TOPIC 7: MAPPING GENES

Transcription

1 Page 1 OPIC 7: MAPPING GENES MAPPING GENES 7.1: uning Cossove Fequencies ino Maps Making a geneic map compises hee pocesses: 1. Couning he numbe of cossove evens beween wo genes 2. Conveing cossove evens ino disances 3. Figuing ou whee he genes ae siuaed elaive o each ohe Suppose wo genes ae descibed. You wan o make a geneic map. We sudy each of hese hee pocesses in un in ode o make he geneic map. Pocess 1: How many cossove evens wee hee? You ae ying o figue ou he posiions of wo genes elaive o one anohe. he wo alleles of one gene, gene R, ae efeed o as R and. he wo alleles of he ohe gene, gene, ae efeed o as and. You coss an RR o an, and all he pogeny will be R. Now, you coss he R o an (a backcoss). Was hee cossing ove? he following diagams show he siuaions of no cossing ove and of cossing ove.

2 OPIC 7: MAPPING GENES / Page 2 No Cossing Ove R R MEIOSIS I R R MEIOSIS II R R Cossing Ove R R R R MEIOSIS I MEIOSIS II R R

3 OPIC 7: MAPPING GENES / Page 3 If no cossing occued ove duing gameogenesis, hen he heeozygoe has made gamees ha ae eihe R o, while he homozygoe has made only gamees. heefoe, he genoypes ha esul when a gamee fom he heeozygoe mees a gamee fom he homozygoe (if cossing ove has no happened) ae R and. If cossing ove did occu duing gameogenesis, hen he heeozygoe also made some gamees ha ae eihe R o. he homozygoe sill made gamees only. So, he genoypes ha esul when hese gamees mee (if cossing ove has occued) ae R and, in addiion o R and. Diploid Adul Diploid Adul RR Haploid Gamees R R R R R R CROSSING OVER OCCURED DURING HE FORMAION OF HESE GAMEES R R R

4 OPIC 7: MAPPING GENES / Page 4 o calculae he fequency of cossoves, you add he numbe of R s plus he numbe of s hen divide by he oal numbe of offsping. EXAMPLE: Genoype Numbe of offsping of ha genoype R R he numbe of offsping in which cossing ove occued is he numbe of R s plus he numbe of s, o = 28. he oal numbe of offsping is = 115. he fequency of cossing ove is 28 evens 115 offsping = 0.24 evens offsping Pocess 2: How fa apa ae he wo genes? I may seem sange, bu o figue ou he disances beween pais of genes, you use fequency daa o deemine lengh. he unis in he compuaion ae also unusual. he good news is ha he pocedue is exacly he same as conveing beween any wo dimensions. You se up you convesion equaion hen solve i. In his siuaion, he fequency you measue is he numbe of ecombinaion evens pe numbe of offsping, and he unis of lengh ae Mogans [M], o, moe usually, cenimogans [cm]. Hee is he convesions shocu, again, as a eminde.

5 OPIC 7: MAPPING GENES / Page 5 CONVERSIONS SHORCU 1. Wie an equaion. 2. Coun unis. 3. Inse ha many facions. 4. Make unis cancelable. 5. Make facions =1, hen calculae. So, o conve numbe of ecombinaion evens pe offsping o cm, you follow he same diecions: 1. numbe of evens offsping =?cm 2. hee diffeen unis 3. numbe of evens offsping =?cm 4. numbe of evens offsping offsping cm = numbe of evens?cm Hee is whee i ges icky. How do you make hese facions equal o one? Wha is he elaionship beween he unis? he elaionship beween he unis is given in he definiion of he cenimogan: 1 cm = 1 even pe 100 offsping = 1 even 100 offsping 1 cm 100 offsping Reaanging ha definiion we find ha = = 1 1 even

6 OPIC 7: MAPPING GENES / Page 6 ha is he elaionship among he unis. Reuning o sep 5 of he convesion shocu, we wie: 5. numbe of evens offsping 1 cm 100 offsping =? cm even As you can see, all you ae doing is muliplying he fequency by 10 2 while conveing unis. I s acually quie simple once you know he ule. EXAMPLE: Conside he same genes R and as in he example above. If hee wee 28 cossove evens ou of 115 offsping, how many cm apa ae he wo genes? 28 evens 115 offsping 1 cm 100 offsping = 24 cm even he gene is 24 cm away fom he R gene. Pocess 3: Whee ae he alleles locaed? his quesion comes up when you ae mapping moe han wo genes. o figue gene posiions elaive o each ohe, you figue he disances beween each pai (which means doing hee backcosses), hen look o see wha ode is consisen wih all hee disances. Le s add gene A, wih is wo alleles, A and a. Hee ae he esuls of he backcosses: Genes Cossoves oal numbe of offsping Cossove fequency cm R o A o R o A

7 OPIC 7: MAPPING GENES / Page 7 Fom his cha, you can ell he disances beween each pai of genes. o go fom he cha o a map akes wo seps: 41 Sep one: Sa he map using he wo genes ha ae sepaaed by he geaes disance. 24 Sep wo: Pu he hid gene beween he ohe wo, he coec disance fom eihe (boh) of he fis wo. ha is you geneic map.

8 OPIC 7: MAPPING GENES / Page 8 MAPPING GENES RY I OU EXERCISE I: he genes AP7B, BRCA2, and RB1 ae found on human chomosome 13. Map hem using hese daa (he daa ae compleely imaginay, he map will be elaively accuae): Genes Evens oal numbe of offsping AP7B and BRCA AP7B and RB BRCA2 and RB EXERCISE II: Map he imaginay genes Hl, Oia, Pvs, and Sdva Genes Evens oal numbe of offsping Hl and Oia Hl and Pvs Hl and Sdva Oia and Pvs Oia and Sdva Pvs and Sdva LINKS O ANSWERS EXERCISE I EXERCISE II

9 OPIC 7: MAPPING GENES / Page 9 RY I OU: ANSWERS EXERCISE I: he genes APB, BRCA2, and RB1 ae found on human chomosome 13. Map hem using hese daa.

10 OPIC 7: MAPPING GENES / Page 10 RY I OU: ANSWERS EXERCISE II: Map he imaginay genes Hl, Oia, Pvs, and Sdva.