Exam 2: Extensions to Mendelian Genetics and Extranuclear Inheritance Flashcards
Explain examples of pleiotrophy.
The production by a single gene of two or more apparently unrelated effects.
Ex: Agouti gene in mice
Phenylketonuria– is caused by a single gene defect but causes multiple effects such as mental retardation, and hyperpigmentation of hair and skin.
Define epistasis
Epistasis is when expression of one gene modifies the effect of a second gene. Phenotypic effects of a gene product can be masked of another gene.
Huntington’s Disease is an example of
A dominant lethal allele
The FUT1 gene is epistatic to
The ABO gene
Define pleiotropy
A single gene impacting multiple phenotypes
What is the phenotypic ratio of a monohybrid Punnett square when there is a recessive lethal allele?
2:1
Lethal alleles help to identify
Essential genes– because they are essential for survival and a homozygous mutation acts as a lethal allele.
Explain the significance of the Agouti gene in mice
The mutant AY allele behaves dominantly when heterozygous but is lethal homozygous recessive.
Why would an individual’s ABO gene be masked phenotypically?
If it was homozygous recessive for the FUT1
Recessive epistasis ratio
9:3:4
Dominant epistasis ratio
12:3:1
Don’t forget the parental phenotypes when calculating
Penetrance
Position Effect
Physical location of gene influences expression
– Translocation or inversion events modify expression
– Gene can be relocated to condensed or genetically inert chromosome (heterochromatin)
Conditional Mutations
Temperature effects: Mutant allele expresses mutant phenotype at one temperature, wild-type phenotype at another
Ex: Siamese cats and Himalayan rabbits. Enzymes lose catalytic function at higher temperature, so the colder parts of the body are darker.
If the deletion is passed on paternally on chromosome 15q11 to 15q13
Prader-Willi syndrome (PWS)
If the deletion is passed on maternally on chromosome 15q11 to 15q13
Angelman syndrome (AS)
Describe extranuclear inheritance.
Transmission of genetic information to offspring through cytoplasm not nucleus, usually from one parent
Examples:
-organelle heredity
-maternal effect
Contrast organelle inheritance and maternal effect.
Organelle inheritance: One variety of extranuclear inheritance. DNA contained in mitochondria or chloroplasts determines certain phenotypic characteristics of offspring (Ex: petite colonies)
Maternal Effect: Nuclear gene products are stored in egg, then transmitted through ooplasm to offspring. Offspring’s phenotype is under control of nuclear
gene products present in egg. Mother’s GENOTYPE determines offspring’s PHENOTYPE.
Relate heteroplasmy to the variation seen in phenotypes governed by organelle inheritance.
Heteroplasmy is the variation of mtDNA.
Often a mutation in one or few mitochondria will be diluted out by many mitochondria that lack mutation and function normally.
Instead of the discrete allele frequencies of 0%, 50%, or 100% observed for alleles of nuclear genes, allele frequencies for mtDNA loci have a continuous distribution ranging from 0% to 100% (“homoplasmy”).
A low-mutation heteroplasmy is unlikely to cause a discernible phenotype, whereas high-mutation heteroplasmy is very likely to cause problems, especially when present in an energy-sensitive organ like the brain or muscles.
Maternal effect
Genotype of the mother determines phenotype of the offspring regardless of its own genotype. Nuclear gene products are stored in egg, then transmitted through ooplasm to offspring.
Nutritional mutations
-Prevent proper synthesis of nutrient molecules in microbes.
– Phenotype expressed or not depending upon diet
Ex: Phenylketonuria
– Loss of enzyme to metabolize amino acid phenylalanine
– Severe problems unless low-Phe diet
Genomic (parental) 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)
Hemizygous
Condition of a gene where only one set of chromosomes from a chromosomal pair is observed.
What are some examples of non-Mendelian inheritance?
-Codominance
-Incomplete dominance
-Multiple alleles
-Pleiotropy
-Epistasis
-Lethal alleles
SNP
Single-nucleotide polymorphism
DNA marker during sequencing
Does crossing over occur between sister chromatids/Harlequin chromosomes?
No
Describe variable expressivity and calculate penetrance
Penetrance- The extent (given as %) to which individuals with a disease genotype manifest at least some degree of the associated phenotype.
Expressivity- The range of expression of mutant phenotype. Result of genetic background differences and/or environmental effects.
Evaluate inheritance patterns that involve parental imprinting
Imprinting occurs when regions of maternal or paternal chromosomes are turned “off” during gametogenesis.
This phenomenon creates a situation where an egg and sperm are required to make a viable embryo
Overall, imprinting results in “monoallelic expression” of certain genes in the embryo.
DNA methylation can cause this.
Ex: PWS vs AS on chromosome 15
Ex: Igf2 – imprinted on the maternal allele such that only the paternal allele is active.
What other organelles besides the nucleus is DNA found?
Mitochondria and chloroplast
Complementation Analysis
Screens number of individual mutations resulting in same phenotype. Can predict total number of genes determining a trait.
“Does each mutant carry a mutation in the same gene, or are there two separate genes involved?”
Allelic Series/Complementation Group
All mutations present in any single gene
If the offspring are recombinant after a complementation analysis (crossing two homozygous mutants), what does it tell you? What if they aren’t?
1) Mutant phenotype (aa x bb = ab)
Non-complementation
Alleles of same gene
2) Wild-type phenotype
Complementation occurs
Alleles are present on different genes
Define sex-linkage
Sex linkage applies to genes that are located on the sex chromosomes. These genes are considered sex-linked because their expression and inheritance patterns differ between males and females.
Truebreeding is a
phenotypic outcome
Homozygous is an
allelic makeup
A sinistral (homozygous recessive) female snail has a heterozygous progeny. Will the progeny be sinistral or dextral?
Sinistral
_________ are the unit of transmission in meiosis, not genes
Chromosomes
________ can not undergo independent assortment
Linked genes
If the mutations are on separate genes
complementation occurs
