Key Area 3-Variation and Sexual Reproduction Flashcards
What is sexual reproduction?
The formation of haploid gametes of 2 parents that fuse nuclei to form diploid zygotes
What are the two main disadvantages of sexual reproduction?
- half of the population are unable to produce offspring (paradox of the existence of men)
- by mixing the genetic information between two individual each parent disrupts a successful genome and only passes in half to each offspring
What is the main benefit of sexual reproduction?
It creates genetic diversity and variation therefore allows evolution
How does genetic variation link to the red queen hypothesis?
It’s essential that genetic variation occurs to allow constant running to stay in the same place
What is asexual reproduction?
The cloning of one single parent to form genetically identical offspring
Where does asexual reproduction occur?
Usually prokaryotes but some eukaryotes such as plants
Where is asexual reproduction particularly successful as a reproductive strategy?
Very narrow niches or when recolonising disturbed habitats
What is the name given to asexual reproduction in plants?
Vegetative propagation
Where is vegetative propagation a useful strategy?
Perennial weeds
What are two examples of eukaryotic organisms reproducing asexually?
Vegetative propagation
Parthenogenesis
What is parthenogenesis?
Where animals are capable of reproducing without fertilisation of eggs
What are the two conditions where parthenogenesis usually occurs?
Cooler climates
Where parasite diversity/density is low
Give two specific examples of parthenogenesis
Komodo dragons where females can reproduce alone in the absence of males and produce male offspring only
Stick insects can reproduce alone in the absence of males and produce female offspring only
What is mitosis?
The process of cell division that results in the formation of new diploid cells
What is meiosis?
The process of cell division that results in the formation of new haploid cells
What is a homologous chromosome?
Homologous chromosomes have the same:
Size
Centromeres in the same place
Carry the same genes at the same Loki
How does meiosis affect variation in organisms?
Increases variation
What are the two divisions in meiosis and the products formed at each division?
Meiosis I - two haploid daughter cells with double the DNA
Meiosis II - four haploid gamete cells
What cell begins meiosis?
A gamete mother cell (2n)
What is the first stage in meiosis I?
Interphase where DNA replicates to form 2 sister chromatids
What occurs in meiosis I after interphase?
Homologous chromosomes line up in pairs along the metaphase plate
How do homologous chromosomes line up along the metaphase plate?
In an independent assortment
What does an independent assortment assure in meiosis?
Further variation
What occurs in meiosis I after homologous chromosomes line up?
The pairs touch at chiasmata and crossing over occurs which results in recombination of alleles (swapped DNA)
What are linked genes?
All genes on one chromosome
What occurs in meiosis I after crossing over?
Spindle fibres separate homologous chromosomes to form two haploid daughter cells that contain double copies of the DNA
How are the two haploid daughter cells in meiosis I created?
Cytokinesis where the cleavage of the cytoplasm separates the cells
What is the crossover frequency?
An indication of the likelihood that two genes will cross over
How is cross over frequency determined?
The further apart the genes are on the chromosome, the greater the chance of crossover
What are the three stages of meiosis II?
- chromosomes line up singly along the metaphase plate
- sister chromatids are separated by spindle fibres with random distribution in gametes
- cleavage furrow develops and four haploid gametes are formed
What happens in meiosis in algae?
Often haploid fells from meiosis divide by mitosis to form haploid individuals, rather than gametes fusing to form zygotes. These haploid individuals produce their gametes through mitosis. These can fuse so the next generation is diploid.
What is a hermaphroditic species?
A species that contains both male and female reproductive systems and gametes
What are two factors that that determine sex and sex ratio?
Genetics
Environmental conditions
How does environmental conditions affect sex determination in crocodiles?
If nest temperature is lower then 31.7 or higher than 34.5 the offspring will be female
Explain a sequential hermaphrodite
Some animals are sequential hermaphrodites which change their sex during their lives due to size, parasitic infection and/or competition
What is the sex bearing chromosome in live-bearing mammals?
X and Y chromosomes
What insect has X and Y sex chromosomes?
Drosophila
Which chromosome has genes related to maleness?
Y chromosome
Why do live-bearing mammals have males that lack some alleles?
Y chromosome lacks pair chromosome that features on X
Why does sex-linked patterns of inheritance occur?
The male lacks homologous alleles for genes on X chromosome
How is a lethal dose of the products of gene expression avoided in females?
To compensate for an extra dose of genes, the portions of the X chromosome that are lacking on the Y are randomly inactivated on one of the homologous X chromosomes in each cell
Why are carries less likely to be affected by deleterious mutations?
Carriers are less likely to be affected by deleterious mutations on these X chromosomes as X chromosome inactivation is random so half of the cells in any tissue sample will have a working copy of the gene
