Chapter 4 Flashcards
1) Epistasis
2) Example
1) A phenomenon when the genotype of one gene can mask the effects of a separate gene
2) The Bombay phenotype. A parent with AB blood has a child with type O blood, for example. Due to the presence of a second gene for blood type (H), which when homozygous recessive prevents expression of the ABO phenotype.
1) lethal allele
2) Example
In human genetics, it is common for dominant disease alleles to be lethal if homozygous.
Ex. Huntington disease is a dominant disorder, but affected individuals are heterozygous, because homozygous individuals died in the womb.
2) Yellow coat allele in mice
1) Klinefelter Syndrome:
2) Karyotype
1) male with feminization (breast development, small penis)
2) 47, XXY (most common)
47, XXY (most common) male with feminization (breast development, small penis)
Klinefelter Syndrome:
1) Turner Syndrome:
2) Karyotype
1) Female, usually distinguishable by presence of webbed neck
2) 45, XO only one X
- 45 chromosomes, only one X. XO
- Female, usually distinguishable by presence of webbed neck
Turner Syndrome:
1) Congenital adrenal hyperplasia (CAH) – AR -
2) mode of inheritance
3) Karyotype
1) Caused when an excess of androgens released from the adrenal glands androgenizes the fetus, including brain, and makes external genitalia ambiguous. The brain androgenization makes this XX individual feel like a boy.
2) Autosomal recessive
3) 46, XX
1) Caused when an excess of androgens released from the adrenal glands androgenizes the fetus, including brain, and makes external genitalia ambiguous. The brain androgenization makes this XX individual feel like a boy.
2) Autosomal recessive
3) 46, XX
1) Congenital adrenal hyperplasia (CAH)
2) mode of inheritance
3) Karyotype
1) 5 α reductase deficiency –
2) mode of inheritance
3) Karyotype
Mutation in the 5αR gene results in an XY fetus that cannot produce dihydrotestosterone (which normally triggers development external male genitalia). Internal male organs develop normally. Common in a certain region of the Dominican Republic, where these boys are often raised as girls. At puberty, testosterone triggers masculization.
2) Autosomal recessive –
3) 46, XY
Mutation in the 5αR gene results in an XY fetus that cannot produce dihydrotestosterone (which normally triggers development external male genitalia). Internal male organs develop normally. Common in a certain region of the Dominican Republic, where these boys are often raised as girls. At puberty, testosterone triggers masculization.
5 α reductase deficiency – AR –
1) Androgen insensitivity
2) mode of inheritance
3) Karyotype
Androgen receptors do not respond to male hormones, resulting in an XY individual who appears to be a tall female, often with ambiguous genitalia and no internal reproductive organs.
2) X-linked recessive
3) 46, XY
Androgen receptors do not respond to male hormones, resulting in an XY individual who appears to be a tall female, often with ambiguous genitalia and no internal reproductive organs.
Androgen insensitivity - XR –
1) Sex-limited traits
2) Example
Some autosomal traits mimic a sex-linked inheritance pattern
Sex-limited traits are expressed only in one sex, not the other, even when the genotypes are the same.
2) Ex. autosomal dominant precocious puberty - expressed in heterozygous males but not in heterozygous females.
Heterozygous females pass the trait on to ½ of male offspring, making this look like a sex-linked gene
1) Sex-influenced traits
2) Example
1) can be expressed in both sexes, but not according to predicted Mendelian ratios, due to the influence of hormones.
2) Some forms of “male pattern baldness” – autosomal dominant in males, autosomal recessive in females. This differential expression is related to interaction with different levels of testosterone (androgens).
X-chromosome inactivation:
The Lyon hypothesis
X chromosome inactivation is really a mechanism to achieve “dosage compensation” - the solution to the problem of genetic dosage of X chromosome genes between sexes
