TECHNIQUE 2 Parental generation (P) Stamens Carpel 3 4 RESULTS First filial generation offspring (F ) 5 2
EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F Generation (hybrids) All plants had purple flowers F 2 Generation 705 purple-flowered plants 224 white-flowered plants 3
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Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers 5
P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p F Generation Appearance: Genetic makeup: Gametes: Purple flowers P p F 2 Generation P Sperm p Eggs P p PP pp 3 6
Phenotype Genotype Purple PP (homozygous) 3 Purple (heterozygous) 2 Purple (heterozygous) White pp (homozygous) Ratio 3: Ratio :2: 7
TECHNIQUE Dominant phenotype, unknown genotype: PP or? Recessive phenotype, known genotype: pp Predictions If PP If or Sperm Sperm p p p p P Eggs P Eggs P p pp pp RESULTS or All offspring purple offspring purple and offspring white 8
EXPERIMENT P Generation YYRR yyrr Gametes YR yr F Generation Predictions Hypothesis of dependent assortment YyRr Hypothesis of independent assortment Predicted offspring of F 2 generation Eggs YR yr Sperm / YR 2 yr YYRR YyRr YyRr yyrr or Eggs YR Yr yr Sperm / YR 4 / 4 Yr / yr 4 yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyrr yyrr 3 Phenotypic ratio 3: yr YyRr Yyrr yyrr yyrr 9 / 6 3 / 6 3 / 6 / 6 RESULTS 35 08 0 32 Phenotypic ratio 9:3:3: Phenotypic ratio approximately 9:3:3: 9
Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm Sperm R r R R R R r Eggs r r r R r 0
P Generation Red C R C R White C W C W Gametes C R C W F Generation Pink C R C W C R Gametes C W F 2 Generation Eggs C R C R Sperm C W C R C R C R C W C W C R C W C W C W
Allele I A I B Carbohydrate i none (a) The three alleles for the ABO blood groups and their associated carbohydrates A B Genotype I A I A or I A i Red blood cell appearance Phenotype (blood group) A I B I B or I B i B I A I B AB ii O (b) Blood group genotypes and phenotypes 2
BbCc BbCc Eggs Sperm BC bc Bc bc BC BBCC BbCC BBCc BbCc bc BbCC bbcc BbCc bbcc Bc BBCc BbCc BBcc Bbcc bc BbCc bbcc Bbcc bbcc 9 : 3 : 4 3
AaBbCc AaBbCc Sperm / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 Eggs / 8 / 8 / 8 / 8 / 8 Phenotypes: Number of / 64 6 / 5 64 / 20 64 / 5 64 / 6 64 / 64 / 64 dark-skin alleles: 0 2 3 4 5 6 4
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Key Male Female Affected male Affected female Mating Offspring, in birth order (first-born on left) st generation (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, and uncles) Ww ww ww Ww Ww ww 3rd generation (two sisters) WW or Ww ww Widow s peak No widow s peak (a) Is a widow s peak a dominant or recessive trait? st generation (grandparents) Ff Ff ff Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff 3rd generation (two sisters) ff FF or Ff Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait? 6
Amniotic fluid withdrawn Fetus Centrifugation Fetus Placenta Uterus Cervix Placenta Chorionic villi Suction tube inserted through cervix Fluid Fetal cells Several hours Several weeks Biochemical tests Several hours Fetal cells Several weeks Karyotyping Several hours (a) Amniocentesis (b) Chorionic villus sampling (CVS) 7
Degree of dominance Description Example Complete dominance of one allele Heterozygous phenotype same as that of homozygous dominant PP Incomplete dominance of either allele Heterozygous phenotype intermediate between the two homozygous phenotypes C R C R C R C W C W C W Codominance Heterozygotes: Both phenotypes expressed I A I B Multiple alleles Pleiotropy In the whole population, some genes have more than two alleles One gene is able to affect multiple phenotypic characters ABO blood group alleles I A, I B, i Sickle-cell disease 8
Relationship among genes Description Example Epistasis One gene affects the expression of another BbCc BC BC bc Bc BbCc bc bc Bc bc Polygenic inheritance A single phenotypic character is affected by two or more genes AaBbCc 9 : 3 : 4 AaBbCc 9