Inheritance, Selection and Speciation

Question 1

Gene

Answer 1

A sequence of bases on a DNA molecule that codes for a protein which results in a characteristic.

Question 2

Allele

Answer 2

One or more alternate versions of the same gene (slightly different order of base sequences).

Most plants and animals have two alleles (one maternal and one paternal)

Question 3

Genotype

Answer 3

Genetic constitution of an organism, i.e. the alleles an organism has.

Question 4

Phenotype

Answer 4

The expression of the genetic constitution of an organism and it’s interaction with the environment, i.e. an organisms’ characteristics.

Question 5

Dominant

Answer 5

An allele whose characteristic appears in the phenotype even when there’s only one copy.

Question 6

Recessive

Answer 6

An allele whose characteristic only appears in the phenotype if two copies are present.

Question 7

Codominant

Answer 7

Alleles that are both expressed in the phenotype - neither are recessive.

Question 8

Locus

Answer 8

Plural loci; The fixed position of a gene on a chromosome.

Alleles of a gene are found at the same locus on each chromosome in a pair.

Question 9

Homozygote

Answer 9

An organism that carries two copies of the same allele.

Question 10

Heterozygote

Answer 10

An organism that carries two different alleles.

Question 11

When is inheritance sex-linked?

Answer 11

When the corresponding allele is located on a sex chromosome.

Question 12

Why are males more likely than females to show recessive phenotypes for sex-linked genes?

Answer 12

Men only have one X-chromosome, thus often have one allele for sex-linked genes, therefore they will express the characteristic of this allele even if it’s recessive.

Question 13

Examples of X-linked disorders:

Answer 13

Colour blindness, Haemophilia etc…

Question 14

What hierarchy does the human ABO blood group system work off?

Answer 14

• A and B are codominant

- O is recessive to both.

Question 15

State the mixed genotypes for the different blood groups:

Answer 15

IAIB = Group AB (only possible genotype)
IAIO = Group A
IBIO = Group B
IOIO = Group O (only possible genotype)

Question 16

Species

Answer 16

A group of similar organisms that can reproduce to give fertile offspring.

Question 17

Population

Answer 17

A group of organisms of the same species living in a particular area.

Question 18

Gene pool

Answer 18

The complete range of alleles present in a population.

Question 19

Allele frequency

Answer 19

How often an allele occurs in a population.

Usually as a percentage of the total population, or a decimal of thus.

Question 20

What does the Hardy-Weinberg Principle predict?

Answer 20

That the frequencies of alleles in a population won’t change from one generation to the next.

Question 21

Under what conditions is the Hardy-Weinberg principle true?

Answer 21

Only if a large population with:

• no immigration
• no emigration
• no mutations
• no natural selection
• random mating

Question 22

Hardy-Weinberg equation for allele frequency:

Answer 22

p + q = 1

p is the frequency of dominant alleles
q is the frequency of recessive alleles

Question 23

Hardy-Weinberg equation for genotype frequency:

Answer 23

p^2 + 2pq + q^2 = 1

p^2 is the frequency of the homozygous dominant genotype
2pq is the frequency of the heterozygous genotype
q^2 is the frequency of the homozygous recessive genotype

Question 24

Explain differential reproductive success:

Answer 24

• Individuals that have an allele that increases their chance of survival are more likely to survive.
• Therefore a greater proportion of the next generation will inherit the beneficial allele.
• They, in turn, are more likely to survive, breed, and pass on their genes.
• Therefore the frequency of the beneficial allele increases from generation to generation.

Question 25

What is differential reproductive success also known as?

Answer 25

Natural selection

Question 26

What is stabilising selection and when does it occur?

Answer 26

When individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce. This reduces the range of possible phenotypes.

Occurs when the environment isn’t changing.

Question 27

What is direction selection and when does it occur?

Answer 27

When individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce.

This could be in response to an environmental change.

Question 28

What two things cause speciation?

Answer 28

Geographical isolation and natural selection.

Question 29

Speciation

Answer 29

The development of a new species, i.e. when populations of the same species become reproductively isolated.

Question 30

Explain how reproductive isolation tends to occur:

Answer 30

1 - A physical barrier divides the population (flood, earthquake etc)
2 - Each population experiences different conditions, e.g. climates
3 - Therefore each population experiences different selective pressures, resulting in different changes in allele frequencies.
4 - This causes changes in phenotype frequencies.
5 - Eventually, there is so much change between individuals of different populations that they won’t be able to breed to produce fertile offspring.