Gene Interaction

Question 1

What is an amorphic (null) mutation?

Answer 1

Complete loss of gene function; no functional protein produced; usually recessive because one wild type allele is sufficient

Question 2

Why is the CFTR ΔF508 mutation recessive?

Answer 2

One wild type CFTR allele maintains the Cl- gradient; disease occurs only when no functional CFTR reaches the membrane

Question 3

What is a hypomorphic (leaky) mutation?

Answer 3

Partial loss of function due to reduced protein amount or activity; usually recessive

Question 4

What is a hypermorphic mutation?

Answer 4

Gain of function mutation increasing protein activity or abundance; usually dominant

Question 5

What is an antimorphic (dominant-negative) mutation?

Answer 5

Mutant protein interferes with wild type protein function, often in multimers (e.g. fibrillin-1 in Marfan syndrome)

Question 6

Why are dominant-negative mutations usually dominant?

Answer 6

Mutant protein disrupts function even when wild type protein is present

Question 7

What is a neomorphic mutation?

Answer 7

Gain of function mutation conferring a novel function; usually dominant

Question 8

What defines a dominant lethal mutation?

Answer 8

Gain of function mutation causing death, usually with late onset (e.g. Huntington’s disease)

Question 9

What is incomplete (partial) dominance?

Answer 9

Heterozygote shows an intermediate phenotype; phenotypic ratios deviate from Mendelian expectations

Question 10

What is codominance?

Answer 10

Both alleles are fully and equally expressed in the heterozygote

Question 11

Why is the ABO blood group an example of codominance?

Answer 11

IA and IB alleles are both expressed in AB individuals

Question 12

How do lethal alleles distort Mendelian ratios?

Answer 12

Homozygous lethal genotypes are absent, altering expected offspring ratios

Question 13

Why are all yellow mice heterozygous?

Answer 13

The yellow allele is dominant for coat colour but recessive lethal for viability

Question 14

Why do homozygous Manx cars not exist?

Answer 14

The tailless allele is dominant for phenotype but lethal when homozygous

Question 15

What is genetic load?

Answer 15

The collection of lethal and deleterious alleles in a population

Question 16

Why do X-linked recessive traits occur more often in males?

Answer 16

Males are hemizygous for the X chromosome

Question 17

What proportion of sons of a carriers female will be affected by an X linked recessive trait?

Answer 17

50%

Question 18

What is a sex-limited trait?

Answer 18

Autosomal trait expressed in only one sex (e.g. milk production)

Question 19

What is a sex-influenced trait?

Answer 19

Autosomal trait whose expression differs between sexes due to hormones (e.g. male pattern baldness)

Question 20

Why is mitochondrial inheritance non-Mendelian?

Answer 20

Mitochondria are inherited exclusively from the mother

Question 21

Why do mitochondrial diseases affect muscle and nerve tissue most?

Answer 21

These tissues have high ATP demands

Question 22

What is penetrance?

Answer 22

The proportion of individuals with a genotype who express the phenotype

Question 23

What is variable expressivity?

Answer 23

Variation in severity or magnitude of phenotype among individuals with the same genotype.

Question 24

What factors influence penetrance and expressivity?

Answer 24

Modifier genes and environmental factors

Question 25

What is epistasis?

Answer 25

Interaction where one gene masks or modifies the phenotypic effect of another gene

Question 26

Why do genes involved in epistasis usually affect the same pathway?

Answer 26

They act sequentially or redundantly to produce a phenotype

Question 27

What phenotypic ratio indicates complementary gene action?

Answer 27

9:7

Question 28

What defines complementary gene action?

Answer 28

Two dominant alleles at different loci are required for the phenotype

Question 29

Why does homozygous recessive at either locus give the same phenotype?

Answer 29

Loss of either enzyme blocks the pathway

Question 30

What phenotypic ratio indicates recessive epistasis?

Answer 30

9:3:4

Question 31

Which genotype is epistatic in recessive epistasis?

Answer 31

Homozygous recessive at one locus masks the other gene

Question 32

Example of recessive epistasis?

Answer 32

Mouse coat colour (albino masks pigment genes)

Question 33

What phenotypic ratio indicates dominant epistasis?

Answer 33

12:3:1

Question 34

What defines dominant epistasis?

Answer 34

A dominant allele at one locus masks the expression of another gene

Question 35

Example of dominant epistasis?

Answer 35

Colour in summer squash

Question 36

What phenotypic ratio indicates duplicate gene action?

Answer 36

15:1

Question 37

What causes duplicate gene action?

Answer 37

Two genes perform the same function; only double recessive shows mutant phenotype

Question 38

What is the purpose of the complementation test?

Answer 38

To determine whether mutations are in the same gene or different genes

Question 39

What does failure to complement indicate?

Answer 39

Mutations are alleles of the same gene

Question 40

What does complementation indicate?

Answer 40

Mutations are in different genes

Question 41

What is a complementation group?

Answer 41

A set of mutations that fail to complement; corresponds to one gene

Question 42

How does the number of complementation groups relate to genes?

Answer 42

Number of complementation groups equals number of genes involved