Chapter 20: Patterns of Variation and Inheritance

Question 1

What is chlorosis and what are the causes?

Answer 1

• Leaves look pale yellow due to cells not producing enough chlorophyll.

Causes:

• Lack of light.
• Mineral deficiencies.
• Viral infections.

Question 2

Define genotype.

Answer 2

• Alleles that code for a characteristic.

Question 3

Define phenotype.

Answer 3

• The physical characteristics.

Question 4

State difference between homozygous + heterozygous genotype.

Answer 4

• Homozygous = two identical alleles for a characteristic.

- Heterozygous = two different alleles for a characteristic where the dominant allele will be expressed.

Question 5

Define monogenic inheritance.

Answer 5

• Characteristic inherited on a single gene.

Question 6

Describe codominance.

Answer 6

• Two different alleles occur for a gene –> both equally dominant.
• Both expressed on phenotype of organism if present.

Question 7

Name an example of a sex linked disease and state the genotypes for males + females.

Answer 7

• Haemophilia.
• X^h Y= infected male
• X^H Y = normal male
• X^H X^H = normal female
• X^H X^h = carrier female.
• X^h X^h = infected female.

Question 8

Define dihybrid inheritance.

Answer 8

• Inheritance of two characteristics controlled by different genes.

Question 9

Name an example of dihybrid inheritance + state genotypes.

Answer 9

• Peas.
• Y = allele coding for yellow seeds.
• y = allele coding for green seeds.
• R = allele coding for round seeds.
• r = allele coding for wrinkled seeds.
• F2 generation –> ratio of 9:3:3:1 for yellow rounded:yellow wrinkled:green rounded: green wrinkled.

Question 10

Why is actual ratio of offspring in F2 gen for peas different to expected 9:3:3:1 ratio.

Answer 10

• Random fertilisation of gametes.
• Genes being studied are on same chromosomes –> linked genes –> inherited together if no crossing over occurs.
• New allele combinations created.
• Sample size too low.

Question 11

Define linkage.

Answer 11

• Genes that code for different characteristics are present at different gene loci on same chromosome.

Question 12

Define recombinant offspring.

Answer 12

• Different combination of alleles than each parent.

Question 13

What does the chi-squared test measure?

Answer 13

• The difference between the expected results and the observed results.
• Tests the null hypothesis.
• Small chi-squared < critical value at 5% –> little difference between observed + expected –> differences due to chance.
• Large chi-squared > critical value at 5% –> reject H0 –> significant difference between observed + expected.

Question 14

Define epistasis.

Answer 14

• Interaction of genes at different loci.

- Multi-step interaction.

Question 15

What is a hypostatic gene?

Answer 15

• Gene that is affected by another gene.

Question 16

What is an epistatic gene?

Answer 16

• Gene that affects the expression of another gene.
• Masks expression –> lack of substrate available to bind to active site of next enzyme in pathway –> less enzyme produced.

Question 17

What are the conditions required for the Hardy-Weinberg Principle to be valid.

Answer 17

• No mutations –> constant allele frequency.
• Isolated pop.
• Random mating within pop.
• Large pop.
• Characteristic being studied is not sex linked.
• No selection pressures of any type.

Question 18

What are the factors affecting evolution.

Answer 18

• Mutation
• Sexual selection = increase in allele frequency of alleles that increase reproductive success.
• Gene flow.
• Genetic drift in small pops.
• Natural selection.

Question 19

Outline stabilising selection.

Answer 19

• Increase in frequency of normal (positive) alleles –> selected for.
• Decrease in frequency of extreme (negative) alleles –> selected against.

Question 20

Outline directional (natural) selection.

Answer 20

• Change in environment –> normal allele no longer most advantageous.
• Extreme allele becomes more advantageous + is selected for.
• Leads to evolution.

Question 21

Outline disruptive selection.

Answer 21

• Extremes selected for and norms selected against.

Question 22

Outline the process of allopatric speciation.

Answer 22

1. Geographical isolation to separate gene pools –> no interbreeding between pops.
2. Variation due to mutation.
3. Natural selection –> different selection pressures –> those with most advantageous alleles survive + reproduce passing advantageous alleles to offspring.
4. Increase in allele frequency of advantageous allele + decrease in disadvantageous allele.
5. Repeats over many generations.

Question 23

Outline sympatric speciation.

Answer 23

• Geographical isolation not needed for reproductive isolation.
• Random mating leads to reproductive isolation.

Post-zygotic barrier:

• After fertilisation –> reduces reproductive viability of offspring –> hybridisation –> infertile offspring.

Prezygotic barrier:

• Prevent fertilisation + formation of zygote.
• E.g. geographical isolation, anatomical, seasonal and behavioural changes.
• Rare because organisms are exposed to same selection pressures.

Question 24

Outline steps of artificial selection/selective breeding.

Answer 24

1. Select organisms with desired traits.
2. Breed them.
3. Monitor offspring + select those who exhibit the desired characteristic.
4. Interbreed to offspring.
5. Repeat for many generations.

Question 25

Issues with selective breeding.

Answer 25

• Reduces gene pool.
• Inbreeding.
• Reduced genetic diversity.
• Loss of potentially useful alleles.
• Increased susceptibility to disease.

Question 26

Issues with inbreeding.

Answer 26

• Many genetic disorders are caused by recessive alleles.
• Decreased genetic diversity + gene pool.
• Large GD –> recessive alleles masked –> heterozygous individuals.
• More closely related individuals same homozygous alleles.
• More likely to get genetic disorders + increased susceptibility to disease.