Emery's rule

Emery's Rule is a principle in genetics and evolutionary biology that concerns the phenomenon of heterogamety in sex determination systems. It states that the heterogametic sex (the sex with two different sex chromosomes, such as XY in mammals or ZW in birds) is more likely to express sex-linked recessive traits than the homogametic sex (the sex with two identical sex chromosomes, such as XX in mammals or ZZ in birds). This rule is named after the geneticist Alfred Henry Sturtevant's student, Arthur Milnes Emery, who first observed this pattern in 1936.

Overview

In species with genetic sex determination, individuals carry two types of sex chromosomes. In mammals, for example, males are typically XY (heterogametic) and females are XX (homogametic). Emery's Rule suggests that because the heterogametic sex has only one copy of each sex chromosome, any recessive alleles on that chromosome will be expressed, as there is no corresponding allele on a second, matching chromosome to potentially mask its expression. This contrasts with the homogametic sex, which would require two copies of the recessive allele (one on each of their two matching sex chromosomes) for the trait to be expressed.

Implications

Emery's Rule has significant implications for the understanding of genetic disorders and sex-linked traits. It helps explain the observation that certain disorders, particularly those linked to the X chromosome in humans (such as hemophilia and color blindness), are more frequently expressed in males than in females. This is because females, being XX, have a second X chromosome that can carry a normal allele to mask the effects of a recessive allele on the other X chromosome, a mechanism not available to XY males.

Evolutionary Significance

From an evolutionary perspective, Emery's Rule highlights the potential for sex chromosomes to influence the differential susceptibility of males and females to genetic disorders. It also suggests a mechanism by which sex-linked traits can spread through populations, as heterogametic individuals effectively act as a test case for the viability of recessive alleles.

Criticism and Limitations

While Emery's Rule provides a useful framework for understanding the expression of sex-linked traits, it is not without its limitations. The rule does not account for phenomena such as dosage compensation, where organisms adjust the expression levels of genes on the sex chromosomes to balance for the difference in chromosome number between sexes. Additionally, the rule may not apply uniformly across all species or for all sex-linked traits, as the complexity of genetic expression and regulation can lead to exceptions.

See Also

• Sex determination and differentiation
• X-linked recessive inheritance
• Y chromosome
• Z chromosome
• Dosage compensation