Chapter 12

 Humans have 46 chromosomes  44 are autosomes  22 pairs of homologous chromosomes  2 are sex chromosomes: X and Y  Females have two X chromosomes  Males have one X and one Y

 Many genes not related to sex are on the X chromosome  They are called sex-linked traits  Example: Drosophila (Fruit fly) eye color

 Red eyes ( X R ) is dominant  White eyes ( X r ) is recessive XrYXrYXRXRXRXR XRXrXRXr XRYXRY XRXR XrXr Y XRXrXRXr XRXR XrXr XRXRXRXR XRXR Y XRYXRY XrXRXrXR XRYXRY XrYXrY XRXrXRXr XRXR XrXr XrXr Y XRXrXRXr XrXrXrXr XRYXRY XrYXrY XrYXrY R = red-eye allele r = white-eye allele

 Most sex-linked human disorders are due to recessive alleles  Examples: hemophilia, red-green color blindness  These are mostly seen in males  Why??? Figure 9.23A

 A male receives a single X-linked allele from his mother, and will have the disorder (X c Y)  A female has to receive the allele from both parents to be affected (X c X c )  A female with the gene for the trait on one of her X chromosomes is called a carrier (X c X).  A carrier may not know she carries the gene for the trait until she has a son with the trait.

 Hemophilia is a deadly disease in which the individual’s blood does not clot properly.  It is caused by a recessive gene on the X chromosome. (X h )  Women can be XX, XX h (carrier), or X h X h (affected).  Men can be XY or X h Y (affected).

 A pedigree is a diagram that shows how a trait is inherited over several generations.  Circles represent women, squares are men  Solid (dark) is affected (with disease, etc.)  Empty (white) is not affected  Half filled is a carrier (heterozygous)

 A high incidence of hemophilia has plagued the royal families of Europe Figure 9.23B Queen Victoria Albert AliceLouis AlexandraCzar Nicholas II of Russia Alexis

 A female has the genotype XX and she marries a man who is color blind (X c Y). What is the probability that they will have a child who is color blind?

 A female who is not color blind, but whose father was color blind marries a color blind male. What is the probability that they will have a child who is colorblind?

 Sex-linked disorders  Affected allele is usually on X chromosome  Autosomal disorders  Can affect men and women equally  Chromosome number disorders  Chromosomes don’t separate evenly in meiosis

 Colorblindness – Cannot distinguish between red & green  Hemophilia – Blood clotting disorder  Muscular Dystrophy – Muscles weaken and eventually break down.

 Albinism  no hair or skin pigment  Cystic Fibrosis  mucus clogs lungs  Tay-Sachs disease  breaks down nervous system

 Sickle-cell anemia  abnormal protein causes RBCs to sickle and clump together  PKU (Phenylketonuria)  Body can’t break down a certain amino acid so toxic substance accumulates  nerve damage, retardation

 Huntington’s Disease  Autosomal Dominant  Weakness and deterioration of brain  No symptoms appear until middle age  A person might not know he or she had the gene until after having children.

 Turner’s Syndrome  X0 (sterile females)  Klinefelter’s Syndrome  XXY (mentally retarded, boy, underdeveloped female characteristics)  Down’s Syndrome  Trisomy 21