A short video explaining gene inheritance and demonstrating how to use a punnett square
Some characteristics are controlled by a single gene, such as fur colour in mice and red-green colour blindness in humans.
Genes might have different forms, and these are called alleles.
Chromosomes are found in the nucleus of a body cell in pairs. One chromosome is inherited from the mother and one is inherited from the father. The chromosomes in each pair carry the same genes in the same location. These genes could be the same, or different versions.
Alleles are different versions of the same gene.
For any gene, a person may have the same two alleles, and is described as homozygous, or two different alleles and is described as heterozygous.
For example, for this gene, with two different alleles, shown as green and red, three combinations are possible:
The genotype of an organism is its genetic description and is based on the collection of alleles that it has.
The genotype is expressed as a phenotype - these are the characteristics that we see.
In the heterozygous individual, A, if the allele that enables the person to produce the pigment in brown eyes is present, it does not matter what the second allele is - the person will be able to produce the pigment, and will therefore have brown eyes.
Alleles may be either dominant or recessive:
A dominantallele is always expressed, even if only one copy is present. Dominant alleles are represented by a capital letter, for example, A.
The allele for brown eyes is dominant. Only one copy of this allele is needed for a person to have brown eyes. Two copies will still give brown eyes.
A recessiveallele is only expressed if the individual has two copies and does not have the dominant allele of that gene. Recessive alleles are represented by a lower case letter, for example, a.
The allele for blue eyes is recessive. Two copies of this allele is needed for a person to have blue eyes.
In a homozygous individual, both alleles are identical for the same characteristic, for example AA or aa.
In a heterozygous individual, the alleles for the same characteristic will be different, for example Aa.
Most characteristics, including eye colour, are controlled by more than a single gene.