Unit 2 KA3 Flashcards
Costs of Sexual Reproduction
Males unable to produce offspring
Only half of each parents genome passed onto offspring
Disrupts successful parental genomes
Fitness in hosts
Hosts better able to resist and tolerate parasitism have greater fitness
Parasites better able to feed, reproduce and find new hosts have greater fitness
Benefits of hosts reproducing sexually
Genetic variability in their offspring reduced the changed they all will be susceptible to infection by parasites
Asexual Reproduction as a successful reproductive strategy
Whole genomes are passed on from parent to offspring
One parent can produced daughter ce,,s and establish a colony of virtually unlimited size over time
Where can maintaining parent genome be advantageous?
Very narrow, stable niches
Recolonising disturbed habitats
Examples of asexual reproduction in eukaryotes in lower plants and animals that lack fertilisation
Vegetative Cloning
Parthenogenesis
Parthenogenesis
Reproduction from a female gamete without fertilisation
Where is parthenogenesis most common?
Cooler climates which are disadvantageous to parasites or regions of low parasite density or diversity
Asexual Reproduction advantage - offspring
Can produce offspring more often and in larger numbers
Disadvantage of asexual reproduction
Not able to adapt easily to changes in their environment
How does variation occur in asexual reproduction?
Mutations which enable some natural selection and evolution to occur
Horizontal gene transfer between individuals
Examples of where horizontal gene transfer occurs
Plasmids of bacteria and yeasts
Prokaryotes - results in faster evolutionary change than in organisms that only use vertical transfer
Meiosis Definition
Division if the nucleus that results in the formation of haploid gametes form a diploid gamete
Chromosomes in diploid cells
Homologous pairs
Homologous Chromosomes
Same size, same centromere position, same sequence of gene at the same loci
Meiosis I - First Step
Chromosomes that have replicated prior to meiosis I each consist of 2 genetically identical chromatids attached at the centromere
Chromosomes condense and the homologous chromosomes pair up
Chiasmata
Chiasmata form at points of contact between the non-sister chromatids of a homologous pair and sections of DNA are exchanged.
Linked genes are those on the same chromosome.
Crossing over can result in new combinations of the alleles of these genes.
Crossing over if DNA is random and produces genetically different recombinant chromosomes.
Meiosis I (consequent steps)
Spindle fibres attach to the homologous pairs and line them up at the equator of the spindle
The chromosomes of each homologous pair are separated and move toward opposite poles
Cytokinesis occurs and two daughter cells are formed
Independent Assortment
Each pair of homologous chromosomes are positioned independently of the other pairs irrespective of their maternal and paternal origin
Meiosis II
Each of the two cells produced in meiosis I undergoes a further division
Sister chromatids of each chromosome are separated producing four haploid cells
SRY gene
The gene on the Y chromosome that determines the development of male characteristics
Heterogametic males (XY)
Lack most of the corresponding homologous alleles on the shorter Y chromosome
Sex linked patterns of Inheritance
Carrier females and affected males
X Chromosome Inactivation
One of the X chromosomes is randomly inactivated at an early stage of development
Prevents a double dose of gene products which could be harmful to cells
Carriers are less likely to be affected by any deleterious mutations on these X chromosomes
As it is inactivated each cell is random, half of the cells in any tissue will have a working copy of the gene
Hermaphrodites
Species that have functioning male and female reproductive organs in each individual
Benefits for Hermaphrodites
If encountering a partner is uncommon there is no requirement that the partner has to be of opposite sex
Environmental sex determination control in Reptiles
Environmental temperature of egg incubation
Factors affecting sex
Size, competition or parasitic infection
How can sex ratio of offspring be adjusted?
In response to resource availability
