Homozygous and Heterozygous
Humans possess 23 pairs of chromosomes, out of which 22 are autosomes and one pair is of sex chromosomes, XX in females and XY in males. Since there are 23 pairs it means there is a grand total of 46 chromosomes in the nucleus of each cell. As we have two copies of each chromosome so we have two copies of each gene. Homozygous and heterozygous are the conditions which describe a gene for organisms, depending on the alleles of a trait.
Concept of Genes and Alleles
All multi-cellular organisms contain two copies of each gene. From which one copy is obtained from the mother and one from the father. These twin copies are often referred to as alleles of a gene. The genes are responsible for all the traits we possess, like height, skin color, eye color etc. All the genes possessed by an organism are called its genotype and the expression of the genotype is called phenotype.
What is meant by homozygous and heterozygous?
As described above we inherit a complete set of chromosomes from our mother and one complete set of chromosomes from our father. The two chromosomes of a pair are called homologous chromosomes which contain same genes located in the same places. There may be two forms of the same gene. These different forms of a gene are called alleles. If an organism carries two identical versions of a gene it is called homozygous for that particular trait. If the gene pair possessed by an organism contains two different versions, it is called heterozygous for that trait. Remember that an organism can not be called homozygous or heterozygous as there are thousands of genes and it can be homozygous or heterozygous at that gene locus only.
Dominant and Recessive Alleles
Alleles can be dominant or recessive. In writing, dominant alleles are usually expressed by a capital letter and recessive alleles by a small letter. e.g ‘A’ or ‘a’. The trait which is expressed by an organism depends on the dominance or recessiveness of the allele. Dominant alleles take priority over the recessive ones. The recessive allele can be expressed only in homozygous condition. For example, the allele for albinism is recessive and for normal pigment is dominant. If an individual carries ‘AA’ or ‘Aa’ it will produce normal pigment. It can show albinism only if it carries both recessive alleles i.e. ‘aa’.
