Sex Linked Genetic Disease

Sex Linked Genetic disease Predicting the mode of inheritance and understanding how a lethal allele is maintained in the gene pool

Sex Chromosomes • Homologous pair (XX) in the female • Heterozygous pair (XY) in the male • Sex of the individual is determined by which chromosome is carried by the sperm during fertilization.

Traits that are sex-linked • X-linked alleles are always expressed in males because males have only one X chromosome. • If a male inherits a recessive trait on the X chromosome it will be expressed. • What about females?

Sex (X) linked Dominant Disorder • Females receive a copy of the mutated chromosome from either their mother or father, as both the mother and the father pass on a copy of the X chromosome to their daughter. • Males receive an X chromosome from their mother and a Y chromosome from their father. • Males can only inherit an X linked Dominant Disorder from his mother. If the father has an X linked dominant mutation he will not pass it to his sons • Sons inherit their father’s Y chromosome and not their X chromosome. However a father with an X linked dominant mutation will pass the mutation to his daughters.

Sex (X) linked recessive disorder • Gene affected is located on the X chromosome and the mutation is recessive. • If a female has a mutated copy of the gene on one X chromosome, but a normal copy on the other X chromosome it means that she will not suffer from the disease, however she will be known as a carrier. • If males inherit one copy of the mutated X chromosome they will develop the disease, as they don’t have another normal X chromosome.

Aneuploidy • Wrong number of chromosomes • Usually results from the non-disjunction (failure to separate) of homologous chromosomes during meiosis. • Two m/c forms are monosomy and trisomy

Aneuploidy in sex chromosomes • Turner Syndrome -person has only one sex chromosome (X), totally lacking another X or Y. - results from nondisjunction during meiosis -karyotype will have only 45 chromosomes -incidence rate is 1 in 5000 live female births

• Klinefelter Syndrome – Person has an extra sex chromosome (X), having XXY. – Karyotype has 47 chromosomes. – Incidence rate is 1 in 1000 live male births with a maternal age effect.

Duchenne muscular dystrophy • Wasting of skeletal muscle • In the U.S., 1 out of 3000 males born with muscular dystrophy. • Defective gene that codes for muscle protein. • MDA videos

Review 1. Why are sex-linked disorders more common in males than in females? 2. How does nondisjunction cause chromosome number disorders? 3. What are two types of sex-linked disorders?