Unit 2: KA3 - Variation and Sexual Reproduction

Question 1

Describe the three costs of sexual reproduction.

Answer 1

• Males cannot produce offspring
• Only half each parent’s genome are passed on
• Successful parental genomes are disrupted

Question 2

Name the benefit of sexual reproduction.

Answer 2

Greater genetic variation within sexually reproducing organisms.

Question 3

Describe why genetic variation is important.

Answer 3

It provides the raw materials for adaptation, giving sexually reproducing organisms a better chance of survival under changing selection pressures.

Question 4

How does the ‘Red Queen Hypothesis’ support the persistence of sexual reproduction?

Answer 4

It states the importance of constant adaptations to survive.

Question 5

What does co-evolutionary interactions between parasites and hosts select for?

Answer 5

Sexually reproducing hosts

Question 6

State the benefits to hosts and parasites of hosts reproducing sexually.

Answer 6

Hosts: The variation allows them to adapt to be less susceptible to the parasite

Parasite: This forces them to adapt themselves.

Question 7

Describe asexual reproduction

Answer 7

Just one parent produces daughter cells, passing on whole genomes.

Question 8

When is asexual reproduction an advantage?

Answer 8

• In very narrow, stable niches

- When recolonising disturbed habitats (for speed)

Question 9

What are 2 examples of asexual reproduction in eukaryotes?

Answer 9

• Vegetative cloning in plants (bulbs)

- Parthenogenesis in lower plants and infertile animals.

Question 10

When is parthenogenesis is more common?

Answer 10

In areas of low parasite density e.g. cooler climates.

Question 11

What is the disadvantage of asexual reproduction?

Answer 11

Organisms who reproduce asexually, are less able to adapt.

Question 12

What are sexually reproducing organisms’ source of variation?

Answer 12

Mutations

Question 13

How can asexually reproducing organisms increase their population’s variiation?

Answer 13

Horizontal gene transfer

Question 14

What organelle do bacteria and yeast use for horizontal gene transfer?

Answer 14

Plasmids

Question 15

Describe meiosis

Answer 15

the process of cell division that results in the formation of new haploid cells from a diploid gametocyte.

Question 16

How do chromosomes appear in diploid cells?

Answer 16

as homologous pairs

Question 17

What 3 similarities do homologous pairs share?

Answer 17

• the same size
• the same centromere position
• genes for the same characteristics at the same ‘loci’

Question 18

describe 2 sources of variation within meiosis?

Answer 18

• Chiasmata: DNA is exchanged between crossed over chromosomes, producing genetically different recombinant chromosomes.
• Independent variation: Orientation of homologous pairs of chromosomes at the equator is irrespective of maternal or parental origin.

Question 19

Define the term: linked gene.

Answer 19

Genes that are found on the same chromosome

Question 20

What happens before meiosis?

Answer 20

interphase: duplication of DNA within the nucleus, chromatids attach at the centromere.

Question 21

Describe the process of meiosis I

Answer 21

• Homologous chromosomes pair up
• Chiasmata form at points of contact between non-sister chromatids
• Spindle fibres attach and independent assortment occurs
• Chromosomes of each homologous pair are separated and move towards the poles
• cytokinesis occurs and two daughter cells form

Question 22

Describe the chromosome count at each point of meiosis.

Answer 22

After interphase: 46 chromosomes; 92 chromatids; 1 cell
After meiosis I: 23 chromosomes; 46 chromatids; 2 cells
After meiosis II: 23 chromosomes; 23 chromatids; 4 cells

Question 23

Describe meiosis II

Answer 23

• Each of the two cells produced in meiosis I undergo a further division.
• The sister chromatids are separated
• four haploid cells are produced.

Question 24

How is sex determined in birds, mammals and some insects?

Answer 24

The presence of sex chromosomes

Question 25

What gene determines the development of male characteristics, and where is it found?

Answer 25

the SRY gene on the Y chromosome

Question 26

Why are males said to be heterogametic?

Answer 26

Their sex chromosomes are dissimilar

Question 27

What do males lack on the Y-chromosome?

Answer 27

Most corresponding homologous alleles

Question 28

Why are males more susceptible to certain recessive conditions?

Answer 28

Where females need two copies of the recessive allele (XbXb), males only need one (XbY)

Question 29

What happens in X-chromosome inactivation?

Answer 29

In homogametic females, one of the X chromosomes in every cell is randomly inactivated in early development

Question 30

What is the benefit of X-chromosome inactivation?

Answer 30

Prevents a double dose of gene products, which could be harmful.

Question 31

Describe homogametic carriers’ chances of being affected by deleterious mutations on X chromosomes.

Answer 31

Carriers are less likely to be affected, as X-inactivation is random, so half of the cells in any tissue will have a working copy of the gene.

Question 32

Define the term: hermaphrodite

Answer 32

a species that have functioning male and female reproductive organs in each individual.

Question 33

How do hermaphrodites interact with each other?

Answer 33

They produce both male and female gametes, and usually have a partner with which to exchange gametes.

Question 34

What is the benefit of hermaphrodites?

Answer 34

If the chance of encountering a partner is an uncommon event, there is no requirement for that partner to be of the opposite sex.

Question 35

Other than genetics, what can determine sex and sex ratio?

Answer 35

environmental factors

Question 36

What can happen to the sex ratio of offspring in some species?

Answer 36

Sex ratio can be adjusted in response to resource availability.

Question 37

What 4 factors may cause sex in individuals to change?

Answer 37

• Size
• Parasitic infection
• Competition
• Temperature