Morgan knew that male and female fruit flies differ in light is scanned across her retina her ability to tell the colour of the light changes as it passes from one group of cells to the next. About half the time, she makes a perfect match but for the rest she is no better at telling red and green apart than is her colour-blind son. Different X chromosomes have been switched off in each colour-sensitive cell, either the normal one or that bearing the instruction for colour blindness.
The inheritance of mitochondria! genes also shows sexual differences. When an egg is h-rlilisrii, much of its contents, including [host- crucial slim tuirs, is passed on to the developing embryo. Mitochondria have a pattern of inheritance quite different from those in the nucleus. They do not bother with sex, but instead are passed down the female line. Sperm are busy little things, with a long journey to make, and are powered by many mitochondria. On fertilisation these are degraded, so that only the mother's genes are passed on. In the body, too, mitochondria are transmitted quite passively, each cell dividing its population among its descendants. Their DNA contains the history of the world's women, with almost no male interference. Queen Elizabeth the Second's mitochondrial DNA descends, not from Queen Victoria (her ancestor through the male line) but from Victoria's less eminent contemporary Anne Caroline, who died in 1881.
Mitochondria, small as they are, are the site of an impressive variety of diseases. Their sixteen and a half thousand DNA bases — less than a hundredth of the whole sequence — were, a century after the death of Anne Caroline, the first to be read off. Every cell contains a thousand or so of the structures. They are the great factories of metabolism; places where food — the fuel of life — is burned. Mitochondrial genes code for just thirteen proteins, and about twice that number of the molecules that transfer information from the DNA to where proteins are made.
They are more liable to error than are others. Some of the mistakes pass between generations, while others build up in the body itself as it ages. Some of the two hundred known faults involve single changes in the DNA, others the destruction of whole lengths of genetic material. Some are frequent: thus, a certain change in one mitochondrial gene is present in about one in seven thousand births.
Mitochondrial disease involves many symptoms: deafness, blindness, or damage to muscles or the brain. Certain forms of diabetes are due to mitochondrial errors, as is an inherited muscle weakness and drooping of the eyelids. Different patients in the same family may have distinct problems; perhaps deafness in one child and brain damage in another. All this conks I mm the role of mitochondria in burning energy and from their random shuffling as cells divide. An egg may cany both normal and abnormal mitochondria. It, in an embryo, ihosc with an error become by chance common in the cell lines that make brain tissue, that organ suffers; if in cells that code for insulin, then diabetes is the result. Mothers pass such genes to sons and daughters, but only daughters pass it to the next generation; a pattern quite different from sex-linked inheritance.
These, then, are the rules of the genetical game. From here on, the rest is molecular biology: mechanics rather than physics. The notion that life is chemistry came first from humans. In 1902, just two years after the rediscovery of Mendelism, the English physician Sir Archibald Garrod noticed that a disease called aikaptonuria — at the time thought to be due to an intestinal worm — was more frequent in the children of parents who shared a recent ancestor than in those of unrelated people. Its symptoms, a darkening of the urine and the earwax, together with arthritis, followed that of a recessive. The disease was, he thought, due to an inherited failure in one of the pathways of metabolism, what he called a "chemical sport' (Darwin's another way. Chromosomes are paired bodies in the cell which appear as dark strands. Most of the chromosomes of the two sexes look similar but one pair-the sex chromosomes — are different. Females have two large X chromosomes; males a single X and a much smaller Y.
Morgan noticed that the pattern of inheritance of eye colour followed that of the X chromosome. Males, with just a single copy of the X (which comes from their mother, the father providing the Y) always looked like their mother. In females, the copy <>l the X chromosome horn the mother was accompanied by a mulching X Irntn ihc father. In a cross between white-eyed mothers and red-eyed fathers, the female offspring have one X chromosome bearing 'white' and another bearing 'red'. Just as Mendel would have expected, they have eyes like only one of the parents, tn this case the one with red eyes.
The eye colour gene and the X chromosome hence show the same pattern of inheritance. Morgan suggested that this meant that the gene for eye colour was actually on the X chromosome. He called this pattern 'sex-linkage'. Chromosomes were already candidates as the bearers of genes as, like Mendel's hypothetical particles, their number is halved in sperm and egg compared to body cells.
Everyone has forty-six chromosomes in each body cell. Twenty-two of these are paired, but the sex chromosomes, X and Y, are distinct. Because the Y carries few genes, in males the ordinary rules of Mendelian dominance and recessivity do not apply. Any gene on the single X will show its effects in a male, whether or not it is recessive in females.
The inheritance of human colour blindness is just like that of Drosophila eye colour. When a colour-blind man marries a normal woman none of his children is affected, but a colour-blind woman whose husband has normal vision passes on the condition to all her sons but none of her daughters. Because all males with the abnormal X show its effects (while in most females the gene is hidden by one for normal vision) the trait is commoner in boys than in girls. Many other abnormalities show the same pattern.
Sex-linkage leads to interesting differences between the sexes. For the X chromosome, females carry two copies of each gene, but males only one. As a result, women contain more genetic information than do men. Because of the two different sensors for the perception of red controlled by a gene on the X chromosome, many women must carry both red receptors, each sensitive to a slightly different point in the spectrum. Males are limited to just one. As a result, some women have a wider range of sensual experience — for colour at least — than is available to any man.
Whatever the merits of smug the world in a different way, women have a potential problem with sex-linkage. Any excess of a chromosome as large as the X is normally fatal. How do females cope with two, when just one contains al! the information needed to make a normal human being (or a male)? The answer is unexpected. In almost every cell in a woman's body one or other of her two X chromosomes is switched off.
Tortoiseshell cats have a mottled appearance, which comes from small groups of yellow and black hairs mixed together. All tortoiseshells are females and are the offspring of a cross in which one parent passes on a gene for black and the other transmits one for yellow hair. Because the coat-colour gene is sex-linked about half the skin cells of the kitten switch off the X carrying the black variant and the remainder that for yellow. The coat is a mix of the two types of hair, the size of the patches varying from cat to cat.