Fundamentals of Genetics 1 Gregor Mendel 1822 1884

Fundamentals of Genetics 1

Gregor Mendel (1822 -1884) Responsible for the Laws governing Inheritance of Traits 2

Gregor Johann Mendel §Austrian monk §Studied the inheritance of traits in pea plants §Developed the laws of inheritance §Mendel's work was not recognized until the turn of the 20 th century 3

Gregor Johann Mendel §Between 1856 and 1863, Mendel cultivated and tested some 28, 000 pea plants §He found that the plants' offspring retained traits of the parents §Called the “Father of Genetics" 4

Site of Gregor Mendel’s experimental garden in the Czech Republic 5

Particulate Inheritance §Mendel stated that physical traits are inherited as “particles” §Mendel did not know that the “particles” were actually Chromosomes & DNA 6

Genetic Terminology § Trait - any characteristic that can be passed from parent to offspring § Heredity - passing of traits from parent to offspring § Genetics - study of heredity 7

Types of Genetic Crosses § Monohybrid cross - cross involving a single trait e. g. flower color § Dihybrid cross - cross involving two traits e. g. flower color & plant height 8

Punnett Square Used to help solve genetics problems 9

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Designer “Genes” § Alleles - two forms of a gene (dominant & recessive) § Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R) § Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r) 11

More Terminology § Genotype - gene combination for a trait (e. g. RR, Rr, rr) § Phenotype - the physical feature resulting from a genotype (e. g. red, white) 12

Genotype & Phenotype in Flowers Genotype of alleles: R = red flower r = yellow flower All genes occur in pairs, so 2 alleles affect a characteristic Possible combinations are: Genotypes RR Rr rr Phenotypes RED YELLOW 13

Genotypes § Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e. g. RR or rr); also called pure § Heterozygous genotype - gene combination of one dominant & one recessive allele (e. g. Rr); also called hybrid 14

Genes and Environment Determine Characteristics 15

Mendel’s Pea Plant Experiments 16

Why peas, Pisum sativum? §Can be grown in a small area §Produce lots of offspring §Produce pure plants when allowed to self -pollinate several generations §Can be artificially cross-pollinated 17

Reproduction in Flowering Plants Pollen contains sperm Produced by the stamen Ovary contains eggs Found inside the flower Pollen carries sperm to the eggs for fertilization Self-fertilization can occur in the same flower Cross-fertilization can occur between flowers 18

Mendel’s Experimental Methods Mendel hand-pollinated flowers using a paintbrush He could snip the stamens to prevent self-pollination He traced traits through the several generations 19

How Mendel Began Mendel produced pure strains by allowing the plants to selfpollinate for several generations 20

Eight Pea Plant Traits Seed shape --- Round (R) or Wrinkled (r) Seed Color ---- Yellow (Y) or Green (y) Pod Shape --- Smooth (S) or wrinkled (s) Pod Color --- Green (G) or Yellow (g) Seed Coat Color ---Gray (G) or White (g) Flower position---Axial (A) or Terminal (a) Plant Height --- Tall (T) or Short (t) Flower color --- Purple (P) or white (p) 21

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Mendel’s Experimental Results 24

Did the observed ratio match theoretical ratio? The theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round : 1 wrinkled Mendel’s observed ratio was 2. 96: 1 The discrepancy is due to statistical error The larger the sample the more nearly the results approximate to theoretical ratio 25

Generation “Gap” Parental P 1 Generation = the parental generation in a breeding experiment. F 1 generation = the first-generation offspring in a breeding experiment. (1 st filial generation) From breeding individuals from the P 1 generation F 2 generation = the second-generation offspring in a breeding experiment. (2 nd filial generation) From breeding individuals from the F 1 generation 26

Following the Generations Cross 2 Results in Cross 2 Hybrids Pure all get Plants Hybrids 3 Tall & 1 Short TT x tt Tt TT, Tt, tt 27

Monohybrid Crosses 28

P 1 Monohybrid Cross Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Wrinkled seeds RR x rr r r R Rr Rr Genotype: Rr Phenotype: Phenotype Round Genotypic Ratio: All alike Phenotypic Ratio: All alike 29

P 1 Monohybrid Cross Review § Homozygous dominant x Homozygous recessive § Offspring all Heterozygous (hybrids) § Offspring called F 1 generation § Genotypic & Phenotypic ratio is ALL ALIKE 30

F 1 Monohybrid Cross Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Round seeds Rr x Rr R RR Rr rr Genotype: RR, Rr, rr Phenotype: Phenotype Round & wrinkled G. Ratio: 1: 2: 1 P. Ratio: 3: 1 31

F 1 Monohybrid Cross Review § Heterozygous x heterozygous § Offspring: 25% Homozygous dominant RR 50% Heterozygous Rr 25% Homozygous Recessive rr § Offspring called F 2 generation § Genotypic ratio is 1: 2: 1 § Phenotypic Ratio is 3: 1 32

What Do the Peas Look Like? 33

…And Now the Test Cross Mendel then crossed a pure & a hybrid from his F 2 generation This is known as an F 2 or test cross There are two possible testcrosses: Homozygous dominant x Hybrid Homozygous recessive x Hybrid 34

F 2 Monohybrid Cross st (1 ) Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Round seeds RR x Rr R RR Rr Genotype: RR, Rr Phenotype: Phenotype Round Genotypic Ratio: 1: 1 Phenotypic Ratio: All alike 35

F 2 Monohybrid Cross (2 nd) Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Wrinkled seeds x Round seeds rr x Rr R r r Rr Rr r rr rr Genotype: Rr, rr Phenotype: Phenotype Round & Wrinkled G. Ratio: 1: 1 P. Ratio: 1: 1 36

F 2 Monohybrid Cross Review § Homozygous x heterozygous(hybrid) § Offspring: 50% Homozygous RR or rr 50% Heterozygous Rr § Phenotypic Ratio is 1: 1 § Called Test Cross because the offspring have SAME genotype as parents 37

Practice Your Crosses Work the P 1, F 1, and both F 2 Crosses for each of the other Seven Pea Plant Traits 38

Mendel’s Laws 39

Results of Monohybrid Crosses Inheritable factors or genes are responsible for all heritable characteristics Phenotype is based on Genotype Each trait is based on two genes, one from the mother and the other from the father True-breeding individuals are homozygous ( both alleles) are the same 40

Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. All the offspring will be heterozygous and express only the dominant trait. RR x rr yields all Rr (round seeds) 41

Law of Dominance 42

Law of Segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring. 43

Applying the Law of Segregation 44

Law of Independent Assortment Alleles for different traits are distributed to sex cells (& offspring) independently of one another. This law can be illustrated using dihybrid crosses. 45

Dihybrid Cross A breeding experiment that tracks the inheritance of two traits. Mendel’s “Law of Independent Assortment” a. Each pair of independently b. Formula: 2 n alleles segregates during gamete formation (n = # of heterozygotes) 46

Question: How many gametes will be produced for the following allele arrangements? Remember: 2 n (n = # of heterozygotes) 1. Rr. Yy 2. Aa. Bb. CCDd 3. Mm. Nn. Oo. PPQQRrss. Tt. Qq 47

Answer: 1. Rr. Yy: 2 n = 22 = 4 gametes RY Ry r. Y ry 2. Aa. Bb. CCDd: 2 n ABCD ABCd a. BCD a. BCd = 23 = Ab. CD ab. CD 8 gametes Ab. Cd ab. CD 3. Mm. Nn. Oo. PPQQRrss. Tt. Qq: 2 n = 26 = 64 gametes 48

Dihybrid Cross Traits: Seed shape & Seed color Alleles: R round r wrinkled Y yellow y green Rr. Yy RY Ry r. Y ry x Rr. Yy RY Ry r. Y ry All possible gamete combinations 49

Dihybrid Cross RY Ry r. Y ry 50

Dihybrid Cross RY RY RRYY Ry RRYy r. Y Rr. YY ry Rr. Yy Ry r. Y ry RRYy Rr. YY Rr. Yy RRyy Rr. Yy Rryy Rr. Yy rr. YY rr. Yy Rryy rr. Yy rryy Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9: 3: 3: 1 phenotypic ratio 51

Dihybrid Cross Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9: 3: 3: 1 52

Test Cross A mating between an individual of unknown genotype and a homozygous recessive individual. Example: bb. C__ x bbcc BB Bb bb = = = brown eyes blue eyes CC = curly hair Cc = curly hair cc = straight hair b. C b___ bc 53

Test Cross Possible results: bc b. C b___ C bb. Cc or bc b. C b___ c bb. Cc bbcc 54

Summary of Mendel’s laws LAW DOMINANCE SEGREGATION INDEPENDENT ASSORTMENT PARENT CROSS OFFSPRING TT x tt tall x short 100% Tt tall x x Tt tall Rr. Gg x Rr. Gg round & green x round & green 75% tall 25% short 9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods 55

Incomplete Dominance and Codominance 56

Incomplete Dominance F 1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties. Example: snapdragons (flower) red (RR) x white (rr) r r RR = red flower rr = white flower R R 57

Incomplete Dominance r R Rr Rr produces the F 1 generation All Rr = pink (heterozygous pink) 58

Incomplete Dominance 59

Codominance Two alleles are expressed (multiple alleles) in heterozygous individuals. Example: blood type 1. 2. 3. 4. type A B AB O = = IAIA or IAi IBIB or IBi I AI B ii 60

Codominance Problem Example: homozygous male Type B (IBIB) x heterozygous female Type A (IAi) IA i IB I AI B I Bi 1/2 = IAIB 1/2 = IBi 61

Another Codominance Problem • Example: male Type O (ii) x female type AB (IAIB) IA IB i I Ai I Bi 1/2 = IAi 1/2 = IBi 62

Codominance Question: If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents? boy - type O (ii) AB (IAIB) X girl - type 63

Codominance Answer: IA IB i i I AI B ii Parents: genotypes = IAi and IBi phenotypes = A and B 64

Sex-linked Traits (genes) located on the sex chromosomes Sex chromosomes are X and Y XX genotype for females XY genotype for males Many sex-linked traits carried on X chromosome 65

Sex-linked Traits Example: Eye color in fruit flies Sex Chromosomes fruit fly eye color XX chromosome - female Xy chromosome - male 66

Sex-linked Trait Problem Example: Eye color in fruit flies (red-eyed male) x (white-eyed female) X r. Y x X r Remember: the Y chromosome in males does not carry traits. Xr Xr RR = red eyed Rr = red eyed r X rr = white eyed Xy = male Y XX = female 67

Sex-linked Trait Solution: Xr XR XR Xr Y Xr XR Xr Xr Y 50% red eyed female 50% white eyed male 68

Female Carriers 69

Genetic Practice Problems 70

Breed the P 1 generation tall (TT) x dwarf (tt) pea plants t t T T 71

Solution: tall (TT) vs. dwarf (tt) pea plants t t T Tt Tt produces the F 1 generation T Tt Tt All Tt = tall (heterozygous tall) 72

Breed the F 1 generation tall (Tt) vs. tall (Tt) pea plants T t 73

Solution: tall (Tt) x tall (Tt) pea plants T t T TT Tt tt produces the F 2 generation 1/4 (25%) = TT 1/2 (50%) = Tt 1/4 (25%) = tt 1: 2: 1 genotype 3: 1 phenotype 74

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