Transcript SY Sy sY
Patterns of Inheritance By observing how traits are passed to the next generation, how can the inheritance patterns be used to understand the principles of heredity? Use of Garden Pea for Genetics Experiments Carpel (female) produces eggs Intact pea flower Stamens (male) produce pollen Flower dissected to show reproductive structures Mendel’s Experiment With Peas Differing in a Single Trait Parental: Smooth seed x Wrinkled seed F1: All smooth seed coats F1 smooth plants x F1 smooth plants F2: 5474 smooth: 1850 wrinkled (3/4 smooth to 1/4 wrinkled) Patterns of Inheritance Mendel needed to explain 1. Why one trait seemed to disappear in the first generation. 2. Why the same trait reappeared in the second generation in one-fourth of the offspring. Mendel’s Proposal 1. Each trait is governed by two factors – now called genes. 2. Genes are found in alternative forms called alleles. 3. Some alleles are dominant and mask alleles that are recessive. Mendel’s Experiment With Peas Differing in a Single Trait Parental: Smooth seed x Wrinkled seed SS ss Homozygous Homozygous Dominant Recessive F1: All smooth seed coats Ss Heterozygous F1 smooth plants x F1 smooth plants Ss Ss Heterozygous Heterozygous F2 Homozygous parents can only pass one form of an allele to their offspring. S S S S Heterozygous parents can pass either of two forms of an allele to their offspring. S s S s Locus: Area on the chromosome where a gene is located. For a heterozygote, homologous chromosomes will have different alleles at the same locus. Additional Genetic Terms Term Genotype Definition Alleles carried by an individual Example SS, Ss, ss Phenotype Physical characteristic smooth or or appearance of an wrinkled individual Mendel’s Principle of Genetic Segregation In the formation of gametes, the members of a pair of alleles separate (or segregate) cleanly from each other so that only one member is included in each gamete. Each gamete has an equal probability of containing either member of the allele pair. Genetic Segregation Parentals: SS x ss S S s s F1 x F1: Ss x Ss s S s S Ss Ss S Ss Ss S s s S S SS Ss Ss ss s s 100% Smooth seeds 75% Smooth seeds 25% Wrinkled seeds Traits Studied by Mendel Seed shape Seed color Pod shape Pod color Flower color Flower location Plant size Mendel’s Experiment With Peas Differing in Two Traits Parental: Smooth Yellow x Wrinkled Green F1: All smooth yellow seed coats F1 plants x F1 plants F2 315 smooth, yellow 9/16 108 smooth, green 3/16 101 wrinkled, yellow 3/16 32 wrinkled, green 1/16 Patterns of Inheritance Mendel needed to explain 1. Why non-parental combinations appeared in the F2 offspring. 2. Why the ratio of phenotypes in the F2 generation was 9:3:3:1. Mendel’s Principle of Independent Assortment When gametes are formed, the alleles of one gene segregate independently of the alleles of another gene producing equal proportions of all possible gamete types. Genetic Segregation + Independent Assortment Parentals: SSYY x SY SY SY SY sy sy sy F1: SY ssyy SsYy 100% smooth, yellow sy sy Genetic Segregation + Independent Assortment F1 x F1 : S s Y y SY Sy sY sy x SsYy SY Sy sY sy Four different types of gametes are formed in equal proportions. F1 x F1 SsYy X SsYy 1 4 SY 1 4 Sy Pollen 1 4 sY 1 4 sy 1 16 1 16 1 16 1 16 1 4 Eggs 1 1 4 Sy 4 sY SY SSYY 1 16 SSYy 1 16 SsYY 1 16 SsYy 1 16 SSYy 1 16 SSyy 1 16 SsYy 1 16 Ssyy 1 16 1 4 sy SsYY 1 16 SsYy SsYy 1 16 Ssyy ssYY 1 16 ssYy ssYy 1 16 ssyy F2 Genotypes and Phenotypes Phenotypes Smooth Yellow Genotypes Smooth Green Wrinkled Yellow Wrinkled Green 1/16 SSyy+ 2/16 Ssyy Total = 3/16 S_yy 1/16 SSYY + 2/16 SSYy + 2/16 SsYY + 4/16 SsYy Total = 9/16 S_Y_ 1/16 ssYY+ 2/16 ssYy Total = 3/16 ssY_ 1/16 ssyy Meiotic Segregation Explains Independent Assortment Two possible orientations Additional Genetic Patterns Mendel’s peas Alternative Pattern Complete Dominance Incomplete Dominance Incomplete dominance: neither allele masks the other and both are observed as a blending in the heterozygote Incomplete Dominance Red RR x White R’R’ Four o’clock flowers R = red, R’ = white Pink RR’ Incomplete Dominance F1 x F1 Pink x Pink RR’ x RR’ ½R ½ R’ ½R ½ R’ ¼ RR ¼ RR’ ¼ RR’ ¼ R’R’ Genotypic Ratio: ¼ RR + ½ RR’ + ¼ R’R’ Phenotypic Ratio: ¼ red + ½ pink + ¼ white Additional Genetic Patterns Mendel’s peas Alternative Patterns Complete Dominance Codominance Two alleles per gene Multiple Alleles Codominance: Neither allele masks the other so that effects of both alleles are observed in heterozygotes without blending Multiple Alleles: Three or more alleles exist for one trait Note: A diploid individual can only carry any two of these alleles at once. Multiple Alleles and Codominance ABO Blood Type in Humans Blood Type Allele Type A A Type B B Type O o A= B > o A and B are codominant. A and B are completely dominant over o. Human ABO Blood Types •Type •Genotype •Antigen on RBCs •Antibodies •Receives •Donates •Freq •A •AA or Ao Type A •B •A or O •A or AB •40% •B •BB or Bo Type B •A •B or O •B or AB •10% •Neither •AB, A, B, O •AB •AB •AB A and B (universal) •O •oo Neither •Both •O (universal) •O,AB, A,B (universal) Codominance is observed for Type AB Blood since the products of both the A and B alleles are found on the cells. •4% •46% Inheritance of Rh Factor Phenotype Genotype* Gene Product Rh Positive RR or Rr Rh Negative rr Antibodies Present Rhesus Protein None None None unless exposed *Although there are multiple R alleles, R1, R2, R3, etc. all are completely dominant over all of the r alleles, r1, r2, r3, etc. ABO Blood Type and Rh Factor are controlled by separate genes. They show independent assortment. Multiple Alleles and Codominance Type A, Rh positive x Type B, Rh negative AoRr AR x Borr Br ABRr Ar ABrr oR BoRr or Borr or Aorr ooRr oorr AoRr Phenotypic Ratio of Offspring 1/8 Type AB positive 1/8 Type AB negative 1/8 Type B positive 1/8 Type B negative 1/8 Type A positive 1/8 Type A negative 1/8 Type O positive 1/8 Type O negative Additional Genetic Patterns Mendel’s peas Alternative Patterns One gene affects one trait Polygenic Inheritance Polygenic Inheritance: Many genes affect one trait Example of Polygenic Inheritance Two genes affecting one trait Number of Skin Color* Dominant (Phenotype) Alleles 0 White Genotypes % Pigmentation* aabb 0-11% 1 Light Black Aabb or aaBb 12-25% 2 26-40% 3 Medium Black AAbb or AaBb or aaBB Dark Black AABb or AaBB 4 Darkest Black AABB 56-78% 41-55% *Based on a study conducted in Jamaica. Example of Polygenic Inheritance Grandma aabb Medium Black Woman X Darkest Black Man (her mother is white) AABB AaBb AB AB Ab AABB AABb Darkest Black Dark Black aB ab AaBB AaBb Dark Black Medium Black ¼ Darkest Black + ½ Dark Black + ¼ Medium Black Additional Genetic Patterns Mendel’s peas Alternative Patterns One gene affects one trait Pleiotropy Pleiotropy: One gene affects many traits Sickle-Cell Anemia One gene affects many phenotypic characteristics Gene Product Cell Shape Disease Conditions SS Hemoglobin A Spherical, slightly concave No anemia SS’ Hemoglobin A Hemoglobin S Some sickling under extreme conditions Sickle Cell Trait Resistance to Malaria S’S’ Hemoglobin S Sickled under low O2 tension Sickle Cell Anemia