Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 15.2 Sex-Linked Genes Exhibit Unique Patterns of Inheritance
Genes located on sex chromosomes follow distinct inheritance patterns. In humans and many animals, the X chromosome carries numerous genes unrelated to sex, while the Y chromosome contains relatively few. The differences in inheritance between males and females create unique patterns for sex-linked traits.
Chromosomal Systems of Sex Determination
- X-Y system (humans, mammals): Sex depends on whether sperm contributes an X (female) or Y (male)
- X-0 system (insects): Females are XX; males are X0
- Z-W system (birds, some fish/insects): Females are ZW; males are ZZ
- Haplo-diploid system (bees, ants): Females are diploid; males are haploid (develop from unfertilized eggs)
The Human Y Chromosome and SRY
- Human embryos are initially undifferentiated in sex
- The SRY gene (Sex-determining Region of Y) on the Y chromosome triggers development of testes
- Without SRY, ovaries develop—even in XY embryos
- The Y chromosome contains few genes, most related to male sex determination and sperm production
X-Linked Genes and Inheritance
- The X chromosome contains about 1,100 genes; the Y chromosome has far fewer
- Fathers pass X-linked alleles to all daughters but not sons
- Mothers pass X-linked alleles to both sons and daughters
- Males are hemizygous for X-linked genes (only one copy), so recessive alleles are always expressed
- X-linked recessive disorders affect males more often than females
Examples:
- Red-green color blindness
- Hemophilia: Impaired blood clotting; famously passed through European royal families
- Duchenne muscular dystrophy: Progressive muscle weakening, affects ~1 in 5,000 males in the U.S.
X Inactivation in Female Mammals
- Female mammals inactivate one X chromosome in each somatic cell early in embryonic development
- Inactive X condenses into a Barr body
- Inactivation is random—either the maternal or paternal X may be inactivated in each cell
- Result: females are mosaics of two cell populations (e.g., calico/tortoiseshell cats)
- Reactivation occurs in germ cells, ensuring all eggs carry an active X
- Inactivation involves the XIST gene, which produces RNA that coats and silences the X chromosome
- Epigenetic modifications like DNA methylation help maintain inactivation
Variations and Complexity in Sex Determination
- Sex is not strictly binary—genetic and developmental factors can vary
- Genes outside the sex chromosomes (e.g., WNT4 on chromosome 1) influence sexual development
- Intersex conditions and chromosome number variations reveal the complexity behind sex determination
- Research continues to uncover how networks of genes shape sexual development and identity
In a Nutshell
Sex-linked genes show unique inheritance patterns due to the differing number of sex chromosomes in males and females. X-linked recessive disorders are more common in males, while females compensate through X inactivation. The diversity in chromosomal systems and gene interactions makes sex determination a complex biological process.