Patterns of Inheritance

Question 1

Define the term “variation”.

Answer 1

The differences in characteristics between organisms.

Question 2

Define the term “interspecific variation”.

Answer 2

The differences between organisms of different species.

Question 3

Define the terms “intraspecific variation”.

Answer 3

The differences between organisms of the same species.

Question 4

Name and describe the two causes of variation.

Answer 4

1) Environmental -
The environment in which the organisms lives causes environmental differences.

2) Genetic -
The differences in the genetic material an organism inherits from its parents leads to genetic variation.

Question 5

Describe 5 causes of genetic variation between individuals within a population.

Answer 5

• Alleles:
  with a gene for a particular characteristic, different alleles produce different effects and different individuals within a species population may inherit different alleles of a gene.
• Mutation:
  Changes to the DNA sequence and therefore to genes can lead to changes in the proteins that are coded for.
• Meiosis:
  Gametes are produced by meiosis. Each gamete receives half the genetic content of a parent cell and the genetic material is mixed up by independent assortment and crossing over.
• Sexual reproduction:
  Offspring produced from two individuals inherits genes from each parent.
• Chance:
  Different gametes are produced and in reproduction, it is chance as to which two combine. This is why individuals differ from siblings.

Question 6

Define the term “phenotype”.

Answer 6

Observable characteristics of an organism.

Question 7

Define the term “genotype”.

Answer 7

The genetic makeup of an organism.

Question 8

Describe how chlorosis in plants is an example of phenotypes influenced by both genetic and environmental factors.

Answer 8

Chlorosis results in the leaves appearing pale or yellow because their cells are not producing the normal amount of chlorophyll. Most plants which show chlorosis have the normal genes coding for chlorophyll production.

Change in phenotype is due to environmental factors:

• Lack of light, the absence of light causes plants to turn off chlorophyll production to save energy.
• Mineral deficiencies, a lack of iron or magnesium. Iron is a cofactor needed by enzymes to produce chlorophyll.
• Virus infections; when viruses infect plants, they interfere with metabolism of cells.

Question 9

Describe how body mass in animals is an example of

a phenotype influenced by both genetic and environmental factors.

Answer 9

• Dramatic variations in weight are generally due to diet, presence of a disease and level of exercise (environmental).
• Obesity can be the results of a mutation in your genes which causes the pattern of fat depositions to be altered

Question 10

Define the term “allele”.

Answer 10

Different version of the same gene.

Question 11

Define the term “dominant allele”.

Answer 11

The version of the gene that will always be expressed if present.

Question 12

Define the term “recessive allele”.

Answer 12

The version of the gene that will only be expressed if two copies are present in the organism.

Question 13

Define the term “homozygous”.

Answer 13

Two identical alleles for a characteristic.

Question 14

Define the term “heterozygous”.

Answer 14

Two different alleles for a characteristic.

Question 15

Define the term “carrier”.

Answer 15

A person who has one copy of a recessive allele coding for a genetically inherited condition.

Question 16

Define the term “monogenic inheritance”.

Answer 16

A characteristic inherited on a single gene.

Question 17

Define the term “dihybrid inheritance”.

Answer 17

A characteristic inherited on two genes.

Question 18

Define the term “autosomal linkage”.

Answer 18

Genes present on the same non-sex chromosome.

Question 19

Define the term “sex linked genes”.

Answer 19

Genes carried on the sex chromosomes.

Question 20

Define the term “codominance”.

Answer 20

When different alleles of a gene are equally dominant and both are expressed in the phenotype.

Question 21

Define the term “epistasis”.

Answer 21

The effect of one gene on the expression of another gene.

Question 22

Describe the 6 steps for drawing a genetic cross diagram

Answer 22

1) State the phenotype of both the parents.
2) State the genotype of both parents. Assign a letter to represent each of the alleles of the genes being studied. Capitalised for dominant and lower case for recessive.
3) State the gametes of each parent and circle them.
4) Use a Punnett Square to show the results of the random fusion of gametes on the edges of the square.
5) State the proportion of each genotype which are produce among the offspring. Can be in the form of a percentage or ratio.
6) State the corresponding phenotype for each of the possible genotypes.

Question 23

Describe what co-dominance is.

Answer 23

Co-dominance occur when two different alleles occur for a gene which are both equally dominant. As a result, both alleles of the gene are expressed in the phenotype of the organism if they are present.
e.g. colour of flowers, red and white made pink.

Question 24

How is co-dominance represented in genetic diagrams?

Answer 24

Upper and lower case letters are not used to represent the genes as this would imply dominant and recessive.
Instead, a letter is chosen to represent the gene, and the different alleles are then represented using a second letter and is shown as a superscript.

Question 25

Describe what happens when there are multiple alleles for a gene.

Answer 25

• Some genes have more than two versions of alleles, they have multiple alleles .
• However, an organism only carries two versions of a particular gene (one on each of the homologous chromosomes) so only two alleles can be present in an individual.
• Blood group is determined by a gene with multiple alleles. This results in many different possible crosses.

Question 26

Describe how sex is determined in humans.

Answer 26

• Genetically determined.
• Humans have 23 pairs of chromosomes, and in 22 of the pairs both chromosomes are the same.
• The 23rd pair, know as the sex chromosomes, are different.
• Human females have two X chromosomes (XX) whereas a male has an X and a Y (XY).
• The X chromosome is large and contains many genes not involved in sexual development. The Y chromosome is very small, and carries almost no genetic info except the genes that causes the embryo to develop into a male.
• Therefore, the sex of an offspring will be determined by whether the sperm fertilising the egg contains an X or a Y chromosome.

Question 27

Describe what sex linkage.

Answer 27

• Sex linked characteristics are characteristics determined by genes carried on the sex chromosomes.
• As the Y is much smaller than X chromosome, there are many genes in the X chromosome that males only have one copy of (because they are no present in the Y chromosome).
• This means that any characteristic caused by a recessive allele on the section of the X chromosome, which is missing in the Y chromosome, occurs more frequently in males.
• This because many females will also have a dominant allele present on their second X chromosome, which overrides the recessive allele.

Question 28

Give an example of a sex-linked genetic disorder.

Answer 28

Haemophilia - the absence of clotting factor that results in pro-longed bleeding following an injury.

It is caused by a recessive allele on the X-chromosome, so most haemophilia suffers are male. A female would have to be heterozygous recessive for it to be expressed.

Question 29

Describe what dihybrid inheritance and how a dihybrid cross would look.

Answer 29

• A dihybrid cross is used to show the inheritance of two different characteristics, caused by two genes, which may be located on different pairs of homologous chromosomes. Each of these genes can have two or more alleles.
• In a dihybrid cross four alleles (two for each characteristic) are shown at each stage instead of two.
• -

Question 30

State the expected phenotypic ratio for dihybrid crosses involving: i) two double heterozygotes, ii) one double heterozygote and one double homozygous recessive.

Answer 30

— ?? See sophie’s, ask guilly.

Question 31

Explain why expected phenotypic ratios may not occur if there is linkage between two genes.

Answer 31

The genes being studied are both on the same chromosome - known as linked genes.
If not crossing over occurs the alleles for the two characteristics will always be inherited individually.

Question 32

Explain why crossing over disrupts autosomal linkage, but only occasionally.

Answer 32

Autosomal linkage refers to genes being on the same chromosome. These genes are inherited as one unit unless they are separated by chiasmata during crossing over.
The closer the genes are on a chromosome the less likely they are to be separated during crossing over and the fewer recombinant offspring are produced.

Question 33

Define the term “recombinant offspring”.

Answer 33

New combination of alleles - different allele combination to their parents.

Question 34

Define the term recombinant frequency.

Answer 34

The proportion of recombinant offspring resulting from a cross.

Question 35

Describe how epistasis can occur.

Answer 35

Epistasis is the interaction of genes at different loci. We talk about the expression of a gene to give a particular characteristic but another gene may interact with that gene to change its expression. Gene regulation e.g the lac operon is an example of epistasis, where a regulatory gene controls the activity a the structural gene.

Question 36

What are the different forms of epistasis?

Answer 36

1) Antagonistic epistasis - one gene may prevent expression of another gene,
2) Complementary epistasis - may allow a second gene to be expressed.

Question 37

Define the term “hypostatic gene”.

Answer 37

A gene that is being effected by another gene.

Question 38

Define the term epistatic gene.

Answer 38

A gene that effects another gene.

Question 39

Define the term epistatic gene.

Answer 39

A gene that effects another gene.

Question 40

Define the term “dominant epistasis”.

Answer 40

Dominant epistasis occurs if a dominant allele results in a gene having an effect on another gene. This would happen if an epistatic gene coded for an enzyme that modified one of the precursor molecules in the pathway. The next enzyme in the pathway would then lack a suitable substrate molecule and so the pigment would again not be produced. All of the genes in the sequence would be effectively masked.

Question 41

Define the term “recessive epistasis”.

Answer 41

If presence of two recessive alleles at a locus led to the lack of an enzyme then it would be called recessive epistasis.

Question 42

Describe when a chi-squared (c2) test would be used to analyse data.

Answer 42

When you are trying to see if the difference between your expected results and the actual outcome is just due to chance or if there the difference is significant - i.e. there is another factor influencing the results.

Question 43

Describe the meaning of each of the symbols in the equation for calculating the c2-value (the test statistic) from a chi-squared (c2) test.

Answer 43

O = observed frequencies (actual outcome). 
E = expected frequencies
x2 = the test statistic
∑ = sum of

Question 44

Describe how to conduct a chi-squared test to determine whether data from a genetic cross supports the hypothesis about how the characteristic(s) is/are inherited.

Answer 44

1) Calculate the expected the phenotype ratios using a punett square.
2) Calculate the expected outcomes using the expected phenotype ratio and the total number of offspring.
3) The observed values will be given in the question.
4) Use the formula to find the test statistic.
5) Find degrees of freedom (n-1).
6) Use degrees of freedom and P value (0.05) to find critical value from table.
7) Compare test statistic to critical value. If test statistic is lower than critical value accept the null hypothesis. If test statistic is higher than critical value then reject the null hypothesis.

Question 45

Define the terms “continuous variation”.

Answer 45

A characteristic that can take any value within a range.

Most continuous variations are controlled by multiple genes.

Question 46

Define the terms “discontinuous variation”.

Answer 46

A characteristic that can only result in discreet values e.g blood type. Most discontinuous variations are controlled by a single gene.

Question 47

Describe the causes of variation that result in discontinuous variation.

Answer 47

Variation determined purely by genetic factors falls into the discontinuous category.

Question 48

Define the term “polygenic”.

Answer 48

A characteristic that is controlled by a group of genes.

Question 49

Define “multifactorial”.

Answer 49

Characteristics which are dependant on a number of factors, genetic or environmental.

Question 50

Describe the causes of variation that result in continuous variation.

Answer 50

Characteristics that show continuous variation are not controlled by a single gene but a number of genes, polygenes, and are often influenced by environmental factors.

Question 51

Define the term “evolution”.

Answer 51

The change in allele frequency within a gene pool over time.

Question 52

Define the term “natural selection”.

Answer 52

The process by which organisms best-suited to their environment survive and reproduce passing on their characteristics to their offspring, through their genes.

Question 53

Define the terms “allele frequency”.

Answer 53

The relative frequency of a particular allele in the population at a given time.

Question 54

Define the term “gene pool”.

Answer 54

The sum total of all the genes in a population at a given time.

Question 55

Define the term “selection pressure”.

Answer 55

Factors that affect an organism’s chance of survival or reproduction.

Question 56

Define the term “selectively neutral allele”.

Answer 56

A variety of a gene that doesn’t provide a selective advantage or disadvantage to the organism.

Question 57

Define the term “advantageous allele”

Answer 57

A variety of a gene that provides a selective advantage to the organism.

Question 58

Describe the steps in the process of adaptations evolving by natural selection

Answer 58

• Organisms within a species show variation in their characteristics which are caused by genetic variation.
• Organisms whose characteristics are best adapted to a selection pressure (e.g predation or competition) have an increased chance of surviving and successfully reproducing.
• Successful organisms pass the allele encoding the advantageous characteristics onto their offspring.
• This is repeated for every generation over time the proportion of individuals with the advantageous adaptation increases.
• Therefore the frequency of the allele that codes for this increases in the population gene pool.
• Over very long periods of time, this process can lead to the evolution of new species.

Question 59

Name 5 factors that can affect the evolution of a species

Answer 59

1) Mutation
2) Sexual selection
3) Gene flow
4) Genetic drift
5) Natural selection

Question 60

Define the term “genetic drift” and describe the consequence of it on the genetic diversity of a population.

Answer 60

Genetic drift occurs in small populations. This is a change in allele frequency due to the random nature of mutation. The appearance of a new allele will have a greater impact (is more likely to increase in number) in a smaller population than in a much larger population where there is a greater number of alleles present in the gene pool.

Question 61

Define the term “genetic bottleneck” and describe the consequence of it on the genetic diversity of a population.

Answer 61

Population bottlenecks are a large reduction in population size which last for at least one generation. The gene pool along with genetic diversity is greatly reduced and the effects will be seen in future generations.

Question 62

Define the term “founder effect” and describe the consequence of it on the genetic diversity of a population.

Answer 62

The founder effect is an extreme example of genetic drift. Its when small populations arise due to the establishment of new colonies by a few isolated individuals. These small populations have much smaller gene pools than the original population and display less genetic variation.
The frequency of any alleles that were rare in the original population will be much higher in the new smaller population if carried across.

Question 63

Define the term “stabilising selection” and describe the consequences of it for a population.

Answer 63

When the norm or average is selected for (positive selection) and the extremes are selected against (negative selection). It results in a reduction in the frequency of alleles at ‘extremes’ and an increased in the frequency of ‘average’ alleles.

Question 64

Define the term “directional selection” and describe the consequences of it for a population.

Answer 64

Occurs when there is a change in the environment and the normal (most common) phenotype is no longer the most advantageous. Organisms which are less common and have more extreme phenotypes are positively selected. The allele frequency then shifts towards the extreme phenotypes and evolution occurs. e.g. peppered moth colours.

Question 65

Define the term “disruptive selection” and describe the consequences of it for a population.

Answer 65

In disruptive selection the extremes are selected for and the norm selected against. Opposite of stabilising selection. An example is Darwin’s finches.

Question 66

State what the Hardy-Weinberg principles says.

Answer 66

In a stable population with no disturbing factors the allele frequency will remain constant from one generation to the next and there will be no evolution.

Question 67

Describe the assumptions made in the Hardy-Weinberg principle.

Answer 67

No selection occurs because all alleles are all equally advantageous.
No mutations are occurring.
Mating is random.
Population is not large.
No migration into or out of the population.

Question 68

Describe what the symbols represent in the Hardy-Weinberg equations .

Answer 68

1) p + q = 1
p= frequency of dominant alleles.
q= frequency of recessive alleles.

2) p2 + 2pq + q2 = 1
p2 = frequency of homozygous dominant genotype in the population.
2pq = frequency of heterozygous genotype in the population.
q2 = frequency of homozygous recessive genotype in the population.

Question 69

Describe what the symbols represent in the Hardy-Weinberg equations .

Answer 69

1) p + q = 1
p= frequency of dominant alleles.
q= frequency of recessive alleles.

2) p2 + 2pq + q2 = 1
p2 = frequency of homozygous dominant genotype in the population.
2pq = frequency of heterozygous genotype in the population.
q2 = frequency of homozygous recessive genotype in the population.

Question 70

Define the term “species”.

Answer 70

A group of organisms that can interbreed to produced fertile offspring.

Question 71

Define the term “speciation”.

Answer 71

The formation of new species through evolution.

Question 72

Define the term allopatric speciation.

Answer 72

Speciation that occurs as a result of a physical barrier between populations.

Question 73

Define the term sympatric speciation.

Answer 73

Speciation that occurs when there is no physical barrier between populations.

Question 74

Outline the stages in the process of speciation.

Answer 74

1) Members of a population become isolated and no longer interbreed with the rest of the population resulting in no gene flow between two groups.
2) Alleles within the groups continue to undergo random mutation. The environment of each group may be different or change (resulting in different selection pressures) so different characteristics will be selected for and against.
3) The accumulation of mutations and changes in allele frequencies over many generations eventually lead to large changes in phenotype. The members of different populations become so different that they are no longer able to interbreed (to produce fertile offspring). They are now reproductively isolated and are different species.

Question 75

Describe how geographical isolation can lead to the evolution of new species.

Answer 75

Allopatric speciation is the more common form of speciation and happens when some members of a population are separated from the rest of the group by a physical barrier such as a river or the sea - they are geographically isolated. The environments of the different groups will often be different and so will the selection pressures resulting in the different physical adaptations.
Separation of a small group will often result in the founder effect leading to genetic drift further enhancing the differences between the populations. e.g finches in the Galapagos.

Question 76

Define the term “pre-zygotic reproductive barrier”.

Answer 76

Prevents fertilisation and formation of a zygote because gametes do not meet.

Question 77

Define the term “post-zygotic reproductive barrier”.

Answer 77

Zygotes are formed but they do not produce mature fertile offspring.

Question 78

Describe 5 types of pre-zygotic reproductive barrier.

Answer 78

1) Geographical isolation: the populations never meet because they inhabit different areas.
2) Ecological isolation: the populations never meet because although they inhabit the same area, they live in different habitats within that area.
3) Temporal isolation: the populations never meet because although they inhabit the same habitat, they are active at different times of the day or reproduce at different times of the year.
4) Behavioural isolation: The populations meet but don’t recognise eachother’s courtship displays, so fail to mate.
5) Mechanical isolation: The populations meet but can’t interbreed because their reproductive parts don’t fit each other.

Question 79

Describe 2 types of post-zygotic reproductive barrier.

Answer 79

1) Hybrid inviability:
The populations interbreed but the hybrid offspring produced fail to live to maturity,
2) Hybrid sterility:
The populations interbreed but the hybrid offspring are sterile.

Question 80

Describe the role of hybridisation and polyploidy in the process of speciation in plants.

Answer 80

It can occur when members of two different species interbreed and form fertile offspring - this often happens in plants. The hybrid formed, which is a new species, will have a different number of chromosomes to either parent and may no longer be able to interbreed with members of either parent population.