2.3 Flashcards
What are the advantages of sexual reproduction?
- greater genetic variation within sexually reproducing organisms
- provides some organisms with ability to adapt to changing conditions or survive new strains of disease
What are the disadvantages of sexual reproduction?
- half of population unable to produce offspring (males)
- each parents only able to pass 50% of genetic material rather than 100%
- gamete formation is random, disrupting successful parental genomes
What gives sexually reproducing organisms a better chance of surviving selection pressures?
genetic variation gives raw material for adaptation
How does sexual reproduction affect co-evolutionary relationship between parasite and host?
If host reproduces sexually, genetic variability in offspring reduces chances that all will be susceptible to parasite infections
What are the advantages of asexual reproduction?
- whole genomes passed from parent to offspring
- maintaining parent genome is and advantage in very narrow, stable niches or when re-colonising disturbed habitats
- faster method, 1 member of species needed
What are disadvantages of asexual reproduction?
- all offspring genetically identical (clones)
- if one gets infected, it spreads quickly
- not able to easily adapt to environmental changes, but mutations can occur, to provide variation and enable natural selection and evolution to occur
What do organisms that reproduce asexually have mechanisms for?
horizontal gene transfer between individuals to increase variation
e.g. plasmids of bacteria and yeasts
What is horizontal gene transfer?
prokaryotes can exchange genetic material within the same generation (horizontally), resulting in faster evolutionary change than in vertically transferring organisms (between generations)
What are examples of asexual reproduction in eukaryotes?
vegetative cloning in plants and parthenogenesis in lower plants and animals that lack fertilisation
What is benefit of asexual reproduction?
offspring can be produced more often and in larger numbers
What is parthenogenesis?
the development of an unfertilised egg into adult organism without egg/sperm fusion
the offspring will be haploid
What is vegetative propagation?
eukaryotic plants have ability to produce offspring from 1 parent plant
Tubers - potatoes
Bulbs - daffodils
Offsets - aloe vera
Runners - strawberries
Where is eukaryotic asexual reproduction more common?
in cooler climates that are disadvantageous to parasite or regions of low parasite density or diversity
What are homologous chromsomes?
chromosomes that are:
- same size
- same centromere position
- same gene sequence at same loci
What is meiosis?
the division of nucleus to form haploid gametes from a diploid gametocyte (not a cycle)
mechanism where variation is increased
Where does meiosis take place in animals?
ovaries and testes
Where does meiosis take place in plants?
ovaries and anthers
What is the first stage of meiosis I
Pairing of homologous chromosomes
Describe the pairing of homologous chromosomes…
chromosomes (replicated prior) consist of two identical chromatids attached at centromere
chromosomes condense and homologous pairs pair up
chiasmata form at points of contact between non-sister chromatids and DNA sections are exchanged
Describe crossing over…
human chromsomes usually have 2/3 chiasmata
crossing over is random, produces genetically recombinant chromosomes which results in new allele combinations
Describe the second stage of meiosis I…
spindle fibres attach to homologous pairs, line up at equator
orientation is completely random, each pair positioned independently irrespective of maternal/paternal origin (independent assortment)
chromosomes of each homologous pair separated to opposite poles, cytokinesis occurs and sister chromatids are no longer identical due to crossing over
Describe meiosis II…
each cell is now haploid (one copy of each homologous chromosomes, though has two sets of genetic info)
nuclear membrane breaks down, each of the two cells produced in meiosis I undergoes further division where sister chromatids of each chromosome are separated
total of 4 haploid cells are produced
What happens after meiosis in human males?
each haploid cell develops to form a sperm cell
What happens after meiosis in human females?
- meiosis I occurs in last 3 months before birth
- only one of the cells develops further
- after egg cell released from ovary it will not undergo meiosis II until sperm nucleus has entered
- nucleus of only one new cell will fuse with sperm nucleus
What is independent assortmen?
the alignment of homologous chromosome pairs are random leading to variation in haploid cells at end of meiosis II
What are the sources of variation in meiosis?
- crossing over
- independent assortment
- sexual reproduction
What are linked genes?
genes on the same chromosome
What creates recombinants?
separated linked genes through crossing over
What is recombination used for?
scientists use frequency of recombination to map chromosomes, working out where each gene is in relation to others, pinpointing exact locations on chromosomes
What does recombination frequency correlate with?
the distance between gene loci on chromosome
How is sex determined?
by presence of sex chromsomes
Explain sex chromosomes in mammals…
in live-bearing mammals (not marsupials) females have 2 large X chromosomes and males have one X chromosome and a smaller y chromosome
this also applies to drosophila melanogaster (fruit fly), which is unusual for insects
What and where is the SRY gene
on the Y chromosome in most mammals, it determines male characteristic development
SRY = Sex determining Region of Y chromosome
What is the default human embryonic development?
female
What are does SRY gene do?
it is a ‘master switch’, produces transcription factor to switch on other genes, many of which are on autosomes but are only transcribed due to SRY control in males
What are some rare congenital intersex conditions?
- XX male due to SRY gene translocated to X-chromosome
- XY female due to SRY deletion from Y-chromosome
What leads to sex linkage?
heterogametic (XY) males lack most of corresponding homologous alleles in shorter Y chromosome
How can X chromosomes mask sex linked conditions?
in XX females, a recessive alleles on one X can be masked by a dominant allele on others
in XY males, a recessive allele on X has no second copy to mask effects
What does homogametic mean?
all gametes produced have same chromosome combination (human females)
What does heterogametic mean?
gamete produced have two possible chromosome combinations (human males)
Name examples of sex linkage…
- red-green colour blind
- haemophilia
- duchenne muscular dystrophy
- rickets (dominant allele)
Explain dosage compensation…
in homogametic females one X is randomly partially inactivated at early stage of development which prevents double dose of gene products, this could be harmful to cells
Explain carrier females…
- carrier less likely affected by deleterious mutations on X chromosome
- carrier unaffected as working gene copy in half of cells in all tissue
- due to random X-inactivation, XX female has mosaic of cells with different X active an inactive
What are mosaics?
only female cats can be tortoiseshell, show mosaic in fur as coats colour gene is found on X
What are hermaphrodites?
species that have functioning male and female reproductive organs in each individual, they produce both male and female gametes which are exchanged with a partner
What is the benefit of being a hermaphrodite?
benefit is that is encountering a partner is uncommon, no requirement for partner to opposite sex
Give examples of hermaphrodites…
- many flowers
- earthworms
- most land molluscs
What affects sex ratio?
temperature, parasites and competition
How does temperature affect sex ratio?
during egg incubation, in some reptiles it determines whether male or female
How do parasite affect sex ratio?
- some arthropods (mainly insects) sex ratio affected by infection of bacteria genus wolbachia
- some have chromosomal sex determination but bacteria kills males or femenizes them
- other entirely rely on parasites
How does competition affect sex ratio?
some species actively change sex ratio of offspring
Give examples of competition affected sex ratio in group living fish…
clown fish live in groups with dominant female and many small males
- if female removed, largest male becomes female
blue-headed wrasse have one male and many females
- removal of male causes largest female to grow twice as large and change colour
How does aromatase influence sex ratio?
an enzyme that control hydrogen : oestrogen ratio
What changes sex as it gets larger?
sea anemone (metridium senile), male -> female as it gets larger
Explain sex change in mayfly through parasitisation…
-nematode parasitises larvae of mayfly
- female becomes sterile
- some males become intersex or female
all parasitised act as females when adult and crawl into water to lay eggs which allows parasite to parasitise next generation.
