Ch 10

• • Chromosomes replicate in interphase before meiosis.
• • Crossing over is the exchange of DNA material between non-sister homologous chromatids.
• • Crossing over produces new combinations of alleles on the chromosomes of the haploid cells.
• • Chiasmata formation between non-sister chromatids can result in an exchange of alleles.
• • Homologous chromosomes separate in meiosis I
• • Sister chromatids separate in meiosis II.
• • Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I
• Diagrams of chiasmata should show sister chromatids still closely aligned, except at the point where crossing over occurred and a chiasma was formed
• • Gene loci are said to be linked if on the same chromosome.
• • Unlinked genes segregate independently as a result of meiosis
• • Variation can be discrete or continuous
• continuous variation is common with polygenic traits (discrete variation is things like blood groups, there are 4 seperate ones )
• • The phenotypes of polygenic characteristics tend to show continuous variation
• NULL hypothesis for chi squared of linked genes (H0) = traits assort independently (status quo)
• alternative hypothesis for linked genes (H1) = traits do NOT assort independently (are linked)
• if calculated X2 greater than critical (in critical region) null is REJECTED
• sum of (o - e)^2 / e
• • Application: Morgan’s discovery of non-Mendelian ratios in Drosophila. (males and females showed different inheritance patterns, didnt fit law of independent assortment -> morgan suggested idea of sex linkage/linked genes)
• • Application: Completion and analysis of Punnett squares for dihybrid traits
• • Application: Polygenic traits such as human height may also be influenced by environmental factors
• • Skill: Calculation of the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.
• • Skill: Identification of recombinants in crosses involving two linked genes
• • Skill: Use of a chi-squared test on data from dihybrid crosses
• • A gene pool consists of all the genes and their different alleles, present in an interbreeding population.
• • Evolution requires that allele frequencies change with time in populations
• • Reproductive isolation of populations can be temporal, behavioural or geographic.
• • Speciation due to divergence of isolated populations can be gradual
• • Speciation can occur abruptly
• • Application: Identifying examples of directional, stabilizing and disruptive selection.
• • Application: Speciation in the genus Allium by polyploidy (polyploidy = more than 2 sets of homologous chromosomes eg 3 )
• • Skill: Comparison of allele frequencies of geographically isolated populations
• Punctuated equilibrium implies long periods without appreciable change and short periods of rapid evolution.
• middle of a chromosome is called a centromere