Non-Mendelian Patterns of Inheritance Quiz

Study Notes

Non-Mendelian Patterns of Inheritance

In the realm of genetics, we typically associate the inheritance of traits with the principles established by Gregor Mendel. However, some patterns of inheritance deviate from Mendel's simple one-to-one relationship between alleles and traits, instead exhibiting more complex dynamics. These non-Mendelian patterns provide us with a more diverse and nuanced understanding of how genes are inherited and expressed.

Codominance

In codominance, both alleles are expressed, contributing equally to the expression of a trait. This deviates from the dominant-recessive model where one allele masks the other. Blood types O and A/B are examples of codominance, as both alleles (O and A/B) are visible in a person with the blood type AB.

Incomplete Dominance

Unlike codominance, where both alleles are expressed equally, incomplete dominance results in a phenotype that is intermediate between the two parental alleles. This means that neither allele is fully dominant over the other. One well-known example of incomplete dominance is the color of flowers in certain species of snapdragons, such as Antirrhinum majus. Flowers with the red allele (R) and white allele (W) will display pink flowers.

In some cases, traits are sex-limited or sex-influenced. Sex-limited traits are only expressed in one sex, whereas sex-influenced traits show varying levels of expression depending on the sex. For example, the gene responsible for red-green colorblindness (RS) is on the X chromosome. Males are more likely to be colorblind because they only have one X chromosome, while females have two, providing them with a backup copy of the gene.

Multiple Alleles

Whereas Mendelian genetics is based on two alleles, non-Mendelian inheritance can involve multiple alleles. In these cases, the number of alleles can range from three to many more. For example, the ABO blood group system has three alleles (A, B, and O). An individual can carry two of these alleles (A-A, A-B, B-B, or A-O). The combination of these alleles determines their blood type.

In the next sections, we will explore these non-Mendelian patterns of inheritance with more specific examples, deepening our understanding of the complexities that DNA presents us with. Remember, however, that these patterns are not exclusive. A single trait can exhibit more than one type of non-Mendelian inheritance. Understanding these patterns and their exceptions allows us to better predict and analyze the inheritance of genetic traits in species across the tree of life.