Sex-linked traits Knowledge-Based Study

Study notes

1. What Are Sex-Linked Traits?

   In many sexually reproducing organisms, biological sex is determined by a distinct pair of chromosomes known as sex chromosomes (allosomes), distinguishing them from the remaining non-sex chromosomes, termed autosomes.

   In humans and many other organisms, females possess two homologous X chromosomes (XX), whereas males possess one X chromosome and one significantly smaller Y chromosome (XY).

   Sex-linked traits are characteristics controlled by genes located specifically on sex chromosomes:

   • X-linked traits: Governed by genes carried on the X chromosome. Because the X chromosome carries thousands of functional genes unrelated to sexual development, most sex-linked traits are X-linked.
   • Y-linked traits (Holandric): Governed by genes found exclusively on the non-homologous region of the Y chromosome, passed exclusively from father to son.

   Patterns of X-Linked Recessive Alleles

   Because human males inherit only a single copy of the X chromosome, they are considered hemizygous for all X-linked loci. This hemizygosity leads to unique inheritance patterns compared to autosomal Mendelian traits:

   • Males express recessive alleles readily: If a male inherits a single recessive allele on his X chromosome ($X^rY$), he manifests the phenotype because there is no corresponding allele on the Y chromosome to mask it.
   • Females can be carriers: A female requires two recessive alleles ($X^rX^r$) to express the condition. A heterozygous female ($X^RX^r$) carries the recessive allele without expressing the trait, acting as a phenotypic carrier.
   • Criss-cross inheritance: Affected fathers cannot pass an X-linked recessive condition directly to their sons, as they contribute their Y chromosome to male offspring. Instead, their daughters become obligate carriers, who may subsequently pass the allele to their sons.

   Common examples in humans include red-green color blindness, hemophilia A and B, and Duchenne muscular dystrophy.

   Using Punnett Squares for Sex-Linked Crosses

   When tracking sex-linked traits, alleles must be written as superscripts attached to the sex chromosome notation (such as $X^B$, $X^b$, and $Y$).

   Example Cross: Carrier Mother and Unaffected Father

   Consider a cross between a carrier female for color blindness ($X^B X^b$) and a normal-vision male ($X^B Y$):

   • Maternal gametes: 50% $X^B$, 50% $X^b$
   • Paternal gametes: 50% $X^B$, 50% $Y$

   Offspring Genotypes and Phenotypes

   • $X^B X^B$ (25%): Unaffected female (homozygous dominant)
   • $X^B X^b$ (25%): Carrier female (phenotypically unaffected)
   • $X^B Y$ (25%): Unaffected male
   • $X^b Y$ (25%): Color-blind male

   Key probability distinction: While 25% of all offspring are color-blind, exactly 50% of the male offspring will express color blindness, while 0% of the female offspring express the condition.

   Other Modes of Sex-Linked Inheritance

   X-Linked Dominant Inheritance

   In X-linked dominant conditions, a single copy of the mutated dominant allele on the X chromosome is sufficient to cause the phenotype in both males and females:

   • An affected male ($X^D Y$) will pass the trait to all of his daughters and none of his sons.
   • An affected heterozygous female ($X^D X^d$) has a 50% chance of passing the condition to any child, regardless of the child's sex.
   • Examples include hypophosphatemic rickets (vitamin D-resistant rickets).

   Y-Linked Inheritance

   Genes located exclusively on the Y chromosome affect only biological males and are transmitted directly from an affected father to 100% of his biological sons. Females are never affected and cannot be carriers of Y-linked traits.