Unit 10 Flashcards
Mendel’s law of independent assortment
Gregor Mendels crosses with pea plants - showed that two alleles of a gene separate into different haploid gametes during meiosis - lay of segregation
law of independent assortment - did crosses where the parents differed by two characteristics controlled by two different genes (dihybrid cross)
Ratios in genetic crosses
genotypic ratio : proportions of the various genotypes produced by the cross
phenotypic ratio : proportions of the various phenotypes
Independent assortment
Pea plants contain two copies of each gene - gametes only contain one copy of each gene - one copy of each gene again is passed on in gametes- 9:3:3:1 ratio shows the the four types of gametes are all equally common - inheritance of the two genes is independant
Predicting ratios in Dihybrid crosses
9:3:3:1 ratio often found when parents that are heterozygous for two genes are crossed together
Can give other ratios if :
- either of the genes has co-dominant alleles
- either of the parents is homozygous for one or both of the traits
- either of the genes is not autosomal
- interactions between genes (epistasis)
Other possible ratios : 3:6:3:1:2:1
3:3:1:1
1:1:2:2:1:2
Unlinked genes
genes that assort independently are unlinked genes - located on different chromosomes and can be explained in terms of chromosome movement during meiosis - pair of homologous chromosomes is called a bivalent
Bivalents orientated randomly on the equator during metaphase 1 of meiosis - orientation of one bivalent does not depend on the other
Linked genes
some pairs of genes do not follow the law of independent assortment and expected ratios for unlinked genes are not found - combinations of genes tend to be inherited together - gene linkage. the scientific name for location of a gene on a chromosome = locus. - linked genes have the same loci on the same chromosome
new combination of allele can only be produced if DNA is swapped between chromatids - recombination and involves process called crossing over. Individuals that have different combination of characters from parents due to crossing over (recombinants)
Mendel and Morgan
Mendel : performed careful dihybrid crosses with meticulous recording of his results. In 20th century - found things that did not fit his theory : Thomas Hunt Morgan - developed the idea of linked genes to account for anomalies - did this by investigating Drosophila where the pattern was different.
Explanation that genes were found on the sex chromosomes
Other anomalies - inheritance was same in both male and female could be explained by same gene being located on the same autosome
Prophase 1 of Meiosis
homologous chromosomes pair up in prophase of meiosis 1. Each homologous chromosome = two sister chromatids as all dna has been replicated in s phase - chromatids of the two different chromosomes in a pair are non sister chromatids
Crossing over
exchange of DNA material between non-sister homologous chromosomes - at one stage in prophase 1 all the chromatids of two homologous chromosomes becomes tightly packed up together (synapsis)
The DNA molecule in one of the chromatids is cut and a second cut is made at the same point of a non-sister chromatid
The DNA of each chromatis is joined up to DNA of the non sister chromatid - has the effect of swapping the sections of DNA
In the later stages - the tight pairing ends but the sister chromatids remain tight - where each cross over. has occurred there is an X shaped structure called a chiasma
Recombination of Linked Genes
without crossing over- it would be impossible to produce new combinations of linked genes
Homologous chromosomes separate in meiosis 1 and sister chromatids in meiosis 11 so each of the four cells produced receives one chromatid for each bivalent
An example of gene linkage and test crossing
bars used to represent chromosomes on which genes are linked
Continuous variation
variation can be discrete or continuous - discrete = every individual fits into one of a number of non-overlapping classes
continuous variation : any level of characteristics possible between the two extremes eg human height
Discrete = usually due to one gene
Continuous = due to combined affect of two or more genes (polygenic)
any polygenic traits also influenced by environmental factors (genes and nutrition/ light exposure)
Gene Pools
concept of the gene pool = all of the genes and their different alleles in an interbreeding population
evolution - change over time in allele frequency in a gene pool
Differences in allele frequency
frequency of allele is the number of that allele there is in a population divided by the total number of alleles of the gene (0-1)
Geographically isolated population have different allele frequencies from the rest of the species : may be due to natural selection or random drift
Types of natural selection
Directional : one extreme in the range of variation is selected for and other extreme selected against
Stabilising: intermediates are selected for and extremes are selected against
Disruptive : extreme types are selected for and intermediates against
