2.3 - Variation and Sexual Reproduction Flashcards
what is sexual reproduction
the production of new living organisms by combining genetic information from two individuals of different types (sexes)
what are the costs of sexual reproduction
males are unable to produce offspring
only half of each parent’s genome passed onto offspring, disrupting successful parental genomes
why do the benefits of sexual reproduction outweigh the costs
due to an increase in genetic variation
what does genetic variation provide
the raw material required for adaptation, giving sexually reproducing organisms a better chance of survival under changing selection pressures
how does the red queen hypothesis explain the persistence of sexual reproduction
the red queen hypothesis postulates that parasite pressure maintains sexual reproduction in the host population by selecting for the ability to reproduce rare genotypes that are resistant to infection
hosts have greater fitness if
it is able to resist and tolerate parasitism
parasites have greater fitness if
it is able to feed, reproduce and find new hosts
if hosts reproduce sexually…
the genetic variability in their offspring reduces the chances that all will be susceptible to infection by parasites
what is asexual reproduction
a type of reproduction by which offspring arise from a single organism, and inherit the genes of that parent only; it does not involve the fusion of gametes
why can asexual reproduction be a successful reproductive strategy
whole genomes are passed on from parent to offspring
when is maintaining the genome of the parent an advantage in asexual reproduction
in very narrow, stable niches or when re-colonising disturbed habitats
examples of asexual reproduction in eukaryotes
vegetative cloning in plants
parthenogenesis in lower plants and animals that lack fertilisation
what is parthenogenesis
asexual reproduction whereby embryos develop from an unfertilised egg
where is parthenogenesis more common
in cooler climates which are disadvantageous to parasites, or regions of low parasite density or diversity
asexually reproducing populations are not able to adapt easily to changes in their environment, what provides some degree of variation
mutations that can occur which enable some natural selection and evolution to occur
what is vegetative progation/cloning
a form of asexual reproduction that takes place on some plants, resulting in offspring that are genetically identical to the parent plant e.g. bulbs and runners
organisms that reproduce principally by asexual reproduction also often have mechanisms for…
horizontal gene transfer between individuals to increase variation, for example the plasmids of bacteria and yeasts
what does prokaryotes exchanging genetic material horizontally result in
faster evolutionary change than in organisms that only use vertical transfer
what is meiosis
a type of cell division where four haploid gametes are produced from one diploid gamete mother cell
how do chromosomes appear in diploid cells
homologous
what are homologous chromosomes
chromosomes of the same size and shape, same centromere position and with the same sequence of genes at the same loci
which stages is meiosis split into
meiosis I
meiosis II
where are reproductive organs produced in animals
ovaries and testes
where are reproductive organs produced in plants
anthers and ovaries
what is the first step of meiosis I
interphase - each undergoes DNA replication to become two identical chromatids
what is the second step of meiosis I
homologous chromosomes pair up and line up along the equator of the cell
what is the third step of meiosis I
crossing over occurs at points called chiasmata
what does chiasmata mean
points of contact between non-sister chromatids of a homologous pair
what does crossing over lead to
it shuffles sections of the DNA between the homologous pairs, allowing the recombination of alleles to occur thus increasing variation
what are linked genes
genes that are on the same chromosome
what is the fourth step of meiosis I
independent assortment - 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
each pair of homologous chromosomes is positioned independently of the other pairs, irrespective of their maternal and paternal origin
what is the fifth step of meiosis I
the chromosomes of each homologous pair are separated and move towards opposite poles
cytokinesis occurs, the division of the parent cytoplasm, dividing the cell into two haploid daughter cells
what happens in meiosis II
each of the two cells produced in meiosis I undergoes a further division during which the sister chromatids of each chromosome are separated
a total of four haploid cells are produced
how is the sex of birds, mammals and some insects determined
the presence of sex chromosomes
what is a sex chromosome
a type of chromosome that participated in sex determination
e.g. male - (XY) female - (XX)
what determines the development of male characteristics
the SRY gene on the Y chromosome
what does SRY stand for
sex determining region of the Y chromosome
what do heterogametic (XY) males lack
most of the corresponding homologous alleles on the shorter Y chromosome
what does heterogametic mean
dissimilar sex chromosomes e.g. mammalian males where the Y chromosome is much smaller than the X chromosome
what does lacking most of the corresponding homologous alleles on the shorter Y chromosome lead to
sex-linked patterns of inheritance as seen with carrier females (XBXb) and affected males (XbY)
what is sex linkage
when males have a greater chance of being affected by certain recessive conditions because females would need two copies of the recessive allele to be affected whereas males only need one
what does homogametic mean
sex chromosomes which do not differ e.g. mammalian females have 2 X chromosomes (XX)
what happens to one of the two X chromosomes present in each cell in females
it is randomly inactivated at an early stage of development
what is X chromosome inactivation
the process by which most of one X chromosome is inactivated
why is X chromosome inactivation important
prevents a double dose of gene products, which could be harmful to cells
why are X carriers less likely to be affected by any deleterious mutations on these X chromosome
as the X chromosome inactivates 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
an organism with both functioning male and female reproductive organs
what is the advantage 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 genetic factors, what determines sex and sex ratio
environmental factors
environmental sec determination in reptiles is controlled by…
environmental temperature of egg incubation
how can the sex of an organism change
as a result of size, competition, or parasitic infection
how can the sex ration of offspring be adjusted
in response to resource availability
