Humans contain two copies of each gene, one from the father and one from the mother, which sometimes are referred to as the alleles of a gene. If a mutation occurs in just one copy of the gene then that individual is considered heterozygous. On the other hand if both copies of a gene are mutated then that individual is homozygous genotype.
Majority of hereditary disorders are harmful if both copies or alleles of a gene are affected, which means protein products from both genes may fail to operate properly. In such cases immediate medical attention is needed so the function of a defected protein can be restored through medication. In heterozygous genotypes one copy of the gene is healthy and can produce fine proteins thus these individuals are usually not affected and are considered just carriers. However in a few hereditary disorders heterozygous individuals may suffer from a milder version of the disease.
Homozygous vs Heterozygous
If we assign a letter such as B to a dominant genetic or hereditary trait then b will be a defective allele:
If homozygous genotypes are represented by BB (normal) and bb (affected) and heterozygous genotypes are represented by Bb (carrier genotype)
Below are examples and probabilities of children born to parents with different genptypes:
Parents BB x BB Two healthy parents
Children 100% BB All children will be normal
Parents BB x Bb One healthy and one carrier parent
Children 50% BB, 50% Bb Children could be half normal and half carrier
Parents Bb x Bb Two carrier parents
Children 25% BB, 50% Bb, 25%bb 25% of children could be normal, half carrier, and another 25% could be affected (mutant)
Parents BB x bb One healthy and one affected parent
Children 100% Bb All children will be carriers
Parents bb x bb Two affected parents
Children 100% bb All children will be affected mutants