Patterns of inheritance and variation

Question 1

What are alleles?

Answer 1

Different variations of the same genes

Question 2

How are gene variants (alleles) produced?

Answer 2

DNA mutations

Question 3

How does sexual reproduction produce new allele combinations in gametes? How does this establish phenotypic variation?

Answer 3

Crossing over
Independent assortment
Random fertilisation

Different alleles in a population code for different polypeptides

Question 4

What is genotype?

Answer 4

Genetic composition of an organism, which describes all the alleles it contains. Genotypes for a particular locus can be heterozygous or homozygous

Question 5

What is phenotype?

Answer 5

An organism’s observable characteristics

Question 6

What is an example of disease which shows codominance?

Answer 6

Sickle cell anaemia (faulty version of haemoglobin molecule produced)
HAHA - normal haemoglobin production
HSHS - disease
HAHS - both versions of haemoglobin produced because alleles show codominance

Question 7

When predicting a phenotypic ration of 9:3:3:1 for a dihybrid cross, what assumptions do you have to make about the two gene loci being studied?

Answer 7

Genes do not interact - no epistasis

Genes are not linked - not on the same chromosome

Question 8

When are 2 genes said to be (autosomally) linked?

Answer 8

When they are located on the same autosome (a chromosome other than X or Y)

Question 9

Why is autosomal linkage important?

Answer 9

Means linked allele combinations will be inherited together (as a single unit)
Only crossing over during meiosis can separate linked allele combinations
The nearer the 2 genes are on a chromosome, the less likely they are to be separated during crossing over

Question 10

What are the two forms of epistasis?

Answer 10

Recessive and dominant

Question 11

What is epistasis?

Answer 11

The effect of one gene on the expression of another

Question 12

What is recessive epistasis?

Answer 12

The epistatic gene (the gene doing the suppressing) needs to be homozygous recessive to prevent the expression of the other gene - e.g. flower colour in Salvia

Question 13

What is dominant epistasis?

Answer 13

The epistatic gene needs at least one dominant allele to prevent the expression of the other gene - e.g. fruit colour in summer squash

Question 14

What is the chi-squared test used for when looking at phenotypes?

Answer 14

To assess whether there is a significant difference between the observed and expected number of offspring phenotypes - in order to see whether there is autosomal linkage or epistasis

Question 15

What is the chi-squared equation?

Answer 15

x2 = E(O-E)2/E

Question 16

How do you calculate degrees of freedom?

Answer 16

Number of phenotypes - 1

Question 17

What does the p value tell us?

Answer 17

The probability of differences being a result of chance

Differences are statistically significant if P = 0.05 (only 5% probability that differences can be attributed to chance)

Question 18

Suggest a likely molecular mechanism or recessive epistasis

Answer 18

epistatic gene produces enzyme
Homozygous recessive genotype results in no enzyme production
Precursor molecule is not converted

Question 19

Suggest a likely mechanism for dominant epistasis

Answer 19

Epistatic gene produces inhibitor
modifies other gene product
affects transcription

Question 20

What phenotypic ratio will recessive epistasis produce?

Answer 20

9:3:4

Question 21

What phenotypic ratios is dominant epistasis likely to produce?

Answer 21

12:3:1 or 13:3

Question 22

When does evolution occur?

Answer 22

When genetic mutations and natural selection result in .a change of allele frequencies in a population

Question 23

How can allele frequencies be calculated?

Answer 23

Using the Hardy-Weinberg equations

Question 24

What is stabilising selection and give an example?

Answer 24

When selection favours average phenotypes - alleles that produce extreme traits are eliminated. An example is mammals having a fur length close to the mean in an environment with a stable temperature. Individuals with short or long fur are less likely to survive and reproduce - these alleles are therefore eliminited

Question 25

What shape is a normal distribution?

Answer 25

Bell-shaped curve

Question 26

What is directional selection and give an example?

Answer 26

Occurs when environmental conditions change - selection favours individuals with extreme phenotypes. An example is mean fur length increasing if the mean temperature decreases, and decreasing if the mean temperature increasing - give higher survival rates so more likely to survive and reproduce. Normal phenotype no longer most favourable.

Question 27

What is disruptive selection and give an example?

Answer 27

When the extremes are selected for and the norms are selected against. E.g. Darwin’s finches, rabbits with black, white or grey fur (black and white rocks only)

Question 28

What is a genetic bottleneck?

Answer 28

A drastic reduction in population numbers (e.g. due to a natural disaster or environmental change) - proportions of alleles in surviving population could be very different to those in the original population

Question 29

What is genetic drift?

Answer 29

A random change in allele frequencies - effects more noticeable in smaller populations - e.g. genetic bottleneck could lead to proportions of alleles in the surviving population being very different to those in the original population

Question 30

What is the founder effect?

Answer 30

Occurs when a small group breaks away from the original large population to form a new colony (e.g. group of birds migrating to a new island) - can be considered a type of genetic bottleneck

Question 31

What are the Hardy-Weinberg equations used for?

Answer 31

To calculate the proportions of alleles and genotypes in a population

Question 32

What is the first Hardy-Weinberg equation and what does it mean?

Answer 32

p + q = 1.0
p is the frequency of the dominant allele, q is the frequency of the recessive allele. If only 2 alleles exist their frequencies must add up to 1.0 (100%)

Question 33

What is the second Hardy-Weinberg equation and what does it represent?

Answer 33

p2 + 2pq + q2 = 1.0
Considers the frequencies of the possible genotypes in a population
p2 = the homozygous dominant genotype; 2pq = the heterozygous genotype, q2 = the homozygous recessive genotype.
The frequencies of the 3 possible genotypes must add up to 1.0 (100%) since no other genotypes are possible

Question 34

What assumptions do the Hardy-Weinberg equations make?

Answer 34

No new mutations, no migration, no natural selection for or against alleles, a large population, random mating
- in reality these conditions rarely exist. Nonetheless provides basis for study of gene frequencies

Question 35

What must occur for a new species to evolve?

Answer 35

Members of a population become isolated from the rest of the population, preventing gene flow between the two groups
Genetic mutations continue to occur in both groups (producing new alleles)
Different groups experience different selection pressures
Different alleles are selected in the two groups
Over generations, the two groups become genetically different and unable to reproduce fertile offspring together

Question 36

What is allopatric speciation?

Answer 36

Members of a population separated by a physical barrier (geographical reproductive isolation) - e.g. migration to different islands, a mountain range, agricultural activity

Question 37

What is sympatric speciation?

Answer 37

Speciation occurs within a population that shares the same habitat - rare and theory is controversial

Question 38

What is an example of temporal reproductive isolation?

Answer 38

Differences in the timing of flowering

Question 39

What is an example of behavioural reproductive isolatioin?

Answer 39

Different mating rituals or calls

Question 40

What is an example of mechanical reproductive isolation?

Answer 40

Incompatible reproductive systems

Question 41

What are prezygotic reproductive barriers?

Answer 41

Prevent fertilisation and formation of a zygote (gametes do not meet)
Geographical, ecological, temporal, behavioural, mechanical isolation

Question 42

What are postzygotic reproductive barriers?

Answer 42

Reduce viability or reproductive potential of offspring (often as a result of hybridisation)
Hybrid inviability - offspring fail to live to maturity
Hybrid sterility - hybrid offspring are sterile

Question 43

Give an example of sympatric speciation

Answer 43

Parasitic ants descended from fungus farming ants (change in behaviour at some point in time)

Question 44

How can selective breeding create problems?

Answer 44

Requires inbreeding (breeding closely related individuals)

• genetic diversity is reduced
• inbred populations less able to adapt to changing environmental conditions
• Homozygous recessive disorders are more likely to occur

Question 45

What is an important word to use in this topic (p.s. you’re a little bitch, little bitch)

Answer 45

Gene flow

Question 46

Give an example of the founder effect

Answer 46

Afrikaner population in South Africa (descended mainly from a few dutch settlers) have abnormally high proportion of people with Huntington’s disease in their population - thought that just one of original settlers carried the disease causing allele
Amish in America (Ellis-van Creveld syndrome) - rarely have children outside their own religion and are therefore a closed community . - high proportion of polydactyl, abnormalities of nails and teeth, hole between upper chambers of the heart - caused by one couple who settled their in the 1700s