Heredity (part 4): Gene, Chromosome Mutations & Natural selection

1. Mutations

2. ***What are mutations? • A permanent change, a structural alteration, in the DNA • Mutations can be caused by many factors including 1) environmental insults such as radiation and mutagenic chemicals 2) random chance events e.g. fault in the replication of the gene or chromosome resulting in a modified gene or altered chromosome

4. Mutagenic agents • The rate of spontaneous mutation is usually very low • However, mutagens present in the external environment greatly increase the rate of mutation • Examples of mutagens: i) ultraviolet light ii) alpha, beta, gamma radiations iii) radioactive dust iv) mustard gas, formaldehyde, ferrous and manganous salts and lysergic acid diethylamide (LSD)

5. 2 types of mutations • Gene mutation i) albinism ii) sickle cell anaemia • Chromosome mutation e.g. Down’s Syndrome

6. 1) Albinism • Caused by the absence of pigments in the skin, hair and eyes of animals - reddish white skin - white hair - iris appears red (colour of blood vessels in it) • It is a recessive mutation, caused by a recessive allele • Sensitive to light and skin is easily sunburnt • Rare mutation: 28 per million gametes produced

7. 2) Sickle-cell anaemia • Results from a slight structural change in the genetic material (DNA) • Mutated gene is recessive so it expresses itself only in the homozygous recessive condition • Sufferers have abnormal haemoglobin in their red blood cells (sickle-shaped RBCs) that are unable to transport O2 efficiently • Affected persons die young

8. Effects of sickle cell anaemia • Pain: The sickle-shaped red blood cells are sticky and have trouble passing through small blood vessels in the body. The cells get stuck, clump together, and block the flow of blood. Blocking off the blood causes pain. This is called a sickle cell crisis. • Anaemia: The sickle-shaped red blood cells die quickly. There are then not enough red blood cells to carry oxygen throughout the body. The result is fatigue, paleness, and shortness of breath. • Delayed growth: Anaemia slows the rate of growth because the body's cells don't get the oxygen they need to grow. Children and adults with sickle cell anaemia often have a smaller build than other people of the same age. • Eye problems: The eye can be damaged by the lack of oxygen; it can be serious enough to cause blindness. • Infections: People with sickle cell anaemia are more vulnerable to infections because of damage to their immune systems from the disease. • Stroke: If the blood flow to a part of the brain is blocked by the sickled cells, a stroke can occur. • Acute chest syndrome: Chest pain and fever caused by sickle cell anaemia can become a life-threatening complication.

9. Sickle-cell anaemia (continued…) • Under normal circumstances, the harmful allele would have been eliminated as the affected individuals would die before reproducing • However in West Africa where malaria is prevalent, individuals who are heterozygous [one dominant (normal) allele and one recessive (sickle-cell) allele] for the sickle-cell allele have apeculiar advantage over those with the normal allele

10. Sickle-cell anaemia (continued…) • Heterozygous individuals do not show the disease although a small percent of their RBCs do show sickling • These heterozygous individuals are more resistant to malaria than individuals who have two copies of the normal haemoglobin allele • As a result these heterozygous individuals have a better chance of survival and reproduction than normal individuals in regions where malaria is present

12. Chromosome mutation • Results in a change in the chromosome number • People suffering from Down’s Syndrome have 47 chromosomes in their body cell (one extra chromosome in the 21st chromosome pair)

13. Are all mutations harmful? • Activity: • In groups of 4, • Name the benefits of such a mutation, • Name the disadvantages of such a mutation

14. Mutation and Selection • Not all mutations are harmful/lethal • Some mutations are beneficial to plants/animals e.g. a mutation that allows the individual to 1) avoid predators because it hasbetter camouflage 2) contain achemical which predators find unpleasant • Such individuals will leave more offspring than individuals without the mutation • Nature ‘selects’ those organisms with more favourable characteristics to survive to reproduce their own kind e.g. more competitive species, more resistant to diseases, better adapted to changes in the environment (natural selection)

16. Evolution • Proposed by Charles Darwin (English Naturalist) • Studied and gathered evidence from nature to support his Theory of Evolution by Natural Selection • e.g. evolution of antibiotic resistance

17. Theory of Evolution • Mutations provide new alleles/genes (variation for natural selection to operate on) • After thousands or millions of years, more and more beneficial qualities may accumulate • The new breed of organisms may become better adapted to their new environment • They may change so much that they become a new species which differs from the original stock from which they evolved

18. Possible one day…?

19. Example of natural selection • Industrial melanism • Light coloured moth with speckled wings • Black moth (melanistic form)

20. Industrial melanism: Natural Selection in action • Before the Industrial revolution in Britain, the speckled variety was more abundant (less likely to be eaten by birds as they were well camouflaged in lichen-rich tree trunks) • The Industrial revolution caused pollution which killed lichens and deposited soot on the tree trunks (therefore black moth was well camouflaged on the sooty bark whilst the specky form were easily seen and eaten by birds) • Therefore in industrial areas, the black variety became more abundant and the speckled form became rare