Unit 2 - Variation and Sexual Reproduction

Question 1

What are the costs of sexual reproduction?

Answer 1

-males 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 reproduction?

Answer 2

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

Question 3

What does genetic variation provide?

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

What might co-evolutionary interactions between parasites select for hosts?

Answer 4

Sexually reproducing hosts

Question 5

What does Parasites choosing sexually reproducing hosts result in?

Answer 5

Hosts better able to resist and tolerate
parasitism have greater fitness. Parasites
better able to feed, reproduce and find new
hosts have greater fitness.

Question 6

What happens if hosts reproduce sexually?

Answer 6

the genetic variability in their offspring reduces the chances that all will be susceptible to infection by parasites

Question 7

What is asexual reproduction?

Answer 7

just one parent can produce daughter cells and establish a colony of virtually unlimited size over time

Question 8

Why is asexual reproduction a successful reproductive strategy?

Answer 8

whole genomes are
passed on from parent to offspring

Question 9

When is maintaining the genome of the parent advantageous?

Answer 9

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

Question 10

What are examples of asexual reproduction in eukaryotes?

Answer 10

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

Question 11

What is parthenogenesis?

Answer 11

Parthenogenesis is reproduction from a
female gamete without fertilisation.

Question 12

What does asexual reproduction allow?

Answer 12

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

Question 13

When is parthenogenesis more common?

Answer 13

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

Question 14

What are asexually reproducing populations not able to do?

Answer 14

Asexually reproducing populations are not
able to adapt easily to changes in their
environment

Question 15

What does occur that increases variation in asexual reproducing populations?

Answer 15

mutations can occur that
provide some degree of variation and enable
some natural selection and evolution to occur

Question 16

How do prokaryotes exchange genetic material?

Answer 16

Prokaryotes can exchange genetic material horizontally,

Question 17

What does asexual reproduction have for organisms that principally do it?

Answer 17

have mechanisms for horizontal gene transfer between individuals to increase variation, for example the plasmids of bacteria and yeast

Question 18

What is meiosis?

Answer 18

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

Question 19

What are the qualities of homologous chromosomes?

Answer 19

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

Question 20

What do chromosomes typically appear as in diploid cells?

Answer 20

In diploid cells, chromosomes typically
appear as homologous pairs

Question 21

What happens during Meiosis 1?

Answer 21

• 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 homologous pair and sections of DNA are exchanged
• This crossing over of DNA is random and produces genetically different recombinant chromosomes
• Each pair of homologous chromosomes is positioned independently of the other pairs, irrespective of their maternal and paternal origin. This is known as independent
  assortment.
• 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 22

What are chiasmata?

Answer 22

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

Question 23

What are linked genes and how are they formed?

Answer 23

Linked genes are those on the same
chromosome. Crossing over can result in
new combinations of the alleles of these
genes.

Question 24

What is crossing over and what does it produce?

Answer 24

This crossing over of DNA is random and
produces genetically different recombinant
chromosomes

Question 25

How are homologous chromosomes postitioned?

Answer 25

The orientation of the pairs of homologous
chromosomes at the equator is random

Question 26

What is independent assortment?

Answer 26

Each pair of homologous chromosomes is
positioned independently of the other pairs,
irrespective of their maternal and paternal
origin. This is known as independent
assortment.

Question 27

What does Meiosis 2 produce?

Answer 27

A total of four haploid cells are produced.

Question 28

What happens during meiosis 2?

Answer 28

Each of the two cells produced in meiosis I
undergoes a further division during which the
sister chromatids of each chromosome are
separated

Question 29

How is the sex of birds, mammals and insects determined?

Answer 29

by the presence of sex
chromosomes

Question 30

What do most male mammals contain?

Answer 30

the SRY gene on the Y
chromosome

Question 31

What does the SRY gene determine?

Answer 31

determines development of
male characteristics

Question 32

What does heterogametic mean?

Answer 32

Have two different sex chromsomes

Question 33

What does homogametic mean?

Answer 33

Have two of the same sex chromosomes

Question 34

What do heterogametic males have and what does that result in?

Answer 34

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

Question 35

What do homogametic females have and what happens?

Answer 35

In homogametic females (XX) one of the two
X chromosomes present in each cell is
randomly inactivated at an early stage of
development

Question 36

What is X chromosome inactivation?

Answer 36

X chromosome inactivation is a process by
which most of one X chromosome is
inactivated

Question 37

Why does X chromosome inactivation occur?

Answer 37

X chromosome inactivation prevents a
double dose of gene products, which could
be harmful to cells

Question 38

What benefit does x chromosome inactivation have?

Answer 38

Carriers are less likely to be affected by any
deleterious mutations on these X
chromosomes

Question 39

What happens when X chromosome inactivation occurs?

Answer 39

As 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 40

What are Hermaphrodites?

Answer 40

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

Question 41

What do hermaphrodites have?

Answer 41

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

Question 42

What is the benefit of hermaphrodites?

Answer 42

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 43

What is environmental sex determination in reptiles controlled by?

Answer 43

controlled by environmental temperature of
egg incubation.

Question 44

What are the factors that can affect the sex of a species?

Answer 44

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

Question 45

What is the benefit of horizontal gene transfer?

Answer 45

Resulting in faster evolutionary
change than in organisms that only use
vertical transfer.