Variation and sexual reproduction Flashcards
Costs of sexual reproduction?
- males unable to produce offspring
- only half of each parents genome passed onto offspring, disrupting successful parental genomes.
In sexual reproduction benefits outweigh 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.
Co-evolutionary interactions between parasites and hosts may select for?
Sexually reproducing hosts.
Hosts that are better at … have greater fitness.
Resisting and tolerating parasitism
Parasites that are better able at … have greater fitness
Feeding, reproducing and finding new hosts.
Asexual reproduction can be a successful reproductive strategy as…
Whole genomes are passed on from parent to offspring
In asexual reproduction only one parent can?
Produce daughter cells and establish a colony of virtually unlimited size over time.
Maintaining the genome of the parent is an advantage particularly in…
Very narrow and stable niches or when recolonising disturbed habitats.
Asexually reproducing populations are not able to … BUT …
Adapt easily to changes in their environment BUT mutations occur that provide some degree of variation and enable some natural selection and evolution to occur
Examples of asexual reproduction in eukaryotes?
Vegative cloning and parthenogenesis in lower plants and animals that lack fertilisation.
What is parthenogenesis?
Reproduction from a female gamete without fertilisation.
Asexual reproduction and offspring?
Offspring can often be reproduced often and in larger numbers
Where is parthenogenesis more common?
Cooler climates which are disadvantageous to parasites, or regions of low parasite density or diversity.
Organisism that reproduce asexually and Horizontal gene transfer?
Organisms that reproduce principally by asexual reproduction also have mechanisms for horizontal gene transfer between individuals to increase variation.
Prokaryotes can exchange genetic material horizontally, resulting in…
Faster evolutionary change than in organisms that only use vertical transfer
What is meiosis?
The division of the nucleus that results in the formation of haploid gametes from a diploid gametocyte
In diploid cells chromosomes typically appear as?
homologous pairs
Homologous chromosomes?
Chromosomes of the same size, same centromere position and with the same sequence of genes at the same loci
The chromosomes which have replicated prior to meiosis I, each consist of…
2 genetically identical chromatids attached at the centromere
Meiosis I step 1
Chromosomes condense and the homologous chromosomes pair up.
Meiosis I step 2
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?
Those on the same chromosome. Crossing over can result in new combinations of the alleles of these genes.
Crossing over?
Crossing over of DNA is random and produces genetically different recombinant chromosomes.
Meiosis I step 3
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.
Independent assortment?
Each pair of homologous chromosomes is positioned independently of the other pairs, irrespective of their maternal or paternal origin.
Meiosis I step 4
The chromosomes of each homologous pair are separates and move towards opposite poles.
Meiosis I step 5
Cytokinesis occurs and 2 daughter cells form.
Meiosis II
Each of the 2 cells produced in meiosis I undergoes a further division during which the sister chromatids of each chromosome are seperated. A total of 4 haploid gametes are produced
The sex of birds, mammals and some insects is determined by?
The presence of sex chromosomes.
What determines the development of male characteristics in most mammals?
SRY gene on the Y chromosome.
What do heterogametic (XY) males lack?
Most of the corresponding homologous alleles on the shorter (Y) chromosome.
What does males lacking most of the corresponding homologous alleles on the shorter (Y) chromosome result in?
Sex linked patterns of inheritance as seen with carrier females (XBXb) and affected males (XbY)
In homogametic females (XX) what happens to the X chromosomes at an early stage of development?
One of the 2 X chromosomes present in each cell is randomly inactivated.
Define X chromosome inactivation?
The process by which most of one X chromosome is inactivated.
What does X chromosome inactivation prevent?
Double dose of gene products which could be harmful to cells.
Which type of chromosome is less likely to be affected by any deleterious mutations on X chromosomes?
Carriers
As 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?
Species that have functioning male and female reproductive organs in each individual.
What type of gametes do hermaphrodites produce?
Both male and female gametes and usually have a partner to exchange gametes.
What is the benefit of hermaphroditism?
The chance of encountering a partner is often an uncommon event, there is no requirement for that partner to be of the opposite sex.
What else can sometimes determine sex and sex ratio?
Environmental factors.
Environmental sex determination in reptiles is controlled by?
Environmental temperature of egg incubation.
Sex change can be as a result of?
Size, competition or parasitic infection.
In some species, sex ratio of offspring can be adjusted in response to?
Resource availability.
What is the sex of birds and mammals determined by?
The presence of sex chromosomes.
