Editing XY sex-determination system (section)

===Maternal===
In humans and many other species of animals, the [[father]] determines the [[sex]] of the child. 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.<ref name="Krackow-1995"/> Maternal influences also impact which sperm are more likely to achieve [[fertilisation|conception]].

Human ova, like those of other mammals, are covered with a thick translucent layer called the [[zona pellucida]], which the sperm must penetrate to fertilize the egg. Once viewed simply as an impediment to [[fertilization]], recent research indicates the zona pellucida may instead function as a sophisticated biological security system that chemically controls the entry of the sperm into the egg and protects the fertilized egg from additional sperm.<ref>{{cite book | vauthors = Wymelenberg S | chapter = Infertility | title = Science and Babies | publisher = National Academy Press | date = 1990 | page = 17 | isbn = 978-0-309-04136-2 }}</ref>

Recent research indicates that human ova may produce a chemical which appears to attract sperm and influence their swimming motion. However, not all sperm are positively impacted; some appear to remain uninfluenced and some actually move away from the egg.<ref>{{cite book | vauthors = Jones RE, Lopez KH | title = Human Reproductive Biology | edition = Third | publisher = Elsevier | date = 2006 | page = 238 | isbn = 978-0-12-088465-0 }}</ref>

Maternal influences may also be possible that affect sex determination in such a way as to produce [[fraternal twins]] equally weighted between one male and one female.<ref>{{cite web | vauthors = Johnson BH | url = http://home.comcast.net/~brucehjohnson/bgtwins.pdf | title = Familial recurrence of gender-balanced twins | archive-url = https://web.archive.org/web/20151002160341/http://home.comcast.net/~brucehjohnson/bgtwins.pdf | archive-date=October 2, 2015 }}</ref>

The time at which insemination occurs during the [[estrus cycle]] has been found to affect the sex ratio of the offspring of humans, cattle, hamsters, and other mammals.<ref name="Krackow-1995">{{cite journal | vauthors = Krackow S | title = Potential mechanisms for sex ratio adjustment in mammals and birds | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 70 | issue = 2 | pages = 225–241 | date = May 1995 | pmid = 7605846 | doi = 10.1111/j.1469-185X.1995.tb01066.x | s2cid = 27957961 }}</ref> Hormonal and pH conditions within the female reproductive tract vary with time, and this affects the sex ratio of the sperm that reach the egg.<ref name="Krackow-1995"/>

Sex-specific mortality of embryos also occurs.<ref name="Krackow-1995"/>