Topic 3

Question 1

What does haplosufficient mean?

What does haploinsufficient mean?

Answer 1

Haploid dose (one) has the ability to produce the wild type phenotype 
Only need one dominant (or wild type) allele to show wild type trait

Haploinsufficient means one wild type dose is not enough to achieve normal levels of function

Slide 7
Slide 9

Question 2

What is a null mutation?

What happens when you combine it with a single wild type allele?

Answer 2

Mutation that produces a non functional protein

Combining null mutation with a single wild type allele would produce well below the minimum, so the heterozygote is a mutant and the mutation is dominant

For most genes. A single wild type copy is good enough for full expressions (these genes are haplosufficient) and their null mutations are fully recessive. Harmful mutations of haploinsufficient genes are often dominant

Question 3

What is a dominant negative mutation?

Answer 3

Produces a polypeptide that interferes with the wild type polypeptide by binding to it and distorting it (interfering with its function)

Mutations in genes that encode units in homo- or heterodimers can behave as dominant negatives

Slide 4
Slide 9

Question 4

What is incomplete dominance?

What is codominance?

Answer 4

The occurrence of an intermediate phenotype for a heterozygous allele pairing

Crossing pure breed red flowers with pride breed white flowers results in pink flowers

Codominance is when more than one allele is dominant
Heterozygote displays both parental phenotypes
Blood types are good examples

Slides 13-19
Pages 219-220

Question 5

What is ABO transferase?

Answer 5

Enzyme with glycotransferase activity

A variant- transfers N-acetyl-galactosamide
B variant- transfers galactose
O mutation results in non functional enzyme (null allele)

Slide 18

Question 6

What is an essential gene?

What is a sub lethal allele?

Answer 6

Essential gene is a gene without which an organism dies
To test to see if a gene is essential, a null allele is tested for lethality

Sublethal allele is an allele where the lethality is expressed in only some but not all of homozygous individual

Pages 221-223
Slides 26-28

Question 7

What is variable penetrance?

Answer 7

The proportion of individuals of the same genotype that will express the determined phenotype (either express it or not)
Same genotype, different phenotypes

Slide 21
Slide 25

Question 8

What is variable expressivity?

Answer 8

The degree or intensity with which a genotype is expressed (100% of individuals shows the consequences of the mutation, but there are many possible degrees of severity)

Same genotype, variable (non-uniform) mutant phenotypes

Slide 23-25

Question 9

What is the one-gene-one-polypeptide hypothesis?

Answer 9

All proteins were found to be encoded by genes

Gene encoded physical structure of protein, which in turn dictates its function

Question 10

What are messenger RNAs (mRNA)?

What are functional RNAs (fRNAs)?

Answer 10

Protein encoding genes are transcribed to mRNA which is then translated to protein

fRNAs are RNAs encoded by a minority of genes that are never transcribed into a protein

Question 11

What is pleiotropic inheritance?

What is polygenic inheritance?

Answer 11

Pleiotropic inheritance- any allele that affects several properties of an organism
One allele, affects 2 or more phenotypes

Polygenic inheritance- many genes (and alleles) affecting the same phenotype

Polygenic inheritance refers to the cumulative effect of 2 or more genes on a single trait
Pleiotropy refers to a single gene that affects multiple traits

Slides 26-40

Question 12

What is the complementation test?

What is complementation?

Answer 12

Performed by inter crossing 2 individuals that are homozygous for different recessive mutations
Then observe if the progeny have the wild type phenotype

If the progeny are wild type, the 2 recessive mutations must be in different genes because the wild type alleles provided wild type function (the 2 mutations are said to be complemented)

Complementation is the production of a wild type phenotype when 2 haploid genomes bearing different recessive mutations are united in the same cell (when 2 independently derived recessive mutant alleles producing similar recessive phenotypes fail to complement, they must be alleles of the same gene)

Slides 41-50
Pages 227-230

Question 13

What is the 9:3:3:1 ratio?
What is the the 9:7 ratio?
What is the 9:3:4 ratio?

Answer 13

9:3:3:1 ratio means there’s no gene interaction

9:7 ratio means genes in the same pathway
Only possible if the double mutant has the same phenotype as the 2 single mutants

9:3:4 ratio means recessive epistasis, double mutant shows the phenotype of one mutation but no the other
Overriding mutation is epistatic
Overridden mutation is hypostatic

Pages 231-234
Slides 80-111

Question 14

What is the 12:3:1 ratio?

Answer 14

Dominant epistasis- when the dominant allele of one gene masks the effects of either (dom/rec) allele of second gene
Dominant allele is epistatic

Compare epistasis and dominance slide 80

Question 15

What is a suppressor?

Answer 15

Mutant allele of a gene that reverses the effect of a mutation of another gene
Results in a wild type or near wild type phenotype

Suppression often confused with epistasis
Difference between them is suppressors cancels the expression of a mutant allele and restored the corresponding wild type phenotype
Only usually 2 phenotypes segregate in suppression where as 3 in epistasis

Pages 236-237
Slide 56

Question 16

What is a modifier?

Answer 16

Modifier mutation at a second locus changes the degree of expression of a mutated gene at the first locus

Page 238

Question 17

What are synthetic lethals?

Answer 17

In some cases when 2 viable single mutants are inter crossed, the resulting double mutants are lethal
Inna diploid F2 this would result in 9:3:3 ratio because double mutant is lethal

Synthetic lethals can point to specific types of interactions of gene products

Slide 57

Question 18

What are the modified F2 ratios for each of these:
No interaction
Genes in same pathway 
Recessive epistasis
Dominant epistasis
Suppressor has no phenotype
Suppressor is like mutant 

Page 239!!

Answer 18

9: 3:3:1- no interaction
9: 7- genes in same path
9: 3:4- recessive epistasis
12: 3:1- dominant epistasis
13: 3- suppressor has no phenotype
14: 2- suppressor is like mutant

Question 19

What is penetrance?

Answer 19

Defined as the percentage of individuals with a given allele who exhibit the phenotype associated with that allele

Question 20

What are the 3 reasons a organism has a genotype and not express the phenotype?

Answer 20

1. The influence of environment- individuals with genotype May show range of phenotypes dependant on environment
2. The influence of other interacting genes- uncharacterized modifiers, epistatic genes, or suppressors in the rest of the genome may act to prevent the expression of the typical phenotype
3. The subtlety of the mutant phenotype- subtle effects brought about by the absence of a gene function may be difficult to measure

Question 21

What is expressivity?

Answer 21

Measure the degree to which a given allele is expressed at the phenotypic level
Measures intensity of the phenotype

Question 22

What are the 3 levels of genetic interaction affecting a phenotype?

Answer 22

1. Allele interaction (intra-genic)
2. Allele interaction with environment
3. Gene interaction

Slides 58-61

Question 23

What is additive gene action?

Answer 23

Alleles of both genes influence the same phenotype
The F1 hybrid has a different phenotype than the parentals (similar time incomplete dominance)

Shoes mendalian like 9:3:3:1 ratio

1 trait, 2 loci, 4 phenotypes

Slides 66-71

Question 24

What is complementary gene action?

Answer 24

Both genes loci have homozygous recessive alleles and both produce identical phenotypes
Both dominant alleles are necessary to produce a different phenotype (CCPP, CcPp, CcPP all work, ccPP and CCpp produce mutant)

2 loci, 1 trait, 2 phenotypes

Slides 72-74

Question 25

What is redundancy (duplicate gene action)?

Answer 25

Dominant alleles of both genes alone produce the same phenotypic effect
Atleast one of the dominant allele is necessary for the phenotypic effect (only aabb gives mutant)

2 loci, 1 trait, 2 phenotypes

Slides 75-78

Question 26

Can genes be separated in meiosis?

Can alleles be separated in meiosis?

Answer 26

Genes that are in the same chromosome travel through meiosis together
Alleles of linked genes can be recombined by crossing over

Linked alleles tend to be inherited together

Slides 5-6 topic 4