A homozygote is an organism which has inherited the same allele twice, once from each parent. This term is used in reference to diploid organisms, organisms which have two complete sets of chromosomes. The opposite of homozygosity is heterozygosity, in which the organism has inherited a different allele from each parent.
Technically, the term “gene” refers to a section of the genetic code which dictates the expression of a particular trait, while the term “allele” is used to discuss various forms of a gene. A locus is an area on the chromosome where a gene appears, and someone who is a homozygote has the same allele at the same loci on both sets of chromosomes. One could talk, for example, about a gene which codes for eye color, and then discuss various alleles such as blue, brown, and hazel. It is important to remember that genetic traits can be influenced by more than one gene, and sometimes interactions between multiple genes are involved, which can make the process of inheritance rather complex.
Alleles are often described in terms of being dominant or recessive. In the case of a dominant allele, only one copy of the allele needs to be inherited for the trait to manifest. With recessive alleles, on the other hand, two copies of the allele must be present for the trait to express in the organisms. When someone is homozygous for a dominant trait, it means that he or she inherited two copies of a dominant allele, and the homozygote will, as a result, pass the dominant trait on to any descendants, which means that the trait should express in those descendants as well.
A homozygote who has two copies of a recessive trait will express the trait and pass a copy of the recessive gene on to descendants. If the other parent is homozygous for the same trait, the child will express the trait, but if the other parent is heterozygous, with one copy of the recessive trait and one copy of a dominant trait which will essentially trump the recessive, the child only has a 50% chance of expressing the recessive trait.
Many genetic conditions are recessive, and people who have these conditions must be homozygous at the locus which determines whether or not the condition will express. Sometimes, multiple loci are involved, which explains why many genetic traits come in varying degrees of intensity, because people may have some of the alleles involved, but not others. Someone who is a homozygote for a recessive genetic condition will pass the problem gene on to a child, which can be a concern for people who do not want their children to develop genetic conditions or to become carriers for such conditions.