The Epigenetics Revolution

This is why we don’t normally inherit acquired characteristics. But there are certain regions of the genome, such as IAP retrotransposons, that are relatively resistant to reprogramming. If we want to work out how certain acquired characteristics – responses to vinclozolin or responses to paternal nutrition, for example – get transmitted from parent to offspring, these IAP retrotransposons might be a good place to start looking.

Chapter 9. Generation X

The sound of a kiss is not so loud as that of a cannon, but its echo lasts a great deal longer.

Oliver Wendell Holmes

At a purely biological, and especially an anatomical level, men and women are different. There are ongoing debates about whether or not certain behaviours, ranging from aggression to spatial processing, have a biological gender bias. But there are certain physical characteristics that are linked unequivocally to gender. One of the most fundamental differences is in the reproductive organs. Women have ovaries, men have testicles. Women have a vagina and a uterus, men have a penis.

There is a clear biological basis to this, and perhaps unsurprisingly, it’s all down to genes and chromosomes. Humans have 23 pairs of chromosomes in their cells, and inherited one of each pair from each parent. Twenty-two of these pairs (imaginatively named chromosomes 1 to 22) are called autosomes and each member of a specific pair of autosomes looks very similar. By ‘looks’ we mean exactly that. At a certain stage in cell division the DNA in chromosomes becomes exceptionally tightly coiled up. If we use the right techniques we can actually see chromosomes down a microscope. These chromosomes can be photographed. In pre-digital days, clinical geneticists literally used to cut out the pictures of the individual chromosomes with a pair of scissors and rearrange them in pairs to create a nice orderly picture. These days the image processing can be carried out by a computer, but in either case the result is a picture of all the chromosomes in a cell. This picture is called a karyotype.

Karyotype analysis is how scientists originally discovered that there were three copies of chromosome 21 in the cells of people with Down’s syndrome. This is known as trisomy 21.

When we produce a human karyotype from a female, there are 23 pairs of identical chromosomes. But if we create a human karyotype from a male, the picture is different, as we can see in Figure 9.1. There are 22 obvious pairs – the autosomes – but there are two chromosomes left over that don’t look like each other at all. One is very large, one exceptionally small. These are called the sex chromosomes. The large one is called X, and the small one is called Y. The notation to describe the normal chromosome constitution of human males is 46, XY. Females are described as 46, XX because they don’t have a Y chromosome, and instead have two X chromosomes.

^{Figure 9.1 Karyotype of all the chromosomes in a male (top) and female (bottom) somatic cell. Note that the female cell contains two X chromosomes and no Y chromosome; the male cell contains one X chromosome and one Y chromosome. Note also the substantial difference in size between the X and Y chromosomes. Photos: Wessex Reg Genetics Centre/Wellcome Images.}

The Y chromosome carries very few active genes. There are only between 40 and 50 protein-coding genes on the Y chromosome, of which about half are completely male-specific. The male-specific genes only occur on the Y chromosome, so females have no copies of these. Many of these genes are required for male-specific aspects of reproduction. The most important one in terms of sex determination is a gene called SRY. SRY proteins activate a testis-determining pathway in the embryo. This leads to production of testosterone, the archetypal ‘male’ hormone, which then masculinises the embryo.