Sex-Linked and Nontraditional Modes of Inheritance

Question 1

Sex linked mutations?

Answer 1

Mutations of the sex chromosomes ( X and Y ).

Question 2

X-Linked diseases?

Answer 2

Caused by genes located on the X chromosome.

Question 3

What is X inactivation?

Answer 3

The Lyon hypothesis: one X chromosome in each cell is randomly inactivated early in the embryonic development of females. This ensures that females, who have two copies of the X chromosome, will produce X-linked gene products in quantities roughly similar to those produced in males (dosage compensation).

Question 4

Define a Barr body.

Answer 4

An inactive X chromosome.

Question 5

Despite X inactivation, people with additional or missing X chromosomes show a different phenotype. Why?

Answer 5

X inactivation is random, fixed, and incomplete. The last feature helps to explain why, despite X inactivation, most persons with abnormal numbers of sex chromosomes have a disease phenotype.

Question 6

What are some characteristics and examples of X-Linked recessive inheritance?

Answer 6

1. Because females have two copies of the X chromosome and males have only one (hemizygosity), X-linked recessive diseases are much more common among males than among females.
2. X-linked recessive inheritance is characterized by an absence of father-to-son transmission, skipped generations when genes are passed through female carriers, and a preponderance of affected males.
3. Recurrence risks for X-linked recessive disorders are more complex than for autosomal disorders. The risk depends on the genotype of each parent and the sex of the offspring.
4. Because X inactivation is a random process, some female heterozygotes experience inactivation of most of the normal X chromosomes in their cells. These manifesting heterozygotes are usually mildly affected.

Examples: Duchenne’s muscular dystrophy, Haemophillia A

Question 7

What are some characteristics and examples of X-Linked dominant inheritance?

Answer 7

X-linked dominant diseases display characteristic patterns of inheritance. They are about twice as common in females as in males ( unless the disease is lethal for males, which it often is ), skipped generations are uncommon, and father-to-son transmission is not seen.
Examples:
incontinentia pigmenti type 1 ( males do not survive )
Rett syndrome ( majority males do not survive )

Question 8

What are Y-Linked genes called?

Answer 8

Holandric genes.

Question 9

What inheritance pattern is observed in Y-Linked traits?

Answer 9

Strictly from father to son.

Question 10

What’s the difference between sex-influenced and sex-related traits?

Answer 10

Sex-limited: Occur in only one sex, due to anatomical differences ( for example testicular defects )
Sex-influenced: Occur in both sexes, but more commonly in one ( for example “male” pattern baldness)

Question 11

What is mitochondrial inheritance and what defines it?

Answer 11

The mitochondria, which produce ATP, have their own unique DNA. Mitochondrial DNA is maternally inherited and has a high mutation rate. A number of diseases are known to be caused by mutations in mitochondrial DNA.

Question 12

How is genomic imprinting characterized?

Answer 12

For some human genes, one of the alleles is transcriptionally inactive (no mRNA is produced), depending upon the parent from whom the allele was received. For example, an allele transmitted by the mother would be inactive, and the same allele transmitted by the father would be active. The normal individual would have only one transcriptionally active copy of the gene. The silenced one is said to be imprinted.