Unformatted text preview: 10/19/09 Transmission of alleles 2 genes each located on a diﬀerent chromosome A1 A2 • 4 diﬀerent allelic combina<ons • Created through independent assortment B1 B2 • What are the genotypes of the gametes this individual can produce? Transmission of alleles • What about the following cross, involving 2 diﬀerent genes, located on the same chromosome? • What are the genotypes of the gametes this individual can produce? • Genes found on same chromosome = LINKAGE (not same as sex‐linkage [means that gene is on X‐chromosome]) A1 A2 B1 B2 1 10/19/09 Linked Genes • Genes on diﬀerent chromosomes = UNLINKED • Alleles of unlinked genes independently assort • Genes on same chromosome = LINKED = syntenic • Do alleles of linked genes independently assort OR are they transmiTed to gametes together (co‐segregate)?? • New nota<on for linked genes • A1B1/A2B2 • “/” indicates alleles on homologous chromosomes Co‐segrega<on of alleles of linked genes Linked Genes • What about the following cross, involving 2 diﬀerent genes, located on the same chromosome? Individual 1 A 1B 1
A1 A2 Only 2 types of gametes? It depends… B1 B2 A 2B 2 • Genes found on same chromosome = LINKAGE 2 10/19/09 Linked Genes vs. Sex‐Linked Genes • Sex‐linked gene is located on the sex chromosomes • Females homozygous or heterozygous for X‐linked genes; • Males are hemizygous for X‐linked genes • Linked genes don’t have to be sex‐linked • All genes on X chromosome are linked AND they are sex‐linked Linked Genes • Will genes undergo independent assortment if they are on the same chromosome (i.e., they are linked)? Informa<ve individual Must be heterozygous Non‐informa<ve individual 3 10/19/09 Linked Genes • Genes found on same chromosome = LINKAGE • How do you ﬁnd out if recombina<on occurred? • Genotype eggs? Linked Genes • Genes found on same chromosome = LINKAGE 4 10/19/09 Linked Genes • What oﬀspring would you look at to see if your predic<ons were correct? 1. 2. 3. 4. All oﬀspring. Just females. Just males. Random selec<on of oﬀspring. Linked Genes • Two X‐linked traits: wy+ w+y wy+ Y Sons & daughters From Mom… Daughters Sons From Dad… 5 10/19/09 Linked Genes • Males have only 1 X chromosome • Get their X chromosome from their mom (Y from dad) Recombina<on between linked genes • Allele combina<ons of linked genes can be broken up through X.O. (recombina<on) • Further apart genes are on same chromosome – experience more recombina<on • Genes closer together – experience less recombina<on This diagram doesn’t show centromeres or the sister chroma<ds, but does show how the non‐sister chroma<ds (originally A1, B1 & A2, B2) would exchange parts…. A1 A2 A1 A2 B2 B1 B2 B1 6 10/19/09 Linked Genes • Further apart genes are on same chromosome – experience more recombina<on • Genes closer together – experience less recombina<on • Use this to map genes on a chromosome! Linked Genes • Genes found on same chromosome = LINKAGE • Genes – arranged linearly along chromosome (you already knew this) • X.O. occurs at random – can occur at any loca<on along length of chromosome • Thus – logical that the shorter the distance b/w any 2 genes on a chromosome, the less likely X.O. is to take place b/w those 2 genes. • X.O. breaks up linked alleles & makes it appear as if independent assortment is occurring. 7 10/19/09 Linked Genes Linked genes • Alleles of syntenic (linked) genes transmiTed to gametes together. • Get only 2 of the 4 possible allele combina<ons in gametes. • X.O. breaking up gene<c linkage. • Get all 4 possible combina<ons of the alleles of the 2 genes in gametes. Linked Genes • Crossing over (X.O.) occurs b/w genes on the same chromosome (e.g., Chr 1). • i.e., b/w genes on homologous chromosomes. • X.O. involves the physical exchange of segments (& thus, any genes within the segment) of homologous chromosomes. • Linked genes are inherited together unless X.O. takes place • When X.O. occurs, gene4c recombina4on takes place. • NB: These are a pair of homologous chromosomes, with the same genes, but diﬀerent alleles. 8 10/19/09 Recombina<on Frequency Recombina<on frequency (Rf) = # Oﬀspring with recombinant phenotypes Total # of oﬀspring • Further apart genes are more recombina<on Parental chromosome Parental chromosome Calculate the recombina<on frequency of the white (eye colour) and yellow (body colour) genes. 1. 2. 3. 4. 98.5% 2.9% 52% 1.4% 9 10/19/09 Ques<on: Recombina<on Frequency Recombina<on frequency (Rf) = # Oﬀspring with recombinant phenotypes Total # of oﬀspring Parental chromosome Parental chromosome Rf = 86 + 44 4292 + 4605 + 86 + 44 Rf = 130 = 0.014 9027 1.4 % recombina<on frequency 1.4% of oﬀspring have recombinant phenotypes recombinant genotypes Gene<c Maps & Recombina<on Frequency Recombina<on frequency (Rf) = # Oﬀspring with recombinant phenotypes Total # of oﬀspring • 1 cen<Morgan (cM) = 1 map unit (m.u.) • 1 cM = physical distance produces 1 % recombinant oﬀspring Rf = 1.4% 1.4 cM (or map units) apart 10 10/19/09 Gene<c Maps & Recombina<on Frequency • 1 cM = 1 map unit (m.u.) = physical distance produces 1% recombinant oﬀspring • If have 3 or more genes can produce a gene<c map with their rela<ve loca<ons. 260 km Toronto 540 km Toronto 540 km 260 km Toronto Kingston 540 km Montreal 280 km Kingston Montreal 260 km Kingston Montreal Toronto Kingston Montreal 11 ...
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This note was uploaded on 01/10/2010 for the course BIOLOGY biol2040 taught by Professor Tamarakelly during the Fall '09 term at York University.
- Fall '09