Chapter 14 Mendel and the Gene Idea

Transcription

1 Chater 14 Mendel and the Gene Idea Histrical Views f Heredity Histrically, what gene6c rinciles accunt fr the assing f traits frm arents t ffsring? Acquired traits Traits gained by arents in their lives are assed t ffsring Blending hythesis Idea that gene6c material frm the tw arents blends tgether Like blue and yellw aint blend t make green The ar6culate hythesis Mendels Thery Idea that arents ass n discrete heritable units (genes) Gregr Mendel Dcumented a ar6culate mechanism thrugh his exeriments with garden eas Discvered the basic rinciles f heredity by breeding garden eas in carefully lanned exeriments 1

2 Mendel s Exerimental, Quan?ta?ve Arach Advantages f ea lants fr gene6c study: TECHNIQUE Many varie6es 1 with dis6nct heritable characters r traits Ma6ng f lants can be cntrlled arental generatin () 2 Stamens Carel Each ea lant 3 Has serm- rducing stamens and egg- rducing carels Crss- llina6n RESULTS First filial generatin ffsring (F 1) 4 5 Can be achieved by dus6ng ne lant with llen frm anther Mendel s Exerimental, Quan?ta?ve Arach Mendel nly tracked characters that varied in an either- r manner used varie6es that were true- breeding rduce ffsring f same variety when self- llinated Mendel mated tw cntras6ng, true- breeding varie6es Called hybridiza?n genera?n Hybrid ffsring f the genera6n Called the F 1 genera?n When F 1 individuals self- llinate, the F 2 genera?n is rduced Mendel The Law f Segrega?n Crssed cntras6ng, true- breeding white and urle flwered ea lants All F 1 hybrids were urle Crssed the F 1 hybrids Many F 2 lants had urle flwers But sme had white Discvered a ra6 f abut three t ne urle t white flwers In F 2 genera6n 2

3 Fig EXERIMENT Generatin (true-breeding arents) urle flwers White flwers Fig EXERIMENT Generatin (true-breeding arents) urle flwers White flwers F 1 Generatin (hybrids) All lants had urle flwers Fig EXERIMENT Generatin (true-breeding arents) urle flwers White flwers F 1 Generatin (hybrids) All lants had urle flwers F 2 Generatin 705 urle-flwered lants 224 white-flwered lants 3

4 Mendel Mendel s Exerimental, Quan?ta?ve Arach Reasned that nly the urle flwer factr was affec6ng flwer clr In the F 1 hybrids Called the urle flwer clr a dminant trait And the white flwer clr a recessive trait Observed the same auern f inheritance In six ther ea lant characters Each reresented by tw traits What Mendel called a heritable factr is what we nw call a gene Mendel s Mdel Mendel develed a hythesis t exlain the 3:1 inheritance auern he bserved in F 2 ffsring Fur cncets can be related t what we nw knw abut genes and chrmsmes 1st: Alterna6ve versins f genes accunt fr varia6ns in inherited characters Alterna6ve versins f a gene are nw called alleles ie - urle r white Each gene resides at a secific lcus n a secific chrmsme Allele fr urle flwers Lcus fr flwer-clr gene Hmlgus air f chrmsmes Allele fr white flwers 2nd: Fr each character an rganism inherits tw alleles One frm each arent Did nt knw the rle f chrmsmes Tw alleles at a lcus n a chrmsme may be iden6cal True- breeding lants Alterna6vely, the tw alleles at a lcus may differ Hybrids Mendel s Mdel 4

5 3rd: If the tw alleles at a lcus differ Mendel s Mdel Then ne determines the rganism s aearance Dminant allele And the ther has n n6ceable effect n aearance Recessive allele In the flwer- clr examle, the F 1 lants had urle flwers because the allele fr that trait is dminant 4th: Mendel s Mdel Law f segrega?n Tw alleles fr a heritable character searate (segregate) During gamete frma6n and end u in different gametes Thus, an egg r a serm gets nly ne f the tw alleles Crresnds t the distribu6n f hmlgus chrmsmes T different gametes in meisis Calcula?ng ssible cmbina?ns Mendel s segrega6n mdel Accunts fr the 3:1 ra6 he bserved in the F 2 genera6n f his numerus crsses ssible cmbina6ns f serm and egg can be shwn Using a unneo square A diagram fr redic6ng the results f a gene6c crss between individuals f knwn gene6c makeu A caital leuer reresents a dminant allele = urle allele Lwercase leuer reresents a recessive allele = white allele 5

6 Fig Generatin Aearance: urle flwers White flwers Genetic makeu: Fig Generatin Aearance: urle flwers White flwers Genetic makeu: F 1 Generatin Aearance: Genetic makeu: urle flwers 1 / 2 1 / 2 Fig Generatin Aearance: urle flwers White flwers Genetic makeu: F 1 Generatin Aearance: Genetic makeu: urle flwers 1 / 2 1 / 2 F 2 Generatin Serm Eggs 3 1 6

7 Hmzygus Gene?c Vcabulary An rganism with tw iden6cal alleles fr the gene fr a character Hmzygus dminant Hmzygus recessive Heterzygus An rganism that has tw different alleles fr a gene fr a character Unlike hmzygtes heterzygtes are nt true- breeding Traits d nt always reveal gene6c cmsi6n Gene?c Vcabulary Different effects f dminant and recessive alleles hentye Gentye hentye urle (hmzygus) 1 hysical aearance Gentye 3 urle (heterzygus) 2 gene6c makeu urle (heterzygus) and lants have the same hentye (urle) 1 White (hmzygus) 1 but different gentyes Rati 3:1 Rati 1:2:1 The Testcrss Finding gentye f individual with dminant hentye TECHNIQUE Must have (at least) ne dminant allele Dminant hentye, unknwn gentye: r? Recessive hentye, knwn gentye: culd be either hmzygus dminant r heterzygus redictins If If r Serm Serm Testcrss: Eggs Eggs Breeding unknwn with hmzygus recessive individual RESULTS If any ffsring dislay the recessive hentye, the mystery arent must be heterzygus All ffsring urle r 1/2 ffsring urle and 1/2 ffsring white 7

8 The Law f Indeendent Assrtment Mendel derived the law f segrega?n by fllwing a single character The F 1 ffsring rduced in this crss were mnhybrids individuals that are heterzygus fr ne character A crss between such heterzygtes is called a mnhybrid crss 8