Gonosomal inheritance in pedigree and experiment, examples of traits in man

Two subtypes:
 * gonosomal (X-linked) recessive, GR
 * gonosomal (X-linked) dominant, GD

Characteristics of GR pedigrees

 * males are much more likely to be affected
 * affected males get the disease from their mothers – healthy carriers
 * no transmission from father to son
 * transmission from an affected grandfather to his grandsons

GR disease/trait examples

 * hemophilia A, hemophilia B,
 * Duchenne muscular dystrophy,
 * color blindness,
 * anhidrotic ectodermal dysplasia

Characteristics of GD (X-linked Dominant) pedigrees

 * Only one copy of a disease allele on the X chromosome is required (and sufficient) for an individual to be susceptible to an X-linked dominant disease
 * Both males and females can be affected, although males may be more severely affected because they only carry one copy of genes found on the X chromosome
 * Some X-linked dominant disorders are (even) lethal in males (in male fetuses).
 * When a female is affected, each pregnancy will have a one in two (50%) chance for the offspring to inherit the disease allele.
 * When a male is affected, all his daughters will be affected, but none of his sons will be affected.
 * Transmission from an affected grandfather to his grandsons

GD disease examples

 * vitamin D-resistant (hypophosphatemic) rickets,
 * incontinentia pigmenti,
 * Alport syndrome,
 * amelogenesis imperfekta (X-linked)

The white (w) locus in Drosophila
A single white-eyed male fly was isolated in the laboratory of T. H. Morgan in 1910, and they studied genetic crosses using this white mutant.
 * 1) When the white male was crossed to wild type (i.e. red-eyed) females, all the progeny were red-eyed. From it they concluded the white mutation was recessive.
 * 2) When the F1 generation members were crossed with one another, 1/4 of the (F2) progeny were white- eyed. But … The white phenotype was only seen in males.

Supposed the white gene is located on the X chromosome, the original male is hemizygous for w allele (genotype XwY). The original cross is represented as XwY x X+X+, and all progeny are wild-type in phenotype.
 * Schedule is explaining the experimental procedure:



If F1 siblings are now crossed, X+Y x XwX+, all females are phenotypically normal, and 1/2 of the males are white (1/4 of total progeny) in F2 generation.



In a backcross (Bc), when white males are crossed to heterozygous females, XwY x XwX+, equal numbers of white males and females are observed in the progeny; 1/2 progeny are white-eyed.



In reverse backcross, when white females are crossed to wild-type males, XwXw x X+Y, white female parents give rise to white progeny males.