2.3 VARIATION AND SEXUAL REPRODUCTION Flashcards
What is a benefit of sexual reproduction?
Huge genetic variability amongst offspring and therefore in population
What is a cost of sexual reproduction?
Only half of each parent’s genome is passed onto offspring disrupting successful parental genome
What is the term given to half of the population not being able to reproduce?
The paradox of the existence of males
Is sexual reproduction worth the cost?
The benefits far outweigh the disadvantages due to the increase in variation within the population
Genetic variation can be described as?
Essential raw material for natural selection (and the Red Queen’s arms race)
What are three benefits of asexual reproduction?
Rapid reproduction, lower metabolic costs, every member of population can reproduce
Where is asexual reproduction found?
In very narrow stable niches
When is asexual reproduction particularly useful?
In the recolonization of disturbed habitats
Give two examples of asexual reproduction in eukaryotes.
Vegetative cloning in plants, parthenogenesis in animals lacking fertilisation
What is vegetative cloning?
When a new plant grows from a fragment of the parent plant or from a specialised reproductive structure
What is the literal meaning of parthenogenesis?
Virgin creation
What is parthenogenesis?
Reproduction from an ovum without fertilisation - producing clones
Can males be produced by parthenogenesis?
Yes; through the deletion of a sex chromosome
Where is parthenogenesis most commonly found?
In cooler climates
Why is parthenogenesis more commonly found in cooler climates?
They are disadvantageous to parasites - regions of low parasite density
What are prokaryotes?
Microscopic single-celled organisms (eg bacteria, archaea)
Name a mechanism that prokaryotes use to move genes between individuals of the same generation.
Horizontal gene transfer
Give an example of horizontal gene transfer.
Plasmids of bacteria and yeast
What are homologous chromosomes?
Pairs of chromosomes of the same size, centromere position and genes at the same loci
How can two individuals produce a new combination of alleles?
Through production of haploid gametes
How are haploid gametes produced?
By meiosis in gamete mother cells
What is meiosis?
The process whereby one diploid gamete mother cell divides into four genetically dissimilar haploid gametes
What happens before meiosis begins?
Interphase
What are the three components of interphase?
Growth, DNA replication, general cell processes
How can meiosis be described?
Reduction, division process
How does sexual reproduction increase variation?
It involves crossing over, independent assortment and combining of two haploid genomes
How many divisions take place during meiosis?
2 (Represented by I and II)
What memory aid can be used to remember the process of meiosis (and also mitosis)
PMAT
What does PMAT stand for?
Prophase, Metaphase, Anaphase, Telophase
What happens during PI (Prophase I)?
Crossing over of chromosomes
What is required for ‘crossing over’ to take place?
Chiasmata must form at random positions between homologous pairs
What is the result of ‘crossing over’?
DNA is exchanged between the homologous pairs forming recombinant chromosomes
What are linked genes?
Genes located on the same chromosome that usually travel together during meiosis
How does ‘crossing over’ affect linked genes?
Linked genes can be separated and swapped
What happens during MI (Metaphase I)?
Chromosomes line up in pairs on the metaphase plate and spindle fibres attach
What is independent assortment?
The random orientation (left/right) in which the homologous pairs line up (irrespective of maternal and paternal origin)
What happens during AI (Anaphase I)?
Microtubules of spindle fibres shorten, pulling chromosomes to opposite ends of the cell
What happens during TI (Telophase I)?
Nuclear membrane forms around chromosomes and cytokinesis separates the two cells
How does PII (Prophase II) begin?
Each cell is haploid with one copy of each homologous chromosome. Sister chromatids are non-identical
What happens during MII (Metaphase II)?
Chromosomes line up on metaphase plate (in single file)
What happens during AII (Anaphase II)?
Microtubules of spindle fibres shorten, separating and pulling the sister chromatids to opposite ends of the cell
What happens during TII (Telophase II)?
Nuclear membrane forms around chromosomes and cytokinesis separates the cells
What is the result of Meiosis II?
Four genetically different haploid cells have been produced
Is meiosis the only way to produce gametes?
No; sometimes mitosis occurs after meiosis producing haploid organisms and gametes are formed later through differentiation
What is a hermaphrodite?
An organism with both male and female reproductive organs
What two factors can determine sex and sex ratio?
Genetic factors, environmental factors
What environmental factor determines sex of some reptiles?
Environmental temperature of egg incubation
Other than temperature, what other environmental factor can determine sex (ratio)
Resource availability
What environmental stimulus could cause an individual to change their sex?
Size, competition, parasitic infection
What is a live-bearing mammal?
A mammal which bears live young rather than laying eggs
What determines maleness in most live-bearing mammals?
The SRY gene on the Y chromosome
In mammals, what do males tend to be?
Heterogametic
What does heterogametic mean?
(Male) has both a X and Y chromosome
What is the result of being heterogametic?
Sex-linked traits are more common (in males)
Why do males tend to be heterogametic?
They lack homologous alleles on the smaller (Y) chromosome
Do sex-linked traits always affect females?
No; if trait is recessive then they can be carriers without symptoms
How can females be carriers?
One X chromosome is randomly inactivated in each cell so half of all cells contain an active copy of gene
Why are X chromosomes randomly inactivated in females?
To prevent a double-dose of gene products which could be harmful to cells
