance due to the random assortment of sperm. In humans, males produce sperm with either an X or a Y chromosome, while females produce eggs with only an X chromosome. If an egg with an X chromosome is fertilized by a sperm carrying an X chromosome, the offspring will be female (XX). Conversely, if fertilized by a Y-carrying sperm, the offspring will be male (XY). If a woman has only daughters, it means that every fertilization event involved an X-carrying sperm.
Detailed Explanation:
Sex Chromosomes:
Humans have 23 pairs of chromosomes. One pair, the sex chromosomes, determines the sex of an individual. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
Sperm and Egg:
During gametogenesis (the formation of sex cells), males produce two types of sperm: half carry an X chromosome and half carry a Y chromosome. Females produce only eggs with an X chromosome.
Fertilization:
The sex of the offspring is determined at fertilization. If an egg with an X chromosome is fertilized by a sperm carrying an X chromosome, the resulting zygote will be XX (female). If an egg with an X chromosome is fertilized by a sperm carrying a Y chromosome, the resulting zygote will be XY (male).
Probability:
While the probability of having a male or female child is 50/50 at each fertilization event, a woman having only daughters indicates that every fertilization event involved an X-carrying sperm.
Future Offspring:
Having only daughters in the past does not guarantee that future offspring will also be daughters. The random selection of sperm at each fertilization event means there’s still a 50/50 chance of having a male or female child in the future.