Heterozygous
Homologous chromosomes are chromosomes contained in a cell which pair together. Zygosity refers to the similarities, or dissimilarities of the genes within the homologous chromosomes of a fertilised egg, which is usually referred to as a zygote. Various forms within the gene are known as alleles, and each organism usually has two alleles for each gene, each of which contains information which has a bearing on the trait of an individual. The alleles are inherited from both parents, and each parent contributes some of their own trait information. If the gene for a particular organism has different alleles, it is said to be heterozygous, and the cell is called a heterozygote.
The trait, such as brown eyes or black hair, is determined by the dominant allele. Sometimes alleles can produce more complicated schemes, so therefore the results of the heterozygosity can themselves be more complex. If the dominant allele within a gene loses its normal function, and the other allele is already inactive, it can result in loss of heterozygosity. This can be important in diseases such as cancer since the inactivity of the dominant cell, which may be caused by a mutation, can then lead to the inaction of a tumour suppressor.
This does not mean that the individual will automatically suffer from cancer, as the other of the functional genes on the other chromosome will still activate the tumour suppressor. However, should this become inactive due to a mutation, this can lead to a cancerous tumour not being suppressed.
