6.1.2 Patterns of inheritance

Question 1

What factors can lead to changes in the frequency of alleles in a population? (4)

Answer 1

Mutations: Allows formation of different alleles and new alleles.

Sexual selection: Increase in alleles coding for characteristics to improve mating process can occur.

Genetic drift: Change in allele due to random nature of mutations. Occurring in small populations.

Natural selection: Alleles with surviving characteristics increase.

Question 2

What factors can lead to changing the size of a population? (8)

Answer 2

Density dependant factors:
Competition
Predation
Parasitism
Communicable diseases

Density independent factors:
Climate change
Nature disaster
Seasonal change
Human activities

Question 3

The bottleneck effect. (4)

Answer 3

Example of a massive change in the population.

Occurs when there is a larger reduction in population size which lasts for at least a generation.

Usually occurs when a natural, random event kills off a larger population.

This reduces the gene pool of a population and can detrimental as a high percentage of alleles can be shared between species.

Problems arise when interbreeding occurs.

Question 4

Founder’s effect. (2)

Answer 4

Establishment of new colonies by a few isolated individuals who are usually unrepresentative of the original population.

Creates smaller gene pool that display less genetic variation.

Question 5

Stabilising selection. (3)

Answer 5

This occurs when the norm/average characteristics in a population is selected for.

The extremes are selected against. This causes a reduction in the frequency of extreme alleles.

An increase in the frequency of average alleles occur.

Question 6

Directional selection.

Answer 6

A change in environment causes extreme phenotypes to be positively selected.

The ‘normal’ phenotype is no longer advantageous.

This causes a shift in the allele frequency, as this increases for extreme characteristics.

This causes evolution.

Question 7

Disruptive selection.

Answer 7

Occurs when the extreme are selected for whilst the norm is selected against.

This forms two peaks in the distribution curve, at the extreme ends.

Frequency of alleles for the norm are the lowest as they are selected against.

Question 8

Genotype

Answer 8

Genetic makeup of an organism due to a combination of alleles.

Question 9

Dominant allele

Answer 9

Only one of the allele needs to be present in order for it to be expressed in the phenotype.

Question 10

Recessive

Answer 10

When both of the recessive allele have to be present in homozygous form in order to be expressed in the phenotype.

Question 11

Co-dominant

Answer 11

When both the dominant and recessive alleles are expressed in the phenotype.

Question 12

Linkage

Answer 12

Occurs when genes are within close proximity in a chromosome.

The closer genes on a different loci are, the more likely their will both be inherited during crossing over in meiosis.

Question 13

Three ways in which meiosis gives rise to genetic variation.

Answer 13

Crossing over: In prophase 1 it occurs between homologous chromosomes. Genetic material is exchanged between maternal and paternal chromosomes. This forms a unique combination of alleles.

Independent assortment of chromosomes: During metaphase 1, the orientation of each chromosome is randomly assigned to different poles of the cell.

Independent assortment of chromatids: During metaphase 2, orientation of chromatids are randomly assigned before separation in anaphase.

Question 14

Monogenic inheritance

Answer 14

Inheritance of a single gene.

Question 15

Dihybrid inheritance and example.

Answer 15

Occurs in the event when two genes are inherited.

For example; peas can be wrinkly and yellow.

Question 16

Epistasis (3)

Answer 16

Involves the interaction of genes at different loci.

Epistatic gene: Able to mask the presence of a gene expressed in a different loci (hypostatic gene).

Hypostatic gene: Has the potential to be masked by an epistatic gene from another loci. Despite the gene being expressed, it is masked.

Question 17

Recessive epistasis

Answer 17

When the presence of recessive alleles for one, epistatic gene masks the effects of the second hypostatic gene.

Question 18

Dominant epistasis.

Answer 18

When the presence of a dominant allele for one, epistatic gene, masks the effects of any alleles of the second, hypostatic gene.

Question 19

Complementary epistasis.

Answer 19

Having a particular allele combination at ANY locus will mask the expression on the other locus.

Question 20

What is the expected phenotypic ratio when two heterozygous are mated? (In a dihybrid cross)

Answer 20

9:3:3:1

Question 21

What are the phenotypic ratios for:
Dominant epistasis
Recessive epistasis
Complementary epistasis

Answer 21

Dominant:
12:3:1 / 13:3

Recessive:
9:3:4

Complementary:
9:7

Question 22

Explain why despite a plant containing normal genes that code for chlorophyll, they could still have pale/yellow leaves. (4)

Answer 22

Chlorosis: As a result of environmental factors.

Lack of sunlight:
Photosynthetic cells turn off/ reduce their production of chlorophyll to save energy when there is constantly low light.

Lack of minerals:
Lack of iron and magnesium in the soil affects chlorophyll production.
Iron is used in a coenzyme that is involved in chlorophyll synthesis.
Magnesium is a component of chlorophyll.

Viral infection:
This disrupts cell metabolism and suppresses the production of chlorophyll.

Question 23

Explain how animal body mass can be both genetically and environmentally influenced.

Answer 23

The mass of an organism can be influenced by the genes they have inherited from their parents.

Some mutations can affect the way digestive molecules are metabolised, causing obesity or being underweight.

Environmentally:
The amount and quality of food eaten.
The amount of exercise done by the animal.
Disease can also influence weight.

Question 24

Describe allopatric speciation.

Answer 24

Occurs when the population of a species become geographically isolated:

This causes a disruption is gene flow between the separate populations

The separated population are exposed to a new environment and face new selection pressures.
This causes them to adapt to their new habitat and gradually obtain features that are different from the original population —> can no longer interbreed with the original population.

Geographic separation causes the founder’s effect, which causes genetic drift—> Alters the frequency of alleles.

Question 25

Describe sympatric speciation

Answer 25

Formation of a new species from an original population in the same habitat.

This is more common in plant when species obtaining a different number of chromosomes interbreed to form a polyploid offspring.

Question 26

Describe the three pre-zygotic isolating mechanisms as examples of reproductive isolation.

Answer 26

Mechanical:
Anatomical differences in genital organs prevent copulation.
In plants: where the morphophysiological characteristics of the flowers prevent pollination.

Temporal:
Species mate at different seasons or live in different habitats.

Behavioural:
Different species have different mating rituals. May not be able to

Question 27

Describe the three post-zygotic isolating mechanisms.

Answer 27

Hybrid breakdown/ zygote mortality:
Despite the F1 generation being fertile, the F2 generation do not develop properly to survive.

Hybrid inviability:
Offsprings are produced, but do not grown into reproductive maturity

Hybrid infertility:
Hybrid has normal viability, but fails to produce functional gametes.

Question 28

Artificial selection (2)

Answer 28

Purposely selecting organisms with desirable characteristics and interbreeding them.

Repeating the process many times with the same species causes a genetic drift which alters the frequency of certain alleles in a population.

Question 29

Example of artificial selection

Answer 29

Breeding of domesticated dogs:

Different sub-species of dogs have been interbred to continuous produce desired characteristics:
i.e small size in chiuauas

Farming:
Cows that produce the best meat and milk quality, and other characteristics, are bred together.

Question 30

Explain problems that artificial selection causes. (2)

Answer 30

Interbreeding limits the gene pool of a population:

• Decreases genetic biodiversity
• Inhibits evolution
• Population is more susceptible to changes in the environment.

Increases the prevalence of recessive alleles:

• Increases the chance of recessive alleles that causes negative conditions
• Produces less biologically fit organisms that can survive and reproduce

Question 31

Why is it difficult to classify species using the ‘biology’ concept.

Answer 31

Biology concept: Species are organisms that can interbreed to produce fertile offspring.

Some members of the same species are unable to interbreed
They are still classified as the same species: i.e dogs of different sizes.

Some members of different species can interbreed to form a fertile offspring: i.e some dogs and wolves

Question 32

Why is it difficult to classify species using the ‘phylogenetic’ concept.

Answer 32

Phylogenetic concept: Species are the same if they come from the same monophyletic group (sharing a common ancestor)

Some organism have very small genetic differences but are classified as different species: dogs and wolves.

One species can have a great phylogenetic variation