BIO325 F2018-Lecture 4a

Lecture 4a - Outline Linkage & Mapping: the idea of using linkage to locate genes Independent assortment vs. linkage Using testcross to measure gene distance: Recombination Frequency (RF) Genetic maps give location of genes/alleles (locus-singular; loci-plural) on chromosomes Linkage: Genes that are next to each other tend to travel together What would be the inheritance pattern for two genes that are linked? Here, two traits are inherited together or not at all Here, two traits are inherited together or individually Investigation of linkage in sweet peas Flower color is determined by gene F dominant allele F = purple recessive allele f = red Pollen shape is determined by gene L dominant allele L = long recessive allele l = round 1. When two genes are on separate chromosomes Parent 1 Parent 2 Write the genotypes of gametes produced by parent #1. Write the genotypes and phenotypes of offspring produced. F l f L f l f l 2. When two genes are close together on the same chromosome Parent 1 Parent 2 Write the genotypes of gametes produced by parent #1. Which are recombinant? Which are parental? Write the genotypes and phenotypes of offspring produced. No crossover F l f L f l f l 3. When two genes are very far apart on the same chromosome Parent 1 Parent 2 Single crossover every time (100% meioses) F l f L X f l f l X Write the genotypes of gametes produced by parent #1. Which are recombinant? Which are parental? Write the genotypes and phenotypes of offspring produced. 4. When two genes are semi-close together on the same chromosome Parent 1 Parent 2 Single crossoversometimes (<100% meioses) F l f L X f l f l X Q1. Which gamete type of Parent 1 will represent greater than 50% if crossing-over occurred in some of all meioses? A) Recombinant B) Parental Two genes on separate chromosomes Two genes very far apart on the same chromosome Two genes close together on the same chromosome Q2: Which gene arrangement results in independent assortment (i.e. P=R)? Pick all that apply. F l f L X F l f L F l f L 100% Between 50% and 100% 50% Less than 50% Q3: What is the percentage of recombinant gametes when two genes assort independently? F l f L X F l f L Linkage and crossing-over Alleles of genes on different chromosomes assort independently = Unlinked (Parentals = Recombinants) Alleles of genes on the same chromosome may be inherited together = Linked (Parentals > Recombinants) But, they will not always be inherited together because non-sister chromatids can CROSS OVER during meiosis to produce recombinant gametes The farther apart two genes are on the same chromosome, the more likely a crossover occurs between them, and the two genes may appear to assort independently (50% Recombinants) You Your Parents Your Grandparents Inheritance of chromosomes that stay intact during meiosis (NO recombination) It doesn’t really happen this way! - except for fruit fly males Your Grandparents Your Parents You In fact, homologous chromosomes recombine during meiosis (gamete formation) You have alleles from all four of your grandparents You Your Parents Your Grandparents A1 B1 A2 B2 A3 B3 A4 B4 A1 B1 A4 B4 Determining which alleles usually stay together allows us to map genes on chromosomes AB/AB x ab/ab P F1 These two alleles came from one parent These two alleles came from the other parent New notation shows alleles that are inherited together from each parent AB/ab Aa Bb AB/ab Ab/aB or Not enough information F1 / separates homologs P F1 F2 New notation shows alleles that are inherited together from each parent AB/ab ab/ab Ab/ab aB/ab Parentals (>50% if A and B are linked) Recombinants (<50% if A and B are linked) What type of cross is this? AB/AB x ab/ab These two alleles came from one parent These two alleles came from the other parent AB/ab AB/ab x ab/ab Crossing-over is essential for pairing of homologs during meiosis I Nonsister chromatids align by DNA sequence identity. Chromosome breakage and exchange result in recombination When do you see this structure? A Recombination is reciprocal! A B B b b a a A A B B b b a a A A a a B b B b A b a B A B a b : P : P : R : R P: %A B = %a b R: %A b = %a B Recombination frequencies are the basis of genetic maps 1% RF = 1 map unit (m.u.) = 1 centiMorgan (cM) # Recombinant offspring Total # of offspring Recombination Frequency (%) = X 100 We will use “m.u.” throughout this class! Using testcross to calculate map distance Fig. 5.4 Using testcross to calculate map distance Fig. 5.4 Are b and c linked? Using testcross to calculate map distance Fig. 5.4 When genes are linked, parental classes are always the majority of progeny! What if P=R? Are b and c linked? Q4. What is the map distance between b and c? Fig. 5.4 77 m.u. 23 m.u. 50 m.u. When genes are linked, parental classes are always the majority of progeny! The a and b loci are 20 m.u. apart. An AB/ab woman mates with an ab/ab man. What is the probability their first child will be Ab/ab? Mapping using 2-point testcrosses # Recombinant offspring Total # of offspring Recombination Frequency (%) = X 100