Most reptiles (and other animals that you would normally think of) are diploid. This means they have two copies of each chromosome (strand of DNA). Because they have two copies of each chromosome and the genes are on the chromosomes, they must have two copies of each gene. One copy comes from each parent. These two copies are called alleles.



Therefore, because you have two copies of each gene (two alleles) you can either have two copies of the same version of the gene or your two copies can be different. If your two copies match, you are homozygous. If your two copies don't, you are a heterozygous.



So for example, 4 o'clock flowers have a gene that affects flower color. They either have a red version or a white version of the gene. Each plant has two copies, therefore the plant can have

- two red copies (homozygous red - produces red flowers)

- two white copies (homozygous white - produces white flowers)

- one red and one white (heterozygous - produces pink flowers)





What the heterozygote looks like depends on the nature of the two alleles. Some alleles are both expressed (like the flowers) while others produce nothing and are therefore hidden by the alternate (dominant) allele. So a cornsnake that is heterozygous for albinism has a copy of the albino allele, but it is hidden by the other normal allele.
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Chris Harrison

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