Why do humans have near-equal numbers of male and female babies, unlike many other animals? A new genetic study looks for clues
A new paper searches huge human data sets for gene variants that throw the 1:1 sex ratio off balance, and test the biological and theoretical rules of sex ratio.
Of course, we now know that sex chromosomes are the real determiners of sex. Females have two X chromosomes; males have a single X and a male-specific Y.
The Y carries a male-determining gene called SRY, which kickstarts the differentiation of a ridge of cells into a testis. The embryonic testis makes male hormones which direct the embryo to develop as a boy. Without SRY, an alternative pathway is activated that makes an ovary, and the embryo develops as a girl.
The 1:1 ratio results from the way the X and Y chromosomes are doled out in sperm and eggs. Our cells all have two sets of chromosomes that constitute our genome, one set from each parent. A special type of cell division makes sperm and eggs with just a single set of chromosomes, so that a fertilised egg once again has two sets (one set from the sperm and the other from the egg).
So sperm all get a single copy of each chromosome – and just one sex chromosome, either an X or a Y. XX females make eggs with a single chromosome set, all of which carry an X.
