Lecture 4

Question 1

define genotype

Answer 1

the genetic constitution of an organism
- defined in relation to a particular gene or gene combinations
- eg Aa, Bb

Question 2

define phenotype

Answer 2

feature of the organism as observed
- used when discussing a trait of an organism that varies
- eg size, fur colour, mRNA expression level

Question 3

define genome

Answer 3

the entirety of an organism’s DNA
- includes genes and non-coding regions

Question 4

state 3 sources of genetic variation

Answer 4

1. mutation
2. independent assortment
3. recombination

Question 5

define a mutation

Answer 5

a stable change in the DNA sequence

Question 6

how often do mutations occur?

Answer 6

at a low rate
- mutation rate varies in ways that are partially predictable

Question 7

what are the different possible effects of mutations on fitness?

Answer 7

• neutral (won’t matter for the fitness of the organism)
• deleterious (weakly detrimental up to lethal)
• beneficial

Question 8

state the 4 main characteristics of mutation

Answer 8

1. mutation is an inevitable phenomenon, despite cellular mechanisms to correct errors during DNA replication
2. mutation is not directed toward an outcome by the organism or by the environment
   - it is random with respect to effects on fitness
   - it is not ‘summoned’ to make things better
3. rate depends on the type of mutation, and can also vary among genes
4. the environment can affect the mutation rate (eg mutagens, high temp)

Question 9

what are the 4 types of mutation?

Answer 9

1. point mutations (a simple substitution of a nucleotide)
2. insertions/deletions (‘indels’) - adding/removing a nucleotide
3. changes in repeat number
4. chromosomal rearrangements (eg inversions)

Question 10

describe a mutation involving a change in repeat number

Answer 10

• when there is a repeated motif (eg ATG) within the gene, the motif may accidentally be added/removed an extra time
• the repeated motif makes it harder for the replicative machinery to replicate

Question 11

describe an example of a chromosomal rearrangement

Answer 11

inversions:
- flipping of the nucleotide sequence
- occurs when there is a double break in DNA strand; when enzymes try to put the two strands back together, it is hard to know which one was forward and which one was backwards

Question 12

give an example of a method that can be used to identify a new mutation?

Answer 12

a trio study:
- two parents (reproducing organisms) are selected
- their genome and the genome of their offspring is sequenced

Question 13

what is the rate of new mutation in humans?

Answer 13

per base pair of DNA: 16 in every billion nucleotides each generation

per individual genome (two copies of our 3 billion base pair genome): approx. 96 new mutations per zygote

for the entire human population (8 billion): every base pair in the genome mutated about 126 times over per generation

Question 14

why do some mutations not lead to amino acid changes?

Answer 14

we have more codons than amino acids - so sometimes mutations don’t make a difference to the amino acid sequence (= silent substitution)

Question 15

give an animal example of how single mutations can cause profound effects on traits

Answer 15

• Antp Hox gene mutations in Drosophila
• results in an extra pair of wings, for example, or a set of legs, instead of antennae, growing from the fly’s head

Question 16

describe G6DP deficiency in humans?

Answer 16

Glucose-6-phosphate dehydrogenase

most common enzyme deficiency in humans
- causes severe anemia
- also protects against malaria
- 2 amino acid replacement mutations in the coding sequence of G6PD are associated with disease allele (A-)

Question 17

what are polymorphisms?

Answer 17

• A gene is said to be polymorphic if more than one allele occupies that gene’s locus within a population.
• each allele must also occur in the population at a rate of at least 1% to generally be considered polymorphic.

Question 18

what are allozymes?

Answer 18

Alloenzymes are variant forms of an enzyme which differ structurally but not functionally from other allozymes coded for by different alleles at the same locus.

Question 19

give a flow chart for how mutations in G6PD lead to genetic variation

Answer 19

DNA (polymorphisms) -> proteins (allozymes) -> phenotype (polyphenisms)

Question 20

define independent assortment

Answer 20

the alleles of two (or more) different genes get sorted into gametes independently of one another.

Question 21

how does independent assortment in segregation during meiosis generate diversity?

Answer 21

• allows different combinations of parental chromosomes
• 2^n, where n=sets of chromosomes

Question 22

how can we tell if a mutation has increased or decreased fitness from a fitness distribution bar chart?

Answer 22

if the mutation leads to a fitness less than 1 (which is the fitness of the ancestor) then they are detrimental

Question 23

describe how recombination during meiosis (metaphase) further contributes to variation

Answer 23

there is synapsis of bivalents, leading to crossing over at chiasmata and recombinant chromosomes.

Question 24

how did scientists believe that heredity worked before Mendel?

Answer 24

performatifs (1700s):
- spermists/ovists believed only one parent contributed to inheritance

theory of blending inheritance (1800s)
- postulated that factors from both parents mix together irreversibly

Question 25

what is the problem with blending inheritance for evolution by natural selection?

Answer 25

there would be no way for a beneficial mutation to increase in frequency across generations

Question 26

describe Mendel’s experiment with diagrams

Answer 26

1. hybrid cross of pure-breeding lines (yellow and green)
2. self fertilised (all yellow)
3. offspring were 3/4 yellow and 1/4 green

Question 27

state the key conclusions from Mendel’s pea experiments

Answer 27

1. inheritance is determined by discrete particles, genes
2. each diploid organism carries two copies (alleles) of each gene.
   - alleles can exhibit dominance/recessivity
   - gametes contain only one allele per gene
3. gametes fuse to make offspring
   - sperm/pollen fuses with egg/ovule
4. offspring inherit one gamete from each parent at random
   - one allele per gene at random from each parent

Question 28

describe phenotypic polymorphisms with simple mendelian genetic causes

Answer 28

• common in nature
• direct correspondence between trait and its genetic basis
• easy to track selection and evolution

Question 29

what are the two types of genetic variation?

Answer 29

discrete variation:
- mendelian genetics
- genes of major effect, dominance, and recessiveness
- involves a spread of alleles and a change in allele frequency

continuous variation
- quantitative genetics
- many genes each with alleles of small effect, important environmental effects
- selection response as change in average trait value

Question 30

define discrete variation

Answer 30

Discrete variation refers to traits with a finite number of phenotypes, often controlled by single genes

Question 31

define continuous variation

Answer 31

Continuous variation, on the other hand, displays an infinite number of phenotypes across a continuum and is usually polygenic.

Question 32

how is the phenotype of a quantitative trait established?

Answer 32

• often affected by many factors
• complex polygenic inheritance
  AND
• environmental factors

Question 33

define partial dominance

Answer 33

a form of Gene interaction in which both alleles of a gene at a locus are partially expressed, often resulting in an intermediate or different phenotype.

Question 34

define a polygenic trait

Answer 34

a characteristic that is influenced by two or more genes.

Question 35

locus

Answer 35

the location of the gene on a region of a chromosome