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
1) Dosage compensation
2) Example
1) All but one X chromosome are inactivated in each cell; which of the X chromosomes is inactivated is random and varies from cell to cell. This ensures the same amount of X-linked gene product in both female and male.
2) Toroiseshell cat and calico cats (mosaic patern)
Barr bodies –
A highly condensed, inactivated X chromosome that appears as a darkly staining body lying against the nuclear membrane of interphase cells.
A highly condensed, inactivated X chromosome that appears as a darkly staining body lying against the nuclear membrane of interphase cells.
Barr bodies –
1) Complete dominance
2) Example
1) Phenotype of the heterozygote is the same as the phenotype of one of the homozygotes.
2) Bb genotype = B phenotype
1) Incomplete dominance
2) Example
1) Phenotype of heterozygote is intermediate between the phenotype of the two homozygotes.
2) Red flower + White flower = pink flower
1) Define co-dominance
2) Example
1) Phenotype of heterozygote includes the phenotype of both homozygotes.
2) ABO blood groups
Incomplete dominance
When a heterozygote has a phenotype INTERMEDIATE between those of two parental homozygotes.
Variable Expressivity
Refers to the degree of severity of the phenotype among persons with the disease genotype. In a group of individuals with a disease genotype, some may exhibit mild (or no) symptoms while others are severe.
Incomplete pentrance and variable expressivity Example.
A dominant allele causes polydactyly, but it exhibits incomplete penetrance and variable expressivity
Gene interaction
Epistasis: a gene interaction where expression of one gene prevents expression of a second gene’s phenotype.
Epistasis
A gene interaction where expression of one gene prevents expression of a second gene’s phenotype.
Complementation Test
Answers the question,
“Does each mutant carry a mutation in the same gene, or are there two separate genes involved?”
If you have two organisms with the same recessive trait, you cross them to determine if they are both homozygous recessive for the same gene, or for 2 different genes.
Procedure: Cross two homozygous mutants.
1) Cytoplasmic” or “Extranuclear” Inheritance
2) Example
1) Trait inherited due to a defective allele in the mitochondria (or chloroplasts); maternal inheritance only.
2) MERRF – myoclonic epilepsy with ragged red fibers (in skeletal muscle)
1) Maternal Effect
2) Example
1) Genotype of the mother determines phenotype of the offspring.
Explanation: Maternal mRNAs or gene products are deposited into the egg.
Maternal effect genes are well-studied in Drosophila, but very few are known in mammals.
2) Snail shell coiling
Genomic Imprinting
Imprinting occurs when regions of maternal or paternal chromosomes are turned “off” during gametogenesis. These regions contain genes vital for embryogenesis. This phenomenon creates a situation where an egg and sperm are required to make a viable embryo (not 2 sperm or 2 eggs).
Overall, imprinting results in “monoallelic expression” of certain genes in the embryo.
Imprinting is not permanent; it is removed from the chromosomes as they pass through the germ-line, and new imprints are added.
What is imprinting?
The inactivation of certain developmental genes on the chromosomes during spermatogenesis and oogenesis, such that only the fusion between an egg and sperm will produce a viable embryo.
1) Epigenetics
2) Examples
Reversible changes to DNA that influence the expression of traits.
Examples: genomic imprinting, methylation of DNA, acetylation of histones.
1) Sex-linked
2) Examples
1) Characteristics determined by genes located on the sex chromosomes.
Because the Y chromosome has little genetic information, most sex-linked are X-linked
2) hemophilia, muscular dystrophy
