Inheritance and Variation

17.1 Variation

Investigating. Measure the height of students or the length of leaves (continuous: plot a histogram, which gives a bell-shaped curve), and compare it with a feature such as blood group or tongue rolling (discontinuous: a bar chart).

17.2 DNA

• DNA controls cell function by controlling the production of proteins, including enzymes.
• The sequence of bases in a gene determines the sequence of amino acids needed to make a specific protein.
• Different sequences of amino acids give different shapes to protein molecules, and the shape determines the protein's function (for example the shape of an enzyme's active site).

A pairs with T and C pairs with G, so the other strand is T A A C G T.

17.3 Inheritance

Terms

Two identical homozygous individuals that are bred together are pure-breeding: all their offspring will be the same.

Monohybrid crosses

A monohybrid cross looks at the inheritance of one gene. Use a genetic diagram or Punnett square.

Example. Tall (T) is dominant to dwarf (t) in pea plants. Cross two heterozygous (Tt) plants.

Offspring genotypes: 1 TT : 2 Tt : 1 tt. Phenotypes: 3 tall : 1 dwarf, a ratio of 3 : 1.

Cross a heterozygous plant (Tt) with a homozygous recessive plant (tt) (a test cross):

Offspring: 2 Tt : 2 tt, so 1 tall : 1 dwarf (a 1 : 1 ratio).

Why do observed ratios differ from expected ratios? The Punnett square gives only the probability. Fertilisation is random, so when the number of offspring is small, the observed ratio can be quite different. With a large number of offspring, it gets closer to the expected ratio.

The child is bb, so each parent passed on a b. The parents are brown-eyed (dominant allele B), so both are Bb (heterozygous).

Codominance: ABO blood groups

The ABO blood group gene has three alleles: $I^A$, $I^B$ and $I^O$. $I^A$ and $I^B$ are codominant, and both are dominant to $I^O$ (recessive).

Sex determination

Humans have 23 pairs of chromosomes, one pair being the sex chromosomes. Females are XX and males are XY.

All eggs carry an X chromosome, and half of the sperm carry an X and half a Y. So there is a 50% chance of a boy (XY) or a girl (XX) at each birth. The sex of the baby is decided by the sperm that fertilises the egg.

Mutation

Example: sickle cell anaemia. A change in one base in the gene for haemoglobin gives a different haemoglobin, which makes the red blood cells a sickle shape, so they carry less oxygen and block capillaries.

Example: Down's syndrome. A person has 47 chromosomes instead of 46 (an extra chromosome 21).

Sources of genetic variation in populations: mutation, meiosis (which shuffles the alleles into gametes), random mating and random fertilisation.

Ionising radiation (such as X-rays, gamma rays and ultraviolet light) and some chemicals (such as chemicals in tobacco smoke) increase the rate of mutation.

17.4 Selection

Natural selection

The steps:

1. There is variation within a population (for example due to mutation).
2. Organisms produce many offspring.
3. There is a struggle for survival, because of competition for resources, predators and disease.
4. Individuals that are better adapted to the environment survive and reproduce.
5. They pass on their alleles to the next generation, so the proportion of those alleles increases.

Over many generations, the inherited features of a population change (evolve) as a result of natural selection.

Antibiotic-resistant bacteria (such as MRSA). In a population of bacteria, a random mutation can make one bacterium resistant to an antibiotic. When the antibiotic is used, the non-resistant bacteria are killed, but the resistant one survives and reproduces, and passes on the allele, so the resistant strain increases. This is an example of natural selection.

Artificial selection (selective breeding)

1. Select animals or plants with the desirable features (for example high milk yield).
2. Cross them (breed them together) to produce the next generation.
3. Select offspring that show the desirable features.
4. Repeat over many generations.

It is used to produce economically important organisms, such as cows with a high milk yield, crops with a high yield or resistance to disease, and fast-growing chickens.