Unit 2 - Variation and Sexual Reproduction Flashcards
What are the costs of sexual reproduction?
-males unable to produce offspring
-only half of each parent’s genome passed onto offspring,disrupting successful parental genomes
What are the benefits of sexual reproduction?
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 might co-evolutionary interactions between parasites select for hosts?
Sexually reproducing hosts
What does Parasites choosing sexually reproducing hosts result in?
Hosts better able to resist and tolerate
parasitism have greater fitness. Parasites
better able to feed, reproduce and find new
hosts have greater fitness.
What happens if hosts reproduce sexually?
the genetic variability in their offspring reduces the chances that all will be susceptible to infection by parasites
What is asexual reproduction?
just one parent can produce daughter cells and establish a colony of virtually unlimited size over time
Why is asexual reproduction a successful reproductive strategy?
whole genomes are
passed on from parent to offspring
When is maintaining the genome of the parent advantageous?
In very narrow, stable
niches or when re-colonising disturbed
habitats
What are examples of asexual reproduction in eukaryotes?
Vegetative cloning in plants and
parthenogenesis in lower plants and animals
that lack fertilisation
What is parthenogenesis?
Parthenogenesis is reproduction from a
female gamete without fertilisation.
What does asexual reproduction allow?
Offspring can be reproduced more often and
in larger numbers with asexual reproduction
When is parthenogenesis more common?
Parthenogenesis is more common in cooler
climates, which are disadvantageous to
parasites, or regions of low parasite density
or diversity
What are asexually reproducing populations not able to do?
Asexually reproducing populations are not
able to adapt easily to changes in their
environment
What does occur that increases variation in asexual reproducing populations?
mutations can occur that
provide some degree of variation and enable
some natural selection and evolution to occur
How do prokaryotes exchange genetic material?
Prokaryotes can exchange genetic material horizontally,
What does asexual reproduction have for organisms that principally do it?
have mechanisms for horizontal gene transfer between individuals to increase variation, for example the plasmids of bacteria and yeast
What is meiosis?
Meiosis is the division of the nucleus that
results in the formation of haploid gametes
from a diploid gametocyte
What are the qualities of homologous chromosomes?
- Homologous chromosomes are chromosomes of
- the same size,
- same centromere position
- the same sequence of genes at the same loci.
What do chromosomes typically appear as in diploid cells?
In diploid cells, chromosomes typically
appear as homologous pairs
What happens during Meiosis 1?
- 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
What are chiasmata?
Chiasmata form at points of contact between
the non-sister chromatids of a homologous
pair and sections of DNA are exchanged
What are linked genes and how are they formed?
Linked genes are those on the same
chromosome. Crossing over can result in
new combinations of the alleles of these
genes.
What is crossing over and what does it produce?
This crossing over of DNA is random and
produces genetically different recombinant
chromosomes
How are homologous chromosomes postitioned?
The orientation of the pairs of homologous
chromosomes at the equator is random
What is independent assortment?
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.
What does Meiosis 2 produce?
A total of four haploid cells are produced.
What happens during meiosis 2?
Each of the two cells produced in meiosis I
undergoes a further division during which the
sister chromatids of each chromosome are
separated
How is the sex of birds, mammals and insects determined?
by the presence of sex
chromosomes
What do most male mammals contain?
the SRY gene on the Y
chromosome
What does the SRY gene determine?
determines development of
male characteristics
What does heterogametic mean?
Have two different sex chromsomes
What does homogametic mean?
Have two of the same sex chromosomes
What do heterogametic males have and what does that result in?
Heterogametic (XY) males lack most of the
corresponding homologous alleles on the
shorter (Y) chromosome
What do homogametic females have and what happens?
In homogametic females (XX) one of the two
X chromosomes present in each cell is
randomly inactivated at an early stage of
development
What is X chromosome inactivation?
X chromosome inactivation is a process by
which most of one X chromosome is
inactivated
Why does X chromosome inactivation occur?
X chromosome inactivation prevents a
double dose of gene products, which could
be harmful to cells
What benefit does x chromosome inactivation have?
Carriers are less likely to be affected by any
deleterious mutations on these X
chromosomes
What happens when X chromosome inactivation occurs?
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 have?
They produce both male and female gametes
and usually have a partner with which to
exchange gametes
What is the benefit of hermaphrodites?
if the chance of encountering a partner is an uncommon event, there is no requirement for that partner to be of the opposite sex
What is environmental sex determination in reptiles controlled by?
controlled by environmental temperature of
egg incubation.
What are the factors that can affect the sex of a species?
Sex can change within individuals of some
species as a result of size, competition, or
parasitic infection
What is the benefit of horizontal gene transfer?
Resulting in faster evolutionary
change than in organisms that only use
vertical transfer.
