*Sex-linked Inheritance

*You already know…- Sex chromosomes determine gender

XX: female XY: male

- In humans the X chromosome is much larger than the Y chromosome and contains thousands of more genes

Very little homologous DNA

- X chromosomes are responsible for many non-sexual traits

- Recessive allele carried on the X chromosome but not on the Y chromosome

- More common in males... WHY?

Males- one X chromosomeOne copy of a recessive allele results in expression of the trait

 Females- two X chromosomes  Heterozygote: recessive allele is hidden so no expression of the trait

BUT… she is a carrier

Homozygote recessive: trait is expressed

Examples: Colour blindness and Hemophilia

X-linked RECESSIVE Inheritance

Colour blindness- Inability to distinguish between certain colours

- Absence of certain cells in the eyes or cells with limited sensitivity

- Approximately 1 out of 12 males and 1 out of 20 females are colour blind

- A person who is colour blind has trouble seeing red, green, blue or mixtures of these colours

most common type: red-green colour blindness

- Ishihara test for red-green colour blindness

- Those with normal colour vision should read the number 74

- Those with normal colour vision should read the number 6

- Those with normal colour vision should read the number 29

- Those with red-green deficiencies read the number 70- Those with total colour blindness can not read any

number

- Those with normal colour vision should not be able to read any number

- Those with red-green deficiencies read the number 5- Those with total colour blindness can not read any

number

Hemophilia- Low levels of blood plasma clotting factors

- The body’s ability to control blood clotting is impaired

- Hemophiliac’s can bleed for a long time

- May also bleed internally: knees, ankles, elbows

- Can range from mild to severe

X-linked DOMINANT Inheritance

Males:- 100% of daughters will be affected- 0% of sons will be affected

Heterozygous female:- 50% of daughters will be affected- 50% of sons will be affected

Homozygous female:- 100% of daughters will be affected- 100% of sons will be affected

Charcot-Marie Tooth Disease- Neurological disorder affecting the peripheral

nervous system

- Periphery nerves control muscles and relay information from our arms and legs to our brain

- Characterized by the progressive loss of muscle tissue and touch sensation across various parts of the body

- Symptoms usually begin in your feet and legs but may eventually affect your hands and arms

- Foot deformities include high arches and hammertoes

Rickets- Hereditary hypophosphate rickets due to low levels

of phosphate

- Also due to deficiency or impaired metabolism of vitamin D, phosphorus or calcium

May be a result of severe malnutrition, little exposure to sunlight, lactose intolerance, digestive disorders, kidney diseases, or liver diseases

- Causes the softening of bones which leads to fractures and deformities

- Other symptoms include bone pain, low calcium blood levels and uncontrolled muscle spasms