Sex-Linked and Sexually Dimorphic Traits: Patterns of Inheritance in Humans

Sex-Linked and Sexually Dimorphic Traits in Humans

Introduction to Sex-Linked and Sexually Dimorphic Traits

Sex-linked traits are determined by genes located on the sex chromosomes (X or Y), while sexually dimorphic traits are influenced by differences in male and female physiology, often due to hormonal effects. Understanding these traits is essential for interpreting patterns of inheritance in human genetics.

• Sex-linked genes are located on either the X chromosome or the Y chromosome.

• Sexually dimorphic traits may not be located on sex chromosomes but are influenced by hormones that differ between males and females (e.g., baldness).

Pedigree Patterns Suggesting Sex-Linked Inheritance

Modes of Sex-Linked Inheritance

Pedigree analysis helps distinguish between different modes of inheritance, especially for sex-linked traits. The three main types are X-linked recessive, X-linked dominant, and Y-linked inheritance.

• X-linked recessive

  • Mutation never passes from father to son.

  • Daughters of affected males are carriers; half of sons of carriers will inherit the trait.

• X-linked dominant

  • Trait is seen in every generation.

  • Affected male will produce affected daughters and unaffected sons.

• Y-linked

  • Trait is seen only in males.

Examples of Sex-Linked Inheritance in Pedigrees

X-Linked Recessive Trait: Hemophilia

Hemophilia is a classic example of an X-linked recessive disorder. Pedigree analysis shows transmission patterns consistent with X-linked recessive inheritance.

• Carrier females (heterozygous) can pass the defective allele to sons (who will be affected) and daughters (who will be carriers).

• Affected males cannot pass the trait to sons, but all daughters become carriers.

Example: The pedigree of Queen Victoria's descendants illustrates the inheritance of hemophilia through several generations, with affected males and carrier females.

X-Linked Dominant Trait: Hypophosphatemia

Hypophosphatemia is an example of an X-linked dominant disorder. Pedigrees show affected individuals in every generation, with both males and females affected, but with distinct transmission patterns.

• Affected males transmit the trait to all daughters but not to sons.

• Affected females can transmit the trait to both sons and daughters.

Example: Pedigrees of families with hypophosphatemia show the trait in every generation, with more females than males affected.

Other X-Linked Dominant Traits

• Hypoplasia: Underdevelopment of tissues or organs.

• Webbing to the tips of the toes: A physical trait seen in some X-linked dominant conditions.

• Constrictional Thrombopathy: Severe bleeding due to lack of blood platelets.

Y-Linked (Holandric) Inheritance

Y-linked traits are transmitted exclusively from father to son and are only expressed in males. Few traits are confirmed to be Y-linked, aside from maleness itself.

• All male descendants of an affected male will exhibit the trait.

• Females neither express nor transmit Y-linked traits.

• Example: The hairy ears trait may be Y-linked, but its inheritance is complex and may involve autosomal genes and/or hormonal effects.

Characteristics of X-Linked Recessive Inheritance

X-linked recessive traits display distinct patterns in pedigrees, which can be used to identify the mode of inheritance.

• Affected fathers transmit the recessive allele to all daughters (carriers), but not to sons.

• Father-to-son transmission of the X-linked allele generally does not occur.

• Many more males than females exhibit the trait because males have only one X chromosome.

• All sons of affected (homozygous recessive) mothers are expected to exhibit the trait.

• A carrier mother will yield half of sons with the trait and half without.

• A carrier female crossed with a normal male will have half carrier and half normal daughters.

Pedigree Analysis: Practice Problems

Evaluating Modes of Inheritance

Pedigree analysis is used to determine whether a trait is autosomal recessive, autosomal dominant, X-linked recessive, or X-linked dominant. The following table summarizes the possible modes for three sample pedigrees:

	Pedigree A	Pedigree B	Pedigree C
Autosomal recessive
Autosomal dominant
X-linked recessive
X-linked dominant

Additional info: Students are expected to fill in the table by analyzing the inheritance patterns in each pedigree.

Summary Tables: Pedigree Patterns Suggesting Sex-Linked Inheritance

X-Linked Recessive Trait

X-Linked Recessive Trait
The trait appears in more males than females because a female must receive two copies of the defective allele to display the phenotype, whereas a hemizygous male with only one copy will show it.
The mutation will never pass from father to son because sons receive only a Y chromosome from their father.
An affected male passes the X-linked mutation to all his daughters, who thus become carriers. Each of those carrier females has a one-half chance to inherit the defective allele and thus the trait.
The trait often skips a generation as the mutation passes from grandfather through a carrier daughter to grandson.
The trait can appear in successive generations when a sister of an affected male is a carrier. If she is, each of her sons has a one-half chance of being affected.
With two rare affected (homozygous) females, all her sons will be affected and all her daughters will be carriers.

X-Linked Dominant Trait

X-Linked Dominant Trait
More females than males show the aberrant trait.
The trait is seen in every generation as long as affected males have female children.
All the daughters but none of the sons of an affected male will be affected. This criterion is the most useful for distinguishing an X-linked dominant trait from an autosomal dominant trait.
The sons and daughters of an affected female each have a one-half chance of being affected.
For incompletely dominant X-linked traits, carrier females may show the trait in less extreme form than males with the defective allele.

Y-Linked Trait

Y-Linked Trait
The trait is seen only in males.
All male descendants of an affected man will exhibit the trait.
Not only do females not exhibit the trait, they also cannot transmit it.

Additional Info: Pedigree Problem-Solving and Team Assignment

Students are encouraged to use positive and negative rules for each type of inheritance to analyze pedigrees. For example, dominant traits cannot arise from unaffected parents, while X-linked traits cannot be passed from father to son. Practice problems include identifying the mode of inheritance and determining genotypes from pedigree diagrams.

• Antagonistic pairs: Dominant: affected children cannot arise from unaffected parents; Recessive: unaffected parents can produce affected children.

• X-linked: X-linked traits cannot be passed from father to son; carrier females of X-linked recessive traits have sons who are affected and daughters who are carriers.

Additional info: Students should refer to representative pedigrees and apply these rules for easier memorization and analysis.