*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