Principles of Heredity What patterns of inheritance can be observed when traits are passed to the next generation?

Mendel’s Experiment With Peas Differing in a Single Trait F1 round plants x F1 round plants 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 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: Round seed x Wrinkled seed F1: All round seed coats RR rr Homozygous Recessive Homozygous Dominant Rr Heterozygous F1 round plants x F1 round plants RrHeterozygous RrHeterozygous F2

RRR R Homozygous parents can only pass one form of an allele to their offspring.

RrRr Heterozygous parents can pass either of two forms of an allele to their offspring. 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 TermDefinitionExample Genotype Alleles carried by an individual RR, Rr, rr Phenotype Physical characteristic or appearance of an individual round or wrinkled

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. 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. Each gamete has an equal probability of containing either member of the allele pair.

Genetic Segregation Parentals: RR x rr R R r r F1 x F1: Rr x Rr R R r r R r R r R r RRRr Rr rr RrRr Rr Rr 100% Round seeds Rr 75% Round seeds 25% Wrinkled seeds

Mendel’s Experiment With Peas Differing in Two Traits F1: All round yellow seed coats Parental: Round Yellow x Wrinkled Green F1 plants x F1 plants 1/16 32 wrinkled, green 32 wrinkled, green 3/ wrinkled, yellow 3/ round, green 9/ round, yellow F2

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. 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: RRYY x r r y y RYryRrYyF1: RY RY RY RY RY RY RY RY ry ry ry ry 100% round, yellow

Genetic Segregation + Independent Assortment F1 x F1 : R r Y y x R r Y y RY Ry rY ry RY Ry rY ry Four different types of gametes are formed in equal proportions. RY Ry rY ry

F1 x F1 RrYy X RrYy RY RyRyRyRy rYrYrYrY ry RY RyRyRyRy rYrYrYrYry Eggs Pollen 1 16 RRYY RRY y RrYY RrY y RRYy RR yy RrYy Rr yy RrYY RrY y rr YY rr Y y RrYy Rr yy rr Yy rryy

F2 Genotypes and Phenotypes PhenotypesGenotypes RoundYellow 1/16 RRYY + 2/16 RRYy + 2/16 RrYY + 4/16 RrYy Total = 9/16 R_Y_ RoundGreen 1/16 RRyy+ 2/16 Rryy Total = 3/16 R_yy WrinkledYellow 1/16 rrYY+ 2/16 rrYy Total = 3/16 rrY_ WrinkledGreen 1/16 rryy

Gene Products Agene is a segment of DNA that directs the synthesis of a specific protein. AllelesAlleles of the same gene direct the synthesis of different forms of the same protein. Dominant Allele Codes for a functional protein Recessive Allele Codes for a non-functional protein or prevents any protein product from forming

Solving Genetics Problems 1.Convert parental phenotypes to genotypes 2.Use Punnett Square to determine genotypes of offspring 3.Convert offspring genotypes to phenotypes

Additional Genetic Patterns Mendel’s peas Other Patterns Complete Dominance Lethal Alleles Codominance Incomplete Dominance Two alleles per gene Multiple Alleles One gene affects one trait One gene affects many traits (pleiotropic effects) One gene affects one trait Many genes affect one trait (polygenic inheritance)

Lethal Alleles Example: Manx cat M L = tailless, lethal in homozygote M = tail Tailless male x Tailless female M L M x M L M M L M L M M L M M L M M L M L M L M L M L M M L M MM MM diestailless taillesstail 2/3 tailless + 1/3 tails X

Multiple Alleles Multiple Alleles: three or more alleles exist for one traitMultiple Alleles: three or more alleles exist for one trait (Note: A diploid individual can only carry two alleles at once.) (Note: A diploid individual can only carry two alleles at once.) Blood Type Allele Type A IAIAIAIA Type B IBIBIBIB Type O i

Codominance Codominance: Neither allele masks the other so that effects of both alleles are observed in heterozygote without blendingCodominance: Neither allele masks the other so that effects of both alleles are observed in heterozygote without blending I A = I B > i I A = I B > i I A and I B are codominant. I A and I B are completely dominant over i.

Codominance PhenotypeGenotypeGeneProduct Type A I A I A or I A i Antigen A Type B I B I B or I B i Antigen B Type O iinone Type AB IAIBIAIBIAIBIAIB Antigen A and and Antigen B Type O Type A Type B Type AB Effects of both alleles observed in phenotype

Inheritance of Rh Factor (another gene with multiple alleles) PhenotypeGenotype*GeneProduct Rh Positive RR or Rr Rhesus Protein Rh Negative rrNone *Although there are multiple R alleles, R 1, R 2, R 3, etc. all are completely dominant over all of the r alleles, r 1, r 2, r 3, etc. ABO Blood Type and Rh Factor are controlled by separate genes. They are inherited independently.

Example of Multiple Alleles and Codominance Type A, Rh positive x Type B, Rh negative I A R I A R I A r I A r iR iR ir x IBirr I A I B Rr I A I B rr I B iRr I B irr I A iRr I A irr iiRr iirr IBrIBrIBrIBrir (mother is Type O, Rh-) (mother is Type O, Rh-) (father is Type O, Rh-) Mom iirr Child with Type AB, Rh negative blood I A I B rr = 1/8 Dadiirr IAiRr ir ir

Incomplete Dominance Four o’clock flowers R = red, R’ = white Red x White RR R’R’ Pink RR’ Neither allele masks the other and both are observed as a blending in the heterozygote Neither allele masks the other and both are observed as a blending in the heterozygote

Incomplete Dominance F1 x F1 Pink x Pink RR’ x RR’ ½ R ½ R’ ½ R ½ R’ ½ R ½ R’ ¼ RR ¼ RR’ ¼ R’R’ Genotypic Ratio: ¼ RR + ½ RR’ + ¼ R’R’ Phenotypic Ratio: ¼ red + ½ pink + ¼ white

Epistasis An allele of one gene masks the expression of alleles of another gene and expresses its own phenotype instead.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 HExample of Epistasis H = enzyme that attaches antigen H to protein on red blood cells to protein on red blood cells h= no enzyme to attach antigen H h= no enzyme to attach antigen H Antigens A and B of ABO blood typingAntigens A and B of ABO blood typing (from alleles I A and I B ) are attached to antigen H. Someone with the genotype hh will have Type O blood irrespective of their genotype for the I allele (from alleles I A and I B ) are attached to antigen H. Someone with the genotype hh will have Type O blood irrespective of their genotype for the I allele Type A H H A A A A A A A A A A A A H H H H H H H H H H I A __H__ Type O ii__ __ or I A __hh or I B __hh

Example of Epistasis I A iHh x I B iHh I A I B HH IAhIAhIAhIAh ih iH iH IAHIAHIAHIAH IBHIBHIBHIBH IBhIBhIBhIBhiHih I A I B Hh I A iHH I A iHh I A I B Hh I A I B hh* I A iHh I A ihh* I B iHH I B iHh iiHH* iiHH* iiHh* iiHh* I B iHh I B ihh* iiHh* iiHh* iihh* iihh* Type A = 3/16 Type B = 3/16 Type AB = 3/16 *Type O = 7/16

Pleiotropic Effects One gene affects many phenotypic characteristics 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 O 2 tension Sickle Cell Anemia

Polygenic Inheritance: Many genes affect one trait Example: Skin color Number of Dominant Alleles Skin Color* (Phenotype)Genotypes % Pigmentation* 0Whiteaabb0-11% 1 Light Black Aabb or aaBb 12-25% 2 Medium Black AAbb or AaBb or aaBB 26-40% 3 Dark Black AABb or AaBB 41-55% 4 Darkest Black AABB56-78% *Based on a study conducted in Jamaica.

Example of Polygenic Inheritance Medium Black Woman X Darkest Black Man (her mother is white) (her mother is white) AABB AaBb AB Ab Ab aB aB ab AABB AABbAaBB AaBb AB Darkest Black DarkBlackDarkBlack MediumBlack ¼ Darkest Black; ½ Dark Black; ¼ Medium Black Grandma aabb ab