1 TECHNIQUE 1 2 Parental generation (P) Stamens Carpel 3 4 RESULTS First filial generation offspring (F1) 5 2 EXPERIMENT P Generation (true-breeding parents) F1 Generation (hybrids) Purple flowers White flowers All plants had purple flowers F2 Generation 705 purple-flowered 224 white-flowered plants plants 3 4 Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers 5 P Generation Purple flowers White flowers Appearance: Genetic makeup: PP pp Gametes: p P F1 Generation Appearance: Genetic makeup: Gametes: Purple flowers Pp 1/ 2 1/ 2 P p Sperm F2 Generation P p PP Pp Pp pp P Eggs p 3 1 6 3 Phenotype Genotype Purple PP (homozygous) Purple Pp (heterozygous) 1 2 1 Purple Pp (heterozygous) White pp (homozygous) Ratio 3:1 Ratio 1:2:1 1 7 TECHNIQUE Dominant phenotype, Recessive phenotype, unknown genotype: known genotype: PP or Pp? pp Predictions If PP Sperm p p P Pp Eggs If Pp Sperm p p or P Pp Eggs P Pp Pp pp pp p Pp Pp RESULTS or All offspring purple 1/2 offspring purple and offspring white 1/2 8 EXPERIMENT YYRR P Generation yyrr Gametes YR F1 Generation YyRr Hypothesis of dependent assortment Predictions yr Hypothesis of independent assortment Sperm or Predicted offspring of F2 generation 1/ 4 Sperm 1/ YR 1/ 2 2 yr 1/ 4 1/ 2 YR 1/ 4 1/ 4 Yr yR 1/ 4 yr YR YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr YR YYRR Eggs 1/ 2 YyRr 1/ 4 Yr Eggs yr YyRr 3/ 4 yyrr 1/ 4 yR 1/ 4 Phenotypic ratio 3:1 1/ 4 yr 9/ 16 3/ 16 3/ 16 1/ 16 Phenotypic ratio 9:3:3:1 RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 9 Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm Sperm 1/ R 2 R 1/ 2 r R R Eggs 4 r 2 r 2 R 1/ 1/ 1/ 1/ 4 r r R r 1/ 4 1/ 4 10 P Generation Red CRCR White CWCW CR Gametes CW Pink CRCW F1 Generation Gametes 1/2 CR 1/ CW 2 Sperm 1/ 2 CR 1/ 2 CW F2 Generation 1/ 2 CR Eggs 1/ 2 CRCR CRCW CRCW CWCW CW 11 Allele IA IB Carbohydrate A B i none (a) The three alleles for the ABO blood groups and their associated carbohydrates Genotype Red blood cell appearance Phenotype (blood group) IAIA or IA i A IBIB or IB i B IAIB AB ii O (b) Blood group genotypes and phenotypes 12 BbCc BbCc Sperm 1/ 4 BC 1/ 4 bC 1/ 4 Bc 1/ 4 bc Eggs 1/ 1/ 1/ 1/ 4 BC BBCC BbCC BBCc BbCc BbCC bbCC BbCc bbCc BBCc BbCc BBcc Bbcc BbCc bbCc Bbcc bbcc 4 bC 4 Bc 4 bc 9 : 3 : 4 13 AaBbCc AaBbCc Sperm 1/ Eggs 1/ 8 1/ 8 1/ 8 1/ 8 1/ 1/ 8 1/ 1/ 8 8 1/ 8 1/ 64 15/ 8 1/ 1/ 8 8 8 1/ 8 1/ 8 1/ 8 8 1/ Phenotypes: 64 Number of dark-skin alleles: 0 6/ 64 1 15/ 64 2 20/ 3 64 4 6/ 64 5 1/ 64 6 14 15 Key Male Female 1st generation (grandparents) Affected male Affected female Ww Mating Offspring, in birth order (first-born on left) ww 2nd generation (parents, aunts, Ww ww ww Ww and uncles) ww Ww Ww ww 3rd generation (two sisters) WW or Ww ww No widow’s peak Widow’s peak (a) Is a widow’s peak a dominant or recessive trait? 1st generation (grandparents) Ff 2nd generation (parents, aunts, FF or Ff ff and uncles) Ff ff ff Ff Ff Ff ff ff FF or Ff 3rd generation (two sisters) Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait? 16 Amniotic fluid withdrawn Centrifugation Fetus Fetus Placenta Uterus Placenta Cervix Fluid Fetal cells BioSeveral chemical hours tests Several weeks Several weeks (a) Amniocentesis Karyotyping Chorionic villi Several hours Suction tube inserted through cervix Fetal cells Several hours (b) Chorionic villus sampling (CVS) 17 Degree of dominance Complete dominance of one allele Example Description Heterozygous phenotype PP same as that of homozygous dominant Pp Incomplete dominance Heterozygous phenotype intermediate between of either allele the two homozygous phenotypes CRCR Codominance Heterozygotes: Both phenotypes expressed CRCW CWCW IAIB Multiple alleles In the whole population, ABO blood group alleles some genes have more IA , IB , i than two alleles Pleiotropy One gene is able to affect multiple phenotypic characters Sickle-cell disease 18 Relationship among genes Epistasis Example Description One gene affects the expression of another BbCc BbCc BC bC Bc bc BC bC Bc bc 9 Polygenic inheritance A single phenotypic AaBbCc character is affected by two or more genes :3 :4 AaBbCc 19