Patterns of inheritance

Question 1

Define the term variation

Answer 1

The differences in characteristics between organisms are called variation

Question 2

Define the term interspecific variation

Answer 2

The differences between organisms of different species

Question 3

Define the term 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. An organism’s genetic material- differences in the genetic material an organism inherits from its parents leads to genetic variation
2. The environment in which the organism lives- this causes environmental variation.

Question 5

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

Answer 5

1. Alleles- with a gene for a particular characteristic, different alleles produce different affects. Individuals in a species population may inherit different alleles of a gene.
2. Mutations- Changes to the DNA sequence and therefore to genes can lead to changes in the proteins that are coded for.
3. Meiosis- gametes are produced by meiosis. Each gamete receives half the genetic content of a parent cell. The genetic material is mixed up by independent assortment and crossing over.
4. Sexual reproduction- offspring produced from tow individuals inherits genes form each of the parents
5. Chance- different gametes are produced and in reproduction it is chance as to which two combine- individuals differ from siblings

Question 6

Define phenotype

Answer 6

Observable characteristics of an organism

Question 7

Define genotype

Answer 7

Genetic makeup of an organism

Question 8

Describe how chlorosis is an example of a phenotype influenced by both genetic and environmental factors.

Answer 8

1. Chlorosis, is when the leaves look pale or yellow, this occurs because the cells are not producing the normal amount of chlorophyll
2. Most plants which show chlorosis have normal genes coding for chlorophyll production. The change in their phenotype is the result of environmental factors:
   - 1) lack of light- in absence of light plants will turn of chlorophyll production to conserve resources
   - 2) Mineral deficiencies- lack of iron or magnesium. Iron is needed as a cofactor by some of the enzymes that make chlorophyll and magnesium is found at the heart of the chlorophyll molecule
   - 3) Virus infections- interfere with the metabolism of cells

Question 9

Describe how body mass is an example of a phenotype influenced by both genetic and environmental factors.

Answer 9

1. In most cases dramatic variations in size such as obesity and being severely underweight are a result of environmental factors
2. e.g. amount of foods eaten and quantity of exercise or presence of disease
3. Occasionally obesity can be a result of the genetic make-up of an organism
4. Mutations can cause a pattern of fat deposition in the body to be altered.
5. Scientific studies have shown that this gene acts in conjunction with other genes that regulate the energy balance in the body.

Question 10

Define allele

Answer 10

Different versions of the same gene

Question 11

Define dominant allele

Answer 11

Version of the gene that will always be expressed if present

Question 12

Define recessive allele

Answer 12

Version of a gene that will only be expressed if two copies of this allele are present in an organism

Question 13

Define homozygous

Answer 13

Two identical alleles for a characteristic

Question 14

Define heterozygous

Answer 14

Two different alleles for a characteristic

Question 15

Define carrier

Answer 15

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

Question 16

Define monogenic inheritance

Answer 16

A characteristic inherited on a single gene

Question 17

Define dihybrid inheritance

Answer 17

A characteristic inherited on two genes

Question 18

Define autosomal linkage

Answer 18

Genes present on the same non-sex chromosome

Question 19

Define sex linked genes

Answer 19

Genes carried on the sex chromosomes

Question 20

Define codominance

Answer 20

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

Question 21

Define 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 code to represent the alleles of the gene being studied. A capital letter should be used to represent the dominant allele and its lowercase form to represent the recessive allele.
3. State the gametes of each parent and circle the letters
4. Use a Punnet Square to show the results of the random fusion of gametes during fertilisation. Remember to label the gametes on the edges of the square.
5. State the proportion of each genotype which are produced among the offspring.
6. State the corresponding phenotype for each of the possible genotypes. It must be clear you know which phenotype results from each genotype

Question 23

Describe what codominance is

Answer 23

1. Occurs when two different alleles occur for a gene- both of which are equally dominant.
2. As a result both alleles of the gene are expressed in the phenotype of the organism
3. e.g colours of flowers red and white make pink
4. When studying codominance upper and lower case letters are not used to represent the alleles
5. Instead a letter is chosen to represent the gene e.g C
   The different alleles are then represented using a second letter which is shown as a superscript

Question 24

Describe what happens when there are multiple alleles for a gene

Answer 24

1. Some genes have more than two versions- multiple alleles
2. However, as an organism carries only two versions of the gene (one on each homologous chromosome) only two alleles can be present in an individual
3. Blood group is determined by a gene with multiple alleles
4. This results in many different crosses

Question 25

Describe how sex is determined in humans

Answer 25

1. Humans have 23 pairs of chromosomes of varying shapes and sizes
2. In 22 pairs both members of the pair are the same but the 23rd pair known as the sex chromosomes are different
3. Human females have two x chromosomes, whereas a male has an X and Y
4. The X chromosome is large and contains many genes not involved in sexual development
5. The Y chromosome is very small containing almost no genetic information, but does carry the gene that causes the embryo to develop as a male

Question 26

Describe what sex linkage is

Answer 26

1. Some characteristics are determined by genes carried on the sex chromosomes- sex linked
2. As the Y chromosome is much smaller than the X chromosome, there are a number of genes in the X chromosome that males have only one copy of.
3. 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.
4. Because females will also have a dominant allele present in their cells

Question 27

Give an example of a sex-linked genetic disorder

Answer 27

1. Haemophillia
2. Patients with haemophilia have blood which clots extremely slowly due to the absence of a protein blood-clotting factor. This can lead to prolonged bleeding with is potentially fatal
3. It is a recessive allele on the X chromosomes so more frequent in males

Question 28

Describe what dihybrid inheritance and how a dihybrid cross would look

Answer 28

1. 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.
2. A dihybrid cross is set out in a very similar format to the one used when studying a monohybrid cross but 4 alleles are shown at each stage instead of two
3. e.g peas can be yellow or green and round or wrinkled

Question 29

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

Answer 29

1. If a true breeding homozygous pea plant with yellow round seeds is crossed with a true breeding homozygous pea plant with green wrinkled seeds. All the F1 generation will have a heterozygous genotype YyRr
2. When the F1 generation are crossed there are 16 possible combinations of alleles and the expected ratio is 9:3:3:1
3. 9 is the characteristics where both are dominant
4. 3 is characteristics where one is dominant and one recessive
5. 1 is phenotype which shows both the recessive characteristics

Question 30

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

Answer 30

1. The fertilisation of gametes is a random process so in a small sample a few chance events can lead to a skewed ratio
2. The genes being studied are both on the same chromosome. These are known as linked genes. If no crossing over occurs the alleles for the two characteristics will always be inherited together.

Question 31

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

Answer 31

1. When genes that are linked are found on one of the oher pairs of chromosomes it is called autosomal linkage.
2. Linked genes are inherited as one unit- no independent assortment during meiosis unless alleles are separated by chiasmata- tend to be inherited together
3. Linked genes cannot undergo the normal random shuffling of alleles and the expected rations will not be produced in the offspring.
4. Occasionally they will be separated- recombinant offspring
5. The closer the genes are on a chromosome the less likely they are to be separated during crossing over and the fewer recombinant offspring produced.

Question 32

Define recombinant offspring

Answer 32

New combination of alleles- different allele combination than either parent.

Question 33

Define recombinant frequency

Answer 33

Proportion of recombinant offspring resulting from a cross

Question 34

Describe how epistasis can occur

Answer 34

1. We talk about a the expression of a gene to give a particular characteristic.
2. Another gene may interact with this gene to change its expression
3. Gene regulation is an example.

Question 35

What are the different forms of epistasis

Answer 35

1. One gene can interact with a second gene
2. One gene may prevent the expression of a second gene- antagonistic epistasis
3. It may allow a second gene to be expressed- complementary epistasis

Question 36

Define hypostatic gene

Answer 36

A gene that is affected by another gene

Question 37

Define epistatic gene

Answer 37

A gene that affects the expression of another gene

Question 38

Define dominant epistasis

Answer 38

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

Question 39

Define recessive epistasis

Answer 39

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

Question 40

Describe when a chi-squared (χ2) test would be used to analyse data.

Answer 40

1. When you are trying to see if differences between the observed and expected value are significant or down to chance.
2. Used to test the null hypothesis
3. The values calculated are used to find the probability of the difference being due to chance alone.

Question 41

Describe the meaning of each of the symbols in the equation for calculating the χ2-value (the test statistic) from a chi-squared (χ2) test

Answer 41

1. O= observed frequencies
2. E= expected frequencies
3. χ^2 = the test statistic
4. df= degrees of freedom and is n-1

Question 42

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 42

1. Calculate the observed and expected frequencies
2. Calculate the test statistic
3. Find the critical value from a table
4. If the test statistic is smaller than the critical value the null hypothesis is accepted and the difference is not significant

Question 43

Define continuous variation

Answer 43

A characteristic that can take any value any value within a range e.g height

Question 44

Define discontinuous variation

Answer 44

A characteristic that can only result in discrete values e.g. blood type

Question 45

Describe the causes of variation that result in discontinuous variation.

Answer 45

Variation determined purely by genetic factors falls into the discontinuous category. Most characteristics are controlled by a single gene.

Question 46

Define polygenic

Answer 46

A characteristic that is controlled by a group of genes

Question 47

Describe the term multifactorial

Answer 47

Characteristics which are dependent on a number of factors.

Question 48

Describe the causes of variation that result in continuous variation.

Answer 48

Characteristics which show continuous variation are controlled by a number of genes and are also often influenced by environmental factors.

Question 49

Define evolution

Answer 49

The change in allele frequency within a gene pool over time

Question 50

Define natural selection

Answer 50

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

Question 51

Define the term allele frequency

Answer 51

The relative frequency of a particular allele in a population in a given time

Question 52

Define the term gene pool

Answer 52

Sum total of all the genes in a population at a given time

Question 53

Define the term selection pressure

Answer 53

Factors that affect an organisms chance of survival or reproductive success

Question 54

Define the term selectively neutral allele

Answer 54

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

Question 55

Define advantageous allele

Answer 55

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

Question 56

Define advantageous characteristic

Answer 56

One form of a part of an organism’s phenotype that provides it with a selective advantage

Question 57

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

Answer 57

1. Organisms within a species show variation in their characteristics that are caused by genetic variation.
2. Organisms whose characteristics are best adapted to a selection pressure- predation, competition- have an increased chance of surviving and successfully reproducing
3. Successful organisms pass the allele encoding the advantageous characteristic onto their offspring.
4. This process is repeated for every generation. Over time, the proportion of individuals with the advantageous adaptation increases. There fore the frequency of the allele that codes for this increases in the population’s gene pool.
5. Over a very long periods of time this process can lead to the evolution of new species

Question 58

Name 5 factors that can affect the evolution of a specie

Answer 58

1. Mutations
2. Sexual selection
3. Gene flow
4. Genetic drift
5. Natural selection

Question 59

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

Answer 59

1. Random change of allele frequency
2. This is change in allele frequency due to random nature of mutation.
3. 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 60

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

Answer 60

1. Large reductions in population size which last for at least one generation are called population bottlenecks
2. The gene pool along with genetic diversity, is greatly reduced and the effects will be seen in future generations.

Question 61

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

Answer 61

1. Small populations can arise due to the establishment of new colonies by a few isolated individuals- founder effect- an extreme example of genetic drift
2. These populations have much smaller gene pools than the original populations and display less genetic variation
3. If carried to the new population, the frequency of any alleles that were rare in the original population will be much higher in the new, smaller population and so they will have a much bigger impact during natural selection.

Question 62

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

Answer 62

1. The norm or average is selected for (positive selection) and the extremes are selected against (negative selection)
2. It results in a reduction of frequency of alleles at the extremes and an increase in the frequency of average alleles.
   e. g. medium baby weight

Question 63

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

Answer 63

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

Question 64

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

Answer 64

1. The extremes are selected for and the norm selected against. Opposite of stabilising selection
2. E.g Darwin finches

Question 65

State what the Hardy-Weinberg principle says

Answer 65

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

Question 66

Describe the assumptions made in the Hardy-Weinberg principle.

Answer 66

1. No selection occurs- all alleles are equally advantageous
2. No mutations are occuring
3. Mating is random
4. Population is not large
5. No migration into or out of population

Question 67

Describe what the symbols represent in the Hardy-Weinberg equations

Answer 67

1. p + q =1
   p= frequency of dominant allele
   q= frequency of recessive allele
2. p^2 +2pq +q^2 = 1
   p^2= frequency of homozygous dominant genotype
   2pq= frequency of heterozygous genotype
   q^2= frequency of homozygous recessive genotype

Question 68

Define speciation

Answer 68

The formation of new species through evolution

Question 69

Define allopatric speciation

Answer 69

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

Question 70

Define sympatric speciation

Answer 70

Speciation that occurs when there is no physical barrier between populations

Question 71

Outline the stages in the process of speciation.

Answer 71

1. Members of a population become isolated an no longer interbreed with the rest of the population resulting in no gene flow between the two groups
2. Alleles within the groups continue to undergo random mutations. 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.
4. The members of different populations become so different they are no longer able to interbreed (to produce fertile offspring). They are now reproductively isolated and are different species.

Question 72

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

Answer 72

1. 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 e.g. river of sea.
2. The environments of the different groups will often be different and so will the selection pressures resulting in difference physical adaptations.
3. 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. Galapagos Islands and finches

Question 73

Define prezygotic reproductive barrier

Answer 73

Prevents fertilisation and formation of a zygote

Question 74

Define postzygotic reproductive barrier

Answer 74

Zygotes are formed but do not produce mature fertile offspring

Question 75

Describe 5 types of prezygotic reproductive barrier and 2 types of postzygotic reproductive barrier.

Answer 75

Prezygotic
1. Geographical isolation- populations never meet because they inhabit different areas
2. Ecological isolation- Inhabit same area but they live in different habitats within that area
3. Temporal isolation- inhabit the same habitat but are active at different times of the day and reproduce at different times of year
4. Behavioral isolation- Populations meet, but don’t recognise each others courtship displays, so fail to mate
5. Mechanical isolation- Can’t interbreed because reproductive parts don’t fit each other
Postzygotic
1. Hybrid inviability- Interbreed but the hybrid offspring produced fail to live to maturity
2. Hybrid sterility- The populations interbreed but the hybrid offspring are sterile.

Question 76

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

Answer 76

1. When members of two different species interbreed and form fertile offspring- often in plants
2. 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.
3. This stops gene flow and reproductively isolated the hybrid organisms.

Question 77

Define artificial selection and describe the process

Answer 77

1. Selection of individuals for breeding with desirable characteristics.
2. Artificial selection or selective breeding is fundamentally the same as natural selection except for the nature of the selection pressure applied.
3. Instead of environment leading to survival of the fittest, it is selection for breeding of plants or animals with desirable characteristics by farmers or breeders
4. Individuals with the desired characteristics are selected and interbred
5. Offspring from this cross showing the best examples of the desired traits are selected to breed.
6. This breeding of closely related individuals is called interbreeding.
7. The process is repeated over many generations resulting in changes to the frequency of alleles within the population and eventually speciation

Question 78

Describe the problems caused by interbreeding

Answer 78

1. Limiting the gene pool and so decreasing genetic diversity reduces the chances of a population of inbred organisms evolving and adapting to changes in their environment
2. Many genetic disorders are caused by recessive alleles, cystic fibrosis.
3. Recessive alleles are not uncommon in mos populations but most individuals will be heterozygous.
4. Organisms that are closely related are genetically similar and likely to have the same recessive alleles- results in offspring which have a greater chance of being homozygous and affected by the genetic disorder
5. OVer time this reduces the ability of these organisms to survive and reproduce.
6. Results in the organisms being less biologically fit- less likely to survive and produce two surviving offspring to replace themselves.

Question 79

Give 1 example of artificial selection in crop plants.

Answer 79

Brassica oleracea is a wild mustard which has been selectively bred for many centuries producing a number of common vegetables.

Question 80

Describe the ethical considerations surrounding the use of artificial selection in dogs (and other species).

Answer 80

1. With the limited gene pool and lack of outbreeding it is inevitable that unwanted traits are selected for also.
2. Big dogs have hip or heart problems.
3. The skull of King charles spaniel is too small to accomodate the brain comfortably leading to pain and discomfort
4. Bull dogs have breathing problems.
5. Diseased dogs have effectively been deliberately interbred leading to a lot of problems for the animals.

Question 81

Define wild type

Answer 81

The allele that codes for the most common phenotype in a natural population

Question 82

Define the term outbreeding

Answer 82

Breeding of distantly related organisms

Question 83

Define seed bank

Answer 83

A store of genetic material from plants in the form of seeds. Store both wild type and domesticated varieties

Question 84

Define gene bank

Answer 84

Store of genetic material- usually frozen

Question 85

Describe what populations characteristics are normally

Answer 85

1. Populations are usually polymorphic (display more than one distinct phenotype).
2. The allele coding for the most common characteristic is called the wild type allele.
3. Other forms of that allele, resulting from mutations are called mutants.

Question 86

Explain the value of seed and gene banks for artificial selection in plants and animals.

Answer 86

1. Owing to problems caused by interbreeding, alleles from gene banks are used to increase genetic diversity in a process called outbreeding
2. Breeding unrelated or distantly related varieties is also a form of out breeding
3. This reduces occurrence of homozygous recessives and increases the potential to adapt to environmental change