Key Area 3 - Variation & Sexual Reproduction Flashcards
Costs & Benefits of Sexual & Asexual
Reproduction
Explain the costs of sexual reproduction
Costs of sexual reproduction: males unable
to produce offspring; only half of each
parent’s genome passed onto offspring,
disrupting successful parental genomes.
Why do benefits outweigh costs?
Benefits outweigh costs due to an increase in genetic variation in the population.
What does genetic variation provide?
Genetic variation provides the raw material
required for adaptation, giving sexually
reproducing organisms a better chance of
survival under changing selection pressures.
What can be used to explain the
persistence of sexual reproduction?
The Red Queen hypothesis to explain the
persistence of sexual reproduction.
What interactions between
parasites and hosts may select for sexually
reproducing hosts?
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
better able to feed, reproduce and find new
hosts have greater fitness.
If hosts reproduce sexually, the genetic
variability in their offspring reduces what
chances?
If hosts reproduce sexually, the genetic
variability in their offspring reduces the
chances that all will be susceptible to
infection by parasites.
Why can asexual reproduction be a successful reproductive strategy?
Asexual reproduction can be a successful
reproductive strategy as whole genomes are passed on from parent to offspring.
Explain asexual reproduction
In asexual reproduction, just one parent can produce daughter cells and establish a
colony of virtually unlimited size over time.
Where is maintaining the genome of the parent an advantage?
Maintaining the genome of the parent is an
advantage particularly in very narrow, stable niches or when re-colonising disturbed habitats.
Name examples of asexual reproduction in eukaryotes
Vegetative cloning in plants and
parthenogenesis in lower plants and animals that lack fertilisation are examples of asexual reproduction in eukaryotes.
Explain parthenogenesis
Parthenogenesis is reproduction from a
female gamete without fertilisation.
Give an advantage of asexual reproduction
Offspring can be reproduced more often and in larger numbers with asexual reproduction.
Where is parthenogenesis more common?
Parthenogenesis is more common in cooler
climates, which are disadvantageous to
parasites, or regions of low parasite density
or diversity.
Explain variation in asexually reproducing populations
Asexually 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.
Organisms that reproduce principally by
asexual reproduction also often have
mechanisms for what type of transfer?
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.
Explain how prokaryotes exchange genetic material
Prokaryotes can exchange genetic material
horizontally, resulting in faster evolutionary
change than in organisms that only use
vertical transfer.
Meiosis
What is meiosis?
Meiosis is the division of the nucleus that
results in the formation of haploid gametes
from a diploid gametocyte.
How do chromosomes appear in diploid cells?
In diploid cells, chromosomes typically
appear as homologous pairs.
What are homologous chromosomes?
Homologous chromosomes are
chromosomes of the same size, same
centromere position and with the same
sequence of genes at the same loci.
Meiosis I
Explain the stages of meiosis I
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.
Linked genes are those on the same
chromosome. Crossing over can result in
new combinations of the alleles of these
genes.
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.
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.
The chromosomes of each homologous pair are separated and move towards opposite poles.
Cytokinesis occurs and two daughter cells
form.
Meiosis II
Explain the stages of meiosis II
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.
Sex Determination
How is the sex of birds, mammals and some insects determined?
The sex of birds, mammals and some insects is determined by the presence of sex chromosomes.
In most mammals what gene determines development of male characteristics?
In most mammals the SRY gene on the Y
chromosome determines development of
male characteristics.
What sex lacks most of the
corresponding homologous alleles on the
shorter (Y) chromosome?
Heterogametic (XY) males lack most of the
corresponding homologous alleles on the
shorter (Y) chromosome.
This can result in sex-linked patterns of
inheritance as seen with carrier females
(XBXb) and affected males (XbY).
What occurs in homogametic females (XX) at an early stage of development?
In homogametic females (XX) one of the two X chromosomes present in each cell is
randomly inactivated at an early stage of
development.
Describe X chromosome inactivation
X chromosome inactivation is a process by
which most of one X chromosome is
inactivated.
What does X chromosome inactivation prevent?
X chromosome inactivation prevents a
double dose of gene products, which could
be harmful to cells.
Carriers are less likely to be affected by any
deleterious mutations on these X
chromosomes.
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.
What are hermaphrodites?
Hermaphrodites are species that have
functioning male and female reproductive
organs in each individual.
What do hermaphrodites produce?
They produce both male and female gametes and usually have a partner with which to exchange gametes.
Explain a benefit for a hermaphrodite
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.
For other species, environmental rather than genetic factors determine sex and sex ratio.
What is environmental sex determination in reptiles controlled by?
Environmental sex determination in reptiles is controlled by environmental temperature of egg incubation.
Sex can change within individuals of some
species as a result of what?
Sex can change within individuals of some
species as a result of size, competition, or
parasitic infection.
The sex ratio of offspring can be adjusted in response to what is some species?
In some species the sex ratio of offspring can be adjusted in response to resource
availability.
