Heterozygous and homozygous are two genetic possibilities in humans. There are of course other exceptional possibilities and even heterozygous or homozygous genes have variations. However, most newborns are either homozygous or heterozygous.
As is the case with the standards of nomenclature in biology, including genetics, homo means one or the same and hetero means two or different. Homozygous is a pair of genes that have the same characteristics. Heterozygous is a pair of genes that have different characteristics.
You are perhaps aware that humans get two genes from their parents, one from the father and one from the mother. These two genes are also known as alleles that go on to form the one gene of the newborn. The two genes from the mother and father respectively may be of similar nature, thus having the same characteristics, or they may be different.
For instance, the mother and the father may be both genetically tall or short, they may have light or dark hair and they may both have black eyes, blue eyes or brown eyes. In every case, the genes of the father and mother can be dominant or recessive. For instance, tall parents have a dominant gene while short parents have a recessive gene. But that doesn’t matter as long as both the genes have the same traits, regardless of being dominant or recessed.
Heterozygous genes are a result of two different genes. Hence, if one gene is dominant then another has to be recessive and vice versa. Two dominant or two recessive genes cannot be heterozygous. They have to be homozygous. To make it simpler, if a tall parent and a short parent have a child, then it would be a heterozygous pair of genes. If both parents are tall or short, they it would be a homozygous pair of genes.
In case of heterozygous genes, the newborn may pick the trait of either parent or they may develop a new trait of their own as a result, which is referred to as mutation. For instance, a child born to a tall parent and short parent may not be short or tall. He or she may be somewhat of an average height. However, there is the possibility of the child being too short or too tall. A similar example can be drawn with eyes. If one parent has blue eyes and the other parent has brown eyes, then it is possible that the child may have blue eyes or brown eyes. It is also possible that the newborn will have green eyes. In this case, the green eyes are a result of the mutation of the two alleles of the gene. Neither blue nor brown eyes get inherited by the child.
In most cases, heterozygous genes offer more possibilities than homozygous genes. In homozygous genes, most kids will pick up the dominant trait of their parents. Tall parents will have tall kids. Both parents with black or brown eyes will have kids with black of brown eyes. Both white parents will have white kids. Both black parents will have black kids. If you replace one dominant trait with a recessive trait, one particular characteristic with another, then you have the possibility of either trait dominating or a new trait forming. Heterozygous genes are at the crux of mutation.
Heterozygous Genetic Traits
Let us imagine a scenario where ‘G’ is the dominant gene and ‘g’ is the recessive gene. Hence, homozygous genes or a homozygous pair of genes will be either GG or gg. A heterozygous pair of the same genes would be Gg or gG. Now, let us consider the possibility of one or both genes being affected by some disease or ailment as well as genetic traits that would be passed down to the newborn.
• If both parents are healthy, that is having GG and GG pairs, then the newborn will be absolutely healthy and normal. The baby will be born with GG. There is no possibility of any other trait.
• If one of the parents has a heterozygous pair, Gg or gG, and the other parent has GG or dominant homozygous pair, then the child has a fifty-fifty chance of going either way. The newborn may have a homozygous dominant pair, GG. The newborn may have a heterozygous pair, Gg or gG.
• If both parents have heterozygous pairs, Gg or gG, then the newborn has a three fourth possibility of having the same traits. The newborn could still be healthy with dominant homozygous pair of genes but that is just 25%.
• If both parents have recessive gene pairs (gg and gg), then the newborn would have recessive genes. If one parent has a homozygous recessive pair and the other parent has a homozygous dominant pair (gg and GG), then the result would be a heterozygous pair, one dominant and one recessive gene.