3 - variation and sexual reproduction

Question 1

Identify the three costs of sexual reproduction

Answer 1

Males unable to produce offspring.
Only half of each parents genome passed onto offspring.
Disrupting successful parental genomes.

Question 2

Why do the benefits outweigh the costs of sexual reproduction?

Answer 2

Benefits outweigh costs due to an increase in genetic variation in the population.

Question 3

Describe the importance of genetic variation

Answer 3

Genetic variation provides the raw material required for adaptation, giving sexually reproducing organisms a better chance of survival under changing selection pressures.

Question 4

Use the red queen hypothesis to explain the persistence of sexual reproduction

Answer 4

Sexual reproduction persists because it enables host species to evolve new genetic defences against parasites that attempt to live off them.

Question 5

Why can asexual reproduction be a successful reproductive strategy?

Answer 5

Asexual reproduction can be a successful reproductive strategy as whole genomes are passed on from parent to offspring.

Question 6

state

Answer 6

Maintaining the genome of the parent is an advantage particularly in very narrow, stable niches or when re-colonising disturbed habitats.

Question 7

Give two examples of asexual reproduction in eukaryotes

Answer 7

Vegetative cloning in plants and parthenogenesis in lower plants and animals that lack fertilisation are examples of asexual reproduction in eukaryotes.

Question 8

What type of reproduction produces more often and in larger numbers?

Answer 8

Offspring can be reproduced more often and in larger number with asexual reproduction.

Question 9

Describe the conditions where parthenogenesis is more common

Answer 9

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

Question 10

state

Answer 10

Asexual reproducing populations are 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 11

state

Answer 11

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 yeast.

Question 12

Explain the term homologous chromosomes

Answer 12

Homologous chromosomes are chromosomes of the same size, same centromere position and with the same sequence of genes at the same loci.

Question 13

state

Answer 13

Meiosis is the division of the nucleus that results in the formation of haploid gametes from diploid gametocytes.

Question 14

Describe the events of meiosis one

Answer 14

• The chromosomes, which have replicated prior to meiosis one, each consists of two genetically identical chromatids attached at the centromere.

-The chromosomes condense up and the homologous chromosomes pair up.

-Chiasmata form at points of contact between the non-sister chromatids of a homologous 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 them 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 15

Describe the events of meiosis two

Answer 15

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

Question 16

state

Answer 16

The sex of birds, mammals and some insects is determined by the presence of sex chromosomes.

Question 17

Describe the role of the SRY gene and the Y chromosome

Answer 17

In most mammals the SRY gene on the Y chromosome determines development of male characteristics.

Question 18

state

Answer 18

Heterogametic (XY) males lack most of the corresponding homologous alleles on the shorter (Y) chromosome and this can result in sex-linked patterns of inheritance as seen with carrier females and affected males.

Question 19

Describe the process of X chromosome inactivation

Answer 19

X chromosome inactivation is the process by which most of one X chromosome is inactivated.

Question 20

state

Answer 20

Carriers are less likely to be affected by any deleterious mutations on these X chromosomes because the X chromosome inactivated in each cell is random; half of the cells in any tissue will have a working copy of the gene in question.

Question 21

Define the term hermaphrodite

Answer 21

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

Question 22

Describe the advantage of hermaphroditism

Answer 22

The benefit to the individual organism is that 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 23

state

Answer 23

For some species environmental rather than genetic factors determine sex and sex ratio.

Question 24

state

Answer 24

Sex can change within individuals of some species as a result of size, competition, parasitic infection or temperature.