How X and Y chromosomes compete with each other to control offspring sex

image of x and y chromosomes labelled in different colours

Under normal circumstances, the laws of genetics ensure that sperm carrying an X or a Y chromosome have an equal chance to fertilise the egg, and so parents have an equal chance of having a daughter or a son.

However, male mice with partial deletions on their Y chromosome (Yqdel males) break this law and produce a distorted sex ratio with many more female than male offspring. Until now, how this occurs has been a mystery.

New research by Dr Peter Ellis and his lab in the School of Biosciences, together with collaborators from the Universities of Cambridge, Essex and Paris Descartes shows that the reason lies in the shape of the sperm and how fast they are able to swim.

The researchers showed that they could correct the sex ratio distortion by performing IVF fertilisation – proving that the Yqdel males can produce equal numbers of male and female offspring once their sperm actually reaches the egg.

Next, high-resolution microscopy was used to show that sperm from Yqdel males are shrunken and distorted. The Y-bearing sperm were more severely affected than X-bearing sperm, suggesting that their ability to swim was impaired. The researchers used FISH labelling to identify the X- and Y-bearing sperm (see colour differences in picture). They also showed that the distorted shape was caused by differences in gene expression on the Y chromosome.

Finally, the researchers carried out “sperm races” to isolate the fastest-swimming sperm. This confirmed that the fastest swimmers were predominantly X-bearing sperm thus explaining the predominance of daughters in Yqdel offspring.

Peter Ellis who led the research, said: ‘We have known for some time that the mouse X and Y chromosomes compete to produce female versus male offspring, with genes on the X favouring the production of daughters and genes on the Y favouring sons. Our research reveals for the first time how this occurs. We have also found that IVF can be used to reverse this imbalance, with important implications for its use to influence the sex of mammal offspring.’