Mendelian Genetics 1 Gregor Mendel 1822 1884 Responsible
Mendelian 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 (heredity) 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 § § § Define these terms: 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 -Can only show the possible results of a genetic cross 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
How Mendel Began Mendel produced pure strains by allowing the plants to selfpollinate for several generations 18
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) 19
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Mendel’s Experimental Results 22
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 23
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 24
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 25
Mendel’s Laws 26
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 27
1. 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) 28
Law of Dominance 29
2. 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. 30
Applying the Law of Segregation 31
3. 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. 32
PRACTICE MONOHYBRID CROSSES 33
Independent Assortment 34
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) 36
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 37
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 38
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 39
Dihybrid Cross RY Ry r. Y ry 40
Dihybrid Cross Rr. Yy RY RY RRYY Ry RRYy r. Y Rr. YY ry Rr. Yy x 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 41
Dihybrid Cross Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9: 3: 3: 1 42
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 43
Test Cross Possible results: bc b. C b___ C bb. Cc or bc b. C b___ c bb. Cc bbcc 44
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 45
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