Sex Linked Traits

In unisexual organisms (like Drosophila and humans), sex is determined by a specific pair of sex chromosomes, which exist alongside autosomes.

• Females possess homomorphic (identical) sex chromosomes: XX.
• Males possess heteromorphic (different) sex chromosomes: XY.
• Any genes located on these sex chromosomes exhibit sex linkage.

• A male passes his X chromosome invariably and only to his daughters.
• A female passes her X chromosomes to both her daughters and her sons.

When a female who is homozygous for a recessive trait mates with a male carrying the dominant allele:

• F1 Females: Will always show the dominant phenotype. They are heterozygous, having received the recessive X from their mother and the dominant X from their father.
• F1 Males: Will always show the recessive phenotype. Because they receive their Y chromosome from their father, their single X chromosome must come from their mother (who only has recessive alleles to give).

A-B. Sex-linked characters: A. transmission of sex chromosome (x) from two parents to their female and male offsprings; B. inheritance of sex-linked characters

• The Concept: Traits controlled by genes located specifically on sex chromosomes.
• The Chromosomes: Females are XX, while males are XY.
• Fathers: Pass their single X chromosome only to their daughters.
• Mothers: Pass an X chromosome to both their sons and daughters.
• Criss-Cross Inheritance: A mother with a recessive X-linked trait will pass that trait to all her sons, because sons inherit their only X chromosome directly from her.