Sex-linked traits

• Sex chromosomes - pair of chromosomes that determine the individual sex. All other chromosomes are called autosomal chromosomes.
• phenotypes that affect one sex more than the other are sex-linked; that is, the alleles controlling them are on the X or Y chromosomes.
• XY - males, XX - females
• Y chromosome - plays the largest role in human sex determination. Carries fewer than 100 genes.
• X chromosome - carries more than 1000 protein-encoding genes, has nothing to dow ith sex determination.
• Thomas Hunt Morgan - first to unravel the unusual inheritance patterns associated with the gene on the X chromosome. He created true-breeding lines of flies with each eye color from the eyes of fruit flies (Drosophila melanogaster) which are usually red.
• 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 CCG repeats. Most common form of inherited mental retardation.
• Hemophilia A - mutant alleles for genes 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. Reduces ability to distinguish red and green.
• Rett syndrome - mutant allele for gene required development of nerve cells. Severe developmental disorders. Almost all affected children are female; affected male embryos cease development before birth.
• Pedigree - orderly presentation of family information. It is the first step in studying the inheritance of traits. Important in predicting genetic risk. May be incomplete due to difficulties collecting information.
• basic symbols of pedigree

  
• Mothers pass their X’s to both sons and daughters while fathers pass their X to daughters only.
• Extra hairiness (congenital generalized hypertrichosis) - mechanism is unknown. Many more hair follicles than normal.
• Hypophophatemic rickets (some forms) - 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.