MyReadinessTest Video: Zygosity

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In this lesson, we will differentiate between the terms "heterozygous" and "homozygous". This is a karyotype, which shows all of the chromosomes from one individual. Recall that our somatic cells contain 23 pairs of chromosomes. The two chromosomes that make up a pair are referred to as homologous chromosomes; and one of them came from mom while the other came from dad. Each of our genes occupies a specific position, or locus, on a specific chromosome. Because our chromosomes are in pairs, a single gene will be found at the same locus on both of the homologous chromosomes in the pair. Here the colored bands represent a single gene on this particular chromosome. Notice that on the first chromosome this gene is indicated with red, whereas on the other chromosome it is indicated in blue. This is to show that, although this is the same gene, there can be variations of that gene. Different versions of a single gene are called alleles. So, for any gene, you may have two of the same alleles or you may have two different alleles. Although we are using color here for simplicity, recall that a gene is a specific set of instructions, such as for making a certain polypeptide or protein. These instructions are represented in a specific sequence of DNA. Thus, alleles differ in their DNA sequences. The term "hetero" refers to different; thus, if the homologous chromosomes contain different alleles for a certain gene, the individual is said to be heterozygous for that gene. In contrast, "homo" refers to the same; so if the homologous chromosomes contain the same allele, the individual is homozygous for that gene. Genotype refers to the specific alleles that an individual has and we typically indicate alleles by using letters. Let's consider the genotypes related to dimples. We will use a big-D for the allele that produces dimples and a little-d for the allele that does not. A person would be heterozygous for this gene if they have both a big-D allele and a little-d allele, which we could write like this. But there are two possibilities for being homozygous. An individual with the genotype of dd is homozygous for the no-dimple allele, and someone with the genotype of DD is homozygous instead for the dimple allele and has dimples.