Genetics and Evolution Flashcards
Pairing / Synapsis
During meiosis, prophase, the homologous chromosomes align besides each other.
bivalents / tetrad
tetrad = the combination composed of four chromatids
bivalents = a pair of homologs
–> tetrads = bivalents
When do chromosomes replicated?
During interphase before meiosis.
What holds homologs together?
a centromere
crossing-over
During prophase I of meiosis breaks in the DNA occur. Following these chromosome breaks, non-sister chromatids bind to the break with their homologous sequence (on the respective locus). These connections are called chiasmata (plural) or chiasma (singular).
- These chiasmata are important for the completion of meiosis as they stabilize bivalents and increase genetic variability.
- Crossing over results in an exchange of DNA between the maternal and paternal chromosomes and can decouple linked combinations of alleles and therefore lead to independent assortment.
- Crossing over produces new combinations of alleles on the chromosomes of the haploid cells.
- Crossing over can occur multiple times and between different chromatids within the same homologous pair.
Segregation & Independent assortment
Segregation = the separation of the two alleles of every gene that occurs during meiosis
In. Ass. = Alleles of one gene segregate independently of the alleles of other genes due to the random orientation of bivalents in meiosis 1.
Unlinked genes segregate independently as a result of meiosis (independent assortment).
Genes found of different chromosomes are unlinked and therefore do segregate independently. Genes on the same chromosome are linked, except when they are far apart on the chromosome. Crossing over between genes occurs more frequently the further the separation of genes. Combinations of genes tend to be inherited together, which is called gene linkage.
Bivalents are orientated randomly on the equator during metaphase I. The orientation of on bivalent does not affect the orientation of other bivalents, (so both poles have equal chance in anaphase 1. For example, when a parent with the genotype AaBb produces gametes, AB, Ab, aB, and ab are all equally probable if genes A and B are located on different chromosomes.
Exceptions to Mendel’s rules
aka. reasons dihybrid crosses can give other ratios
Sex linkage / gene linkage / linked loci on the same chromatid
Gene linkage leads to offspring that carries more parental combinations (similarity) than in an inheritance with unlinked genes, which would follow an expected/typical ratio of 9:3:3:1 (for unlinked genes) and have more recombinants.
other reasons:
- either of the genes has co-dominant alleles
- either of the parents is homozygous for one or both of the genes
- either of the genes is not autosomal — in other words if it is sex-linked
- interaction between genes occurs (epistasis): albino mouse with colour genes present but not expressed
Variation: discrete or continues
Blood types are an example of discrete variation (no in-between categories).
The phenotypes of polygenic characteristics tend to show continuous variation. Polygenic characteristic is where two or more genes affect the same character and have an additive effect. Different skin color can be explained if there are two unlinked genes, with co-dominant alleles.
Gene pools (genetic equilbrium & change of time)
all genes and corresponding alleles in an interbreeding population
In a typical interbreeding population, some alleles are more common than others; evolution always involves a change over time in allele frequency in a population’s gene pool.
Genetic equilibrium exists when all members of a population have an equal chance of contributing to the future gene pool.
Differences in allele frequency (calcuation)
The frequency of an allele is the number of that allele in a population divided by the total number of alleles of the gene, ranging from 0 to 1, where the total frequency of all alleles is 1.
Geographically isolated populations often have different allele frequencies from the rest of a species, which may be due to either differences in natural selection or random shift.
Types of natural selection
Directional — one extreme in the range of variation is selected for; the other extreme is selected against.
Example: in the bird species Parus major, breeding success has been greater with birds that breed earlier because the peak availability of prey is now easier in the year as a consequence of climate change.
Stabilizing — intermediates are selected for and extremes are selected against.
Example: in the bird species Parus major, breeding success is greatest with intermediate clutch sizes (number of eggs) because in large clutches the offspring have lower survival rates and in small clutches there are fewer offspring with no greater chance of survival than in intermediate clutches.
Disruptive — extreme types are selected for and intermediates are selected against.
Example: in the bird species Passerine amoena, year-old males with the dullest and brightest plumage (feathers) are more successful than males with intermediate plumage at obtaining high-quality territories, pairing with females and siring offspring.
fitness of a genotype or phenotype
the likelihood that it will be found in the next generation
Speciation and reproductive isolation
Speciation is the formation of new species - divergence.
New species are formed when a pre-existing species splits. This usually involves one population not interbreeding with any other populations of its species — reproductive isolation.
If natural selection acts differently on this separated population, it will gradually diverge from the other populations of this species. Eventually the population will be incapable of interbreeding with the rest of the species and has become a new species.
Types of speciation
gradualism – divergence over thousands of years
or
punctuated equilibrium – long periods without appreciable change and short periods of rapid and abrupt evolution
