The genotype is the genetic constitution of an organism. The phenotype is the expression of this genetic constitution and its interaction with the environment. Alleles are one or more alternative versions of the same gene. The alleles at a specific locus may be either homozygous or heterozygous. Alleles may be dominant, recessive or codominant. There may be multiple alleles of a single gene. Candidates should be able to use fully labelled genetic diagrams to predict the results of • monohybrid crosses involving dominant, recessive and codominant alleles • crosses involving multiple alleles and sex-linked characteristics.

An allele is a form of a gene (e.g. brown hair).

A genotype is all the alleles that an organism possesses in its genetic code.

A phenotype is the allele that gets expressed in proteins. Phenotype also accounts for environmental factors, so if you die your hair.

A locus is an area of DNA which codes for a protein, so where a gene is stored.

In an organism there are two sets chromosomes, one from each parent, and on a pair of chromosomes at the same locus there will be two alleles for the same characteristic; you could either have two of the same allele (homozygous), or you could have two different alleles (heterozygous).

In general only one of the two alleles will be expressed, so how does the body decide which one? Well, some alleles are dominant and some are recessive, this means that a dominant characteristic will always be produced if it is present, and a recessive one only gets produced when both the alleles are recessive.

On the last pair of chromosomes (23 in humans) the two chromosomes may not be identical, one may be shorter than the other and so not have a matching gene. This short version of the chromosome is called the Y chromosome and it is what makes humans male. So a female is XX and a male is XY. Having only one version of a gene is dangerous because it means that you are more likely to have only recessive genes, and often diseases are caused by recessive alleles, for example colour blindness, this means men are more susceptible to certain problems. Women can still get these problems, but it is less likely as they would have to get 2/2 recessive genes.

In some genes there is co-dominance, this means that some alleles will both be expressed if they are both present. For example if a cow has two white alleles it will be white, and if it has two black it will be black, but if it has one of each they will both work so it will come out with bits of black and white.

In general there are only two alleles for a gene, one dominant and one recessive, but in some cases there are many more alleles. In this case, to decide which one gets produced there is a hierarchy of alleles. For example rabbit fur is determined by four alleles:

The table shows the alleles in order of dominance, with grey being dominant over everything and albino being recessive to everything. So if we have a chinchilla allele with a grey allele we will get a grey rabbit; but if we have a chinchilla allele with any other allele, chinchilla, himalayan or albino, we will get a chinchilla rabbit.