Genetic dominance refers to the differences in strengths of physical expression of one gene over another. Traits, such as hair or eye color are determined by genes, and genes are frequently available in different variations (alleles). The reason why there are more brunettes than redheads, and more brown eyes than blue eyes is a matter of their dominance relationship. Some alleles, such as those for brown hair/eyes can dominate other "recessive" alleles. Although variations of dominance exist, the basic difference between dominant and recessive genes is that if the dominant gene is present it will mask the presence of the recessive gene.
Types of Dominance
Basic complete dominance occurs when there is one dominant gene and one recessive gene within the genome (genetic makeup). In this case, the organism has either two dominant alleles, two recessive alleles, or one of each. If the individual has at least one dominant allele they will express the dominant trait. Besides simple dominance, there are many other types that impact what you see in every day life (Miller, p 273).
Incomplete Dominanceheterozygous organism with a combination phenotype (Miller, p 273). Snapdragons can show incomplete dominance in their color. If a homozygous red snapdragon is bred with a homozygous white snapdragon the result will be a pink snapdragon. The offspring was a mixture or blend of its two parents .
In codominance, again, neither allele is completely dominant over the other one or they can be equally dominant. The phenotype looks different from that of incomplete dominance. There isn't a blending of the traits, but rather both alleles are present in the phenotype. An example of codominance is the human blood type AB. Someone with the blood type AB has both the A antigens and B antigens on the surface of their cells .
Polygenic traits are controlled by more than just the dominant allele. They result from the relationship between many different alleles at different loci . In these traits there are more than two possible phenotypes. A good example of a polygenic trait is a human's height. It is based around a number of different factors like bone size, and those are determined by many genes. More examples of polygenic traits are human eye color, skin color, hair color, and animal fur.(Miller, p 273) (F.liu et.al 2010)
Autosomal Dominant Gene
Autosomal dominance is one of the ways that a disease, disorder, or trait can be passed on from generation to generation . In an autosomal disease, the abnormal gene is associated with the non sex chromosomes. This means that males and females are at the same risk. .
With an autosomal disease or disorder, only one abnormal gene is needed to be passed down from a parent to child for the child to inherit the gene and disease. When this defective gene is paired up with another gene it "dominates" over its counterpart. Even if one parent passes down a normal gene, the offspring would still inherit the disease. This means that if a parent carries this defective gene their children have a 50% chance of inheriting the disease .
Gregor Mendel, often known as the father of genetics, was an Austrian monk responsible for his monastery's garden. He became curious as to how traits were passed on through generations of hybrid plants. Mendel started to experiment with the peas in his garden (Miller, p 264).
He crossbred two homozygous (purebred) plants with opposite traits. For example, he crossed a purebred tall pea plant with a purebred short pea plant. His results showed that every hybrid was tall. Mendel came to the conclusion that some traits are dominant and others are recessive. Mendel repeated his experiment, but crossbred two hybrid plants instead of two purebreds. This resulted in three quarters of the plants displaying the dominant gene, tallness, and one quarter showing the recessive gene, shortness. He concluded that the dominant trait had only masked the recessive trait before and the responsible inherited units segregated independently, thus, making it possible for a recessive trait to show up in the second generation (Miller, pp 265-266).
Punnett SquaresPunnett square is a tool used to determine the possible gene outcomes of a cross between two parents when their genetic combinations are known. It was named after Reginald Punnett, an English geneticist .
First off, the Punnett square starts off as a square divided into four different boxes. The types of gametes produced by one parent are put on the top side of the square and the other parent's possible gametes are put on the left side of the square. The dominant alleles are written with a capital letter and the recessive alleles are written with the same lower case letter. The genetic outcomes can then be predicted by matching up the alleles in the different squares. Those organisms that have the same two of the same alleles for a particular trait are called homozygous, and those that have two different traits are called heterozygous (Miller, p 269).
- Biology, Miller/Levine, copyright 2008, published by Pearson Prentice Hall, p. 262-285
- Dominance College of DuPage
- Autosomal dominant by Chad Haldeman-Englert. Medline Plus. 06/24/07.
- Exceptions to Simple Inheritance by Dennis O'neil. April 08, 2008.
- Liu F, Wollstein A, Hysi PG, Ankra-Badu GA, Spector TD, et al. Quantification of Human Eye Color Highlights Genetic Association of Three New Loci. 2010. PLoS Genet 6(5): e1000934. doi:10.1371/journal.pgen.1000934