NBME 4 Block 4 Q5 - vitf

[ArchivalUser]

A acouple who have no children seek genetic councelling bcoz the husbands 2 brothers died in their teens of duchene muscular dystrophy. what is husbands risk of being carrier for duchene ms dys?
a.0
b.1/4
c.1/3
d.1/4
e.3/4

[ArchivalUser]

aa

[ArchivalUser]

yes

[ArchivalUser]

aa

[ArchivalUser]

yes a

[ArchivalUser]

it is X-linked recessive (XR)
males get the disease females carry it .....................Do not they always

[ArchivalUser]

that means male will never be a carrier , right??

[ArchivalUser]

yes (Therefore, no male-to-male transmission in X-linked disorders can occur. On the other hand, a male must contribute his one X chromosome to all daughters )

X-Linked Inheritance......................



X-linked inherited disorders are caused by defects in genes located on the X chromosome. Because females have two X chromosomes, they may carry either one mutant allele (heterozygote) or two mutant alleles (homozygote).
The trait may therefore display dominant or recessive expression. Males have a single X chromosome (and one Y chromosome). Consequently, a male is expected to display the full syndrome whenever he inherits the abnormal gene from his mother. Hence, the terms X-linked dominant and X-linked recessive apply only to the expression of the gene in females. Because a male must pass on his Y chromosome to all male offspring, he cannot pass on a mutant X allele to his sons. Therefore, no male-to-male transmission in X-linked disorders can occur. On the other hand, a male must contribute his one X chromosome to all daughters .
All females receiving a mutant X chromosome are known as carriers, and those who become affected clinically with the disease are known as manifesting female carriers. Table 81“2 lists the characteristic features of X-linked inheritance. Examples of X-linked disorders of the heart include X-linked cardiomyopathy, Barth syndrome, and Duchenne, Becker, and Emery-Dreifuss muscular dystrophies.

[ArchivalUser]

thankx.. thankx.. thankx..

[ArchivalUser]

Characteristic Features of Patterns of Inheritance

A. Autosomal dominant transmission
1. Each affected individual has an affected parent unless the disease occurred because of a new mutation or the heterozygous parent has low expressivity.
2. Equal proportions (i.e., 50“50) of normal and affected offspring are likely to be born to an affected individual.
3. Normal children of an affected individual bear only normal offspring.
4. Equal proportions of males and females are affected.
5. Both sexes are equally likely to transmit the abnormal allele to male and female offspring, and male-to-male transmission occurs.
6. Vertical transmission through successive generations occurs.
7. Delayed age of onset.
8. Variable clinical expression.
B. Autosomal recessive transmission
1. Parents are clinically normal (in alternate generations) but genetically are heterozygotes.
2. Alternate generations are affected, with no vertical transmission.
3. Both sexes are affected with equal frequency.
4. Each offspring of heterozygous carriers has a 25 percent chance of being affected, a 50 percent chance of being an unaffected carrier, and a 25 percent chance of inheriting only normal alleles.
C. X-linked transmission
1. No male-to-male transmission.
2. All daughters of affected males are carriers.
3. Sons of carrier females have a 50 percent risk of being affected and daughters have a 50 percent chance of being carriers.
4. Affected homozygous females occur only when an affected male and carrier female have children.
5. The pedigree pattern in X-linked recessive traits tends to be oblique because of the occurrence of the trait in the sons of normal carriers but not in the sisters of affected males (i.e., uncles and nephews affected).
D. Mitochondrial transmission
1. Equal frequency and severity of disease for each sex.
2. Transmission through females only, with offspring of affected males being unaffected.
3. All offspring of affected females may be affected.
4. Extreme variability of expression of disease within a family (may include apparent nonpenetrance).
5. Phenotype may be age-dependent.
6. Organ mosaicism is common.