Genetic Diversity

Question 1

gene mutation

Answer 1

change in the base sequence of chromosomes

Question 2

when do gene mutations arise?

Answer 2

spontaneously during DNA replication

Question 3

what increases the rate of gene mutation?

Answer 3

mutagenic agents

Question 4

2 types of gene mutation

Answer 4

substitution, deletion

Question 5

why may a base substitution mutation not affect the sequence of encoded amino acids?

Answer 5

as the genetic code is degenerate, so some amino acids are codes for by multiple DNA base triplets. The altered triplet can therefore code for the same amino acid as the original, and the encoded amino acid sequence remains unchanged.

Question 6

base deletion mutation effects

Answer 6

Decrease in number of bases present, causes a frameshift to the left. All the following triplets are read differently and code for different amino acids.

Question 7

how do mutations in the number of chromosomes arise?

Answer 7

spontaneously by chromosome non-disjunction during meiosis. This is when chromosomes fail to separate properly.

Question 8

Process of meiosis

Answer 8

• before meiosis, DNA replicates and condenses.
  meiosis 1 - chromosomes line up n their homologous pairs and one from each homologous pair goes into each daughter cell.
  meiosis 2 - pairs of sister chromatids that form each chromosome are separated as the centromere divides and drawn into different daughter cells.

Question 9

How does independent segregation of chromosomes during meiosis lead to genetic variation in offspring?

Answer 9

Homologous pairs of chromosomes line up randomly in M1 and are then separated. Which chromosome - maternal or paternal - enters each daughter cell here is determined by chance - so daughter cells are genetically unique with different combinations of alleles.

Question 10

How does crossing over of chromosomes in meiosis lead to genetic variation in offspring?

Answer 10

• M1: homologous chromosomes pair up
• chromatids twist around each other and equivalent portions break off and are exchanged.
• chromatids contain different combinations of alleles.
• 4 daughter cells and hence potential offspring have different combinations of alleles.

Question 11

What happens during crossing over of chromosomes - meiosis?

Answer 11

Homologous chromosomes line up in their pairs and chromatids twist around each other. equivalent portions are exchanged.

Question 12

Expression to calculate the number of possible different chromosome combinations following meiosis (without crossing over)

Answer 12

2^n

Question 13

what does meiosis produce?

Answer 13

4 genetically different haploid gametes.

Question 14

How does random fertilisation of gametes lead to increased genetic variation?

Answer 14

2 genetically unique gametes fuse and form a zygote from which the new organism develops, which has a unique combination of alleles from both parents.

Question 15

what is a diploid cell?

Answer 15

a cell that has 2 complete sets of chromosomes - 1 from each parent

Question 16

What is a haploid cell?

Answer 16

a cell that only contains 1 complete set of chromosomes, produced by meiosis

Question 17

when does meiosis occur in a lifecycle?

Answer 17

when the chromosome number halves (to n), production of gametes

Question 18

genetic diversity

Answer 18

the number of different alleles of genes in a population

Question 19

what does genetic diversity enable to occur?

Answer 19

natural selection

Question 20

What are the principles of natural selection?

Answer 20

1) random mutations occur, resulting in new alleles of genes
2) many mutations are harmful, but in some environments the new allele may be beneficial, meaning its possessor is more likely to survive and reproduce.
3) the advantageous allele is inherited by members of the next generation.
4) over many generations, allele frequency of the advantageous allele increases in the population.

Question 21

3 types of selection

Answer 21

directional, stabilising, disruptive

Question 22

directional selection

Answer 22

individuals with alleles for a phenotype at 1 of the extremes are more likely to survive and reproduce. This could be due to environmental change

Question 23

stabilising selection

Answer 23

individuals with alleles for phenotypes around the mean are more likely to survive, reproduce and pass their advantageous alleles on to the next generation.
This occurs when the environment isn’t changing
the range of phenotypes decreases

Question 24

example for stabilising selection

Answer 24

human birth weights
small babies are less likely to survive - as they have difficulty in keeping body temperature constant.
large babies are difficult to give birth to so are also less likely to survive.
medium-sized babies are most likely to survive reproduce, passing their advantageous alleles for size on to offspring - so frequency of medium-sized babied increases in the next generation

Question 25

example for disruptive selection

Answer 25

antibiotic resistance in bacteria

• some bacteria have mutations in certain alleles, giving them resistance to an antibiotic..
• exposure to the antibiotic provides a selection pressure, and only bacteria with alleles for resistance are able to survive and reproduce. non-resistant bacteria are killed.
• resistant bacteria pass on allele for resistance to the next generation, so allele frequency increases

Question 26

what does natural selection result in?

Answer 26

species which are better adapted to their environment

Question 27

types of adaptation

Answer 27

anatomical, physiological, behavioural

Question 28

selection - changes in normal distribution curve shape

• stabilising
• directional

Answer 28

stabilising - curve becomes taller and narrower

directional - curve shifts in direction of favoured phenotype

Question 29

Explain the role of independent segregation in meiosis

Answer 29

• provide genetic variation

- allows different combinations of maternal and paternal alleles to form