Entre el Estatuto y el Tribunal Internacional de Justicia (1973-1974)

Many large towns and cities in the UK have sickle-cell anaemia clinics.

Sickle-cell anaemia is caused by a mutant allele of the gene controlling the production ofβ-globulin, one of the polypeptides in a haemoglobin molecule (Figure 6). Although the mutant allele changes only one amino acid in theβ-globulin chain, its effect is striking.

When the concentration of oxygen is low, haemoglobin molecules with the mutantβ-globulin chains have abnormally low solubility and form fibres within red blood cells. This causes the red blood cells to change from disc-shaped cells to sickle-shaped cells. You can see these differences in Figure 7. The sickle shape reduces the surface area of red blood cells.

Sickle cells are also targeted for destruction by the immune system, so have a much shorter life span than normal red cells (about 15 days compared with the normal 120 days). Both these differences result in anaemia. Sickle cells are also liable to get stuck in capillaries, so that nearby tissues become starved of oxygen.

1 The mutant allele changes only one amino acid in theβ-globulin chain. It is likely that the mutation involves a base substitution and not a base deletion. Explain why.

2 Sickle cells have a smaller surface area and shorter life span than normal red blood cells. Explain why both these differences result in anaemia.

The production ofβ-globulin is controlled by a single gene that has two alleles. We will call theβ-globulin gene Hb and its alleles Hb^Aand Hb^S. Table 2 shows the possible genotypes and phenotypes for this characteristic.

Figure 6 A haemoglobin molecule consists of four polypeptides, twoα-globulin chains and twoβ-globulin chains, and four haem groups that bind to oxygen.

α-chain

α-chain β-chain β-chain

haem groups

Figure 7 Red blood cells from a person suffering sickle-cell anaemia. Sickle cells have haemoglobin molecules withβ-globulin chains that differ by one amino acid from those in normal disc-shaped cells.

Genotype Phenotype

Hb^AHb^A All red blood cells have normal haemoglobin and are disc shaped.

Hb^AHb^S

Almost all red blood cells have normal haemoglobin; a few have abnormal haemoglobin and become sickled at low oxygen concentrations. People with this genotype have the sickle-cell trait,

but are healthy and usually show no symptoms of sickle-cell anaemia.

Hb^SHb^S

All red blood cells contain abnormal haemoglobin, which causes all red blood cells to become sickled in low oxygen concentrations. People with this genotype have sickle-cell anaemia, the

complications of which can shorten their lives.

Table 2 The genotypes and phenotypes associated with sickle-cell anaemia.

Stabilising selection occurs on birth mass in humans. Babies with very low or very high birth masses have a higher infant mortality rate than those with a birth mass near the mode of the range.

sickle-shaped red blood cell

normal red blood cell with biconcave disc shape

Speciation

3 What does Table 2 tell you about the Hb^Aand Hb^Salleles?

4 Sickle-cell anaemia can shorten people’s lives. Explain why you might expect natural selection to reduce the frequency of this allele in human populations.

Despite its apparent disadvantages, sickle-cell anaemia is common among people of African, Middle Eastern or Southern Mediterranean descent. To explain this, we need to look at how malaria is spread.

Malaria is common in Africa and in parts of the Middle East and the Southern Mediterranean. Malaria is caused by a single-celled organism, called Plasmodium falciparum. This parasite is transmitted when a female Anopheles mosquito bites and sucks blood from an infected person and then injects saliva, containing several parasites, as she bites another person. Figure 8 shows a female Anopheles mosquito feeding on human blood. Once in the blood system of a human host, the parasites enter the host’s red blood cells where they use oxygen within the red blood cells for their own respiration.

5 Suggest how the behaviour of a red blood cell that is infected by P. falciparum will differ in someone who suffers from sickle-cell anaemia and someone who does not.

The reduction in oxygen concentration caused by respiration of P. falciparum causes the red blood cells of a sickle-cell anaemia sufferer to become sickled. This causes them to be destroyed by the body’s immune system within about 15 days. As they are destroyed, the P. falciparum within them is also destroyed. This makes people with the sickle-cell trait less susceptible to malaria than people with no sickle-cell condition.

6 In an area where malaria is endemic, which genotype, Hb^AHb^A, Hb^AHb^Sor Hb^SHb^S, will be at a selective advantage? Explain your answer.

7 What is the probability that two parents, both of whom have the sickle-cell trait, will have a child that suffers from sickle-cell anaemia? Use a genetic diagram to justify your answer.

Figure 8 A female Anopheles mosquito must have a blood meal before she lays her eggs. She pierces human skin with her sharp mouthparts and sucks blood. If she carries the malarial parasite (Plasmodium falciparum) she will infect the human she has bitten.

Speciation

Figure 9 on the next page shows three closely related species in the taxonomic family Canidae. The three dogs belong to different breeds but they are all the same species called Canis familiaris. Different breeds of dog originated when humans bred ancestral dogs that had useful characteristics.

Jack Russell Terriers (c) were originally bred for hunting foxes, Border Collies (d) were originally bred for herding sheep and Bullmastiffs (e) were originally bred to find and immobilise poachers. When humans breed animals or plants for their useful characteristics, we call this artificial selection. The other animals in Figure 9, the wolf (Canis lupus) and the jackal (Canis aureus), are naturally occurring species. They arose by natural selection. The formation of new species by natural selection is called speciation.

You learnt in your AS Biology course that organisms belong to the same species if they breed together in their natural habitat and produce fertile young. So how can one species give rise to another during speciation?

Some people imagine that a female produces one group of young, some of which belong to a different species from herself. This is possible, but it is thought to occur mainly in plants. For example, a failure of homologous Canidae is the name of a

taxonomic group, called a family.

Name the taxonomic group represented by a)Canis and b)familiaris.

Q 5

Figure 9 These animals belong to the same taxonomic family, Canidae. a) is a wolf; b) is a jackal; c), d) and e) are three breeds of dog – c) is a Jack Russell Terrier; d) is a Border Collie; e) is a Bullmastiff. The different species resulted from natural selection and the different breeds of dogs resulted from artificial selection by humans.

Figure 10 This male fruit fly has very short (vestigial) wings, which make it less successful in courtship than males that have long wings.

chromosomes to separate during meiosis in plants can result in offspring that contain multiple copies of the haploid complement of chromosomes.

This condition is called polyploidy. Polyploid plants are unable to breed successfully with plants that have the normal, diploid complement of chromosomes. In our definition of a species, these polyploid plants belong to a new species that is different from the diploid plants from which they originated. However, this rarely, if ever, happens in animals.

The accumulation of genetic differences is vital for speciation to occur by natural selection. Figure 10 shows a fruit fly, belonging to the genus Drosophila.You might have seen fruit flies like this on over-ripe fruit in your home or around your dustbin in summer. There are many different species of fruit fly. The differences between them are so small that you would probably not be able to tell one from another. However, the flies can tell.

Before mating, male and female fruit flies undergo a courtship ritual. The male performs a dance, during which he vibrates his wings, changes his body position and licks the female. The whole sequence, including the response of the female, is controlled by many genes and is species-specific.

If a male does not have the correct courtship dance, the female does not allow him to mate. The male in Figure 10 is homozygous for a mutant allele of the gene for wing length. As a result, he has very short (vestigial) wings.

This will not kill him, but it means that his courtship dance does not attract a female to mate with him. He will not pass on his alleles of the gene for wing length.

As a result of gene mutation, the male fruit fly in Figure 10 will die without leaving any offspring. His mutated genes will not be passed on. However, suppose these genetic changes occurred not in an individual but in an entire population that was isolated from another population of the same species. We now have conditions in which speciation can occur.

a)

d) e)

b)

c)