X-Linked Recessive Inheritance has these characteristics:
The mutated gene occurs only on the X chromosome.
Heterozygous females are phenotypically normal. Males are affected because they have only one
allele for the trait (on the X chromosome) and it can be recessive.
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 female and a normal male, all daughters will be carriers and all sons will be affected.
Color blindness is an example of an X-linked
recessive trait that is not very serious.
This three generation pedigree for color
blindness demonstrates some of the distinctive characteristics of an X-linked recessive trait. These include:
more affected males than affected females
affected grandfather to affected grandson transmission through a carrier female intermediate
no male to male transmission.
A serious X-linked recessive condition is hemophilia A
, the inability of the blood to clot because the genes do not code for the necessary clotting
agent(s).
It was common in the European royal families. .
Fragile X syndrome is a recessive disorder that causes mental retardation in males. There is a
mutant allele (with repeated segments of DNA) for a gene that specifies a protein required for
brain development.
REVIEW: Hemophilia
a. is rare in the human population.
b. is more common among men.
c. was common in English royalty.
d. is an X-linked recessive trait.
e. all of these
REVIEW: Genetic disorders are caused by _____ .
a. altered chromosome number
b. altered chromosome structure
c. mutation
d. all of the above
REVIEW: A recognized set ot symptoms that characterize a specific disorder is a _____ .
