Genetic Linkage and Mapping Notation Aa Diploid Adult Haploid gametes single chromatid A a A A a a Two homologous chromosomes four chromatids total 1 2 A 1 2 a Dihybrid Cross AA BB Female A B a b P aa bb Male A Gametes B a Egg A B A b Sperm a b B a b F1 adult after the S phase A F1 B a b A F1 a A A 4 gamete types equally frequent B a a B b b B b F2 of dihybrid cross when genes not linked What proportion of the gametes of the double heterozygote are recombinant But what if the genes are on the same chromosome What if the genes are linked They do not assort independently at meiosis Two linked genes Female A Male B b P a A Gametes B a b Egg A B Sperm a b A a B b A B F1 a A b B a No crossing over between markers b A B a b When there is no crossing over between markers all gametes are A B F1 a A b B a Crossing over in 4 strand stage b A b a B When there is a crossover of gametes are parental and are recombinant No crossover A Crossover A B F1 aA bB a b A B a b B F1 aA bB a b A b a B When genes are on the same chromosome but far apart Crossing over between them will be common approaching 100 of the time In this case what proportion of all F1 gametes will be recombinant have a combination of alleles that was not present in the original true breeding parents A B F1 a A b B Crossing over in 4 strand stage a b A b a B Genes that are so far apart on the same chromosome that crossing over occurs between them in 100 of meioses behave as if they are unlinked parental combinations recombinants What if genes are close together Say that crossing over occurs between the genes in 20 of meioses What proportion of F1 gametes will be recombinant A 80 B A aA bB a b A B a b B F1 20 F1 aA bB a b A b a B If crossing over between genes occurs in 20 of meiosis of gametes will be recombinant Genes with 50 recombination are on the same chromosome Genes that have 50 recombination independent assortment are either on nonhomologous chromosomes or The frequency of crossing over between two genes on the same chromosome is proportional to the distance between the genes This is the basis of genetic mapping At small distances crossing over is rare and few gametes are recombinant r is near 0 As distance between genes increases crossover frequency increases and more gametes are recombinant r At greater distances crossover nearly always occurs and r How would you tell if a gamete produced by a double heterozygote is recombinant or parental How would you determine the genotypes of gametes produced by a doubly heterozygous individual Mate double heterozygote to an individual that is homozygous recessive for both traits Two linked genes Female bw Male hv bw Gametes hv P bw hv bw hv Egg A B Sperm a b bw bw hv hv bw hv F1 bw X hv hv bw hv bw bw hv Par Par hv bw bw hv Rec Rec bw bw hv hv If 15 of the offspring of the testcross are recombinant what proportion of meioses had crossover between the brown and the heavy veined genes If 15 of the gametes are recombinant the genes are said to be map units apart on a genetic map bw hv map units Unit of distance in linkage map is a map unit 1 map unit is equal to 1 percent recombination A map unit is also equivalent to the physical distance along a chromosome which will experience 1 crossover event in every meiotic divisions Two X linked genes Female w Male m Gametes m P w wEgg A B m w m Sperm a b w m w m F1 w X m m w m w m w m w w m Par Par w Rec Rec m 62 8 non recombinant offspring 37 2 recombinant offspring How far apart in map units are the genes map units If two genes are 50 map units apart what proportion of recombinant offspring would you observe What would you conclude about these two genes
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