Principles of Heredity What patterns of inheritance can be observed when traits are passed to the next generation?
Parental: Round seed x Wrinkled seed F1: All round seed coats F2: 5474 round: 1850 wrinkled (3/4 round to 1/4 wrinkled) Mendel’s Experiment With Peas Differing in a Single Trait F1 round plants x F1 round plants
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: Round seed x Wrinkled seed F1: All round seed coats F1 round plants x F1 round plants Rr Rr Heterozygous Heterozygous Mendel’s Experiment With Peas Differing in a Single Trait RR rr HomozygousDominant HomozygousRecessive Rr Heterozygous F2
R R R R 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. R r R r 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.
R R r r R r R r R r Genetic Segregation Parentals: RR x rr F1 x F1: Rr x Rr r r R r RR Rr R R Rr Rr Rr Rr rr Rr 100% Round seeds 75% Round seeds25% Wrinkled seeds
Parental: Round Yellow x Wrinkled Green F1: All round yellow seed coats F1 plants x F1 plants 315 round, yellow 9/16 108 round, green 3/16 F2 101 wrinkled, yellow 3/16 32 wrinkled, green 1/16 Mendel’s Experiment With Peas Differing in Two Traits
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.
RY RYRYRY ry ryryry ry RY Genetic Segregation + Independent Assortment Parentals: RRYY x r r y y F1: RrYy 100% round, yellow
RY RyrY ry RY RyrYry Genetic Segregation + Independent Assortment F1 x F1 : R r Y y x R r Y y Four different types of gametes are formed in equal proportions.
Eggs F1 x F1 RrYy X RrYy 14 14 14 14 RY Ry rY ry RY 14 116 116 116 116 RRYY RRYy RrYY RrYy Ry 14 116 116 116 116 RRYy RRyy RrYy Rryy Pollen rY 14 116 116 116 116 RrYY RrYy rrYy rrYY ry 14 116 116 116 116 RrYy Rryy rrYy rryy
Gene Products • A gene is a segment of DNA that directs the synthesis of a specific protein. • Alleles of the same gene direct the synthesis of different forms of the same protein.
Solving Genetics Problems • Convert parental phenotypes to genotypes • Use Punnett Square to determine genotypes of offspring • Convert offspring genotypes to phenotypes
2/3 tailless + 1/3 tails Lethal Alleles Example: Manx cat ML = tailless, lethal in homozygote M = tail Tailless male x Tailless female ML M MLM x MLM X MLML MLM ML M dies tailless MM MLM tailless tail
Multiple Alleles • Multiple Alleles: three or more alleles exist for one trait (Note: A diploid individual can only carry two alleles at once.)
Codominance • Codominance: Neither allele masks the other so that effects of both alleles are observed in heterozygote without blending IA =IB > i IA and IB are codominant. IA and IB are completely dominant over i.
Type A Type B Type O Type AB Codominance Effects of both alleles observed in phenotype
Inheritance of Rh Factor(another gene with multiple alleles) *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 are inherited independently.
ir ir IBr ir IAIBrr = 1/8 Momiirr Dad iirr Example of Multiple Alleles and Codominance Type A, Rh positive x Type B, Rh negative (mother is Type O, Rh-) (father is Type O, Rh-) IAiRr x IBirr IAR IAriRir IAIBrr IBiRr IBirr IAIBRr IAirr iiRr iirr IAiRr Child with Type AB, Rh negative blood
Red x White RR R’R’ Four o’clock flowers R = red, R’ = white Pink RR’ Incomplete Dominance Neither allele masks the other and both are observed as a blending in the heterozygote
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
Type A A A A A A A A A A A A A IA__H__ Type O H H H H H H H H H H H H ii__ __ or IA__hh or IB__hh Epistasis • An allele of one gene masks the expression of alleles of another gene and expresses its own phenotype instead. • Example of Epistasis H = enzyme that attaches antigen H to protein on red blood cells h= no enzyme to attach antigen H • Antigens A and B of ABO blood typing (from alleles IA and IB) are attached to antigen H. Someone with the genotype hh will have Type O blood irrespectiveof their genotype for the I allele
IBH IBh ih iH IAH IAh iH ih Example of Epistasis IAiHh x IBiHh IAIBHH IAiHH IAIBHh IAiHh Type A = 3/16 Type B = 3/16 Type AB = 3/16 *Type O = 7/16 IAIBhh* IAihh* IAIBHh IAiHh iiHh* IBiHH IBiHh iiHH* IBihh* iiHh* IBiHh iihh*
Pleiotropic Effects One gene affects many phenotypic characteristics
Polygenic Inheritance: Many genes affect one trait Example: Skin color *Based on a study conducted in Jamaica.
ab Grandma aabb Example of Polygenic Inheritance Medium Black Woman X Darkest Black Man (her mother is white) AABB AaBb AB AbaBab AABb AaBB AaBb AB AABB Dark Black Dark Black DarkestBlack Medium Black ¼ Darkest Black; ½ Dark Black; ¼ Medium Black