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Meiosis
a type of cell division division that produces sex cells/gametes with half the number of chromosomes as the somatic cell. Produces genetically unique daughter cells.
3 sources of variation in gametes
independent assortment
segregation
Crossing over
Independent assortment
where the homologous pairs line up randomly during meiosis.
Alleles are reshuffled and new combinations of chromosomes are created.
Segregation
where homologous chromosomes pull apart and migrate to the cell poles during meiosis.
Only one chromosome from each homologous pair is placed into the new gametes made.
Crossing Over
the exchange of alleles / segments of chromosomes between homologous pair chromosomes. Crossing over can create new allele combinations of unlinked genes.
Homologous chromosomes
Each carries the same genes in the same order, but the alleles for each trait may not be the same. One is inherited from each parent.
Unlinked genes
found on different chromosomes. Genes that are found further apart are more likely to be unlinked, because they will not stay together during crossing over.
Incomplete Dominance
type of relationship between alleles, with a heterozygote phenotype intermediate between the two homozygote phenotypes, is called incomplete dominance.
Meiosis steps
Interphase
Prophase 1
Metaphase 1
Anaphase 1
Telophase 1
Cytokinesis 1
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2
Cytokenesis 2
Interphase
DNA replication
Prophase 1
Chromosomes condense, spindles form, nuclear envelope breaks down, Crossing over occurs
Metaphase 1
Pairs of homologous chromosomes move to the equator of the cell, Independent assortment occurs
Anaphase 1
Homologous chromosomes move apart to opposite poles of cell, Segregation occurs
Telophase 1
chromosomes gather at poles of the cell
Cytokinesis 1
Cytoplasm divides forming 2 daughter cells
Prophase 2
Chromosomes condense, new spindle form around chromosomes, nuclear envelope breaks down
Metaphase 2
Chromosomes move to the equator of the cell
Anaphase 2
sister chromatids are separated and pulled to opposite ends of the cell
Telophase 2
Chromosomes gather at the poles of the cell and the nuclear envelope reforms
Cytokinesis 2
Cytoplasm divides forming 4 unique daughter cells with half the number of chromosomes
Linked Genes
located on the same chromosome-inherited together
What do linked genes cause
reduce genetic diversity and produce less-diverse phenotypes
why do linked reduce diversity of genotypes
they cannot independantly assort and segreggate
Linked genes CAN NOT
independently assort and seggragate
Multiple alleles
Genes with more than two allele forms.
Locus
Specific location of a gene on a chromosome.
Lethal alleles
a condition when an allele is able to produce a phenotype that kills the organism, it results in a nonfunctional protein. Dominant lethal alleles are usually eliminated rapidly in the population as their expression is fatal.
Co-dominance
A genetic scenario where both alleles in a heterozygote are fully expressed.
Incomplete dominance
A genetic situation where neither allele is completely dominant, resulting in a blended phenotype.
Gene Pool
A gene pool refers to the combination of all the genes (including alleles) present in a reproducing population or species
Allele Frequency
the number of times that an allele occurs in a population
Population
interbreeding organisms of the same species in a particular area
4 things that change the frequencies of alleles in a gene pool
Mutation-the source of all new alleles
Gene Flow-entering or exiting the gene pool
Natural selection
Genetic drift (bottleneck, founder effect)
Migration
the movement of individuals from one population to another
When individuals migrate…….
alleles are removed from the original population and added to the new population
Gene flow (migration) DOES NOT…..
create new alleles in the species
Gene flow (migration) DOES…
change allele frequencies
Genetic Drift
the change in frequency of an existing gene variant in the population due to random chance. Genetic drift may cause gene variants to disappear completely and thereby reduce genetic variation.
Founder effect
the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population, seperate from the original population
Bottleneck effect
a phenomenon in which a population is reduced in size due to natural disasters, habitat loss, or overhunting, then recovers. Even though the population size may increase, the variation is still reduced as many alleles are lost
Consequence of bottleneck, founder effect/ Genetic drift
the loss of variation in a population caused by the reduction of allele frequencies.
Natural selection
the process where individuals that posses the ‘best’ phenotype in an environment survive and reproduce, passing their alleles which increases the frequency of this allele in population
Nature selects….
Phenotype
Organisms with less favourable phenotype…..
will not pass on their alleles as often, and therefore the frequency of their alleles in the gene pool decreases
high diversity means
more alleles in the gene pool
Types of evolutionary/selection pressures
Biotic
predation and disease
competition
Abiotic
Availability of resources
Climate
non-random events cause
Natural Selection
Random events cause
Genetic Drift
Gene fixation
the change in a gene pool from a situation where there exists at least two alleles of a gene to a given situation where only one of the alleles remains. This remaining allele is fixed and the one that does not is lost.
Gene flow
the movement of alleles from population to population
The advantages of multiple alleles
more phenotypes are expressed, and therefore more phenotypes can respond to natural selection and increase the survival chances of the species.
What do linked genes cause
reduce diversity of genotypes and produce less diverse phenotype.