6.1.2 Patterns of inheritance

Question 1

Define genotype.

Answer 1

The alleles of a gene possessed by an individual.

Question 2

Define phenotype

Answer 2

The observable characteristics of an organism which is influenced by its genotype.

Question 3

Define gene

Answer 3

A length of DNA that codes for a single polypeptide or protein.

Question 4

Define locus

Answer 4

The position of a gene on a chromosome.

Question 5

What is co-dominance?

Answer 5

Where two alleles of the same gene are expressed in the phenotype at the same time.

Question 6

Define allele

Answer 6

Form of a gene

Question 7

Define dominant

Answer 7

An allele that when present will always express itself in the phenotype.

Question 8

Define recessive

Answer 8

An allele that will only express itself when no dominant allele is present.

Question 9

Define homozygous

Answer 9

The alleles of a gene are both the same.

Question 10

Define heterozygous

Answer 10

The two alleles of a gene are different.

Question 11

How do we express co-dominance?

Answer 11

The gene is symbolised as a capital letter and the alleles are represented by superscript letters.

Question 12

Give a human example of co-dominance.

Answer 12

Blood groups.

Question 13

What are the two types of linkage?

Answer 13

Sex-linkage
Autosomal linkage

Question 14

Which sex chromosomes do women have?

Answer 14

XX

Question 15

Which sex chromosomes do men have?

Answer 15

XY

Question 16

What is sex linkage?

Answer 16

Some genes are found on a region of a sex chromosome (e.g X) that are not present on the other sex chromosomes (e.g Y).
As the inheritance of these genes is dependent on the sex of the individual, they are called sex-linked genes.

Question 17

Which chromosome are sex linked genes usually on?

Answer 17

The X chromosome.

Question 18

Why are males more likely to show sex-linked recessive conditions?

Answer 18

Sex-linked traits in humans appear more often in males than in females due to the presence of only one X chromosome in males. Females have two X chromosomes which means if one copy of the gene is mutated, there is a chance that the other copy will be normal thus ensuring a normal phenotype.

Question 19

Why can only females be disease carriers?

Answer 19

Females have two X chromosomes so the presence of one affected X chromosome can be masked by another healthy one. Males only have one X chromosome so they would show the disease in their phenotype.

Question 20

What is a carrier?

Answer 20

A person who has inherited a disease recessive allele however does not show the disease themselves. Their offspring have a 50% chance of acquiring the disease.

Question 21

What is autosomal linkage?

Answer 21

When two or more genes on the same chromosome (non-sex chromosome) do not assort independently during meiosis and so stay together.

Question 22

If an individual had the genotype FFGG how would it be written if there was autosomal linkage?

Answer 22

(FG)(FG).

Question 23

What ratio would we expect in a usual F2 Dihybrid cross?

Answer 23

9:3:3:1

Question 24

What is epistasis?

Answer 24

When the presence of one gene inhibits the expression in the phenotype of another gene.

Question 25

During sexual reproduction, genetic variation is caused by what three processes?

Answer 25

• Crossing over during prophase I
  -Independent assortment during metaphase I
  -Random fusion of gametes during fertilisation.

Question 26

What is crossing over?

Answer 26

When non-sister chromatids exchange alleles during meiosis I at the chiasma. The crossing over places stress on the DNA molecules and as a result this section of a chromatid may break and region with the chromatid from the other chromosome.
The swapping of alleles is significant as it can result in a new combination of alleles on the two choromses.

Question 27

What else other than process involved in sexual reproduction (meiosis) can cause variation between two individuals?

Answer 27

Mutations.

Question 28

What is independent assortment?

Answer 28

The production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I. The different combinations of chromosomes pair up and are pulled towards the equator of the spindle. Each pair can be arranged with either chrome on top. The chromosomes are then separated and pulled to different poles.

Question 29

What is the random fusion of gametes?

Answer 29

During fertilisation, any male gamete can fuse with any female gamete to form a zygote. This random fusion of gametes at fertilisation creates genetic variation between zygotes as each will have a unique combination of alleles.

Question 30

Phenotypic variation can either be _____ or ______.

Answer 30

Genetic
Environmental

Question 31

Variation in phenotypes based on ___ factors cannot be inherited by the organisms offspring.

Answer 31

Environmental.

Question 32

What are selection pressures?

Answer 32

Environmental factors which affect the chance of survival of an organism.

Question 33

What are the two types of selection?

Answer 33

Stabilizing
Directional

Question 34

What is stabilising selection?

Answer 34

Natural selection keeps allele frequencies relatively constant over generations.
This means things stay as they are unless their is a major chance in the environment. Individuals with alleles towards the middle of the range are more likely to survive.

Question 35

What is directional selection?

Answer 35

Natural selection that produces a gradual change in allele frequencies over several generations.

Question 36

When would directional selection occur?

Answer 36

When there has been a change in environment or a new allele has appeared advantageous.

Question 37

What is genetic drift?

Answer 37

When chance affects which individuals in a population survive, breed and pass on their alleles. For example, when a population is significantly small, chance can affect which alleles get passed on to the next generation. Overtime, some alleles will be lost or favoured purely by chance. This is less likely to have an affect on large populations as chance variations would even out due to natural selection.

Question 38

What is a genetic bottleneck?

Answer 38

Occurs when a previously large population suffers a dramatic fall in numbers. E.g a major environmental effect. The surviving individuals end up breeding and reproducing with close relatives which reduces genetic diversity.

Question 39

What is the founder effect?

Answer 39

When a small number of individuals in a large parent population start a new population with only s few alleles in the initial gene pool. Which alleles end up in the founding population is due to chance and so as a result, the changes in allele frequencies may occur in a different direction than the larger parent population

Question 40

What is interspecific variation?

Answer 40

Variation between species.

Question 41

What is intraspecific variation?

Answer 41

Variation within species

Question 42

What is discontinuous variation?

Answer 42

Qualitative differences in the phenotypes of individuals within a population.

Question 43

Give an example of discontinuous variation.

Answer 43

Blood groups

Question 44

Give some features of discontinuous variation.

Answer 44

-Distinct classes or categories.
-characteristics cannot be measured over a range.
-individuals cannot have features that fall between categories.

Question 45

Does discontinuous variation occur due to genetic, environmental factors or both?

Answer 45

Just genetic.

Question 46

Does continuous variation occur due to genetic, environmental factors or both?

Answer 46

Both genetic and environmental.

Question 47

Features of continous variation.

Answer 47

-No distinct classes or categories exist.
-Characteristics can be measured and fall within a range between two extremes.

Question 48

When does continuous variation occur?

Answer 48

When there are quantitative differences in the phenotypes of individuals in a population.

Question 49

What does the hardy weinberg principle state?

Answer 49

If certain conditions are met then the allele frequencies of a gene within a population will not change from one generation to the next.

Question 50

What conditions are needed for the hardy weinberg principle?

Answer 50

-Mating is random
-Large population
-No immigration or emigration
-No mutations
-No natural selection.

Question 51

Give the two hardy weinberg equations.

Answer 51

p^2 + q^2 + 2pq = 1
p + q = 1

Question 52

What is artificial selection?

Answer 52

The process by which humans choose organisms with desirable traits and selectively breed them together to enhance the expression of these traits over many generations.

Question 53

Explain the process of artificial selection via selective breeding.

Answer 53

1. The population shows phenotypic variation - there are individuals with different phenotypes (i.e. different characteristics)
2. A breeder (the person carrying out the artificial selection) selects an individual with the desired phenotype
3. Another individual with the desired phenotype is selected. The two selected individuals should not be closely related to each other
4. The two selected individuals are bred together
5. The offspring produced reach maturity and are then tested for the desirable trait. Those that display the desired phenotype to the greatest degree are selected for further breeding
6. The process continues for many generations: the best individuals from the offspring are chosen for breeding until all offspring display the desirable trait

Question 54

Give a commonly used example today of selective breeding.

Answer 54

Domestic dog breeding.

Question 55

Give some ethical considerations surrounding artificial selection/selective breeding.

Answer 55

Selective breeding can lead to ‘inbreeding’ which occurs when the ‘best’ animals or plants which are closely related to each other are bred together. This results in reduction in the gene pool (inbreeding depression). This means that there is a higher chance of an organism inheriting harmful genetic defects and organisms being vulnerable to new diseases.

Question 56

What are the two types of speciation?

Answer 56

Sympatric
Allopatric

Question 57

When does allopatric speciation occur?

Answer 57

when there is geographic isolation e.g a barrier separating the species such as a body of water or a motor way.

Question 58

Explain allopatric speciation.

Answer 58

The barrier separating the species creates two populations of the same species who are reproductively separated from each other so as a result no genetic exchange occurs. If there are sufficient selection pressures acting to change both of the gene pools then the populations will diverge and become separate species.

Question 59

Outline sympatric speciation.

Answer 59

Takes place with no geographical barrier. A group of organisms could be living in the same place but there must be two populations within the wider group and no gene flow occuring between them. Something must happen to split or separate the populations.

Question 60

Give some examples as to why sympatric speciation may occur.

Answer 60

populations are separated because they live in different environments within the same area.
E.g soil pH has a major effect on plant growth so a population growing in soil with a specific pH may flower at a different time to another population so there is genetic isolation of the two populations.

Populations are seperated because they have different behaviours. E.g differences in feeding.

Question 61

What is genetic isolation?

Answer 61

When two populations of the same species become reproductively isolated from each other.

Question 62

What is speciation?

Answer 62

The formation of new species from pre-existing species over time as a result of changes in gene pools.