How did the giraffe get its long neck? People often make the mistake of thinking that giraffes stretched up to reach high leaves and the exercise built up their necks. The trouble with this idea is that when a giraffe does build up its neck, it cannot pass on the change to its offspring. If you go to the gym and build up your muscles, it will not mean that your children will be stronger.

Ancient ancestors of the giraffes did not have such long necks.

Among the precursors of giraffes, there was variation in neck length. Those who happened to have slightly longer necks would have had an advantage, especially in times of drought.

Mind you, there is a competing theory that long necks were developed for sexual display. The idea is that females would have been attracted to males with longer necks. Part of the courtship ritual of modern giraffes is a kind of “necking”, in which prospective mates intertwine and rub necks. It is also true that males have longer necks than females. However, this theory is not widely believed.

Sickle Cell Anemia Sickle cell anemia is a serious genetic disease. It appears in individuals who have two copies of an allele c, but not in those whose genotype is CC or Cc. The disease is caused by the recessive allele c. The disease reduces the life expectancy of people who have it. Ordinarily, we would expect that such an allele would become rare because there is selection against it.

However, something else interferes in this case. The individuals with the Cc genotype have a resistance to malaria. So in regions where malaria is prevalent, the c allele is “protected” in individuals with the Cc genotype, who actually fare better than those with the CC genotype. The upshot, unfortunately, is that in regions where malaria is common, so is sickle cell anemia.

Applications of Genetics

DNA Fingerprinting Police can analyze the DNA of tissue samples found at a crime scene and identify the person who produced them. The idea is that they find the DNA sequence of the sample, and try to match it to those of the possible suspects. If they find any part of the DNA which is different from that of a certain individual, then that person could not have been the source of the sample. But suppose all the sections of DNA they look at from the sample do match the corresponding sections of the suspect’s DNA. How confident can we be that the suspect is responsible?

Suppose there is a section of the genome in which there is variation in the population: say, half the population has A in that part of the genome, and the other half has some other sequence. Suppose the sample has sequence A. Then there is a 50% chance that a person chosen at random will have the same sequence. Now suppose there is another section where half the population has the sequence B which is found in the sample. The probability that a randomly selected individual will also have this sequence is ½. The probability that a randomly selected suspect will have both these sequences in common with the sample is ½ x ½ = ¼.

Now suppose there are n such sections of the genome. So the probability that a randomly selected suspect will have the same sequence as the sample in each of the n sections is ½ x ½ x … x ½ = (½) n If two samples agree at sufficiently many places on the genome, the probability is extremely low that they come from different people. The probability that a randomly selected suspect will share the sequence in the sample in any one of them is ½.

For this discussion, we have assumed that in each of the sections of the genome considered, the sample has a sequence which occurs in exactly half the population. But in any case, it is not difficult to calculate the probability that the suspect might match the sample at all these places by chance. If two samples agree at sufficiently many places on the genome, the probability is extremely low that they come from different people. In reality, the frequencies in the population vary. The best scenario is to find sections of the genome in which the sample has a sequence which is comparatively rare.

However, these kinds of results have to be used with care in a legal setting. There is always the possibility of samples being contaminated. If the two individuals being compared both come from the same ethnic background, then the fair comparison would involve the frequencies of the sequences in that population. And there is always the problem that a match can at best show that a suspect had been at the scene at some time in the past. Showing that he or she was there at the time of the crime, let alone that he or she was involved, is quite another matter. If the two individuals are related, then it is even more delicate.

Genetic Diversity

Genetic diversity refers to the presence in a population of a variety of alleles for different genes. It is important because it allows the population to adapt to changing circumstances. If all the individuals have exactly the same alleles, then they will all be equally vulnerable to any disease or environmental threat. If there are different alleles present, then a stressful condition may generate selection, favouring those alleles which help individuals to resist the condition. The presence of genetic diversity allows a population to evolve in response to changes.

An important principle is that in small populations, some alleles will tend to be lost. This happens because of random fluctuations. If you think of the graphs pertaining to the binomial distribution, they show that for small populations, it is more likely that the allele frequencies will change significantly. In large populations, they are almost certain to remain nearly the same, unless some outside influence intervenes. n = 12 n = 100

It is not unusual for a species to become “bottlenecked”. This refers to a situation in which the population is reduced to very small numbers for a while, whether because of disease or natural disasters or climatic factors, etc. In this situation, some of the genetic diversity of the population is likely to diminish. At some loci, some of the alleles will disappear. The smaller the population and the longer it stays small, the more pronounced this effect is. In extreme cases, almost every locus will have only a single allele. Once an allele is lost, it cannot come back, except by mutation, no matter how well the population rebounds to a larger size.

The elephant seal was hunted to near extinction in the nineteenth century. At one point, there may have been as few as twenty left. They survived only because they lived on a remote island which the hunters could not reach. Nowadays the elephant seal has made a comeback, and large numbers live in California. However, those who survive have almost no genetic diversity.

Another example is the cheetah. There is evidence to suggest that cheetahs were bottlenecked not once but at least three times. If a cheetah ever needs a heart transplant, there will not be a problem with rejection. Modern cheetahs are essentially all clones of one another. But the population is vulnerable to disease, because they will all be equally susceptible.

The moose is not native to Newfoundland. In 1904, somebody decided it would be a good idea to introduce them. Six moose were shipped over from the mainland. Two died in transit. A third was shot shortly after arriving; there were no laws against hunting moose.

We do not know if there were two males and one female or vice versa. But we do know both sexes must have been represented, because they proliferated.

Newfoundland now has approximately 400,000 moose. And 500,000 people. And all these moose came from three ancestors. There is essentially no genetic diversity at all.

Newfoundland’s moose are vulnerable to an epidemic. And without genetic variability, they will not be able to adapt to changing conditions, such as global warming.

When conservationists want to preserve a wild population, they are aware that if the population is too small, it will lose its genetic diversity. They hope to save a population of sufficient size to preserve genetic variability. How big is a “sufficient size”? There is no simple answer, but a common rule of thumb is that a population should contain at least two hundred individuals. Even this can be too little depending on various factors. For example, a large elephant seal becomes the “beach master” and monopolizes a harem of perhaps ten females. Many other males do not mate at all. This scheme has its advantages, but it does reduce the genetic diversity.

In a population of twenty, for instance, there might be ten females, all of whom mate with a single male, the beach master. This means that half of all the genetic material comes from a single male. The other nine males do not get to contribute at all.

Apples are an ancient fruit. They have been extremely popular throughout history.

In Norse mythology, apples were distributed to the gods by Brita, bringing them fertility and longevity.

Alexander the Great brought apples back to Macedonia. This apple, which grows in Afghanistan, may be the wild ancestor of some European species. Apples appear to have originated in Turkey or perhaps Romania. They have travelled around the world and have adapted to many different kinds of conditions. As a result, many of their genes have large numbers of alleles. In this situation, most individuals have distinct alleles at most loci.

What happens when apples produce seeds? Consider two trees, both with genotype AB at some locus. The possible genotypes of the offspring are AA, AB, and BB. The probability that a seed will have the AA genotype is ¼. The probability that a seed will have the BB genotype is ¼. The probability that a seed will have the AB genotype is ½. If we consider many loci, at each of which the parent trees have a mixed genotype with two different alleles, then the probability of the offspring being the same as the parents at any one locus is ½. The probability that they will be the same at a large number of loci is extremely small.

Apple trees are delicate things. Unless they have exactly the right genetic makeup, they do not produce good apples. We have just seen that seeds produced by two parent trees with the same DNA are most unlikely to be the same as the parents. For this reason, apple trees are propagated not by seeds, but by cloning. A cutting is taken from the tree and allowed to take root. It will eventually grow into a genetically identical tree and produce good apples. WAIT! What about the story of Johnny Appleseed?