Variation and Sexual Reproduction 2.3 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
why do the benefits of sexual reproduction outweigh the costs?
due to an increase in genetic variation in the population
what does genetic variation provide?
the raw material required for adaptation, giving sexually reproducing organisms a better chance of survival under changing selection pressures
what is used to explain the persistence of sexual reproduction?
The Red Queen hypothesis
what may be selected for sexually reproducing hosts?
Co-evolutionary interactions between parasites and hosts
what happens if a host reproduces sexually?
the genetic variability in their offspring reduces the chances that all will be susceptible to infection by parasites
how can asexual reproduction be a successful reproductive strategy?
as whole genomes are passed on from parent to offspring
what is maintaining the genome of the parent an advantage particularly in?
very narrow, stable niches or when re-colonising disturbed habitats
what are examples of asexual reproduction in eukaryotes?
- vegetative cloning in plants
- parthenogenesis in lower plants
- animals that lack fertilisation
how can offspring be reproduced in asexual reproduction?
more often and in larger numbers
why is parthenogenesis more common in cooler climates?
it is disadvantageous to parasites, or regions of low parasite density or diversity
Asexually reproducing populations are not able to adapt easily to changes in their environment, but what can occur that provides some degree of variation and enables some natural selection and evolution to occur?
mutations
what do organisms that reproduce principally by asexual reproduction also often have mechanisms for and what is an example of this?
for horizontal gene transfer between individuals to increase variation
for example the plasmids of bacteria and yeasts
what is meiosis?
the division of the nucleus that results in the formation of haploid gametes from a diploid gametocyte
what do chromosomes typically appear as in diploid cells?
homologous pairs
what happens in 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
- 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
what happens in 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
what is the sex of birds, mammals and some insects determined by?
presence of sex chromosomes
In most mammals the SRY gene on the Y chromosome determines the development of what?
male characteristics
what do heterogametic (XY) males lack most of the corresponding homologous alleles on and what can this result in?
the shorter (Y) chromosome
results in sex-linked patterns of inheritance as seen with carrier females (XBXb) and affected males (XbY)
in homogametic females (XX) what is randomly inactivated at an early stage of development?
one of the two X chromosomes present in each cell
what does the X chromosome inactivation prevent and how does this affect the carriers?
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 what?
have a working copy of the gene in question
what are hermaphrodites ?
species that have functioning male and female reproductive organs in each individual
what do hermaphrodites produce?
both male and female gametes and usually have a partner with which to exchange gametes
what is a 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
For other species, environmental rather than what determine sex and sex ratio
genetic factors
what can sex change within individuals of some species be a result of?
- size
- competition
- parasitic infection
what can the sex ratio of offspring in some species be adjusted to in response of?
resource availability
how do prokaryotes exchange genetic material and what is the result of this?
exchange genetic material horizontally
results in faster evolutionary change then in gametocyte
what are homologous chromosomes?
chromosomes of the same size, same centromere position and with the same sequence of genes at the same loci
what are linked genes?
those on the same chromosome and crossing over can result in new combinations of the alleles of these genes
what happens through recombination?
crossing over of DNA is random and produces genetically different chromosomes
what is independent assortment?
where each pair of homologous chromosomes is positioned independently of the other pairs, irrespective of their maternal and paternal origin.
