1. Chapter 12.7 Examples of X-Linked Inheritance Patterns AP Biology Fall 2010

2. X-Linked Inheritance Characteristics  The mutated recessive gene occurs only on the X chromosome  Heterozygous females are phenotypically normal  Males are more often affected because the single recessive allele (on the X chromosome) is not masked by a dominant gene on any other chromosome

3. X-Linked Inheritance Characteristics  A normal male mated with a female heterozygote has a 50% chance of producing carrier daughters and a 50% chance of producing affected sons  In the case of a homozygous recessive female and a normal male, all daughters will be carriers and all sons affected  Can you perform these crosses?

4. Hemophilia A  A serious X-linked recessive condition is hemophilia A (affecting 1 in 7,000 males)  Is the inability of the blood to clot because the genes do not code for the necessary clotting agent(s)  The disorder frequency was high among the royal families of Europe and Russia in the nineteenth century  Why?

5. Red-Green Colour Blindness  Colour blindness: is the inability to distinguish some or all of the colours in the visible light spectrum  Mutated genes change the light- absorbing capacity of sensory receptors of the eyes  Red-green colour blind people can see fewer than 25 of the 150 differences in colour seen normally  It is 12 times more common in men than women

6. Red-Green Colour Blindness  Ishihara plates:  Do you see a 7 or 29?  Do you see a 3 or 8?  29 and 8 are normal, 7 and 3 are colour blind

7. Duchenne Muscular Dystrophy  Duchenne Muscular Dystrophy (DMD): is characterized by rapid degeeration of muscles early in life  One in 3,500 males  Is caused by a recessive allele for a structural protein called dystrophin  When dystrophin is absent or abnormal, muscle cells progressively die (in an unstoppable manner)  Diagnosed between the ages 3 to 7, the affected boy generally dies before he reaches 25

8. X-Linked Inheritance  More than 300 connections to known genetic disorders have been identified on the X chromosome  Does this mean women are superior? ; )