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 Stamens(male)producepollen Carpel(female)produceseggs Flower dissected to showreproductive structures Intact pea flower
Parental: Smooth seed x Wrinkled seed F1: All smooth seed coats F2: 5474 smooth: 1850 wrinkled (3/4 smooth to 1/4 wrinkled) Mendel’s Experiment With Peas Differing in a Single Trait F1 smooth plants x F1 smooth plants
Patterns of Inheritance • Mendel needed to explain • 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 • 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.
Parental: Smooth seed x Wrinkled seed F1: All smooth seed coats F1 smooth plants x F1 smooth plants Ss Ss Heterozygous Heterozygous Mendel’s Experiment With Peas Differing in a Single Trait SS ss HomozygousDominant HomozygousRecessive Ss Heterozygous F2
S S S S Homozygous parents can only pass one form of an allele to their offspring.
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.
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.
S S s s S s S s S s Genetic Segregation Parentals: SS x ss F1 x F1: Ss x Ss s s S s SS Ss S S Ss Ss Ss Ss ss Ss 100% Smooth seeds 75% Smooth seeds25% Wrinkled seeds
Traits Studied by Mendel Seed shape Seed color Pod shape Pod color Flower color Flower location Plant size
Parental: Smooth Yellow x Wrinkled Green F1: All smooth yellow seed coats F1 plants x F1 plants 315 smooth, yellow 9/16 108 smooth, green 3/16 F2 101 wrinkled, yellow 3/16 32 wrinkled, green 1/16 Mendel’s Experiment With Peas Differing in Two Traits
Patterns of Inheritance • Mendel needed to explain • 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.
SY SY SY SY sy sy sy sy sy SY Genetic Segregation + Independent Assortment Parentals: SSYY x s s y y F1: SsYy 100% smooth, yellow
SY Sy sY sy SY Sy sY sy Genetic Segregation + Independent Assortment F1 x F1 : S s Y y x S s Y y Four different types of gametes are formed in equal proportions.
Eggs F1 x F1 SsYy X SsYy 14 14 14 14 SY Sy sY sy SY 14 116 116 116 116 SSYY SSYy SsYY SsYy Sy 14 116 116 116 116 SSYy SSyy SsYy Ssyy Pollen sY 14 116 116 116 116 SsYY SsYy ssYY ssYy sy 14 116 116 116 116 SsYy Ssyy ssYy ssyy
Meiotic Segregation Explains Independent Assortment Two possibleorientations
Additional Genetic Patterns Incomplete dominance: neither allele masks the other and both are observed as a blending in the heterozygote
Red x White RR R’R’ Four o’clock flowers R = red, R’ = white Pink RR’ Incomplete Dominance
Incomplete Dominance ½ R ½ R’ F1 x F1Pink x Pink RR’ x RR’ ¼ RR ¼ RR’ ½ R ½ R’ 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 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 A=B> o A and B are codominant. Aand B are completely dominant over o.
Type A Type B A and B Neither • Type • Genotype • Antigen on RBCs • Anti-bodies • Re-ceives • Donates • Freq Human ABO Blood Types • A • AA or Ao • B • A or O • A or AB • 40% • B • BB or Bo • A • B or O • B or AB • 10% • AB • AB • Neither • AB, A,B, O(universal) • AB(universal) • 4% • O • oo • Both • O • O,AB,A,B(universal) • 46% Codominance is observed for Type AB Blood since the products of both the A and B allelesare found on the cells.
Inheritance of Rh Factor *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.
Br or Type A, Rh positive x Type B, Rh negative Multiple Alleles and Codominance AoRr x Borr AR Ar oR or ABrr BoRr Borr ABRr Aorr ooRr oorr AoRr Phenotypic Ratio of Offspring 1/8 Type AB positive 1/8 Type A positive 1/8 Type AB negative 1/8 Type A negative 1/8 Type B positive 1/8 Type O positive 1/8 Type B negative 1/8 Type O negative
Additional Genetic Patterns Polygenic Inheritance: Many genes affect one trait
Example of Polygenic Inheritance Two genes affecting one trait *Based on a study conducted in Jamaica.
Grandma aabb Example of Polygenic Inheritance Medium Black Woman X Darkest Black Man (her mother is white) AABB AaBb AB Ab aB ab AABb AaBB AaBb AB AABB Dark Black Dark Black DarkestBlack Medium Black ¼ Darkest Black + ½ Dark Black + ¼ Medium Black
Additional Genetic Patterns Pleiotropy: One gene affects many traits
Sickle-Cell Anemia One gene affects many phenotypic characteristics