KA2.3 - Variation and Sexual Reproduction

Question 1

State the two costs of sexual reproduction

Answer 1

1. Males unable to produce offspring (directly)
2. Disrupts sucessful genomes

Question 2

Explain why the two main costs of sexual reproduction?

Answer 2

1. Males unable to produce offspring. In any population where sexual reproduction is the reproductive strategy only half of the population are able to produce offspring
2. Sexual reproduction involves the mixing of genetic information between two individuals with each parent disrupting a successful genome and only passing half onto each offspring.

Question 3

Why do the benefits of sexual reproduction outweigh the costs?

Answer 3

due to the increase in genetic variation in the population

Question 4

What does genetic variation provide?

Answer 4

the raw material required for adaptation

Question 5

How does genetic variation benefit sexual reproducing organisms?

Answer 5

Increases their chance of survival under changing selection pressures

Question 6

Explain how the red queen hypothesis helps explain the persistence of sexual reproduction?

Answer 6

If hosts reproduce sexually, the genetic variability in their offspring reduces the chances that all will be susceptible to infection by parasites

Question 7

Why can asexual reproduction be a successful reproductive strategy?

Answer 7

• Whole genomes are passed on from parent to offspring.
• one parent can produce daughter cells and establish a colony of virtually unlimited size over time

Question 8

When is maintaining the parent’s genome in asexual reproduction an advantage?

Answer 8

very narrow, stable niches or when re-colonising disturbed habitats

Question 9

Describe examples of asexual reproduction in eukaryotes

Answer 9

• Vegetative cloning in (some) plants
• Parthenogenesis in (some) animals

Question 10

Define ‘partenogenesis’

Answer 10

reproduction from a female gamete without fertilisation

Question 11

Where is parthenogensis more common?

Answer 11

• Cooler climates which are disadvantageous to parasites
• Regions of low parasite density

Question 12

Describe asexual reproduction in prokaryotes?

Answer 12

Can exchange genetic material horizontally, resulting in faster evolutionary change than in organisms that only use vertical transfer

Question 13

Explain the advantage of horizontal gene transfer in prokaryotes?

Answer 13

• Faster evolutionary change than vertical gene transfer
• Increases variation (more than would be possible otherwise)

Question 14

How do asexually reproducing populations adapt to their environments?

Answer 14

Random mutations that provide variation and allow natural selection and evolution to occur

Question 15

Define ‘gametocyte’

Answer 15

The precursor cells to gametes are diploid and divide by meiosis to generate haploid gametes.

Remember gametes are sex cells and are haploid

Question 16

What are the key features that makes two chromosomes homologous?

Answer 16

• Same size
• Same centromere position
• Same genes at the same loci (location)

Question 17

Are the genes on homologous chromosomes identical?

Answer 17

No

The genes are the same but they can have different alleles.

e.g. brown hair gene rather than black hair gene

Question 18

Define ‘meiosis’

Answer 18

the division of the nucleus that results in the formation of haploid gametes from a diploid gametocyte.

Question 19

How many rounds of nuclear division are involved in meiosis?

Answer 19

Two - Meiosis I and Meiosis II

Question 20

Why does meiosis occur twice - meiosis I and meiosis II?

Answer 20

Meiosis I = separates homologous chromosomes
Meiosis II = separates sister chromatids

Question 21

Define ‘centromere’

Answer 21

site which links pair of sister chromatids together

Question 22

Describe Step 1 (interphase) of meiosis?

Answer 22

the DNA is replicated, so each homologous chromosome replicates to become two genetically identical chromatids attached at the centromere

Question 23

Describe Step 2 (prophase) of meiosis?

Answer 23

The chromosomes condense and the homologous chromosomes pair up.

Question 24

Describe Step 3 (metaphase) of meiosis?

Answer 24

Spindle fibres attach to the homologous pairs and line them up at the equator of the spindle.

Question 25

Describe Step 4 (**Anaphase**) of meiosis?

Answer 25

The chromosomes of each homologous pair are separated and move towards opposite poles.

Question 26

Describe Step 5 (**Telophase**) and Cytokinesis of meiosis?

Answer 26

Cytokinesis occurs and two daughter cells form. | Cytokinesis is the splitting of the cytoplasm and cell membrane

Question 27

Describe what happens during Meiosis II

Answer 27

Each of the **two** cells produced in meiosis I undergoes a further division during which the sister chromatids of each chromosome are separated. A total of **four** haploid cells are produced.

Question 28

State the **two** main events in meiosis which increase variation?

Answer 28

1. Random crossing over 2. Independent assortment

Question 29

Define 'chiasmata'

Answer 29

Points where two non-sister chromatids of homologous chromosomes physically cross | Non-sister = non-identical

Question 30

Define 'random crossing over' in the context of meiosis?

Answer 30

Chiasmata form at points of contact between the non-sister chromatids of a homologous pair and sections of DNA are exchanged

Question 31

Explain why random crossing over in meiosis increases variation?

Answer 31

It results in new combinations of alleles. This crossing over of DNA is random and produces genetically different recombinant chromosomes.

Question 32

What stage of Meiosis does random crossing over occur?

Answer 32

Prophase / Stage 2

Question 33

Define 'linked genes'

Answer 33

genes on the same chromosome

Question 34

Define 'recombinant'

Answer 34

the rearrangement of genetic material

Question 35

Define 'independent assortment' in the context of meiosis

Answer 35

1. During meiosis I **homologous** chromosomes pair up along the equator of the cells in a random and independent manner. 2. Each pair of homologous chromosomes is positioned independently of the other pairs, irrespective of their maternal and paternal origin. | This means the cell each chromsome ends up in is random

Question 36

What determines the sex of birds, mammals and some insects?

Answer 36

Presence of sex chromosomes

Question 37

What is the SRY gene?

Answer 37

A gene on the Y chromosome determines the development of male characteristics in most mammals

Question 38

Define 'homogametic'

Answer 38

produce all gametes with the same combination of chromosomes, e.g. females

Question 39

Define 'heterogametic'

Answer 39

produce gametes with two possible combinations of chromosomes, e.g. males

Question 40

What causes sex-linked patterns of inheritance?

Answer 40

Heterogametic (XY) males lack most of the corresponding homologous alleles on the shorter (Y) chromosome

Question 41

What is the genotype of a carrier female of a sex-linked recessive condition?

Answer 41

X_BX_b

Question 42

What is the genotype of a affected male of a sex-linked recessive condition?

Answer 42

X_bY_b

Question 43

Explain how X-linked recessive inheritance affects males and females

Answer 43

* Affected Males: Need only one copy of the recessive allele (X_bY) * Affected Females: Need two copies of the recessive allele (X_bX_b) * Female carriers: Have one copy of the recessive allele, so are largely unaffected (X_BX_b)

Question 44

Define 'X-chromosome Inactivation' in females?

Answer 44

When one of the two X chromosomes present in each cell is **randomly** inactivated at an early stage of development

Question 45

Why does X-chromosome inactivation happen in females?

Answer 45

Prevents a double dose of gene products being produced, which could be harmful to the cell

Question 46

Explain how X-chromsome inactivation affects female carriers of sex-linked conditions?

Answer 46

The X chromosome inactivated in each cell is random, half of the cells in any tissue will have a working copy of each gene. | X_BX_b still has a working allele

Question 47

Define 'hermaphrodite'

Answer 47

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

Question 48

Explain the benefit of hermaphroditism?

Answer 48

If the chance encountering of a partner is uncommon, then there is no requirement for the partner to be of the opposite sex | Increasing chance of reproduction

Question 49

What are the **two** main ways species determine sex?

Answer 49

1. Genetic factors 2. Environmental factors

Question 50

Define 'temperature-dependent sex determination' (TSD)

Answer 50

where the temperature experienced by the embryo/egg during the temperature-sensitive period of incubation determines its sex

Question 51

Which species if TSD most common in? | Temperature-dependent sex determination (TSD)

Answer 51

Reptiles and some species of birds and fish

Question 52

What factors can cause sex change in some species?

Answer 52

* Size * Competition * Parasitic infection

Question 53

What factor affects the sex ratio of some species?

Answer 53

Resource availability

Question 54

Define 'sequential hermaphrodite'

Answer 54

an organism changes its sex at some point in its life