MCAT Content / Mendelian Concepts / Incomplete Dominance Leakage Penetrance Expressivity

Incomplete dominance, leakage, penetrance, expressivity

Topic: Mendelian Concepts

Incomplete dominance is a form of inheritance when two different alleles express a new allele as an offspring; leakage is the gene movement from one species to another; penetrance refers to the probability of a gene or trait being expressed, while expressivity determines how well genes are expressed.

Incomplete dominance is the expression when the offspring expresses a phenotype that is intermediate between one parent’s dominant trait and the other parent’s recessive trait. In humans, incomplete dominance occurs with one of the genes for hair texture. When one parent passes a curly hair allele (the incompletely dominant allele) and the other parent passes a straight-hair allele, the effect on the offspring will be intermediate, resulting in hair that is wavy.

Leakage is the gene flow from one species to another or the introduction of new alleles from an outside source. For instance, a sika deer introduced into Western Europe reproduced easily with the native red deer. This translocation of Sika deer has led to the transfer of genetic information. There are no longer “pure” red deer in the region, and all can be classified as hybrids.

The probability of a gene or trait being expressed is referred to as penetrance. Penetrance is the extent to which a gene is expressed in the phenotypes of individuals carrying it, measured by the proportion of carriers showing the characteristic phenotype. One example of this is polydactyly (extra fingers and/or toes) in humans. A dominant allele produces polydactyly in humans, but not all humans with the allele display the extra digits.

On the other hand, expressivity refers to variation in phenotypic expression when an allele is a penetrant. It quantifies the proportion of individuals of a given genotype that will express a specified phenotype. Back to the polydactyly example, an extra digit may occur on one or more appendages. The digit can be full size or just a stub. Hence, this allele has reduced penetrance as well as variable expressivity.


Practice Questions

https://jackwestin.com/khan-academy-mcat/a-family-history-of-marfan-syndrome


Key Points

• Incomplete dominance is a form of inheritance in which two contrasting alleles are inherited such that the individual displays an intermediate phenotype or expresses a new allele.

• Leakage is the gene flow from one species to another or the introduction of new alleles from an outside source.

• Penetrance refers to the probability of a gene or trait being expressed; it is measured by the proportion of carriers showing the characteristic phenotype.

• Expressivity refers to variation in phenotypic expression when an allele is a penetrant; it quantifies the proportion of individuals of a given genotype that will express a specified phenotype.


Key Terms

incomplete dominance: a condition in which the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes

penetrance: the extent to which a particular gene or set of genes is expressed in the phenotypes of individuals carrying it, measured by the proportion of carriers showing the characteristic phenotype

expressivity: the relative capacity of a gene to affect the phenotype of the organism of which it is a part

phenotype: the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment

allele: one of several alternative forms of the same gene occupying a given position on a chromosome

gene: a section of DNA that codes for a characteristic

recessive: is the term to describe the allele which is not expressed as a phenotype

dominant: the genotype is the stronger of the two alleles and is expressed



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