What’s more, the Y chromosome has degenerated rapidly, leaving females with two perfectly normal X chromosomes, but males with an X and a shriveled Y. If the same rate of degeneration continues, the Y chromosome has just 4.6m years left before it disappears completely. This may sound like a long time, but it isn’t when you consider that life has existed on Earth for 3.5 billion years.
The Y chromosome hasn’t always been like this. If we rewind the clock to 166 million years ago, to the very first mammals, the story was completely different. The early “proto-Y” chromosome was originally the same size as the X chromosome and contained all the same genes. However, Y chromosomes have a fundamental flaw. Unlike all other chromosomes, which we have two copies of in each of our cells, Y chromosomes are only ever present as a single copy, passed from fathers to their sons.
For example, a recent Danish study, published in PLoS Genetics, sequenced portions of the Y chromosome from 62 different men and found that it is prone to large scale structural rearrangements allowing “gene amplification” — the acquisition of multiple copies of genes that promote healthy sperm function and mitigate gene loss.
The study also showed that the Y chromosome has developed unusual structures called “palindromes” (DNA sequences that read the same forwards as backwards — like the word “kayak”), which protect it from further degradation. They recorded a high rate of “gene conversion events” within the palindromic sequences on the Y chromosome — this is basically a “copy and paste” process that allows damaged genes to be repaired using an undamaged back-up copy as a template.
However, the interesting thing about humans is that while the Y chromosome is needed for normal human reproduction, many of the genes it carries are not necessary if you use assisted reproduction techniques. This means that genetic engineering may soon be able to replace the gene function of the Y chromosome, allowing same-sex female couples or infertile men to conceive. However, even if it became possible for everybody to conceive in this way, it seems highly unlikely that fertile humans would just stop reproducing naturally.
Although this is an interesting and hotly debated area of genetic research, there is little need to worry. We don’t even know whether the Y chromosome will disappear at all. And, as we’ve shown, even if it does, we will most likely continue to need men so that normal reproduction can continue.