Presentation on theme: "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."— Presentation transcript:
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.