Like a Virgin, Prasad, Aarathi [free children's ebooks pdf .TXT] 📗

sperm and male fertility. Lifestyle and other factors of biology can also affect the integrity of the genes carried by sperm: exposure to radiation, heat, cigarette smoke, airborne pollutants, or chemotherapy drugs; sexually transmitted infections; ageing; a high body mass index; and medical conditions such as insulin-dependent diabetes – all can degrade the quality of DNA.

But even for perfectly formed sperm, there are many obstacles to overcome. In order to reach the egg, they must survive the acidic atmosphere of the vagina and avoid getting trapped in the sticky mucus in the cervix, the gateway to the womb. If they make it through those hurdles, they will then have to navigate the narrow entrance into the cervix, dodge cells of the immune system that will try to target and destroy them, swim upwards against the current, and escape a final molecular process that will screen and eliminate the vast majority of whichever sperm have survived. Sperm will do all that for only one reason. On a scale that is about one thousand times smaller than a mustard seed, the head of the sperm carries the genetic instructions to start making a baby – an essential ingredient of sexual reproduction. In fact, somewhat as Aristotle suspected two millennia ago, what the semen delivers into the egg will contribute to the form of a resulting child – its looks and its general genetic make-up – but not the ‘matter’. This is because, if and when a sperm makes contact with an egg, only its head penetrates, so that it can release its DNA-containing package into the awaiting receptacle. This DNA is its only contribution.

In contrast, at one hundred times larger than a sperm, the egg is mostly composed of a large amount of cytoplasm, or cellular fluid. Cytoplasm is a repository of miniature organs, such as mitochondria, which produce energy for the cell. An early embryo developing in the womb will need all of the egg’s resources to grow, until its own cells are able to perform these functions itself. And the growing foetus will indeed take ‘matter’ from its mother to build itself – calcium from her teeth to build its bones; nutrients and oxygen from her blood. But this is where any resonances with Aristotle’s intuitions end. Like sperm, the egg also has its own unique complement of DNA, its own set of instructions. The DNA that was carried inside the head of the successful sperm must join with that of the egg, if there is to be any chance of creating a new human being. Together, the male and female genes sculpt the body of the future child.

As we have seen, in a normal human cell DNA is packaged into forty-six separate chromosomes, but sperm and eggs contain only twenty-three. One of these twenty-three chromosomes is the sex chromosome, chromosome X or chromosome Y. While an egg can only ever carry an X chromosome, sperm may carry either an X or a Y. If a Y-carrying sperm makes it to the egg, their forty-six coiled chromosomes will have one X in one strand and one Y in the other, making a boy. If, instead, an X-carrying sperm fertilizes the egg, the resulting child will be XX, a girl. On a chromosomal level, mothers can never determine the sex of a child – no matter what King Henry II and his suffering spouse Catherine de Medici might have believed. The sex of a child simply comes down to whether sperm from the father is loaded with an X or a Y.

There may, however, be other means by which mothers can influence the sex of their children. It is still controversial, but there is some evidence that, even before an egg comes anywhere near sperm, it may already be programmed to ‘prefer’ only an X- or only a Y-carrying sperm. If this were true, then an egg selected to accept only sperm carrying a Y chromosome would not develop into a baby even if it were fertilized by an X-carrying sperm. This preference of an egg for a Y-chromosome-carrying sperm seems to be influenced by higher levels of the hormone testosterone in the egg’s immediate environment. And even after fertilization happens, the development of males could also be promoted by higher levels of glucose in a mother’s body. Experiments with mice show that mothers on a very high saturated fat diet have significantly more male offspring than those on a diet with restricted fat.

Whatever other secrets the human egg holds, it is clear that it is more than just a passive recipient of semen. In fact, the ability of the eggs of insects and sea urchins to create new life entirely without sperm is no isolated occurrence. The more we look, it seems, the more we are finding that virgin births are happening throughout the animal kingdom, sometimes in the most unexpected places.

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THE STORY OF SAFE SEX

I am particularly glad that you are ruminating on the act of fertilisation: it has long seemed to me the most wonderful & curious of physiologicalproblems.

Charles Darwin, letter to T. H. Huxley, November 1857

We all know how sex works, right? If you’re a young couple in your twenties (or younger), and you’re planning to have a baby, or you had a baby around that age, you probably did not think too much about the process. If you’re in your thirties or older, a bit more organization might be required: ‘romantic’ evenings in, planned around an ovulation predictor, possibly with the help of fertility drugs. You have ‘unprotected’ sex with your partner – throwing aside birth control pills, condoms, and other barriers to fertilization. Then you wait to see what happens.

You wait two weeks, maybe a month, and a period never arrives. So you visit your local pharmacist