Question

Genetic Inheritance Patterns: Retinitis pigmentosa (RP) can be autosomal recessive, autosomal dominant, or x-linked. Apparently the dominant forms are often less severe. Usher’s Syndrome is an autosomal recessive inheritance (i.e., you must get a copy of the defective gene from your Mom and one from your Dad). Autosomal means it is not carried on one of the chromosomes that determines sex. One website (http://www.emedicine.com/oph/topic704.htm) says that Usher’s Syndrome Type II has been mapped to chromosome arm 1qe. Usher’s Type II is recessive, so for Eric this means that both his Mom and Dad are carriers of this condition

1) Draw a pedigree of Eric's family showing possible genotypes and chances of having RP. Within this pedigree, include Eric's parents, Eric himself, Eric's brother Dirk, and a daughter of Eric's. Be sure to follow all standard pedigree conventions.

2) Imagine you are a genetics counselor. Based upon the pattern of inheritance, what advice would you give Eric's brother Dirk about the chances of Dirk's children having RP? (Hint: Assume that there is a 1:80 chance that Dirk's wife is a carrier.)

3) How would your advice as a genetics counselor change if Dirk's wife has the disease?

4) Should Eric have children? Imagine he has a daughter (he has two, but both are adopted). What advice would you give a genetic daughter from your professional viewpoint?

Answer 1

There are five basic modes of inheritance for single-gene diseases: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and mitochondrial. Genetic heterogeneity is a common phenomenon with both single-gene diseases and complex multi-factorial diseases.

The genotype is determined by alleles that are received from the individual's parents (one from Mom and one from Dad). These alleles control if a trait is “dominant” or “recessive”. Additionally, the location of the alleles in the genome determine if a trait is “autosomal” or “X-linked”.

The most common inheritance patterns are: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, multifactorial and mitochondrial inheritance. "Autosomal" refers to traits determined by the genes located on the autosomes.

Patterns of Inheritance

Autosomal Dominant

• Each affected person has an affected parent

• Occurs in every generation

Autosomal Recessive

• Both parents of an affected person are carriers

• Not typically seen in every generation

X-linked Dominant

• Females more frequently affected

• Can have affected males and females in same generation

X-linked Recessive

• Males more frequently affected

• Affected males often present in each generation

Mitochondrial

• Can affect both males and females, but only passed on by females

• Can appear in every generation

Genetic Counseling

• checking family medical history and medical records
• ordering genetic tests
• evaluating the results of these tests and records
• helping parents understand and reach decisions about what to do next

Genetic tests are done by analyzing small samples of blood or body tissues. They determine whether you, your partner, or your baby carry genes for some inherited disorders.

Genetic counselors can help:

• identify and interpret the risks of an inherited disorder
• explain inheritance patterns
• suggest testing
• lay out possible scenarios