Unit 2: KA3 - Variation and Sexual Reproduction Flashcards
Describe the three costs of sexual reproduction.
- Males cannot produce offspring
- Only half each parent’s genome are passed on
- Successful parental genomes are disrupted
Name the benefit of sexual reproduction.
Greater genetic variation within sexually reproducing organisms.
Describe why genetic variation is important.
It provides the raw materials for adaptation, giving sexually reproducing organisms a better chance of survival under changing selection pressures.
How does the ‘Red Queen Hypothesis’ support the persistence of sexual reproduction?
It states the importance of constant adaptations to survive.
What does co-evolutionary interactions between parasites and hosts select for?
Sexually reproducing hosts
State the benefits to hosts and parasites of hosts reproducing sexually.
Hosts: The variation allows them to adapt to be less susceptible to the parasite
Parasite: This forces them to adapt themselves.
Describe asexual reproduction
Just one parent produces daughter cells, passing on whole genomes.
When is asexual reproduction an advantage?
- In very narrow, stable niches
- When recolonising disturbed habitats (for speed)
What are 2 examples of asexual reproduction in eukaryotes?
- Vegetative cloning in plants (bulbs)
- Parthenogenesis in lower plants and infertile animals.
When is parthenogenesis is more common?
In areas of low parasite density e.g. cooler climates.
What is the disadvantage of asexual reproduction?
Organisms who reproduce asexually, are less able to adapt.
What are sexually reproducing organisms’ source of variation?
Mutations
How can asexually reproducing organisms increase their population’s variiation?
Horizontal gene transfer
What organelle do bacteria and yeast use for horizontal gene transfer?
Plasmids
Describe meiosis
the process of cell division that results in the formation of new haploid cells from a diploid gametocyte.
How do chromosomes appear in diploid cells?
as homologous pairs
What 3 similarities do homologous pairs share?
- the same size
- the same centromere position
- genes for the same characteristics at the same ‘loci’
describe 2 sources of variation within meiosis?
- Chiasmata: DNA is exchanged between crossed over chromosomes, producing genetically different recombinant chromosomes.
- Independent variation: Orientation of homologous pairs of chromosomes at the equator is irrespective of maternal or parental origin.
Define the term: linked gene.
Genes that are found on the same chromosome
What happens before meiosis?
interphase: duplication of DNA within the nucleus, chromatids attach at the centromere.
Describe the process of meiosis I
- Homologous chromosomes pair up
- Chiasmata form at points of contact between non-sister chromatids
- Spindle fibres attach and independent assortment occurs
- Chromosomes of each homologous pair are separated and move towards the poles
- cytokinesis occurs and two daughter cells form
Describe the chromosome count at each point of meiosis.
After interphase: 46 chromosomes; 92 chromatids; 1 cell
After meiosis I: 23 chromosomes; 46 chromatids; 2 cells
After meiosis II: 23 chromosomes; 23 chromatids; 4 cells
Describe meiosis II
- Each of the two cells produced in meiosis I undergo a further division.
- The sister chromatids are separated
- four haploid cells are produced.
How is sex determined in birds, mammals and some insects?
The presence of sex chromosomes
What gene determines the development of male characteristics, and where is it found?
the SRY gene on the Y chromosome
Why are males said to be heterogametic?
Their sex chromosomes are dissimilar
What do males lack on the Y-chromosome?
Most corresponding homologous alleles
Why are males more susceptible to certain recessive conditions?
Where females need two copies of the recessive allele (XbXb), males only need one (XbY)
What happens in X-chromosome inactivation?
In homogametic females, one of the X chromosomes in every cell is randomly inactivated in early development
What is the benefit of X-chromosome inactivation?
Prevents a double dose of gene products, which could be harmful.
Describe homogametic carriers’ chances of being affected by deleterious mutations on X chromosomes.
Carriers are less likely to be affected, as X-inactivation is random, so half of the cells in any tissue will have a working copy of the gene.
Define the term: hermaphrodite
a species that have functioning male and female reproductive organs in each individual.
How do hermaphrodites interact with each other?
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.
Other than genetics, what can determine sex and sex ratio?
environmental factors
What can happen to the sex ratio of offspring in some species?
Sex ratio can be adjusted in response to resource availability.
What 4 factors may cause sex in individuals to change?
- Size
- Parasitic infection
- Competition
- Temperature
