Variation and Sexual reproduction

Question 1

what are the costs of sexual reproduction

Answer 1

males are unable to produce offspring
only half of each parent’s genome passed onto offspring
disrupting successful parental genomes

Question 2

what are the benefits of sexual selection

Answer 2

increases in genetic variation int he population, providing the raw material for adaptation, giving sexually reproducing organisms a better chance of surviving under changing selection pressures

Question 3

use the red queen hypothesis to explain the persistence of sexual selection

Answer 3

• co-evolutionary interactions between parasites and hosts may select for sexually reproducing hosts
• hosts better able to resist and tolerate parasitism have greater fitness
• parasites able to feed, reproduce, and find new hosts have a greater fitness
• if hosts sexually reproduce, the genetic variability in their offspring reduces the chances that all will be susceptible to infection by parasites

Question 4

benefits of asexual reproduction

Answer 4

• whole genomes are passed on from parent to offspring
• maintaining the genome of the parent is an advantage particularly in very narrow, stable niches or when re-colonising disturbed habits
• offspring can be reproduced more often and in larger numbers

Question 5

give examples of asexual reproduction in eukaryotes

Answer 5

vegetative cloning in plants
parthenogenesis in lower plants and animals that lack fertilisation

Question 6

what is parthenogenesis and when is it more common

Answer 6

parthenogenesis is reproduction from a female gamete without fertilisation

it is more common in cooler climates, which are disadvantageous to parasites, or regions of low parasite density

Question 7

disadvantage of asexual reproduction

Answer 7

• not able to adapt easily to changes in their environment, but mutations can occur that provide some degree of variation and enable some natural selection and evolution to occur

Question 8

describe how asexual reproduction relates to horizontal gene transfer

Answer 8

organisms that reproduce principally by asexual reproduction also often have mechanisms for horizontal gene transfer between individuals to increase variation, for example the plasmids of bacteria and yeasts

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

Question 9

what are homologous chromosomes

Answer 9

chromosomes of the same size
same centromere position
same sequence of genes at the same loci

Question 10

what is meiosis

Answer 10

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

Question 11

how do chromosomes typically appear in diploid cells

Answer 11

as homologous pairs

Question 12

describe the process of meiosis I

Answer 12

• the chromosomes, which have replicated prior to meiosis I, each consist of two genetically identical chromatids attached at the centromere
• the chromosomes condense and the homologous chromosomes pair up
• chiasmata form at points of contact between the non-sister chromatids of a homologs pair and sections of DNA are exchanged
• this crossing over of DNA is random and produces genetically different recombinant chromosomes
• spindle fibres attach to the homologous pairs and line up at the equator of the spindle
• the orientation of the pairs of homologous chromosomes at the equator is random
• the chromosomes of each homologous pair are separated and move towards opposite poles
• cytokinesis occurs and two daughter cells form

Question 13

describe how variation occurs in meiosis I

Answer 13

• chiasmata form at points of contact between the non-sister chromatids of a homologs pair and sections of DNA are exchanged
• linked genes are those on the same chromosome, crossing over can result in new combinations of the alleles of those genes
• the orientation of the pairs of homologous chromosomes at the equator is random
• each pair of homologs chromosomes is positioned independently of the other pairs, irrespective of their maternal and paternal origin. this is known as independent assortment

Question 14

describe meiosis II

Answer 14

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 15

how is the sex of birds, mammals, and some insects determined

Answer 15

by the presence of sex chromosomes

Question 16

what determines the development of male characteristics in most mammals

Answer 16

the SRY gene on the Y chromosome