MyReadinessTest Video: Allele Interactions

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In this lesson we will look at interactions between Alleles. Our genes determine our traits or phenotype. Because we have 23 pairs of chromosomes, we have two copies of each gene, one on each of the homologous chromosomes in a chromosome pair. There are often variations within genes. Different versions of a single gene are called "alleles". Recall that a gene is a specific set of instructions, such as for making a specific polypeptide or protein. These instructions are represented in a specific sequence of DNA, thus alleles differ in their DNA sequences and may code for different instructions. Let's consider dimples. We will use a big-D for the allele that produces dimples and a little-d for the allele that does not. There are three genotype possibilities: homozygous for the dimple allele; homozygous for the no dimple allele; and heterozygous. It is easy to understand that if someone is homozygous for the big-D allele that leads to dimples, he or she will likely have dimples. Similarly someone who is homozygous for the little-d allele will not have dimples. But what about someone who is heterozygous, meaning that they have one of each allele? Individual alleles may be dominant, recessive, incompletely dominant, or codominant. A fully-dominant allele will override any other allele in the individual's genotype. The allele that is suppressed is called recessive. For dimples, the big-D allele is dominant to the recessive little-d allele. Thus, someone who is heterozygous, in other words their genotype is Dd, will likely have dimples because the big-D allele is dominant over the little-d allele and will override it. A single copy of a dominant allele produces the dominant phenotype - in this case, dimples. Thus, dimples can occur with two copies of the dominant allele, which is the homozygous dominant genotype, big-D big-D, and also with the heterozygous genotype, big-D little-d. The only way a recessive phenotype occurs is if there is no dominant allele present to override it. That means only the homozygous recessive geneotype, little-d little-d, can lead to no dimples. However, sometimes alleles are incompletely dominant. This means neither allele is fully dominant or fully recessive. As a consequence, the heterozygous genotype results in a blending or intermediate version of the traits produced by the two alleles. An example in humans is hair shape. Two different alleles code for straight hair and curly hair. Let's designate the straight hair allele as "S" and the curly hair allele as "C". Homozygous SS produces straight hair and homozygous CC produces curly hair. But the heterozygous SC produces an intermediate phenotype, wavy hair. Another type of allele interaction involves codominance. Codominant alleles are equally expressed. Neither dominates the other. For example, our blood types are determined by three alleles, usually indicated as IA and IB, which are codominant alleles, and also little "i", a recessive allele. Genotype IAIA and IAi both produce type A blood, because the IA allele is dominant to the recessing little-i allele. Similarly, people with either IBIB or IBi have type B blood, because the IB is dominant to the little-i. Type O blood is only found in individuals who are homozygous recessive, little-i little-i. But individuals who are heterozygous for both of the codominant alleles IA and IB have type AB blood. Both alleles are equally expressed. To summarize, dominant alleles override recessive alleles to produce the dominant phenotype. Incomplete dominance occurs in heterozygotes, when neither allele is fully dominant, producing an intermediate phenotype. And codominant alleles are equally expressed, resulting in both phenotypes.