Mendel - Pisium sativum self fertilisation, made crosses between flower colour, flower position, seed colour etc Two gene copies from each parent segregate away from each other during gametogenesis Each gamete contains one copy of each gene Alleles are alternative forms of genes Genotype - combination of alleles present Phenotype - final form in the individual Independent segregation - first generation all heterozygous, second generation after self-pollination 3:1 ratio (genotype 1AA:2Aa:1aa) Use chi squared to check the ratios Mitosis - asexual 2n -> 4n -> 2n + 2n Meiosis - sexual 2n -> 4n -> 4 x In (each combine with another to get 2n) First meiotic division - reductive Second - segregative Humans - 23 chromosomes per gamete, 23000 genes Pisum sativium - 7 chromosomes Drosophila melanogaster - 4 chromosomes, 16770 genes Thomas Hunt Morgan used Drosophila melanogaster, found that chromosomes are responsible for genetic inheritance, noticed fly with white eye instead of red (normal) Then crossed the white eyed male with normal females - led to discovery of sex linkage F1 generation - vast majority red eyes males and females F2 generation - roughly 3:1 ratio red:white Backcross - crossing parent with child white eyed male x F1 red eyed daughters: roughly 1:1:1:1 w1 (first) mutant allele for white eyes is on X chromosome (w+ = wild type) Males inherit Y chromosome from father, must get wild type copy of white from mother Females are heterozygous for white, w1 is recessive so F1 progeny have wild type eyes F2 generation - males all inherit Y chromosome from father, 1:1 ratio within males of red:white Males are hemizygous for X chromosome (only one copy) Backcrossing found that white eyes can exist in females and is not lethal to them Flies examined that had extra Y chromosome (XXY fertile females) In flies sex determined by ratio of X chromosomes to autosomes Morgan found first mutant in 1910, published first map of genes on X chromosome: genes are in linear order on chromosome and are in the same position in all individuals 1
Recombination - crossing over, exchange of genetic information between homologous chromosomes, if the recombination is between homologous chromosomes the result is formation of a hybrid chromosome The closer together genes are located on a chromosome, the more likely they are to segregate together Three factor cross - double recombinants, tell us the other of genes on chromosome Recombination frequency = number of recombinant progeny / total number of progeny x100 (units centiMorgans cM or map units) Recombination mapping - used to identify a gene responsible for a trait, narrows down the region on which a gene is linked 95% confidence interval: CI = 1.96 M sqrt(pq/N) M = distance in map units between flanking markers p = frequency of recombination between locus and left marker q = frequency of recombination between locus and right marker N = total number of crossovers analysed SNP - single nucleotide polymorphism (occurs in more than one variant in the population) - molecular