Exam 2: Extensions to Mendelian Genetics and Extranuclear Inheritance

Question 1

Explain examples of pleiotrophy.

Answer 1

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.

Question 2

Define epistasis

Answer 2

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.

Question 3

Huntington’s Disease is an example of

Answer 3

A dominant lethal allele

Question 4

The FUT1 gene is epistatic to

Answer 4

The ABO gene

Question 5

Define pleiotropy

Answer 5

A single gene impacting multiple phenotypes

Question 6

What is the phenotypic ratio of a monohybrid Punnett square when there is a recessive lethal allele?

Answer 6

2:1

Question 7

Lethal alleles help to identify

Answer 7

Essential genes– because they are essential for survival and a homozygous mutation acts as a lethal allele.

Question 8

Explain the significance of the Agouti gene in mice

Answer 8

The mutant AY allele behaves dominantly when heterozygous but is lethal homozygous recessive.

Question 9

Why would an individual’s ABO gene be masked phenotypically?

Answer 9

If it was homozygous recessive for the FUT1

Question 10

Recessive epistasis ratio

Answer 10

9:3:4

Question 11

Dominant epistasis ratio

Answer 11

12:3:1

Question 12

Don’t forget the parental phenotypes when calculating

Answer 12

Penetrance

Question 13

Position Effect

Answer 13

Physical location of gene influences expression
– Translocation or inversion events modify expression
– Gene can be relocated to condensed or genetically inert chromosome (heterochromatin)

Question 14

Conditional Mutations

Answer 14

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.

Question 15

If the deletion is passed on paternally on chromosome 15q11 to 15q13

Answer 15

Prader-Willi syndrome (PWS)

Question 16

If the deletion is passed on maternally on chromosome 15q11 to 15q13

Answer 16

Angelman syndrome (AS)

Question 17

Describe extranuclear inheritance.

Answer 17

Transmission of genetic information to offspring through cytoplasm not nucleus, usually from one parent

Examples:
-organelle heredity
-maternal effect

Question 18

Contrast organelle inheritance and maternal effect.

Answer 18

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.

Question 19

Relate heteroplasmy to the variation seen in phenotypes governed by organelle inheritance.

Answer 19

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.

Question 20

Maternal effect

Answer 20

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.

Question 21

Nutritional mutations

Answer 21

-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

Question 22

Genomic (parental) imprinting

Answer 22

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)

Question 23

Hemizygous

Answer 23

Condition of a gene where only one set of chromosomes from a chromosomal pair is observed.

Question 24

What are some examples of non-Mendelian inheritance?

Answer 24

-Codominance
-Incomplete dominance
-Multiple alleles
-Pleiotropy
-Epistasis
-Lethal alleles

Question 25

SNP

Answer 25

Single-nucleotide polymorphism

DNA marker during sequencing

Question 26

Does crossing over occur between sister chromatids/Harlequin chromosomes?

Answer 26

No

Question 27

Describe variable expressivity and calculate penetrance

Answer 27

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.

Question 28

Evaluate inheritance patterns that involve parental imprinting

Answer 28

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.

Question 29

What other organelles besides the nucleus is DNA found?

Answer 29

Mitochondria and chloroplast

Question 30

Complementation Analysis

Answer 30

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?”

Question 31

Allelic Series/Complementation Group

Answer 31

All mutations present in any single gene

Question 32

If the offspring are recombinant after a complementation analysis (crossing two homozygous mutants), what does it tell you? What if they aren’t?

Answer 32

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

Question 33

Define sex-linkage

Answer 33

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.

Question 34

Truebreeding is a

Answer 34

phenotypic outcome

Question 35

Homozygous is an

Answer 35

allelic makeup

Question 36

A sinistral (homozygous recessive) female snail has a heterozygous progeny. Will the progeny be sinistral or dextral?

Answer 36

Sinistral

Question 37

_________ are the unit of transmission in meiosis, not genes

Answer 37

Chromosomes

Question 38

________ can not undergo independent assortment

Answer 38

Linked genes

Question 39

If the mutations are on separate genes

Answer 39

complementation occurs