Patterns of inheritance

Question 1

What is chlorosis

Answer 1

when plants leaves look pale or yellow because the cells aren’t producing the normal amount of chlorophyll reducing a plants ability to photosynthesise

Question 2

How do environmental factors affect plants phenotype?

Answer 2

o Mineral deficiencies: lack of iron or magnesium. Iron is needed as a cofactor by some enzymes that make chlorophyll and Magnesium is found at the heart of the chlorophyll molecule
o Virus infections: When viruses infect plants, interfering with the metabolism of cells so they can no longer support the synthesis of chlorophyll
o Lack of light: in absence of light, plants turn off chlorophyll production to conserve resources

Question 3

How is body mass affected by environmental factors and genetic factors

Answer 3

• Environmental factors include: amount of food eaten, amount of exercise or presence of disease.
• Occasionally, obesity is a result of the genetic makeup of an organise e.g. the obese mouse has a mutation on chromosome 7 causing the pattern of fat deposition in its body to be altered

Question 4

Define genotype and phenotype

Answer 4

• Genotype: combination of alleles an organism inherits for a characteristic
• Phenotype: observable characteristics of an organism

Question 5

Define the dominant allele

Answer 5

• Dominant allele: Version of the gene that will always be expressed if present in an organism
o Means an individual showing the dominant characteristic could have one or two copies of the dominant gene

Question 6

Define the recessive allele

Answer 6

• Recessive allele: only expressed if two copies of this allele are present meaning if an individual has a recessive phenotype, you also know the genotype

Question 7

Define homozygous and heterozygous

Answer 7

• Homozygous: two identical alleles for a characteristic

* Heterozygous: they have two different alleles for a characteristic

Question 8

What is continuous variation

Answer 8

a characteristic that can take any value within a range, caused by genetic and environmental factors and controlled by a number of genes
- e.g. leaf surface area, animal mass, skin colour

Question 9

What is discontinuous variation

Answer 9

a characteristic that can only appear in specific (discrete) values, caused by mostly genetic factors and controlled by one or two genes
- e.g. blood group, albinism and round and wrinkled pea shape

Question 10

What is true breeding

Answer 10

• organisms that contain homozygous alleles for a particular gene

Question 11

What is codominance

Answer 11

• Occurs when two different alleles occur for a gene – both of which are equally dominant so both alleles of the gene are expressed in the phenotype

Question 12

When are F1 offspring formed and when are F2 offspring formed

Answer 12

F1 = homozygous genetic cross
F2 = heterozygous genetic cross

Question 13

How is sex determined

Answer 13

• Sex is genetically determined
• 23rd pair of chromosomes are known as sex chromosomes – females have two X chromosomes whereas males have an X and a Y
• X chromosome: large and contains many genes involved in sexual development
• Y chromosome: smaller, containing almost no genetic information but does carry gene causing an embryo to develop as a male

Question 14

What is sex linkage

Answer 14

• Characteristics determined by genes carried on sex hormones are genes that are sex linked
• Y chromosome is much smaller than X so there are a number of genes in the X chromosome that males have only one copy of – means that any characteristic caused by a recessive allele on the section of the X chromosome which is missing in the Y chromosome
o Many females also have a dominant allele present in their cells

Question 15

Describe how haemophilia is an example of a sex-linked genetic disorder

Answer 15

• If a male inherits the recessive allele that codes for haemophilia they cannot have a corresponding dominant allele on their Y chromosome and so develop the condition
• Females who are heterozygous for the haemophilia coding gene are carriers

Question 16

How is a dihybrid cross used and how is it different to a monohybrid cross

Answer 16

• 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
• Same as monohybrid cross but 4 alleles instead of two
• You can produce an expected ratio of the four different phenotypes but as with all genetic crosses the actual ratio of offspring produced can differ from the expected as:
o Fertilisation of gametes is a random process so in a small sample a few chance events can lead to a screwed ratio
o Genes being studied are both on the same chromosome, linked genes. If no crossing over occurs the alleles for the two characteristics will always be inherited together

Question 17

Define autosomal linkage

Answer 17

when genes that are linked are found on one of the other pairs of chromosomes

Question 18

Describe how linked genes differ from normal genes

Answer 18

• Linked genes are inherited as one unit – no independent assortment during meiosis unless the alleles are separated by chiasmata – tend to be inherited together
• Linked genes cannot undergo normal random shuffling of alleles during meiosis and expected ratios will not be produced in the offspring

Question 19

Define recombinant offspring

Answer 19

• have different combinations of alleles than either parent

Question 20

Define recombinant frequency and describe how it is calculated

Answer 20

• 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
• Recombinant frequency: measure of the amount of crossing over that has happened in meiosis
o Number of recombinant offspring total number of offspring
o RC of 50% indicates that there is no linkage and the genes are on separate chromosomes
o 50% indicates there is gene linkage and the random process of independent assortment has been hindered
o As the degree of crossing over is determined by how close the genes are on the chromosome – closer they are the less likely they will be separated during crossing over

Question 21

What is epistasis

Answer 21

• The interaction of genes at different loci e.g. gene regulation (regulatory genes controlling the activity of structural genes – lac operon)

Question 22

Define hypostatic and epistatic

Answer 22

• Hypostatic: gene that is affected by another gene

* Epistatic: gene that affects the expression of another gene

Question 23

When does dominant apostasies occur and what influence does this have

Answer 23

• Dominant epistasis occurs if a dominant allele results in a gene having an effect on another gene
o happens if an epistatic gene coded for an enzyme that modified one of the precursor molecules in the pathway and the next enzyme in the pathway would lack a suitable substrate molecule and so the pigment would again not be produced
o all of the genes in the sequence would be masked

Question 24

How is allele frequency calculated

Answer 24

p^2 + 2pq + q^2 = 1
p+q=1
p^2 is frequency of homozygous dominant
q^2 is frequency of homozygous recessive
2pq is frequency of heterozygous

Question 25

What conditions need to be met to predict allele frequencies in a population

Answer 25

o Large population
o No migration
o Random mating (for the characteristic in question)
o All genotypes have equal reproductive success
o No mutation

Question 26

Define allele frequency

Answer 26

o Allele frequency: how often an allele occurs in a population

Question 27

What factors affect evolution

Answer 27

• Mutation necessary for the existence of different alleles in the first place and the formation of new alleles leads to genetic variation
• Sexual selection leads to an increase in frequency of alleles which code for characteristics that improve making success
• Gene flow (movement of alleles between populations) and immigration and emigration result in changes of allele frequency within a population
• Genetic drift occurs in small populations (change in allele frequency due to random nature of mutation) – the appearance of a new allele will have a greater impact in a smaller pop as there are less alleles in the gene pool
• Natural selection leads to an increase in the number of individuals that have characteristics that improve their chances of survival – reproduction rates will increase as well as the frequency of the alleles coding for the characteristics –this is how evolution happens

Question 28

What factors limit the size of a population

Answer 28

o Density-dependent factors: dependant on population size e.g. competition, predation, parasitism and communicable disease
o Density-independent factors: affect populations of all sizes e.g. climate change, natural disasters, seasonal change, human activities (deforestation)

Question 29

What is a population bottleneck

Answer 29

• large reductions in population size which least for at least one generation – gene pool greatly reduced e.g. northern elephant seals
o Positive: beneficial mutation has a greater impact and leads to quicker development of a new species (thought to play a role in evolution of early humans)

Question 30

What is the founder effect

Answer 30

• Founder effect: when small populations arise due to the establishment of new colonies by a few isolated individuals – extreme example of genetic drift
• These small populations have a small gene pool and display less genetic variation e.g. Amish in America

Question 31

What are the three types of evolutionary forces

Answer 31

• Stabilising selection: natural selection that favours average phenotypes – results in a reduction in the frequency of alleles at the extremes and an increase in the frequency of average alleles
• Directional selection: natural selection that favours one extreme phenotype
o Allele frequency shifts toward extreme phenotype and evolution occurs e.g. changes seen in peppered moths in industrial revolution
• Disruptive selection: natural selection that favours both extremes of a given phenotype
o E.g. finches observed by Darwin in the Galapagos
o Examples of disruptive selection are relatively rare

Question 32

What is speciation

Answer 32

• Formation of new species through the process of evolution

Question 33

What leads to speciation

Answer 33

o Members of a population become isolated and no longer interbreed with the rest of the population – no gene flow between the two groups
o Alleles within the groups continue to undergo random mutations – environment of each group may be different or change so different characteristics will be selected for and against
o Accumulation of mutations and changes in allele frequencies over many generations lead to large changes in phenotype - members of the different populations become so different that they are no longer able to interbreed – reproductively isolated

Question 34

What is allopatric speciation (give an example)

Answer 34

• More common form of speciation that happens when some members of a population are separated from the rest of the group by a physical barrier e.g. sea
• Separation of a small group will often result in the founder effect leading to genetic drift further enhancing the differences in populations
• E.g. finches inhabiting the Galapagos Islands in the Pacific Ocean
o Finches are separated by the islands and have formed new colonies on different islands
o Finches have evolved and adapted to the different environments (particularly food sources)
o Example of adaptive radiation: rapid organism diversification takes place – finches unable to breed with each other and new species have evolved

Question 35

What is sympatric speciation

Answer 35

• Occurs within populations that share the same habitat – happens less frequently and more common in plants than animals
• Can occur when members of two different species interbreed and form fertile offspring

Question 36

What are reproductive barriers including pre and post zygotic barriers

Answer 36

• Reproductive barriers: barriers to successful interbreeding can form within populations before or after fertilisation has occurred
o Prezygotic reproductive barriers prevent fertilisation and the formation of a zygote
o Postzygotic reproductive barriers are often produced as a result of hybridisation and reduce the viability or reproductive potential of offspring

Question 37

Define polymorphic

Answer 37

display more than one distinct phenotype for most characteristics

Question 38

Define wild type allele

Answer 38

allele coding for the most common, or normal, characteristic

Question 39

Define mutants

Answer 39

other types of allele that result from mutations

Question 40

Define inbreeding and provide an example

Answer 40

breeding of closely related individuals
E.g. domesticated dogs from the grey wolf species 10,000s of years ago and so different characteristic traits were selected for depending on whether they were used for hunting. Fighting, herding or even as status symbols and so a range of dogs with different characteristics evolved

Question 41

What are the problems with inbreeding

Answer 41

• Limiting the gene pool and so decreasing the genetic diversity and reducing chances of a population of inbred organisms evolving and adapting to their environment
• Many genetic disorders caused by recessive alleles and aren’t uncommon in most populations and organisms that are closely related are genetically similar and are likely to have the same recessive alleles therefore inbreeding offspring have a greater chance of being homozygous for these recessive traits and being affected by these disorders
o Over time reduces the ability of these organisms to survive and reproduce resulting in organisms being less biologically fit

Question 42

What are gene banks

Answer 42

• Seed banks keep samples of seeds from both wild type and domesticated varieties – important genetic resource
• Gene banks store biological samples e.g. sperm, seeds or eggs (usually frozen)
• Problems of inbreeding combated by increasing genetic diversity with alleles from gene banks – outbreeding
• Breeding unrelated or distantly related varieties is also a form of outbreeding reducing the occurrences of homozygous recessives and increases potential to adapt to environmental change