Final answer:
In absence of Y chromosome, female development occurs is the true statement regarding human sex determination. The Y chromosome's presence, particularly the functional SRY gene, is key for male development, while its absence defaults development to female. Extra X chromosomes can cause mild effects due to a process known as X chromosome inactivation.
Step-by-step explanation:
Among the given statements about human sex determination, the true statement based on what we know from sex chromosome aneuploidies is: In absence of Y chromosome, female development occurs.
This is because the Y chromosome contains genes that determine sex, primarily the SRY gene. The presence of an SRY gene, which stands for Sex-determining Region of the Y chromosome, is crucial for an embryo to develop into a male. Without a functioning SRY gene, the default development of an embryo is towards female characteristics. Additionally, the aneuploidies conditions, such as Klinefelter syndrome (XXY) and Turner syndrome (XO), demonstrate how variations in sex chromosomes can lead to differing developmental outcomes.
It is incorrect to say that extra copies of X have no effect on development in both sexes, as an additional X can cause physical and mental effects, albeit the impacts are relatively mild compared to autosomal chromosome aneuploidies due to X chromosome inactivation. A non-functioning SRY gene does affect male phenotype, as it is the key genetic element driving male sexual development. Female determination is not directed by the presence of a Y chromosome, rather its absence is what defaults embryonic development to female. Lastly, genes affecting fertility are not exclusively found on the X chromosome; both sex chromosomes play roles in fertility.
The reason the Y chromosome is smaller and has fewer genes is due to a great deal of its genetic material being nonessential for survival and not related to sexual determination. Through evolutionary processes, the Y chromosome has endured gene loss and has primarily retained genes key to male sex determination and sperm production.
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