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In the XY sex-determination system, the female-provided ovum contributes an X chromosome and the male-provided sperm contributes either an X chromosome or a Y chromosome, resulting in female (XX) or male (XY) offspring, respectively. Hormone levels in the male parent affect the sex ratio of sperm in humans. [25]
The human twin birth rate in the United States rose 76% from 1980 through 2009, from 9.4 to 16.7 twin sets (18.8 to 33.3 twins) per 1,000 births. [5] The Yoruba people have the highest rate of twinning in the world, at 45–50 twin sets (90–100 twins) per 1,000 live births, [6] [7] [8] possibly because of high consumption of a specific type of yam containing a natural phytoestrogen which may ...
However, when an extra DAX1 gene is placed on the X chromosome, the result is a female, despite the existence of SRY, since it overrides the effects of SRY. [11] Even when there are normal sex chromosomes in XX females, duplication or expression of SOX9 causes testes to develop.
In the absence of a Y chromosome, the fetus will undergo female development. This is because of the presence of the sex-determining region of the Y chromosome, also known as the SRY gene. [5] Thus, male mammals typically have an X and a Y chromosome (XY), while female mammals typically have two X chromosomes (XX).
A U.K. woman who was told she would never have children because of a rare condition recently gave birth to twins. When Hayley Haynes was 19, she went to the doctor because she hadn't gotten her ...
The sex chromosome in a human egg is always an X chromosome since a female only has X sex chromosomes. In sperm, about half the sperm have an X chromosome and half have a Y chromosome. [2] If an egg fuses with sperm with a Y chromosome, the resulting individual is male.
There’s a “third form of twinning,” say researchers. For premium support please call: 800-290-4726 more ways to reach us
Occasionally a human birth is anything but ordinary. A recent report highlighted a rare case from 2010, in which a baby girl born in Hong Kong was found to be carrying twin fetuses. The study ...