Lesson 4.1: Sex-Linked Traits

• • Most species of animals and plants carry a pair of chromosomes that determine the individual sex. 
  • All other chromosomes are called autosomal chromosomes.


  Chromosome (Sex Chromosomes)
• Phenotypes that affect one sex more than the other are sex-lined; that is, the alleles controlling them are on the X or chromosomes

  • Determines the sex of many species including humans.

  XX - Females (Complementary)

  XY - Male (Non-complementary)

  Sex-Linked
• The Y chromosome plays the largest role in human sex determination. 
• • The Y chromosome carries fewer than 100 genes. There are few Y-linked disorders; most involve defects in sperm production. 
• The human X chromosome carries more than 1000 protein-encoding genes
• Most human sex-linked traits are therefore X-linked; That is, they are controlled by genes on the X chromosome.
• • X-linked recessive disorders affect more males than female
• Thomas Hunt Morgan


  • He was the first to unravel the unusual inheritance patterns associated with genes on the X chromosome.
  • The eyes of fruit flies (drosophila melanogaster) are usually red.
  • To study the inheritance of this odd phenotype, he created true-bleeding lines of flies with each eye color.
• Duchenne muscular dystrophy

  • Mutant allele for gene encoding dystrophin. Rapid muscle degeneration early in life
• Fragile X syndrome 


  • Unstable region of X chromosome has unusually high number of CGG repeats
  • Most common form of inherited mental retardation
• Hemophilia A

  • Mutant allele for gene encoding blood clotting protein (factor VIII)
  • Uncontrolled bleeding, easy bruising
• Red-Green color blindness 

  • Mutant alleles for genes encoding receptors for red or green (or both) wavelengths of light 
  • Reduce ability to distinguish red and green
• Rett syndrome

  • Mutant allele for gene required for development of nerve cells
  • Severe developmental disorders. Almost all affected children are female; affected male embryos cease development before death.
• Pedigree Analysis

  Is an orderly presentation of family information

  • First step in studying the inheritance of traits
  • Important in predicting genetic risks
  • May be incomplete due to difficulties collecting information
• Sex-linked dominant


  • Mothers pass their X’s to both sons and daughters
  • Father’s pass their X to their daughters only.
• Sex-linked Recessive


  • Males get their X from their mother
  • Fathers pass their X to daughters only
  • Females express it only if they get a copy from both parents.
  • Expressed in males if present
  • Recessive in females
• Extra hairiness

  • Mechanism unknown
  • Many more hair follicles than normal
• Hypophophathemic rickets

  • Mutant allele for gene involved in phosphorus absorption
  • Low blood phosphorus level causes defective bones
• Retinitis pigmentosa

  • Mutant allele for cell-signaling protein; mechanism unknown: 
  • Defects in retina causes partial blindness