Word Relationships
Over the last few weeks, we've been learning a lot of new terms and ideas about genetics. Because of this, it is important to make sure I understand the relationships between individual terms. Below, I have a few sets of words and their corresponding sentences.
A hybrid (or heterozygous) trait is a pair of a dominant and a recessive allele - whenever there is a hybrid trait, the dominant trait always dictates the phenotype.
A Punnett square can be used to determine the genotype of the offspring of two organisms.
A recessive trait is always masked by a dominant trait and is therefore only expressed in a phenotype when paired with itself.
In Mendelian inheritance, genes come in dominant and recessive alleles.
Inheritance Patterns
The past few weeks we've studied three main patterns of inheritance: dominance/recessiveness. (Mendelian inheritance), incomplete dominance, and codominance (both non-Mendelian inheritance). An additional pattern of inheritance we studied briefly was sex-linked traits.
Dominance/recessiveness is the most straightforward method of inheritance. In this pattern, each allele has two copies, dominant (A) and recessive (a). Whenever the dominant allele is present (when the genotype is either AA or Aa) it is expressed in the phenotype. The recessive allele is only expressed when there is no dominant allele present (aa). An example of dominance/recessiveness is dwarfism. Dwarfism is a dominant trait, and therefore whenever it is present, even in just one copy of an allele, it is expressed.
Incomplete dominance is a bit more complicated. When incomplete dominance occurs, the two alleles are both partially dominant. This results in the two blending to create a new variety. For example, if a tall horse and a short horse mated, incomplete dominance would result in a medium-sized horse.Codominance is when both alleles are fully dominant and are both expressed. An example of this is in blood types. If two parents have AA and BB blood, their offspring will be AB, a codominant blend of their parent's blood types.
Sex-linked traits work differently than all of the above. They are inherited through a child's sex chromosomes, usually the X chromosome. Because of this, whether or not a sex-linked trait is expressed can depend on the sex of the offspring. An example of this is colorblindness. Colorblindness is more common in males simply because of the fact that they have only one X chromosome. Females have two, therefore if only one chromosome has the mutation, it is not expressed - they have to have the mutation in both of their X chromosomes to be affected.
Eye Color And Polygenic Traits
A polygenic trait is a trait controlled by multiple genes. Due to their complexity, polygenic traits are not fully understood yet.
An example of a polygenic trait is eye color. In the article, the author says that "In the simplest models of eye color, there are two genes involved." Each gene has two possibilities - for one, brown or blue; for the other, green or blue. These variants have their own pattern of dominance: the brown is dominant over the green and blue, the green is dominant over the blue, and the blue is recessive to all. Once this is understood, the the genes work themselves out so that the most dominant gene is expressed.
