Week 4 Flashcards
Allele variation, DNA structure, and mutations
What is the wild type allele?
The most common allele for a trait
How are wild type alleles often represented?
With a superscript +
Any allele other than the wild type is considered __________.
Mutant
What is a polymorphism?
The presence of two or more variant forms of a specific DNA sequence/allele that affect the phenotype of different individuals.
What must be true for an allele to be considered a polymorphism?
The allele/trait must appear at appreciable frequencies (at least 1%) in a population to be considered a polymorphism
T or F: Different mutations in a gene can cause the same disorder
True
T or F: Different mutations in a gene can cause different phenotypes
True
What do recessive alleles almost always involve?
A mutation in a gene that results in at lease some loss of protein function
Why do phenotypic mutations rarely occur for heterozygous individuals?
Because one “good copy” of the gene is sufficient to make the protein required for the “normal” biological process to occur
What are the two types of mutations?
- Complete loss of function
- Partial loss of function
What type of allele causes a complete loss of function?
A null allele
What type of allele causes a partial loss of function?
A hypomorphic allele (aka neomorphic)
What are the three types of dominance?
- Complete
- Incomplete
- Codominant
What is complete dominance?
The phenotype of a heterozygote dominant for the trait is the same as a homozygote dominant for the trait
What are two ways in which complete dominance from an allele can alter function?
- Gain of a new function
- Complete loss of a function
What is incomplete dominance?
The phenotype of a heterozygote falls in between the phenotypes of two homozygotes
What is the ratio of inheritance for incomplete dominance when both parents are heterozygous?
1:2:1 (dominant:”in between”:recessive)
What is Codominance?
The phenotype of a heterozygote includes the phenotypes of both homozygotes
What is meant by a multi-allelic system?
More than two allele variations exist at a population level, and different individuals in the population may have different pairs of these alleles (ex. blood types)
What are two additional factors at a single locus that can affect phenotype?
- Penatrance
- Expressivity
What does penetrance describe?
The percentage of individuals having a particular genotype that express the expected phenotype
What is complete penetrance?
100% of individuals with a certain genotype express the expected phenotype
What is incomplete penetrance?
Individuals do not express a trait even though they have the genotype
How can penetrance be calculated?
By dividing the total number of individuals with the expected phenotype by the total number of individuals with the specific genotype but don’t necessarily express the phenotype
What is expressivity?
The degree to which a character is expressed within a given genotype
What are three potential explanations for incomplete penetrance and variable expressivity?
- Environmental factors
- Epigenetics
- Maternal age
What are epigenetics?
Changes in gene expression, but not due to mutations
What is a temperature sensitive allele?
An allele whose product is only functional at certain temperatures
What are the three components of a nucleotide?
A sugar, a nitrogenous base, and a phosphate group
What is the main difference between ribose and deoxyribose?
Ribose sugar contains a hydroxyl group on the 2’ carbon and deoxyribose does not
What are the two types of nitrogenous bases?
- Purines
- Pyrimidines
How does the structure of purines and pyrimidines differ?
- Purines: a six-member ring attached to a five member ring
- Pyrimidines: a six member ring
What are the two types of purines?
- Adenine
- Guanine
What are the two types of pyrimidines in DNA?
- Cytosine
- Thymine
What are the two types of pyrimidines in RNA?
- Cytosine
- Uracil
Which nitrogenous bases pair in DNA?
- Adenine and Thymine
- Guanine and Cytosine
Which nitrogenous bases pair in RNA?
- Adenine and Uracil
- Guanine and Cytosine
How are the polynucleotide strands arranged in DNA?
Antiparallel to each other
What is meant when it is said that polynucleotides in DNA run antiparallel to each other?
They run in opposite directions (one strand runs 5’ to 3’ and the other runs 3’ to 5’)
How many hydrogen bonds hols T and A together?
Two
How many hydrogen bonds hold C and G together?
Three
How are the two polynucleotide strands held together in DNA?
Hydrogen bonds between complementary nitrogenous base pairs
How are adjoining nucleotides connected in polynucleotide strands?
A phosphodiester linkage connects the 5’ phosphate group of one nucleotide to the 3’ hydroxyl group of the other
What are changes in DNA that result in a new allele called?
Mutations/variants
Why can mutations be beneficial?
They are the source of genetic variation and provide the raw material for evolution
Why might it be useful to identify mutant genes that alter development?
They are useful for identifying the function of that gene
Which type of genetic mutation can be passed to members of the next generation?
Germ-line mutations
Where do somatic mutations occur?
Nonreproductive cells
Where do germ-line mutations occur?
In cells that give rise to gametes
What are the three main categories of gene mutations?
- Base substitutions/point mutations
- Insertions and deletions
- Expanding nucleotide repeats
What are the two types of base substitution/point mutations?
- Transition
- Transversion
What are the two types of insertions and deletions?
- Frameshift mutations
- In-frame insertions and deletions
What is the result of an expanding nucleotide repeat?
Increase in the number of copies of a set of nucleotides
What is a transition mutation?
- When one purine replaces another purine (ex. A replaces G)
- When one pyrimidine replaces another pyrimidine (ex. T replaces C)
What is a transversion mutation?
- When a purine replaces a pyrimidine (ex. A replaces C)
- When a pyrimidine replaces a purine (ex. T replaces A)
What is a frameshift mutation?
When an insertion or deletion alters the reading frame and therefore the codons
What is an in-frame insertion or deletion?
When the insertion or deletion of nucleotides occurs in groups of three, so the reading frame and codons are not affected
T or F: A frameshift due to insertion or deletion will not cause a stop codon to be read.
False. Frameshifts may result in stop codons being read after the mutation.
What causes an expanding nucleotide repeat mutation?
- A hairpin forms on a newly synthesized strand of DNA, causing part of the template strand to be replicated twice
- When it is used as the template for replication, the resulting DNA will have those additional copies
What is a forward mutation?
Mutation causing a wild type allele to become a mutant allele
What is a reverse mutation?
Mutation causing a mutant allele to become a wild type allele
What is a neutral mutation?
Mutation that causes no change in function
What is a missense mutation?
When the new codon encodes for a different
amino acid as the original (change in amino acid sequence)
What is a nonsense mutation?
When the new codon encodes for a stop codon/nonsense codon (premature termination of translation)
What is a silent mutation?
When the new codon encodes for the same
amino acid as the original (no change in the amino acid sequence)
