What Do We Inherit?

A Little History Of Science: What Do We Inherit?

Who do you look most like – mum or dad? Or perhaps a grand- father or aunt? If you are good at football or play the guitar or flute very well, does someone else in your family have these characteristics too? It has to be someone you are biologically related to and from whom you could have inherited these things, not just a relative by marriage, like a stepmother or stepfather. These relatives can do wonderful things for you, but you cannot inherit any of their genes.

We know now that things like the colour of our eyes or hair are controlled and passed from one generation to the next through our genes. Genetics is the study of our genes. Heredity or inheritance are the words we use to describe how the information our genes possess is passed on. Our genes determine an awful lot about who we are. So how did people realise these tiny things were so important? Let’s go back to Charles Darwin for a moment. Heredity was central to Darwin’s work. It was vital to his ideas on

the evolution of species, even if he hadn’t worked out how heredity occurs. Biologists continued to debate how it happens long after his book On the Origin of Species was published in 1859. In particular, they were interested in whether ‘soft’ heredity can sometimes happen. Soft heredity was an idea associated with a French naturalist, Jean-Baptiste Lamarck (1744–1829), who also believed in the development of species by evolutionary change. Think about a giraffe’s long neck: how had that evolved over time? Lamarck said it was because as giraffes continually stretch upwards to reach the leaves on the tallest trees so this slight change will be passed on to their offspring generation after generation. Given enough time and enough stretching, a shorter-necked animal would eventually become a longer-necked one. The environment would interact with the organism, shaping or adapting it, and that would be passed on to the following generations.

Trying to prove soft heredity experimentally was very difficult. Darwin’s cousin, Francis Galton (1822–1911), performed a careful series of experiments, in which he introduced the blood of black rabbits into white ones. The offspring of the transfused rabbits showed no sign of being affected by the blood. He cut off rats’ tails for generations on end, but did not produce a race of tailless rats. Circumcising young boys had not had any effect on future generations of male babies.

Arguments for and against were bandied about until the early 1900s. Then two things convinced most biologists that the traits plants or animals had simply acquired during their life are not passed on to their offspring. First came the rediscovery of the work of a monk from Moravia (now part of the Czech Republic), Gregor Mendel (1822–84). In the 1860s, Mendel had published (in a little- read journal) the results of his experiments in the monastery garden. He had become fascinated with peas, even before Galton was cutting off the tails of his rats. Mendel wondered what happened when pea plants with certain characteristics were care- fully ‘crossed’ (that is, plants with differently coloured peas were bred together), to provide the next generation of pea plants. Peas were good to work with because they grew fast, so it was quick and easy to move from one generation to the next.

And, in the pod they also had clear differences – the peas were either yellow or green, in wrinkled or smooth skins. He discovered that these traits were inherited with mathematical precision, but in ways that could be easily overlooked. If a plant with green peas (its seeds) was crossed with a yellow one, all the first generation of peas were yellow. But when he crossed these first-generation plants with each other, in the second generation three of every four plants would have yellow peas and one would have green. The yellow trait had dominated in the first generation, but in the second, the ‘recessive’ trait (the green) showed itself again. What did these strong patterns mean?

Mendel concluded that heredity is ‘particulate’, that is, that plants and animals inherit traits in separate units. Rather than the little- by-little changes of soft heredity, or some average of the attributes of the two parents, heredity was something quite definite. Peas were either green or yellow, and not some shade in between.

While Mendel’s work lay unnoticed, August Weismann (1834– 1914) provided the second critical assault on soft heredity. Where Mendel was mostly concerned with his religious life, Weismann was first and foremost a determined scientist. A brilliant German biologist, he strongly believed that Darwin’s evolutionary views were correct. But he could see that the lack of a good explanation for heredity was a problem. He turned his own fascination with cells and cell division into a solution. A few years before Mendel’s experiments with his peas, Rudolf Virchow had announced his ideas about cell division.

In the 1880s and 1890s Weismann saw that to make an egg or a sperm cell, ‘mother’ cells of the reproductive system divided in a way that was different from cell division in the rest of the body. It was this difference that was the key. Known as the process of meiosis, here the chromosomes divided and half of the chromosomal material went into each of the resulting ‘daughter’ cells. In all the other body cells, the ‘daughter’ cell has the same amount of the chromosome material as the ‘mother’. (If you’re confused, remember that a ‘mother’ cell is just any existing cell and that it splits into two ‘daughter’ cells. They are found throughout the body and have nothing to do with real mothers and daughters.) So when the egg and sperm cells fused, the two halves of the chromosomal material would make up the full amount again in the fertilized egg.

These reproductive cells were different to all the other cells of the body. Weissman argued that it did not matter what else happened to the cells of the muscles or bones or blood vessels or nerves: only these reproductive cells contained what would be inherited by the individual’s offspring. So in the case of the giraffe’s neck the supposed stretching would have no effect on the egg and sperm cells, and it was these cells that contained what he termed the ‘germ plasm’. It was the germ plasm, on the chromosomes of the egg and sperm cells, which was inherited, and he called his idea of heredity the ‘continuity of the germ plasm’.

In 1900 not one but three separate scientists dusted off copies of the journal with Mendel’s article in it. They alerted the scientific world to the results of Mendel’s pea experiments. Biologists realised that Mendel had provided the best experimental evidence yet for Weismann’s ‘continuity of the germ plasm’ and that ‘Mendelism’, as it was soon called, had a sound scientific basis.

The scientific community was soon split into two groups, the ‘Mendelians’ and the ‘biometricians’. The biometricians, led by the statistics expert Karl Pearson (1857–1936), believed in ‘continuous’ inheritance. They thought that what we inherit is an average of the attributes of our parents. They conducted important fieldwork in measuring very small differences in sea creatures and snails. They showed that such small differences could play a significant role in determining how many offspring survived – what is termed the reproductive success of species. The Mendelians were led by the Cambridge biologist William Bateson (1861–1926). He coined the term ‘genetics’. Mendelians emphasised the inheritance of the sort of discrete (separate) traits that the monk had illustrated. They argued that biological change occurred by leaps, rather than the slow, continuous changes of the biometricians. Both groups accepted the fact of evolution: they merely argued about how it happened.

These arguments were fierce for about twenty years. Then, in the 1920s, several people showed that each group was right and wrong at the same time. They were just looking at two different sides of the same problem. Many biological characteristics are inherited in a ‘blending’, ‘biometrical’ fashion. A tall father and a short mother will have offspring that average out or ‘blend’ their heights. Some of the children may be as tall as the father (or even taller), but the average height will tend to be midway between the two parents.

Other characteristics, such as human eye colour, or the colour of peas, are inherited in an either/or, not a both/and fashion. The differences between the Mendelians and the biometricians were resolved when they measured whole populations, and then appled mathematical reasoning to the problem. These new biologists, such as J.B.S. Haldane (1892–1964), appreciated the brilliance of Darwin’s original insights. They realised that in any population there is random variation that can be inherited. If it gives an advantage, those plants and animals that have it will survive and other kinds of variation will die out.

How we inherit what we do is also vitally important too. This was the next part of the puzzle. Much of the early work was carried out in the laboratory of Thomas Hunt Morgan (1866–1945), at Columbia University in New York City. He began his career looking at how animals begin life and develop as embryos. He never completely lost his interest in embryology, but his attention shifted in the early 1900s to the new science of genetics. Morgan’s lab was no ordinary place. Nicknamed the ‘Fly Room’, it became home to thousands of generations of the common fruit fly (Drosophila melanogaster). The fruit fly is a convenient experimental animal. These flies have only four chromosomes in the nuclei of their cells, and it was the role of the chromosomes that Morgan wanted to understand: how important were chromosomes in passing on hereditary traits? The fruit fly chromosomes are large and easy to see on microscopic slides. Fruit flies also breed very quickly – leave out a plate of fruit and watch what happens. Many generations can be studied in a short space of time, to see what happens when flies with certain characteristics are bred with other flies. Imagine doing this kind of work with elephants and you can see why they chose fruit flies.

Morgan’s fly room became famous, attracting both students and other scientists. It was a forerunner of the way much science is done today: a group of researchers working under a ‘boss’ – Morgan – who helps define the problems. The boss supervises the work of his or her team of younger researchers, who do the actual experiments. Morgan encouraged everyone to talk and work together so it has been hard to sort out exactly who did what. (When Morgan won his Nobel Prize, he shared the money with two of his younger colleagues.) Almost by chance, Morgan made a crucial discovery. He noticed that one fly from a recent hatching had red eyes, rather than the usual white ones. He isolated this fly before breeding it with ordinary white-eyed flies. When he looked at the red-eyed offspring of that fly, he discovered first that all his red-eyed flies were female.

That suggested that the gene was carried on the sex chromosome, the chromosome that determines whether the offspring will be male or female. Second, the inheritance patterns of eye colour followed the same rules as Mendel’s peas – the eyes were either white or red, but never pink, or some colour in between. Morgan looked at other patterns of the tiny flies’ inherited traits, such as wing size and shape. He and his colleagues examined their chromosomes under the microscope and began to develop maps of each chromosome, showing where the units of heredity (the ‘genes’, as they had been called) were located. Mutations (changes), such as the sudden appearance of the red eyes, could help locate where the gene was, as they carefully analysed what the chromosomes did during cell division. One of Morgan’s students, H.J. Muller (1890– 1967), discovered that X-rays caused faster mutations. Muller won his own Nobel Prize in 1948, and his work alerted the world to the dangers of radiation from atomic bombs and even from the X-rays being used medically. Morgan also showed that chromosomes sometimes exchange material when they are dividing. This is called ‘crossing over’, and it is another way in which nature increases the amount of variation in plants and animals.

Morgan and his group, as well as many others around the world, made genetics one of the most exciting sciences between about 1910 and 1940. The ‘gene’ was increasingly recognised as some material substance. Located on the chromosomes of the cells, the genes are passed, via a female egg fertilised by a male sperm, to the offspring, each parent contributing equally. Mutations were shown to be the thing that drove evolutionary change. They created the variation and they occurred naturally as well as by the artificial methods that Muller studied. The new genetics was central to evolutionary thinking. Even though what exactly the ‘gene’ was remained undefined, its reality was now beyond doubt.

This new genetic thinking had a darker side in society. If there was no soft heredity – so that eating better food, playing sports or being good, could not change the genes of your children – different methods would have to be used if you wanted future generations to improve. Darwin’s ‘artificial selection’ had been practised for centuries, by livestock and plant breeders who tried to improve on the desirable characteristics of whatever they were breeding. Cows could be bred to yield more milk, tomatoes to be even juicier. In 1904, Francis Galton (Darwin’s cousin) founded a ‘eugenics’ laboratory. He had coined the term ‘eugenics’, meaning ‘good birth’.

Here he had tried to change the reproductive habits of human beings. If intelligence, creativity, criminality, insanity or laziness could be shown to run in families (and Galton believed they could), it made sense to encourage the ‘good’ to have more children (‘positive’ eugenics), and to prevent the ‘bad’ from having so many (‘negative’ eugenics). Positive eugenics was the most common form in Britain. Campaigns encouraged educated middle-class couples to have more children, on the assumption that these couples were somehow ‘better’ than a casual labourer and his wife.

In the late 1890s, the government had been frightened by the poor condition of recruits for the Boer War in South Africa. A large number of volunteers were rejected as physically unfit, unable even to carry a rifle. Then the First World War, from 1914 to 1918, saw mass slaughter in the battlefields of Europe. Many assumed that it was mostly the best who had been lost. Every nation throughout the Western world worried about the quality and strength of its population.

Negative eugenics was more sinister. Many assumed it was sensible to lock away people who were mentally disturbed or ‘subnormal’, criminals, even disabled people and others at the margins of society. In the USA, many states passed laws enforcing sterilisation, to prevent these people from having children. From the 1930s until their defeat in the Second World War in 1945, the Nazis in Germany practised the worst atrocities. In the name of the State, they first incarcerated, and then murdered millions of people they decided were unfit to live. Jews, Gypsies, homosexuals, the mentally disturbed or deficient, criminals: all were herded up and either sent to concentration camps or executed.

The Nazi period made ‘eugenics’ a dirty word. As we shall later see, some people believe that eugenics could return through the back door, as scientists learn more and more about what we inherit, and how it affects who we are. We all need science, but we must all make sure that it is used for good.