6.1.2 Patterns of inheritance

Question 1

Explain how an organisms characteristics are related to their genes

Answer 1

The genome of an organism contains their genes

Sexually reproducing organisms have two copies of each gene.

The combination of alleles they have for a particular gene is called their genotype

Different alleles and combinations of alleles result in different gene expression

This is results is particular characteristics of an organism called the phenotype

Question 2

Summarise the variation that can occur between organisms

Answer 2

The variation in characteristics between different species is called interspecific variation

The variation in characteristics within a species is called intraspecific variation

Intraspecific variation can be caused by genetic factors and/or environmental factors

Intraspecific variation can be continuous or discontinuous

Question 3

Use eye colour colour to explain how genetics contribute to intraspecific variation

Answer 3

Eye colour is determined by the amount of the pigment melanin in the cells of the iris

Melanin is produced by a metabolic pathway which involves many enzymes

Each enzyme is a result of the expression of particular genes

Depending on the genotype of an individual, the amount of melanin produced will be different

This results is people with different genotypes having different eye colour

Question 4

Describe how the environment can affect phenotype

Answer 4

Environmental factors can also affect the characteristics of organisms

These may affect members of the populations differently

For example water availability and access to sunlight will affect the height of plants

Those with more access to water and light will grow taller

Environmentally affected variation cannot be passed onto offspring (inherited)

Lifestyle choices such as diet and exercise are environmental factors

Question 5

Describe how chlorosis in plants is an example of environmental variation

Answer 5

A lack of mineral ion supply (from the soil) such as magnesium ions and iron ions

Cause a reduced production of chlorophyll

A magnesium ion is part of the chlorophyll molecule

Iron ions are cofactors for the enzymes that synthesise chlorophyll

Plants with poor access to mineral ions will have yellow leaves

Not as a result of their genotype, but due to their environment

Question 6

Describe how etiolation in plants is an example of environmental variation

Answer 6

Plants deprived of light have abnormal production of auxin

This causes weak cell walls

And causes long spindly leaves and stems

As well as a lack of chlorophyll production (chlorosis)

This is caused by an environmental factor, not by the plant’s genotype

Question 7

Summarise how sexual reproduction can lead to genetic variation in a species

Answer 7

Genetic variation is caused by the range and combinations of alleles

New alleles are produced as a result of mutation (which will result in proteins functioning differently, and new phenotypes)

Meiosis results in gametes with unique and different genetic compositions

And random fertilization produces a zygote or offspring with different genotype to both parents

Question 8

Describe how events during meiosis increase genetic variation

Answer 8

Adult cells are diploid and have two alleles for each gene

Meiosis produces haploid cells which each have either one of those alleles for each gene

Independent assortment results in further increase in the genotypes of the gametes (2n combinations, where n = number of chromosome pairs)

Crossing over events also increase the possible gamete genotypes

Question 9

Describe how fertilisation during sexual reproduction contributes to genetic variation in a species

Answer 9

Both parent organisms have their unique genotypes (and phenotypes)

However their gametes (sperm and egg) have only half their alleles

Also the gametes are genetically different from each other

Fertilisation is a random fusion of one sperm and one egg

The genotype of the zygote/offspring will be different from both parents

Question 10

Describe the keys steps in using genetic diagrams to show patterns of inheritance

Answer 10

State the phenotypes of both parents

state/assign letters to show the genotype of both parents (lowercase, upper case and superscript)

Show the genotypes of all possible gametes for both parents

THEN use a Punnet square to show all the possible offspring phenotypes resulting from random fusion events

Write the proportions of the offspring genotypes (ratios, percentages)

Write the proportions of the offspring phenotypes (these will not always be the same as the genotype proportions)

Question 11

Describe the features of a monohybrid cross

Answer 11

Monohybrid cross is the analysis of inheritance of one gene, and one characteristic

There will be one phenotype or characteristic being analysed

Each parent will have a genotype referring to one gene

Each genotype will have two letters indicating the alleles present on each of the homologous pair of chromosomes

Each gamete should only have one allele/letter

A 2x2 Punnet square is used

Question 12

Describe the features of a homozygous cross

Answer 12

Homozygous crosses use individuals whose genotypes are known

They are called ‘pure’ or ‘true’ breeding because they are homozygous for their genes

One parent is homozygous recessive and the other is homozygous dominant

This always results in all the offspring being heterozygous

This offspring is called the F1 generation

Question 13

Describe the features of a heterozygous cross

Answer 13

The F1 generation (two heterozygous individuals) can be crossed

The offspring are called the F2 generation

The ratio for the dominant phenotype to the recessive phenotype is 3:1

The ratio of offspring produced in the F1 and F2 generation can give information about the alleles

It can tell us which allele/phenotype is dominant and which is recessive

Question 14

Describe the features of a dihybrid cross

Answer 14

Dihybrid crosses follow the inheritance of two different genes and characteristics

We begin with the assumption that both genes are on separate chromosomes

Each parent will have a phenotype describing two characteristics (hair colour, eye colour)

Each parent will have a genotype of four letters, two for each gene location, using different letters for different genes (e.g. EeHH)

Gametes must have one allele/letter for both genes, and must show all possible allele combinations (e.g. EH, eH)

Offspring should have four-letter genotypes, pairing together alleles of the same gene (EeHh)

Punnets squares will be larger to accommodate more variety in the gametes

Question 15

Describe how you would use parental genotypes to determine gamete genotypes in a dihybrid cross

Answer 15

Question 16

Describe how the presence of multiple alleles affects the inheritance of phenotypes

Answer 16

Multiple alleles is the idea that the population (gene pool) can have more than two alleles for a given gene

And that an individual can have any two of those alleles

This results in more genetic variation

There are more possible genotypes

And more possible phenotypes

Question 17

Describe how codominance affects inheritance of phenotypes

Answer 17

Occurs when a gene has more than one dominant allele

both alleles contribute to the overall phenotype

So as well as the recessive phenotype, there may be more than one dominant phenotype

Codominant alleles are represented by capital letters

With different alleles represented by superscript letters

For example FB and FW could be dominant alleles for fur colour

Question 18

Explain what sex linked genes are

Answer 18

The 23rd pair of chromosomes are the sex chromosomes (XX or XY)

They are non homologous (they don’t have identical genes)

Alleles on the X don’t have corresponding alleles on the Y and vice-versa

The Y chromosome contains genes that make the organism a male.

Genes for any other characteristics that are on these chromosomes are called sex-linked genes

The non-sex chromosomes are called autosomal

Question 19

Describe the features of the inheritance of sex-linked phenotypes

Answer 19

Females have two possible alleles for the genotype, whereas males only have one

In genetic diagrams, the genotypes show the sex chromosomes involved as well as the alleles for the phenotype (XAXa)

The Y chromosome of males is assumed not to carry corresponding genes of X chromosomes

In females the phenotype as a result of both alleles is considered

In males, only the allele on the X chromosome affects the phenotype

When reporting the offspring phenotypes, state the sex of the offspring as well as their phenotype

Question 20

Describe epistasis and its effect on the inheritance of phenotype

Answer 20

A gene at a second, different chromosomal location

Can be expressed that affects the phenotype controlled by the first gene

This is called epistasis

For example two different genes involved in the same metabolic pathway

Or one gene being a transcription factor for another gene

Question 21

State the four stages of carrying out a chi-squared test

Answer 21

Making a null hypothesis: stating that the differences between observed and expected are due to chance

Calculating a test statistic based on the actual results/observations. You may need to calculate expected numbers based on phenotype ratios.

Calculate degrees of freedom (no. of categories - 1) to determine the critical value at 5% significance. Compare the test statistic to the critical value to determine the likelihood of the result occurring by chance (less or more than 5%).

Make a conclusion, that (a) accepts or rejects null hypothesis and (b) refers to what that means in terms of the question scenario

Question 22

Explain the genetic influence on discontinuous variation

Answer 22

Discontinuous variation has only a few very distinct phenotypes (eg blood group)

This is because such characteristics are controlled by single or just a few genes

There are not that many different allele combinations possible

Therefore there are not that many different phenotypes produced

However, the possible phenotypes can be very distinct, with no intermediates

(there is little environmental impact)

Question 23

Explain the genetic influence on continuous variation

Answer 23

Continuous variation has many possible phenotypes, each slightly different from the next (eg height, weight)

This is because multiple genes are affecting the phenotype

There are so many unique allele combinations possible

That this causes many slightly different phenotypes

(because the environment can also influence, this causes even more variation)

Question 24

Review the key steps in the process of evolution

Answer 24

Evolution occurs by natural selection

There must (genetic) variation in a population

A selection pressure causes a struggle to survive (not all the population will survive)

Certain genetic variations result in some members of the population having a better chance of survival and reproduction than others

Over time, the alleles for these advantageous characteristics (adaptations) become more common in the population (and the population becomes better adapted to survive the selection pressure)

Question 25

List the factors that can affect the evolution of a species

Answer 25

Anything that causes changes in allele frequency is affecting evolution
Stabilising selection

Directional selection

Genetic drift

Genetic bottleneck

Founder effect

Question 26

Describe stabilising selection

Answer 26

Stabilising selection is when the selection pressure (environment) stays the same

The most common characteristics (alleles) are still the most advantageous

These become even more common

The extreme phenotypes are selected against

Those alleles become less common

Question 27

Describe directional selection

Answer 27

When the environment changes

One of the extreme phenotypes may now result in the best chance of survival

The alleles for the extreme phenotype become more common

While alleles for the other phenotypes become less common

Question 28

Describe genetic drift

Answer 28

Genetic drift is simply a change in frequency of alleles

Due to random, chance events (not selection pressure or adaptation)

This may cause allele frequency to decrease, and even disappear entirely

Or others to increase

Genetic drift is strongest in small populations

Question 29

Describe genetic bottleneck

Answer 29

A large reduction in population size for at least one generation

This causes a lot of alleles to be lost (randomly)

The surviving members reproduce and rebuild the population

But with significantly lower genetic diversity

Mutation may result in creation of new alleles but this will take time

(and environmental changes may take place faster)

Question 30

Describe founder effect

Answer 30

Occurs when a small number of individuals from an original population become reproductively separated

And continue to reproduce in isolation

Isolated populations have smaller gene pools (many of the alleles in the original population are not present in the isolated population)

Less genetic variation

Alleles with low frequency may become high frequency in the new population

Question 31

Explain the significance of calculating allele frequencies

Answer 31

Estimate allele and genotype frequencies in population

This can tell us about the genetic variation in a population

We can estimate how many people in a population may have genetic disorder

Observe changes in allele frequencies

Changes in allele frequency indicate evolution

Question 32

Recall the assumptions required for a Hardy-Weinberg population

Answer 32

mutation is not occurring

natural selection is not occurring

the population is infinitely large

all members of the population breed

all mating is totally random

everyone produces the same number of offspring

there is no migration in or out of the population

Question 33

Recall the two parts of the Hardy-Weinberg equation (these will be given to you in an exam)

Answer 33

p2 +2pq+q2 =1
p+q=1

Question 34

Recall what p, q and the elements of the Hardy-Weinberg represent

Answer 34

p: frequency of dominant allele
q: frequency of recessive allele
p2: frequency of homozygous dominant individuals
2pq: frequency of heterozygous individuals
q2: frequency of homozygous recessive individuals

Question 35

Describe the process of speciation generally

Answer 35

A subpopulation becomes isolated so that it does not interbreed with the rest of the population

The two populations continue to evolve due to natural selection, accumulating changes in alleles and allele frequencies

Because they do not interbreed (no gene flow) genetic differences begin to add up

Eventually they may no longer be genetically compatible enough to interbreed successfully

They are now two different species

Question 36

Describe allopatric speciation

Answer 36

Geographical barrier causes separation, and thus, reproductive isolation

The two geographic areas will have different climatic conditions, or other biotic/abiotic factors, therefore there are different selection pressures

Different characteristics will be advantageous in the two areas

Different changes in alleles/allele frequency will occur

There is no gene flow so the gene pools of the two groups become more and more different

Until they cannot (could not) produce fertile offspring

Question 37

Describe sympatric speciation

Answer 37

A subpopulation may become reproductively isolated

(due to disruptive selection)

They have the same selection pressures

But due to no gene flow (reproduction), they accumulate different adaptations

These differences add up over time, and eventually the two populations become too different to produce fertile offspring

Question 38

Outline the principle of artificial selection

Answer 38

Is the application of selection pressure

That is not natural (biotic/abiotic)

But rather, for characteristics that are desirable for human interests

Question 39

Describe the process of selective breeding

Answer 39

Select individuals with desirable characteristics

These are bred, and offspring produced

Select offspring that have the desired characteristics and continue over many generations (inbreeding)

This will increase allele frequencies for desired characteristics in the population

And potentially result in speciation

Question 40

Describe how genetic variety is maintained in selectively bred plants and animals

Answer 40

Seed banks are stores of wild and domesticated varieties of plants

Gene banks are frozen stores of sperm and eggs (animals)

They are used to preserve genetic variations (individuals with certain alleles) that may otherwise get lost due to artificial selection, genetic drift or some catastrophic event

Outbreeding with these varieties reintroduces genetic variation back into populations

Making populations more heterozygous (less homozygous)

Question 41

State the ethical arguments in favour of the use of selective breeding

Answer 41

Improved quality of life for humans (pets as emotional support, or helping blind people to travel)

Animals that provide high yields lowering prices for consumers

Plants that are the same size can be harvested through automation (lowering cost of production, reducing requirement for manual labour)

The quality (taste/appearance) of food products can be improved

Question 42

State the ethical arguments against the use of selective breeding

Answer 42

Inbreeding may cause loss of certain alleles, a reduction in genetic variation called inbreeding depression

Highly related individuals are more likely to have the same alleles and more likely to be homozygous. Greater chance of recessive genetic disorders (suffering of the animal)