Lesson 4.1: Sex-linked traits

• Most human sex-linked traits are X-linked, controlled by genes on the X chromosome
• Thomas Hunt Morgan: 'He was the first to unravel the unusual inheritance patterns associated with genes on the X chromosome'
• The Y chromosome carries fewer than 100 genes; most Y-linked disorders involve defects in sperm production
• Sex-Linked Phenotypes

  Alleles controlling them are on the X or Y chromosomes
• 1. linked recessive disorders in humans

  1. Duchenne muscular dystrophy: Mutant allele for gene encoding dystrophin

  2. Fragile X syndrome: Unstable region of X chromosome with high number of CGG repeats

  3. Hemophilia A: Mutant allele for gene encoding blood clotting protein

  4. Red-Green color blindness: Mutant alleles for genes encoding receptors for red or green wavelengths of light

  5. Rett syndrome: Mutant allele for gene required for development of nerve cells
• 1. linked dominant disorders in humans

  1. Extra hairiness: Mechanism unknown

  2. Hypophosphatemic rickets: Mutant allele for gene involved in phosphorus absorption

  3. Retinitis pigmentosa: Mutant allele for cell-signaling protein
• Sex-linked dominant: Mothers pass X's to both sons and daughters; Fathers pass X to daughters only
• The human X chromosome carries more than 1000 protein-encoding genes
• Chromosome (Sex Chromosomes)


  Most species of animals and plants carry a pair of chromosomes that determine the individual sex. All other chromosomes are called autosomal chromosomes
• 1. linked recessive disorders affect more males than females
• The eyes of fruit flies (drosophila melanogaster) are usually red
• The Y chromosome plays the largest role in human sex determination
• Sex-linked recessive: Males get X from mother; Fathers pass X to daughters only; Females express it only if they get a copy from both parents
• Pedigree Analysis


  1. Is an orderly presentation of family information

  2. First step in studying the inheritance of traits

  3. Important in predicting genetic risks

  4. May be incomplete due to difficulties collecting information