Inheritance & Selection

Question 1

Define: phenotype

Answer 1

The physical characteristic of an organism which is due to its genotype and its environment

Question 2

Define: genotype

Answer 2

The alleles present within an organisms DNA

Question 3

What is sex-linked inheritance

Answer 3

Where the alleles are inherited on the sex-chromosomes

Question 4

Why are sex-linked alleles usually inherited on the X chromosome?

Answer 4

The X chromosome is longer so contains more genes

Question 5

What sex chromosomes to males and females have?

Answer 5

Males: XY

Females: XX

Question 6

What is codominance?

Answer 6

Where both alleles are expressed in the phenotype

Question 7

What is multiple alleles?

Answer 7

Where a gene has more than two possible alleles

Question 8

What does the Hardy-Weinberg Principle predict?

Answer 8

Allelic frequencies will remain the same between generations if:

1. No mutations arise
2. The population is isolated
3. No selection
4. The population is large
5. Mating is random

Question 9

What are the Hardy-Weinberg equations?

Answer 9

p + q = 1

p² + 2pq + q² = 1

Question 10

What do the different terms in the Hardy-Weinberg equations represent?

Answer 10

• p = frequency of dominant allele
• q = frequency of recessive allele
• p² = frequency of homozygous dominant
• 2pq = frequency of heterozygous dominant (carriers)
• q² = frequency of homozygous recessive

Question 11

How does natural selection occur?

Answer 11

• The ecosystem cannot support all of the offspring
• Organisms compete with each other but there is variation within a population
• Organisms with advantageous alleles are more likely to survive
• This means they are more likely to reproduce, passing on their alleles
• Over time, the frequency of advantageous alleles increases

Question 12

What are the two types of selection?

Answer 12

1. Directional
2. Stabilising

Question 13

What is directional selection?

Answer 13

Selection favouring individuals one direction from the mean:

• Changes the characteristic of a population
• Occurs when environmental conditions change
  
  • This changes the “optimal” phenotype

Question 14

What is stabilising selection?

Answer 14

Selection favouring individuals closest to the mean:

• Preserves a characteristic in a population
• Occurs when environmental conditions remain stable

Question 15

What is speciation?

Answer 15

The evolution of a new species from an existing species

Question 16

How does speciation occur?

Answer 16

1. Two populations are geographically isolated (no interbreeding occurs)
2. Genetic variation of the populations (or mutation)
3. The populations have different selection pressures (habitats/niches/food etc)
4. Organisms with advantageous alleles survive and reproduce, passing on their alleles
5. The frequency of the advantageous alleles increase
6. The gene pools of the two populations diverge and they become different species