MAJOR GENES: A gene may work completely on its own to cause a characteristic to develop, in which case we call it a major gene system. This type of gene system can act in different ways, the gene locus may be occupied by a number of different variants of the basic gene. These variants are called alleles. Alleles may be dominant or recessive in their influence upon a particular characteristic. In a diploid organism there are pairs of chromosomes so there are two alleles in each cell for most of the characteristics the organism possesses. A dominant allele is one which produces an affect upon the phenotype no matter what other gene allele is present with it. A recessive allele will only have an affect upon the phenotype if it is on its own or with another recessive.

Some alleles show complete dominance, ie the genotypes AA and Aa are exactly the same. In the case of many alleles, when they are in heterozygous condition the phenotype is completely like the dominant allele. For example, in the guinea pig the genotype c^kc^d gives sepia coat not a colour somewhere between sepia and cream.

In other cases the Aa type is somewhere between the AA and aa extremes and we refer to this as incomplete dominance.

In some examples the different alleles have different biochemical products within the organism and these can affect a characteristic so that it develops a mixture of the phenotypes caused by the two alleles, ie the phenotype caused by A¹A² is a mixture of the phenotypes found in A¹A¹ and A²A²types.

When the heterozygous condition is precisely half-way between the homozygous recessive and homozygous dominant phenotypes we refer to the alleles as codominants.

It should be remembered that the terms dominant and recessive refer to the action of the allele upon a particular phenotype, it is not a statement about the allele itself. Indeed an allele can be dominant in respect of one phenotype but also recessive when considered in respect of another phenotype which it also affects.

Example: Suppose that at a gene locus we have two possible alleles called X and x. The gene controls the production of an enzyme which produces a blue pigment in the petals, the mutant allele x produces a protein of slightly different shape which just happens to prevent the activity of growth substances in the plant. If we consider the phenotypes of the various genotypes we find:

Clearly X is dominant for colour but recessive for height, while x is dominant for height but recessive for colour!

In some cases there are many possible alleles for a particular gene locus and we use the term multiple alleles to describe this condition. For example, in the guinea pig the coat colour is determined by a gene which has a number of variants, or alleles. In homozygous condition they cause the following phenotypes:

Sometimes multiple alleles show different levels of dominance over each other and we refer to this as a dominance series, eg A¹ > A² > A³......> Aⁿ.

In the guinea pig example There is a dominance series of:

Thus in heterozygous individuals c^bc^k, c^bc^d, c^bc^a, we get black coats.

 In heterozygous individuals c^kc^d, c^kc^a, we get sepia coats.

 In heterozygous individuals c^dc^a we get cream coat.

 Note that c^a is recessive to all other alleles in this particular system.

Two genes may work together to influence a single characteristic, this is an oligogenic system.

Example: Comb shape in chickens.

There are two gene loci involved in the comb character. They are referred to as the P and R loci. There are two alleles at each of the loci P and p, R and r. The two loci inter-act in the following way:

If three or more genes act together to influence a single characteristic then we call it polygenic. Most of the metric or continuous phenotypes are controlled by polygenic systems.

Consider a phenotype controlled by 3 loci, A, B and C. We can have the following genotypes:

AABBCC, AaBBCC, AABbCC, AABBCc, aaBBCC, etc. If A, B and C all control the size of the organism then we could get the phenotypes shown below.

We get three different sized organisms, corresponding to one dominant locus, two dominant loci and three dominant loci.

If genes are found upon the same chromosome we say that they are linked genes. When genes are linked on the same chromosomes then they are less likely to be separated from each other during reduction division than loci on separate chromosomes. This can affect the frequency of new combinations of these genes appearing. The only way in which these linked genes can be recombined is by splitting of the chromosome and swapping of chromosome sections.

Clearly the closer the gene loci are on the single chromosome then the less likely it is that they will be separated during chiasmata formation. The recombinant chromosomes in gametes will allow new combinations of alleles to be brought together in the offspring of the next generation.

Linked genes that are so close on the chromosomes that they hardly ever get swapped during chiasmata formation and which affect the same characteristic are called supergenes, because they are inherited as if they were a single unit called a haplotype.

Some genes can act together upon a characteristic in such a way that each dominant allele amongst the group of genes will add a certain amount to the basic amount of characteristic and these are called additive gene systems. So that if the basic length of a seed was 10mm and each dominant adds 1mm then with the genotype aabb the seed would be 10mm long but with the genotype AaBb, or AAbb, or aaBB the seed would be 12mm in length. This leads to an increase in the number of phenotypic classes. With an additive system the number of phenotypic classes is 2n+1 where n is the number of additive gene loci in the system. If we look at an additive system with a single gene locus then we have the possible genotypes aa, Aa, AA which correspond to basic, basic + 1 unit, basic + 2 units.The table below makes this more obvious:

With two gene loci we can tabulate the genotypes and phenotypes like this:

The example in the diagram above is for flower colour controlled by an additive system with two gene loci, giving 5 classes of phenotype. The numbers in the boxes refer to the number of dominant alleles present in the individuals.