6.20 - Patterns of inheritance

Question 1

Chlorosis

Answer 1

• where a plants leaves look pale or yellow
• occurs because not enough chlorophyll is being produced
  Mostly due to environmental factors (variation):
• mineral deficiencies (iron and magnesium)
• lack of light
• viral infections (TMV affects cell metabolism)
  Even though genetic factors in a plant are likely to code for green leaves, the environment plays a key role in the final leaf appearance/phenotype

Question 2

Animal body mass

Answer 2

• determined by a combination of genetic (mutation or variation in fat storage) and environmental factors
• dramatic variations are usually a result of environmental factors (calorific intake, exercise, disease)

Question 3

homologous chromosomes

Answer 3

the chromosomes from each parent that contain equivalent genes

Question 4

homozygous

Answer 4

an organism has inherited the same alleles coding for a particular characteristic

Question 5

heterozygous

Answer 5

• an organism has inherited different alleles coding for a particular characteristic
• the phenotype depends on which allele is dominant

Question 6

genotype

Answer 6

the combination of alleles in genes

Question 7

phenotype

Answer 7

the appearance/expression of the genotype

Question 8

causes of genetic variation

Answer 8

• sexual reproduction
• genotype and phenotype
• dominant and recessive alleles
• homozygous and heterozygous genotypes

Question 9

continuous variation

Answer 9

• a characteristic that can take any numerical value within a range
• caused by genetic and environmental variation
• polygenetic (controlled by a number of genes)
• graduation in values from one extreme to the other (continuum)
• usually plotted as a histogram
• e.g. height, mass, leaf surface area, skin colour

Question 10

Discontinuous variation

Answer 10

• a characteristic that can only appear in specific (discrete) values
• mostly genetic
• controlled by one (monogenetic) or two genes
• normally presented in a bar graph or pie chart
• e.g. biological sex, bacteria shape, human blood groups

Question 11

codominance

Answer 11

• when two different alleles that code for a gene are both equally dominant
• as a result both alleles of the gene are expressed in the phenotype of the organism if present
• e.g. if alleles for red and white flowers are both present (the plant is heterozygous) pink flowers are produced

Question 12

Sex linkage

Answer 12

• some characteristics are determined by genes carried on sex chromosomes (sex linked genes)
• as the Y chromosome is a lot smaller than the X chromosome, there are a number of genes in the X chromosome that males only have one copy of
• this means that any characteristic caused by a recessive allele on the section of the X chromosome that is missing in the Y chromosome occurs more frequently in males
• e.g. haemophilia, colour blindness
• this is because many females will also have a dominant allele on their other X chromosome

Question 13

haemophilia

Answer 13

• a sex linked genetic disorder
• the absence of a blood-clotting factor means injury can result in prolonged bleeding
• if a male inherits the recessive allele that codes for haemophilia on their X chromosome, they cannot have a corresponding dominant allele on their Y chromosome, so develop the condition
• the vast majority of haemophilia sufferers are male
• females who are heterozygous for the haemophilia coding gene are known as carriers as they do not suffer from haemophilia, but may pass the recessive allele on to their offspring

Question 14

dihybrid cross

Answer 14

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

Question 15

expected dihybrid inheritance ratio

Answer 15

9:3:3:1

Question 16

why can the actual ratios observed in dihybrid crosses differ from those expected

Answer 16

• the fertilisation of gametes is a random process, so in a small sample a few chance events can lead to a skewed ratio
• the genes being studeied are on the same chromosome (autosomal linkage). If no crossing over occurs, the alleles for the two characteristics will be inherited together

Question 17

autosomal linkage

Answer 17

the two genes that are linked are on the same non-sex chromosome (autosome)
- the closer the gene loci are to each other, the more likely they are to be inherited together as it is less likely crossing over will have an effect
- linked genes cannot undergo the normal random shuffling of alleles during meiosis, so the expected dihybrid ratios will not be produced in the offspring as the linked genes are effectively inherited as a single unit

Question 18

recombinant offspring (autosomal linkage)

Answer 18

• have different combinations of alleles that either parent
• this is due to crossing over
• the closer the genes are on a chromosome, the fewer recombinant offspring produced

Question 19

recombination frequency

Answer 19

• a measure of the amount of crossing over that has happened in meiosis
• number of recombinant offspring / total number of offspring
• 50% = there is no linkage, the genes are on separate chromosomes
• <50% = there is gene linkage, the random process of independent assortment has been hindered
• a recombinant frequency of 1% relates to a distance of one map unit apart on a chromosome

Question 20

Chi-squared test

Answer 20

• a statistical test that measures the size of the difference between the observed results an the expected results
• helps determine whether differences in the expected and observed results are significant or not
• if x² is more than the 5% critical value, there is a significant difference
• if the difference between the observed and expected inheritance ratio is significant, the difference is due to autosomal linkage

Question 21

epistasis

Answer 21

the interaction of genes at different loci. A gene locus masks of modifies the the phenotype of a second gene, for example in gene regulation

Question 22

hypostatic gene

Answer 22

a gene affected by another gene

Question 23

epistatic gene

Answer 23

a gene that affects the expression of another gene

Question 24

Epistasis in Labrador colours

Answer 24

• one gene codes for the production of melanin and has the alleles B (dominant, black pigment produced) and b (recessive, brown pigment produced)
• a second gene codes for where the pigment is deposited and has the alleles E (dominant, pigment deposited in the skin and fur) and e (recessive, pigment deposited in the skin only)
• the colour of a labrador varied depending on which alleles are present at each locus
• the gene at the E locus is epistatic to the hypostatic gene at the B locus
• the yellow coat is an example of recessive epistasis and ranges from gold to blond

Question 25

factors affecting the rate of evolution

Answer 25

• Genetic drift
• Gene flow is the movement of alleles between populations.
• Natural selection
• Sexual selection leads to an increase in frequency of alleles which code for characteristics that improve mating success.

Question 26

Population genetics.

Answer 26

Investigates how allele frequencies within populations change overtime.

Question 27

Density dependent limiting factors.

Answer 27

Dependent on population density
- competition
- predation
- parasitism
- communicable diseases

Question 28

Density independent limiting factors.

Answer 28

Affects populations of all sizes in the same way
- Climate change
- natural disasters
- Seasonal change
- Human activities.

Question 29

Population bottlenecks

Answer 29

Large reductions in population size, which last for at least one generation.
- The gene pool run genetic diversity is greatly reduced.
- The remaining individuals may not be representative of the original population.

Question 30

Population bottlenecks in cheetahs

Answer 30

• Experienced multiple population bottlenecks
• Have a very low population size and low genetic diversity.
• Problems with fertility because of inbreeding
• Share 99% of alleles

Question 31

Positive aspects of genetic bottlenecks.

Answer 31

• A beneficial mutation will have a much greater impact and lead to the quicker development of a new species.
• It is thought to have played a role in the evolution of early humans.

Question 32

The founder effect.

Answer 32

Small populations can arise due to the establishment of new colonies by a few isolated individuals. An extreme example of genetic drift. The small population have a much smaller gene pool than the original population and display less genetic variation. Frequencies of alleles that were rare in the original population can be a lot higher in the new population.

Question 33

The founder effects in the Amish people in America.

Answer 33

• Amish people in America descended from 200 Germans who settled in Pennsylvania in the 1700s. - They rarely marry and have children outside their religion.
• They have unusually high frequencies of alleles that cause Ellis Van Creveld syndrome. (shortness, polydactyly, hole between atria, abnormalities of nails and teeth)
• This is usually a very rare genetic disease.
• It was caused by one couple who settled in Pennsylvania in 1744.

Question 34

The founder effects in the Afrikaner population in South Africa.

Answer 34

• Mainly descended from a few Dutch settlers.
• It is thought that one of the satellites carried the allele that causes Huntington’s disease.
• Now the population has an unusually high frequency of that allele.

Question 35

The Hardy Weinberg principle equilibrium

Answer 35

A population’s allele and genotype frequencies are constant unless there is some type of evolutionary force acting upon them.
Assumes that:
- there is no selection, mutation, or migration taking place.
- there is a large population and meeting is random.

Question 36

The Hardy Weinberg principle. (formula)

Answer 36

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

Question 37

Stabilising selection.

Answer 37

• Occurs when there are two selection pressures working against two extremes of a trait.
• the intermediate or middle trait is selected for
• e.g. Robins typically lay for eggs. Larger clutches may result in malnourished chicks, while smaller clutches may result in no viable offspring.

Question 38

Directional selection.

Answer 38

• There is a change in environment so that the normal most common phenotype is no longer the most advantageous.
• More extreme phenotypes are positively selected for
• e.g. peppered moths during the Industrial Revolution.

Question 39

Diversifying selection.

Answer 39

• The extremes are selected for and the norm is selected against.
• Opposite to stablising selection.
• E.g. feather colour in male lazuli buntings, lots of competition between male birds to establish territories for mating meant the brown buds were seen as non threatening whereas bright blue birds were seen as intimidating. Both the Brown and blue birds are therefore left alone, but birds of intermediate colour were attacked by adult birds and so failed to mate or establish territories.

Question 40

Speciation.

Answer 40

The formation of different species through the process of evolution occurs as a result of isolation.

Question 41

Allopatric speciation.

Answer 41

• The most common form of speciation.
• When members of a population are separated as a result of a physical barrier
• Environments of different groups will often be different resulting in evolution
• e.g. Galapagos finches. Unique beaks were adapted to the type of food available on the different islands.(geographical isolation)

Question 42

Sympatric speciation.

Answer 42

• No physical barrier between individuals of the same population.
• Can occur in plants when interbreeding forms fertile offspring.
• Can be reproductive, temporal, behavioural isolation or hybridization.
• E.g. fungus farming ants. Parasitic ants eat the funky and reproduce. They are sometimes killed or ignored by worker ants. Genetic analysis has shown they have evolved from the worker ants as a result of a change in behaviour.

Question 43

Events leading up to speciation.

Answer 43

• Members of the population become isolated and no longer interbreed with the rest of the population, resulting in no gene slow between the two groups.
• 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.
• The accumulation of mutations and changes in allele frequencies over many generations eventually lead to large changes in phenotype.
• The members of the different populations become so different that they are no longer able to enter feed to produce fertile offspring. They are now reproductively isolated and are different species.

Question 44

Reproductive barriers.

Answer 44

• Barriers to successful interbreeding within populations before or after fertilisation has occured.
• prezygotic reproductive barriers prevent fertilisation and the formation of a zygote.
• postzygotic reproductive barriers reduced the viability or reproductive potential of offspring.

Question 45

Adaptive radiation.

Answer 45

Where rapid organism diversification takes place.

Question 46

Artificial selection/ selective breeding

Answer 46

-The selection for breeding of plants and animals with desirable characteristics by farmers or breeders.
- The same as natural selection, except for the nature of the selection applied.

Question 47

Gene banks and seed banks.

Answer 47

• Gene banks store frozen biological samples, such as sperm or eggs.
• Seed banks keep samples of seeds from both wild type or domesticated varieties.
• Both are an important genetic resource used to increase genetic biodiversity in inbred populations. This is called outbreeding.

Question 48

Advantages of selective breeding.

Answer 48

• Produces individuals with desired characteristics.
• Can increase agricultural yields
• time and cost efficient.
• Fewer ethical issues than genetic engineering.
• Produces individuals to suit a certain environment.

Question 49

Disadvantages and ethical issues of selective breeding.

Answer 49

• rare diseases and health issues can be unknowingly selected for, for example, deafness in Dalmatians. This can happen as a result of autosomal linkage.
• Reduced genetic diversity from inbreeding means that the populations are more susceptible to change.
• selecting for exaggerated characteristics can produce health issues in animals, such as hip dysplasia in dogs.
• Access to genes and breeding pairs are expensive