1 Mendelian Genetics

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

3 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 20th century

4 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"

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

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

7 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

8 Designer “Genes”  Alleles – one of the 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)

9 Genetic Terminology  Monohybrid - cross involving a single trait e.g. flower color  Dihybrid – cross involving two traits e.g. flower color & plant height  True Breeding –  True Breeding – all offspring will have the same characteristic generation after generation

10 Genotypes  Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr); also called pure  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

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: GenotypesRR Rrrr PhenotypesRED RED YELLOW

13 Genotypes  Parental Generation (P1) -  Parental Generation (P1) - the parental generation in a breeding experiment  First Filial generation (F1) –  First Filial generation (F1) – First generation created by parents  Second Filial generation (F2) - the second-generation offspring in a breeding experiment. 

14 Mendel’s Pea Plant Experiments

15 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

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18 Mendel’s Experimental Results

19 Did the observed ratio match the 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 the theoretical ratio

Looking closer at Mendel’s work true-breeding purple-flower peas true-breeding white-flower peas X 100% purple-flower peas F 1 generation (hybrids) 25% white-flower peas F 2 generation 75% purple-flower peas 3:1 P self-pollinate

Test cross Cross-breed the dominant phenotype — unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele x pp is it PP or Pp?

x PPpp Test cross pp P P PpPp PpPpPpPp PpPp pp P p PpPp pp PpPp x PpPp 100% 50%:50% 1:1

23 Mendel’s Laws

Mendel’s laws of heredity (#1) Law of segregation –when gametes are produced during meiosis, homologous chromosomes separate from each other –each allele for a trait is packaged into a separate gamete PP P P pp p p PpPp P p

Law of Segregation What meiotic event creates the law of segregation? Meiosis 1

Monohybrid cross Some of Mendel’s experiments followed the inheritance of single characters –flower color –seed color –monohyb rid crosses Other of Mendel’s experiments followed the inheritance of 2 different characters –seed color and seed shape –dihybrid crosses

Dihybrid cross true-breeding yellow, round peas true-breeding green, wrinkled peas x YYRRyyrr P YyRr 100% F 1 generation (hybrids) yellow, round peas self-pollinate F 2 generation 9/16 yellow round peas 9:3:3:1 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas Y = yellow R = round y = green r = wrinkled

Dihybrid cross YyRr YRYryR yr YR Yr yR yr YYRRYYRrYyRRYyRr YYRrYYrrYyRrYyrr YyRRYyRryyRRyyRr YyRrYyrryyRryyrr x 9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled

Mendel’s laws of heredity (#2) Law of independent assortment –each pair of alleles segregates into gametes independently 4 classes of gametes are produced in equal amounts –YR, Yr, yR, yr only true for genes on separate chromosomes YyRr Yr yR YR yr

Law of Independent Assortment What meiotic event creates the law of independent assortment? Meiosis 1

The chromosomal basis of Mendel’s laws… Trace the genetic events through meiosis, gamete formation & fertilization to offspring

Review: Mendel’s laws of heredity Law of segregation –monohybrid cross single trait –each allele segregates into separate gametes established by Meiosis 1 Law of independent assortment –dihybrid (or more) cross 2 or more traits –each pair of alleles for genes on separate chromosomes segregates into gametes independently established by Meiosis 1

Genetics & Probability Mendel’s laws: –segregation –independent assortment reflect same laws of probability that apply to tossing coins or rolling dice

Probability & genetics Calculating probability of making a specific gamete is just like calculating the probability in flipping a coin –probability of tossing heads? 50% –probability making a P gamete… PP P P PpPp P p 50% 100%

Calculating probability Pp x Pp Pp male / sperm P p female / eggs PP Pppp Pp spermegg 1/2 offspring =x1/4 PPPP 1/2 =x1/4 PpPpPp 1/2 =x1/4 pppp 1/2 =x1/4 1/2 pP

Chance that 2 or more independent events will occur together –probability that 2 coins tossed at the same time will land heads up –probability of Pp x Pp  pp Rule of multiplication 1/2 x 1/2 = 1/4

Calculating dihybrid probability Rule of multiplication also applies to dihybrid crosses –heterozygous parents — YyRr –probability of producing yyrr? probability of producing y gamete = 1/2 probability of producing r gamete = 1/2 probability of producing yr gamete = 1/2 x 1/2 = 1/4 probability of producing a yyrr offspring = 1/4 x 1/4 = 1/16

NOW TO PRACTICE! 38