Question

How is an X-linked trait different from an autosomal trait? (page \(481)\)

Short answer

X-linked traits are determined by genes located on the X chromosome, with males being hemizygous and females being either homozygous or heterozygous. X-linked recessive traits are more likely to be expressed in males than in females. Autosomal traits, however, are determined by genes on non-sex chromosomes and have the same inheritance pattern in both males and females. The expressivity of autosomal traits is also equal in both sexes. The difference between X-linked and autosomal traits lies in their location on chromosomes, inheritance patterns, and expressivity in males and females.

Step-by-step solution

Definition of X-linked traits

X-linked traits are those that are determined by genes located on the X chromosome. Since females have two X chromosomes (XX), they can be either homozygous (having two copies of the same allele) or heterozygous (having one copy of each allele) for a given X-linked trait. Males, on the other hand, have only one X chromosome (XY), which means they have just one copy of any given gene on the X chromosome. Thus, males are hemizygous for X-linked traits and express the trait regardless of whether the allele is dominant or recessive.

Definition of Autosomal traits

Autosomal traits are determined by genes located on the non-sex chromosomes, also known as autosomes. There are 22 pairs of autosomes in humans. For these traits, the inheritance pattern is the same in both males and females, as both have two copies of each autosome (one from each parent). An individual can be homozygous dominant, heterozygous, or homozygous recessive for a given autosomal trait.

Inheritance Patterns

The inheritance pattern for X-linked traits is different from that of autosomal traits. For X-linked traits, the inheritance pattern is determined by the sex of the parent carrying the gene and the sex of the offspring. Female carriers can pass on the X-linked gene to both their sons and daughters. In contrast, males can only pass on the X-linked gene to their daughters. For autosomal traits, the inheritance pattern is not influenced by the sex of the parent or the offspring. Both parents contribute one allele each, and the offspring's phenotype is determined by the combination of these alleles (dominant or recessive).

Expressivity in Males and Females

X-linked recessive traits are more likely to be expressed in males than in females. This is because males have only one X chromosome, and if it carries the recessive allele, they will express the trait. Females, on the other hand, need two copies of the recessive allele to express the trait. Since females have two X chromosomes, they are less likely to express X-linked recessive traits. Autosomal traits, however, are equally likely to be expressed in both males and females, as the inheritance pattern is the same for both sexes. In summary, X-linked traits are different from autosomal traits because of their location on the X chromosome, resulting in distinct inheritance patterns and differences in the way these traits are expressed in males and females.

Key concepts

X-linked traits

X-linked traits refer to characteristics determined by genes located specifically on the X chromosome. Because of their unique placement, these traits exhibit distinct patterns of inheritance between males and females. Females (XX) possess two X chromosomes, which means they can carry two identical alleles or two different alleles for an X-linked trait. This allows them to be homozygous or heterozygous. Males, however, have only one X chromosome (XY), making them hemizygous for any X-linked gene. As a result, males express the trait solely based on the one allele they have on their X chromosome, irrespective of whether it's dominant or recessive. This particular arrangement leads to males often expressing X-linked traits more frequently or differently than females.

Autosomal traits

Autosomal traits are influenced by genes on any of the 22 pairs of non-sex chromosomes called autosomes. For autosomal genes, the inheritance pattern is consistent, irrespective of gender, because both males and females have two copies of each autosome. People can inherit and express autosomal traits if they have combinations such as homozygous dominant, heterozygous, or homozygous recessive alleles. This uniform inheritance pattern signifies that males and females are equally likely to express autosomal traits because both have the same potential for passing on the genes to their children, unaffected by sex chromosomes' variation.

Inheritance patterns

Inheritance patterns reveal how traits are passed down from parents to offspring, highlighting specific pathways for different types of traits. X-linked traits have an inheritance pattern highly influenced by the sex of both the parent passing on the gene and the child receiving it. Female carriers can transmit X-linked genes to both daughters and sons, while male carriers can only pass these genes down to daughters. Conversely, autosomal traits follow a simpler pattern where both parents contribute equally, and the outcome depends on the dominant or recessive nature of the alleles. The sex of the parent or child does not influence how autosomal traits are inherited, providing a straightforward understanding of how they manifest in offspring.

Sex-linked inheritance

Sex-linked inheritance refers broadly to traits where gene expression is affected by the location of genes on sex chromosomes. Predominantly, this involves X-linked inheritance due to the more numerous and impactable traits carried on the X chromosome. For X-linked recessive traits, males are more prone to expressing the trait as they need only a single recessive allele on their lone X chromosome. Females, requiring two copies of a recessive allele to express the same trait, are generally less affected. Thus, sex-linked inheritance creates variances in trait expression between sexes, with a particular focus on males typically experiencing more pronounced manifestations of X-linked recessive traits than females.