Quiz 8

Question 1

allele

Answer 1

variants in the genetic sequence at a particular locus

Question 2

chromosome

Answer 2

a single, uninterrupted segment of DNA that is associated with proteins, containing centromeres, telomeres and origins of replication

Question 3

dominant

Answer 3

an allele that produces the same phenotype in an organism regardless if the organism is homozygous or heterozygous for it; the allele that dictates the phenotype of interest

Question 4

gene

Answer 4

a sequence of DNA or RNA that codes for a molecule that has a function

Question 5

genome

Answer 5

all of the genetic material of an individual

Question 6

genotype

Answer 6

a description of a cells alleles and/or variants for a set of loci that determine a characteristic of interest

Question 7

environment

Answer 7

the condition that a set of alleles/variants of interest experience

Question 8

to express

Answer 8

to transcribe, and if applicable, translate

Question 9

expressivity

Answer 9

the “severity” of a phenotype of interest; the extent to which a phenotype is exhibited

Question 10

locus

Answer 10

a specific location on a chromosome

Question 11

mutant allele

Answer 11

a variant that is very rare (frequently below 1%)

Question 12

mutant (noun)

Answer 12

an organism or new genetic character that results due to an instance of mutation

Question 13

mutant phenotype

Answer 13

an extremely rare phenotype

Question 14

phenotype

Answer 14

observative characteristics resulting from both genotype and the environment of the organism and the genes (not necessarily inherited)

Question 15

phenotypic variation

Answer 15

any difference in characteristics between two individuals

Question 16

penetrance

Answer 16

the percentage of individuals of a given genotype that have the phenotype that is usually associated with that genotype

Question 17

recessive

Answer 17

a gene that produces a specific phenotype in the organism only if its allele is identical

Question 18

transcript

Answer 18

the product of transcription in the form of RNA

Question 19

wild-type

Answer 19

the most common variant for a locus

Question 20

In the human body, what cells have the tyrp1 gene, which encodes one of the enzymes in the eumelanin biosynthesis pathway? Do all humans have this gene?

Answer 20

All cells in an organism contain all of the organism’s genes and thus, all cells in the human body have the tyrp1 gene
All humans have the gene, but they may have different alleles

Question 21

Consider a Drosophila cell with the following genotype: Cy+/Cy-; Sb+,cv-/Sb-,cv+
Draw the relevant chromosomes of this individual, labelling the regions that correspond to the alleles in question. You may arbitrarily assign their location along the chromosomes as long as you do not contradict the notation given.

Answer 21

• draw two pairs of homologous chromosomes for a total of four chromosomes
• one pair of chromosomes have Cy alleles and the other pair has Sb and cb alleles
• one chromosome of pair 1 will have Cy+ while the other chromosome of pair 1 will have Cy-
• one chromosome of pair 2 will have Sb+ and cv-, while the other chromosome will have Sb- and cv+
• make sure that alleles are in the same loci of the corresponding homologous chromosome

Question 22

Describe the phenotype at the molecular (mRNA) level of a mouse that is homozygous for a missense mutation in the last exon of the gene that encodes the primase enzyme.

Answer 22

• there will be a single base change in the mRNA that will result in the production of a different amino acid during translation

Question 23

Describe the phenotype at the biochemical level of a mouse that is homozygous for a missense mutation in the last exon of the gene that encodes the primase enzyme.

Answer 23

• one amino acid in the polypeptide sequence will be different from what is expected if it were the wild type
• this may lead to a decrease in function of the primase enzyme or cause it to become entirely dysfunctional

Question 24

Describe the phenotype at the cellular/physiological level of a mouse that is homozygous for a missense mutation in the last exon of the gene that encodes the primase enzyme.

Answer 24

• this may cause a metabolic pathway (ie. DNA replication) that relies on the enzyme to stop, creating an accumulation of substrate and no product
• the enzyme’s role may be impaired and might not be able to produce primers

Question 25

Describe the phenotype at the macroscopic/morphological level of a mouse that is homozygous for a missense mutation in the last exon of the gene that encodes the primase enzyme.

Answer 25

• Replication will occur differently than in the wild type
• Cells may be unable to perform mitosis/replicate DNA
• In an embryo, the mouse will not grow
• In an adult, the tissue may not grow properly

Question 26

Given the genotype of a multicellular organism, it is relatively straightforward to predict its phenotype at the RNA level with reasonable certainty, but almost impossible to predict its overall morphological or physiological phenotype. Explain why this is the case.

Answer 26

The phenotype at the RNA level refers to what mRNA can be found in the organism. The mRNA is directly related to the genotype, as the mRNA is transcribed from the DNA sequence.
However, morphological and physiological phenotype is dependent on more than just the mRNA. It also depends on the environment that the mRNA, cells, and the entire individual is subjected to. There is also some randomness involved. Thus, we cannot predict the morphological and physiological phenotype from genotype alone.

Question 27

Manx cats have a congenitally very short tail compared to typical domestic cats. A major contributor to this phenotype is a particular mutant allele of the T gene; cats that are heterozygous with one wild-type and one such mutant T alleles are born with very short tails. However, homozygosity for the T mutant allele in question is lethal. Based on this information, what is the dominance relationship between the wild-type and the T mutant alleles? Briefly justify your answer.

Answer 27

Depends on what phenotype you are interested in.

Recessive if you are looking at “death” and unknown if you are looking at tail length.

Question 28

Predict the mature fruit colour phenotype of a pepper plant (Capsicum annuum) that is heterozygous for a large deletion of the promoter of the CaSGR gene. Clearly describe your logic and reasoning.

Answer 28

• there will be insufficient quantities of functioning SGR enzyme present to degrade all of the pigments, so it will appear a purple brown colour as some chlorophyll remains

Question 29

Sally got her genome sequenced and found that she has a rare mutant allele of the gene coding for the TYRP1 protein, which carries a large deletion in intron 2 of the gene (her other tyrp1 allele is wild-type). Predict her phenotypes in terms of the tyrp1 mRNA and TYRP protein that her cells produce. If you wanted to experimentally test your predictions, what cells would you collect from Sally?

Answer 29

• the mRNA produced may be different in length if the deletion in the intron affects mRNA splicing (ie. removing the splice acceptor site)
• TYRP1 protein may have an altered amino acid sequence depending on how the mRNA was altered; this may affect the proteins function
• To test, collect skin or hair samples because tyrp gene codes for skin, fur, and eye pigment

Question 30

Sally’s friend Francis also had his genome sequenced and found that he is homozygous for a particular variant of a locus that has a causative association with the development of adrenal cancer during mid-adulthood. Assuming that the sequencing result is accurate, what important information should Francis obtain before deciding whether he should be worried, and how should he use this information?

Answer 30

• get information about the penetrance of the gene to see how often the phenotype is observed when the mutation is present
• get screening done for other signals indicative of adrenal cancer
• research about the impact of other factors from the environment on the occurrence of the phenotype and try to prevent cancer

Question 31

How can we show that variation in a specific phenotype of interest is caused by variation in DNA sequence at a particular locus?

Answer 31

• try to expose cells to the same environment to prove that any differences in phenotype are due to genotype and not the environment
• cause a mutation in a specific locus or sequence the genome of the different groups to compare the differences in alleles