Steps in solving heredity problems:

Transcription

1 Seps in solving herediy problems: Monohybrid crosses I. Boh prens re heerozygous The llele of brown eyes is inheried s n uosoml dominn llele. If boh prens re heerozygous, wh is he probbiliy h hey cn hve child wih blue eyes? Wih brown eyes? 1. Assign symbol for ech llele A cpil leer is used for dominn llele nd smll leer for recessive llele. Use he symbol A for brown eyes nd for blue eyes. llele genoype phenoype A AA, A Brown eyes Blue eyes 2. Deermine he genoype of ech pren nd indice ming Becuse boh prens re heerozygous, he mle genoype is A, femle genoype is lso A. 3. Deermine ll he possible kinds of gmees ech pren cn produce Becuse of gmees re hploid, hey cn hve only one llele insed of he wo presen in he diploid cell. As he mle hs boh he brown-eyed llele nd he blue-eyed llele, hen hlf of his gmees will conin he brown-eyed llele nd he oher hlf will conin he blue-eyed llele. Becuse he femle hs he sme genoype, her gmees will be he sme s his. To solve geneic problems, Punne squre is used. Femle gmees Mle gmees T T 4. Deermine ll he gene combinions h cn resul when gmees copule To deermine he possible combinions of lleles h could occur s resul of his ming, simply fill in ech of he empy squres wih he lleles h cn be doned from ech pren. Femle gmees T Mle gmees T TT T T

2 5. Deermine he phenoype of ech possible gene combinion In his problem, hree of he offsprings, TT, T nd T, hve brown eyes. One progeny, hs blue eyes. Therefore, he nswer o he problem is h he probbiliy of hving offspring wih brown eyes is ¾ (75%) nd wih blue eyes is ¼ (25%). All monohybrid problems cn be solved using his mehod II. One pren is heerozygous nd he oher is homozygous for ri: If one pren is heerozygous for eyes color nd he oher is homozygous for his ri, wh is he probbiliy h hey will hve child wih brown eyes? Blue eyes? Sep1: Use he symbol A for brown eyes nd for blue eyes. llele genoype phenoype A AA, A Brown eyes Blue eyes Sep 2: A x Sep3: Sep4: A A A Sep 5: In his problem, ½ (50%) of he progeny will hve brown eyes nd ½ (50%) will hve blue eyes. A dihybrid cross A dihybrid cross is sudy in which wo pirs of lleles re followed from he prenl generion o he offspring. This problem is solved in bsiclly he sme wy s he monohybrid cross. The min difference is h in he dihybrid cross you re working wih wo differen chrcerisics from ech pren. I is necessry o use Mendel`s lw of independen ssormen when considering double-fcor problems. Recll h ccording o his lw, members of one llelic pir sepre from ech oher independenly of he members of oher pirs of lleles. This hppens during meiosis when he chromosomes segrege (Mendel`s

3 lw of independen ssormen pplies only if he wo pirs of lleles re loced on sepre chromosomes). Problem: In humns, he llele for free erlobes is dominn over he llele for ched erlobes. The llele for drk hir domines over he llele for ligh hir. If boh prens re heerozygous for erlobes nd hir color, wh ypes of offspring cn hey produce, nd wh is he probbiliy for ech ype? Sep 1; Use he symbol: E - free erlobes e - ched erlobes D - drk hir d - ligh hir. EE, Ee ee DD, Dd dd Free erlobes Ached erlobes Drk hir Ligh hir Sep 2: Deermine he genoype for ech pren nd show ming: EeDd X EeDd Sep 3: Deermine ll he possible gmees ech pren cn produce nd wrie he symbols for he lleles in Punne squre. Becuse here re wo pirs of lleles in double-fcor cross, ech gmee mus conin one llele from ech pir. Thus, ech pren cn produce four differen kinds of gmees. To deermine he possible gene combinions in he gmees, selec one llele from one of he pirs of lleles nd mch i wih one llele from he oher pir of he lleles. Then, mch he second llele from he firs pir of lleles wih ech of he lleles from he second pir. This my be done in following wy: ED Ed ed ed ED Ed ed ed Sep 4: Deermine ll he gene combinions h cn resul when hese gmees unie. Fill in he Punne squre:

4 ED Ed ed ed ED EEDD EEDd EeDD EeDd Ed EEDd EEdd EeDd Eedd ed EeDD EeDd eedd eedd ed EeDd Eedd eedd eedd Sep 5: Deermine he phenoype of ech possible gene combinion. In his problem here re 16 possible wys in which gmees cn combine o produce n offspring. There re four possible phenoypes in his cross. They re represened in he following chr: E-D- (EEDD, EEDd, EeDd, EeDD) - free erlobes, drk hir E-dd (EEdd nd EeDD) free erlobes, ligh hir eed- (eedd nd eedd) - ched erlobes, drk hir eedd - ched erlobes, ligh hir The probbiliy of hving given phenoype is: 9/16 free erlobes, drk hir 3/16 free erlobes, ligh hir 3/16 ched erlobes, drk hir 1/16 ched erlobes, ligh hir Codominnce Problem: If one pren is heerozygous for A blood group nd he oher is heerozygous for B blood group, wh phenoype cn heir child hve? Sep 1: I A A blood group, I B B blood group In he AB0 sysem, A nd B show codominnce when hey re ogeher in he sme individul; bu boh re dominn over he 0 llele. I A I A, I A I 0 I B I B, I B I 0 I A I B A blood group B blood group AB blood group Sep 2: I A I 0 X I B I 0 Sep 3: Gmees: I B I 0 I A I O Sep 4: I B I 0 I A I A I B I A I 0

5 I 0 I B I 0 I 0 I 0 Sep 5: Thus, his cross resuls in four differen phenoypes 0, A, B, nd AB blood groups. 1. Seps in solving herediy problems: X-linked Problem: In humns he gene for norml color vision is dominn nd he gene for color deficiency is recessive. Boh genes re X-linked. People who re color blind re no relly blind, bu should more ppropriely be described s hving color defecive vision. A mle who hs norml vision mes wih femle who is heerozygous for norml color vision. Wh ype of children cn hey hve in erms of hese ris, nd wh is he probbiliy for ech ype? Sep 1: This condiion is linked o he X chromosome, so i hs become rdiionl o symbolize he llele s he superscrip on he leer X. Becuse he Y chromosome does no conin homologous llele, only he leer Y is used. X D norml color vision X d color deficien Y no gene resen X D Y X d Y X D X D X D X d X d X d Mle, norml color vision Mle, color deficien Femle, norml color vision Femle, norml color vision Femle, color deficien Sep 2: Mle`s genoype - X D Y (norml color vision), femle`s genoype - X D X d (norml color vision). X D X d X X D Y Seps 3, 4: The genoype of he gmees re lised in he Punne squre: X D X D X D X D X D X d Y X D Y X d Y Sep 5: The phenoype of he offspring re deermined: X D X D, X D X d - dughers wih norml color vision (100%) X D Y - sons wih norml color vision (50%) X d Y - sons wih color blindness (50%) X d