Lesson 6. Different varieties of a gene

In this lesson we learn that each individual has two copies of the same gene. These copies may be the same or different. We will also learn that these copies can influence each other's expression.

Although there can be up to dozens of different copies (alleles) of a single gene in a population, a single individual can only have two alleles of each gene: one on the paternal chromosome and one on the maternal chromosome. These alleles are called a gene pair. So, one of the alleles in a gene pair comes from the father and the other from the mother.

A gene pair can consist of two identical alleles, or it can consist of two different alleles. If an individual has two identical alleles of a particular gene, it is said to be homozygous for that gene. If it has two different alleles, it is heterozygous.

A homozygous individual can pass only one type of allele of the gene to its offspring. A heterozygous individual can pass on both alleles, but only one to each offspring. There is a 50 % chance of either happening.

Three pairs of chromosomes, each carrying a different gene pair: AA, Ab and aa.

All the different alleles of the gene affect the same trait, usually in slightly different ways. Genes also interact with each other and with environmental factors.

Both alleles of a gene pair can be expressed equally in an individual. This type of gene effect is called additive. An additive gene effect refers to the presence of plus and minus alleles that increase and decrease a trait. For traits that are mainly regulated by additive gene effects, the phenotype of the offspring will be the average of the parents. For example, withers height and many other conformation traits are additive.

Often the alleles of a gene pair also influence each other’s expression in some way. One allele can block the expression of another allele, either completely or partially. Such an allele is called dominant. An allele whose effect is blocked is called recessive.

A dominant allele expresses itself, i.e., produces a dominant phenotype when an individual has either one or two copies of the allele. A recessive allele produces a recessive phenotype only in individuals with two copies of the allele, i.e. one from each parent.

The dominant allele is indicated by a capital letter and the recessive allele by a lower-case letter.

An individual’s alleles at a particular locus are called the genotype. The outwardly visible trait is called the phenotype.

Examples

Long coat. Long hair in cats and most dogs is caused by recessive mutations in a gene called FGF5. The original wild-type allele is dominant and always results in a short coat. A long-haired individual must have two long-hair alleles (recessive).
Coat colour. In dogs, cats, horses, and many other species, the MC1R gene (E locus) determines whether the coat can be dark or not. The original allele is dominant (E) and the mutated allele is recessive (e). The E allele allows an individual to produce dark pigment (eumelanin), and all individuals carrying this allele are dark brown or black. The genotype of these individuals is either E/E or E/e. The e allele is the result of a mutation that has resulted in a non-working gene. This only happens in individuals with an e/e genotype. No dark colour is produced, instead a yellow pigment (pheomelanin) is produced.

The genotype of a black dog is either E/E or E/e.

The phenotype is black.


What have we learned?

  • Although a single gene can have up to dozens of different alleles in an animal population, a single individual has only two alleles of each gene. These alleles make up a gene pair.
  • If the alleles of a gene pair are identical, the individual is said to be homozygous. If the alleles are different, the individual is heterozygous.
  • One allele in a gene pair can block the expression of another allele in the pair. Such an allele is called dominant. An allele that is blocked is called recessive.

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