Inheritance

The individuals of any species are never exactly the same. This is partly because they have different genes and partly because they have had different experiences in their lives or lived in different environments. For example, a single gene determines the colour of a persons eyes and the environment has no effect. In contrast, any scars a person has are only the result of the environment. Some things are a mixture of both; for instance a tall person may have genes that help make them tall, but they also need to eat the correct food to help them grow.

The difference between continuous and discontinuous variation often confuses people. Continuous variation means that a characteristic can be almost anything between two extremes. For example, some people are very tall and some people are very short. These are the extremes. Other people can be ANY size in between. On the other hand, discontinuous variation means that there are only specific categories that you can belong to. An example is blood group. A person is either A, B, AB, or O and can be either positive or negative. You cannot be half way between the two, you are either completely one or completely another.

The nucleus in cells contains a chemical called DNA. This is found in long strands called chromosomes. Humans have 46 chromosomes, whilst other organisms have more, less or even the same number as us. The DNA contains sets of coded instructions for making proteins. These proteins will help determine the characteristics of an organism. A set of coded instructions to make a protein is called a gene. Humans have about 100,000 different kinds of gene (we actually have up to two versions of each gene). Almost all of the cells in a person?s body will have exactly the same 46 chromosomes containing the same 100,000 genes. Eye colour is controlled by a single gene, because the colour is caused by a single protein. Some more complicated features, such as the brain, require many proteins so are controlled by many genes.

The only cells that are different are the red blood cells, because they don?t have a nucleus so don?t have any DNA, and the sex cells (sperm and eggs). Each sperm cell only contains half of a man?s chromosomes (23). Equally, each egg contains only half of a woman?s chromosomes. This is important because it means that when an egg and a sperm join together during fertilization, the new zygote that forms will have the correct number of 46 chromosomes; half of which were obtained from each parent.
Earlier I mentioned that we have up to two copies of each gene. One copy of each of the 100,000 genes we received from our father and the other copy of each of the 100,000 genes we received from our mother. This means that we might receive a different version of a gene from each parent. The different versions of a gene are called the alleles. There is a singe gene that controls our eye colour. However, there are several different alleles that we can have, ie blue, brown, green etc. We might get, say, a brown allele from one parent and a blue allele from the other parent. Eye colour has discontinuous variation, so we can only be one colour or the other ? we cannot be a mixture of the two. What happens is that one allele is DOMINANT to the other allele, which we call RECESSIVE. In humans, the brown eye colour allele is always dominant to the blue eye colour allele. In this case the child would have brown eyes, even though they also carry the allele for blue eyes. In this case we say they are HETEROZYGOUS, because the two alleles for that gene are different. A person who receives the allele for blue eyes from both parents will have blue eyes. Both of the alleles are the same, so we say they are HOMOZYGOUS. A good way to remember which is which is to remember that a homosexual fancies people from the same sex and a heterosexual fancies people from a different sex. We can see from this that by looking at the type of features a person has (their PHENOTYPE) we can not necessarily tell what type of genes a person has (their GENOTYPE).

Geneticists are able to predict what genotypes and phenotypes will occur in the offspring when two organisms are bred together. Any mating of this kind we call a genetic cross. It can be represented using a punnett square. For example, in pea plants, tall plants are dominant to short plants. In this case tall is dominant so we will use the letter T to represent tall and t to represent short (this is the standard way of writing it using the capital letter of the first word to describe the dominant allele and the same letter in lower case to represent the recessive allele ? we do not use T and S). If the male plant had the genotype Tt and the female plant had the genotype tt, then the possible pollen that can be made by the male are T or t. When the female plant produces ovules it will produce only t and t.

We can use a punnett square as follows to show the proportions of each genotype and phenotype that we would expect to find in the offspring.






We can see from this cross that half of the genotypes are heterozygous Tt. The T is dominant to the t, so we can say that half of the offspring will have tall phenotypes. Half of the genotypes are homozygous tt. In this case the recessive characteristic will result in a short phenotype. The ratio of tall plants to short plants in this case would be 1:1.

You need to be able to conduct crosses like this using a punnett square and be able to work out the expected ratios of the different genotypes and phenotypes. Remember that they are only ?expected? ratios. If we actually did the cross shown above with real pea plants and had 1000 offspring, we would expect about 500 to be tall and 500 to be short. However, it would not always be exact because it depends which pollen grain fertilizes which ovule so you could easily find that the numbers are a little bit out, say 485 tall and 515 short.

Sometimes a piece of DNA can be mutated. This means that it becomes slightly changed. Sometimes this occurs naturally, but it is especially common wherever there is radiation, because radioactive particles moving at incredibly high speeds can collide with the complicated DNA structure and alter it. Usually, if a normal body cell is mutated in this way it does not cause too many problems. The exception is when it causes the cell to start dividing uncontrollably into a ball of cells called a tumor. This is called cancer. Another problem is caused when the DNA of a sex cell is damaged. This is worse because the damage is passed on from one generation to the next. We call this a genetic disease.

Sickle-cell anaemia is a genetic disease that affects the red blood cells. When the oxygen levels are low the cells form into a sickle shape and can block the capillaries causing pain and in severe cases severe damage or even death. You might then ask the question ?if its so harmful, why hasn?t sickle cell anaemia disappeared through evolution?? The answer is that it can also be an advantage to have the sickle cell allele. People who are heterozygous do not suffer much from the disease, but they have the advantage that they are immune to Malaria, because the parasite cannot live in their blood. This is why in most parts of the world the sickle cell allele is very rare, but in areas with lots of malaria it is very common.

Genetic engineering is a relatively new method whereby scientists are able to take a gene from one organism and put it into another. One famous example is where the gene to make human insulin was removed from a human cell and inserted into a bacteria cell. These bacteri now make human insulin for diabetics to use. Many crop plants and farm animals can also be altered in this way. There can be problems if mistakes are made which cause harmful results that were not expected.

Modern cloning techniques allow the production of plants with identical genes. This has benefits for agriculture, because farmers know exactly how to look after their crop because there is no variation between the individual plants.