Genetics Mendel and the Gene Idea 1 Heredity

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Genetics • Mendel and the Gene Idea 1

Genetics • Mendel and the Gene Idea 1

Heredity • • • What genetic principles account for the transmission of traits from

Heredity • • • What genetic principles account for the transmission of traits from parents to offspring? One possible explanation of heredity is a “blending” hypothesis - The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green. An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea - Parents pass on discrete heritable units, genes. 2

Gregor Mendel • Documented a particulate mechanism of inheritance through his experiments with garden

Gregor Mendel • Documented a particulate mechanism of inheritance through his experiments with garden peas. Gregor Mendel’s monastery garden. Fig. 2. 2 3

Themes of Mendel’s work • • Variation is widespread in nature Observable variation is

Themes of Mendel’s work • • Variation is widespread in nature Observable variation is essential for following genes Variation is inherited according to genetic laws and not solely by chance Mendel’s laws apply to all sexually reproducing organisms 4

Mendel’s Experimental, Quantitative Approach • • • Mendel used the scientific approach to identify

Mendel’s Experimental, Quantitative Approach • • • Mendel used the scientific approach to identify two laws of inheritance Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments Mendel chose to work with the garden pea (Pisum sativum) – Because they are available in many varieties, – easy to grow, – easy to get large numbers – Because he could strictly control which plants mated with which 5

1 Removed stamens from purple flower 2 Crossing Pea Plants Transferred spermbearing pollen from

1 Removed stamens from purple flower 2 Crossing Pea Plants Transferred spermbearing pollen from stamens of white flower to eggbearing carpel of purple flower Parental generation (P) 3 Pollinated carpel Carpel (female) Stamens (male) matured into pod 4 Planted seeds from pod 5 Examined First generation offspring (F 1) offspring: all purple flowers 6

Mendel’s experimental design • • Statistical analyses: – Worked with large numbers of plants

Mendel’s experimental design • • Statistical analyses: – Worked with large numbers of plants – counted all offspring – made predictions and tested them Excellent experimentalist – controlled growth conditions – focused on traits that were easy to score – chose to track only those characters that varied in an “either-or” manner 7

Mendel’s Studied Discrete Traits 8

Mendel’s Studied Discrete Traits 8

Genetic Vocabulary • • Character: a heritable feature, such as flower color Trait: a

Genetic Vocabulary • • Character: a heritable feature, such as flower color Trait: a variant of a character, such as purple or white flowers Each trait carries two copies of a unit of inheritance, one inherited from the mother and the other from the father Alternative forms of traits are called alleles. 9

Antagonistic traits Dominant Recessive 10

Antagonistic traits Dominant Recessive 10

Mendel’s experimental design • Mendel also made sure that he started his experiments with

Mendel’s experimental design • Mendel also made sure that he started his experiments with varieties that were “true-breeding” X X X 11

Genetic Vocabulary • • • Phenotype – observable characteristic of an organism Genotype –

Genetic Vocabulary • • • Phenotype – observable characteristic of an organism Genotype – pair of alleles present in and individual Homozygous – two alleles of trait are the same (YY or yy) Heterozygous – two alleles of trait are different (Yy) Capitalized traits = dominant phenotypes Lowercase traits = recessive phenotypes 12

Genetic Vocabulary • • Generations: – P = parental generation – F 1 =

Genetic Vocabulary • • Generations: – P = parental generation – F 1 = 1 st filial generation, progeny of the P generation – F 2 = 2 nd filial generation, progeny of the F 1 generation (F 3 and so on) Crosses: – Monohybrid cross = cross of two different truebreeding strains (homozygotes) that differ in a single trait. – Dihybrid cross = cross of two different truebreeding strains (homozygotes) that differ in two traits. 13

Phenotype vs Genotype Phenotype Purple 3 Purple Genotype PP (homozygous) 1 Pp (heterozygous) 2

Phenotype vs Genotype Phenotype Purple 3 Purple Genotype PP (homozygous) 1 Pp (heterozygous) 2 Pp (heterozygous) Purple White 1 Figure 14. 6 Ratio 3: 1 pp (homozygous) 1 Ratio 1: 2: 1 14

Phenotype vs Genotype Dominant & recessive alleles (Fig. 10. 7): 15

Phenotype vs Genotype Dominant & recessive alleles (Fig. 10. 7): 15

Mendel’s Experimental Design • • In a typical breeding experiment Mendel mated two contrasting,

Mendel’s Experimental Design • • In a typical breeding experiment Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are called the P generation The hybrid offspring of the P generation are called the F 1 generation When F 1 individuals self-pollinate the F 2 generation is produced 16

Mendel’s Observations • • • When Mendel crossed contrasting, truebreeding white and purple flowered

Mendel’s Observations • • • When Mendel crossed contrasting, truebreeding white and purple flowered pea plants all of the offspring were purple When Mendel crossed the F 1 plants, many of the plants had purple flowers, but some had white flowers A ratio of about three to one, purple to white flowers, in the F 2 generation 17

 P Generation (true-breeding parents) Purple flowers White flowers F 1 Generation (hybrids) All

P Generation (true-breeding parents) Purple flowers White flowers F 1 Generation (hybrids) All plants had purple flowers F 2 Generation 18

Mendel’s Rationale • • In the F 1 plants, only the purple trait was

Mendel’s Rationale • • In the F 1 plants, only the purple trait was affecting flower color in these hybrids Purple flower color was dominant, and white flower color was recessive Mendel developed a hypothesis to explain the 3: 1 inheritance pattern that he observed among the F 2 offspring There are four related concepts that are integral to this hypothesis 19

Heredity Concepts 1. Alternative versions of genes account for variations in inherited characters, which

Heredity Concepts 1. Alternative versions of genes account for variations in inherited characters, which are now called alleles Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers 2. For each character an organism inherits two alleles, one from each parent, A genetic locus is actually represented twice 3. If the two alleles at a locus differ, the dominant allele determines the organism’s appearance 4. The law of segregation - the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes 20

Law of Segregation • Mechanism of gene transmission Gametogenesis: alleles segregate Fertilization: alleles unite

Law of Segregation • Mechanism of gene transmission Gametogenesis: alleles segregate Fertilization: alleles unite 21

Mendelian Genetics • Classical Punett's Square is a way to determine ways traits can

Mendelian Genetics • Classical Punett's Square is a way to determine ways traits can segregate Parental P 0 cross F 1 cross Determine the genotype and phenotype 23

Mendelian Genetics • Classical Punett's Square is a way to determine ways traits can

Mendelian Genetics • Classical Punett's Square is a way to determine ways traits can segregate Parental P 0 cross F 1 cross Determine the genotype and phenotype 24

Mendelian Genetics • Classical Punett's Square is a way to determine ways traits can

Mendelian Genetics • Classical Punett's Square is a way to determine ways traits can segregate Parental P 0 cross F 1 cross Determine the genotype and phenotype 25

P Generation Appearance: Genetic makeup: Purple flowers PP Gametes: Mendel’s Law Of Segregation, Probability

P Generation Appearance: Genetic makeup: Purple flowers PP Gametes: Mendel’s Law Of Segregation, Probability And The Punnett Square White flowers pp p P F 1 Generation Appearance: Genetic makeup: Gametes: Purple flowers Pp 1/ 2 1/ P 2 p p P F 1 sperm F 2 Generation P Pp PP F 1 eggs • p pp Pp 3 : 1 26

Mendel’s Monohybrid Cross White (pp) Purple (Pp) p Gametes p Purple (PP) P Gametes

Mendel’s Monohybrid Cross White (pp) Purple (Pp) p Gametes p Purple (PP) P Gametes p Purple (Pp) P Gametes P Pp Pp P F 1 generation All purple Gametes p PP Pp Pp pp F 2 generation ¾ purple, ¼ white 27

Smooth and wrinkled parental seed strains crossed. • Punnett square – – F 1

Smooth and wrinkled parental seed strains crossed. • Punnett square – – F 1 genotypes: 4/4 Ss F 1 phenotypes: 4/4 smooth 28

Mendel Observed The Same Pattern In Characters 29

Mendel Observed The Same Pattern In Characters 29

The Testcross • • In pea plants with purple flowers the genotype is not

The Testcross • • In pea plants with purple flowers the genotype is not immediately obvious A testcross 1. Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype 2. Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait 30

Test Cross • To determine whether an individual with a dominant phenotype is homozygous

Test Cross • To determine whether an individual with a dominant phenotype is homozygous for the dominant allele or heterozygous, Mendel crossed the individual in question with an individual that had the recessive phenotype: Alternative 1 – Plant with dominant phenotype is homozygous PP ? Dominant Phenotype Gametes Recessive phenotype pp Dominant Phenotype P p Gametes p Alternative 2 – Plant with dominant phenotype is heterozygous ? Pp Gametes P P Recessive phenotype p p pp Gametes p 31

Test Cross • To determine whether an individual with a dominant phenotype is homozygous

Test Cross • To determine whether an individual with a dominant phenotype is homozygous for the dominant allele or heterozygous, Mendel crossed the individual in question with an individual that had the recessive phenotype: Alternative 1 – Plant with dominant phenotype is homozygous PP ? Dominant Phenotype Gametes Recessive phenotype pp Dominant Phenotype P P p Pp Pp Gametes If all offspring are purple; unknown plant is homozygous. Alternative 2 – Plant with dominant phenotype is heterozygous ? Pp Gametes Recessive phenotype pp Gametes P p p Pp pp If half of offspring are white; unknown plant is heterozygous. 32

The Testcross APPLICATION An organism that exhibits a dominant trait, such as purple flowers

The Testcross APPLICATION An organism that exhibits a dominant trait, such as purple flowers in pea plants, can be either homozygous for the dominant allele or heterozygous. To determine the organism’s genotype, geneticists can perform a testcross. TECHNIQUE In a testcross, the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent. Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp If PP, then all offspring purple: If Pp, then 2 offspring purple and 1⁄2 offspring white: p 1⁄ p p p Pp Pp pp pp RESULTS P Pp P p Pp Pp 33

The Law of Independent Assortment • Mendel derived the law of segregation by following

The Law of Independent Assortment • Mendel derived the law of segregation by following a single trait – – • 2 alleles at a single gene locus segregate when the gametes are formed The F 1 offspring produced in this cross were monohybrids, heterozygous for one character Mendel identified his second law of inheritance by following two characters at the same time – – Mendel was interested in determining whether alleles at 2 different gene loci segregate dependently or independently Crossing two, true-breeding parents differing in two characters produces dihybrids in the F 1 generation, heterozygous for both characters 34

Dihybrid Cross • • With his monohybrid crosses, Mendel determined that the 2 alleles

Dihybrid Cross • • With his monohybrid crosses, Mendel determined that the 2 alleles at a single gene locus segregate when the gametes are formed. With his dihybrid crosses, Mendel was interested in determining whether alleles at 2 different gene loci segregate dependently or independently. 35

Dihybrid Cross • • For example, in pea plants seed shape is controlled by

Dihybrid Cross • • For example, in pea plants seed shape is controlled by one gene locus where round (R) is dominant to wrinkled (r) while seed color is controlled by a different gene locus where yellow (Y) is dominant to green (y). Mendel crossed 2 pure-breeding plants: one with round yellow seeds and the other with green wrinkled seeds. 36

Dihybrid Cross • • For example, in pea plants seed shape is controlled by

Dihybrid Cross • • For example, in pea plants seed shape is controlled by one gene locus where round (R) is dominant to wrinkled (r) while seed color is controlled by a different gene locus where yellow (Y) is dominant to green (y). Mendel crossed 2 pure-breeding plants: one with round yellow seeds and the other with green wrinkled seeds. 37

Dependent Segregation • If dependent segregation (assortment) occurs: – – Alleles at the 2

Dependent Segregation • If dependent segregation (assortment) occurs: – – Alleles at the 2 gene loci segregate together, and are transmitted as a unit. Therefore, each plant would only produce gametes with the same combinations of alleles present in the gametes inherited from its parents: Parents Parental Gametes r y R Y Rr Yy F 1 Offspring’s Gametes rr yy RR YY R Y r y What is the expected phenotypic ratio for the F 2? 38

F 2 With Dependent Assortment: R Y r y R Y RR YY Rr

F 2 With Dependent Assortment: R Y r y R Y RR YY Rr Yy r y Rr Yy rr yy Ratio is 3 round, yellow : 1 wrinkled, green 39

Independent Segregation • Alleles at the 2 gene loci segregate (separate) independently, and are

Independent Segregation • Alleles at the 2 gene loci segregate (separate) independently, and are NOT transmitted as a unit. Therefore, each plant would produce gametes with allele combinations that were not present in the gametes inherited from its parents: Parents Parental Gametes F 1 Offspring’s Gametes R Y R y RR YY rr yy R Y r y Rr Yy r Y r y What is the expected phenotypic ratio for the F 2? 40

Mendelian Genetics Dihybrid cross - parental generation differs in two traits example-- cross round/yellow

Mendelian Genetics Dihybrid cross - parental generation differs in two traits example-- cross round/yellow peas with wrinkled/green ones Round/yellow is dominant What are the expected phenotype ratios in the F 2 generation? round, yellow = round, green = wrinkled, yellow = wrinkled, green = 41

F 2 with independent assortment: RY Ry r. Y ry ry RR YY RR

F 2 with independent assortment: RY Ry r. Y ry ry RR YY RR Yy Rr YY Rr Yy RR yy Rr Yy Rr yy Rr YY Rr Yy rr YY rr Yy Rr yy rr Yy rr yy Phenotypic ratio is 9 : 3 : 1 42

A Dihybrid Cross • How are two characters transmitted from parents to offspring? –

A Dihybrid Cross • How are two characters transmitted from parents to offspring? – – • As a package? Independently? A dihybrid cross – – Illustrates the inheritance of two characters Produces four phenotypes in the F 2 generation P Generation EXPERIMENT Two true-breeding pea plants— one with yellow-round seeds and the other with green -wrinkled seeds—were crossed, producing dihybrid F 1 plants. Self-pollination of the F 1 dihybrids, which are heterozygous for both characters, produced the F 2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant. YYRR yyrr Gametes F 1 Generation YR Hypothesis of dependent assortment yr Yy. Rr Hypothesis of independent assortment Sperm 1⁄ YR 1⁄ yr 2 2 Eggs 1⁄ YR F 2 Generation 2 YYRR Yy. Rr (predicted offspring) 1 ⁄ yr 2 Yy. Rr yyrr 3⁄ CONCLUSION The results support the hypothesis ofindependent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. 4 1⁄ 1⁄ 4 YR 1⁄ 4 Yr 1⁄ 4 y. R 1⁄ 4 yr Eggs 1⁄ 1⁄ 4 YR 4 Yr 4 y. R 4 yr 9⁄ 16 1⁄ YYRR YYRr Yy. RR Yy. Rr YYrr Yy. Rr Yyrr Yy. RR Yy. Rr yy. RR yy. Rr 4 Phenotypic ratio 3: 1 1⁄ Yy. Rr 3⁄ 16 Yyrr yy. Rr 3⁄ 16 yyrr 1⁄ 16 Phenotypic ratio 9: 3: 3: 1 315 108 101 32 Phenotypic ratio approximately 9: 3: 3: 1 43

Dihybrid cross • F 2 generation ratio: 9: 3: 3: 1 44

Dihybrid cross • F 2 generation ratio: 9: 3: 3: 1 44

Law of Independent Assortment • • Mendel’s dihybrid crosses showed a 9: 3: 3:

Law of Independent Assortment • • Mendel’s dihybrid crosses showed a 9: 3: 3: 1 phenotypic ratio for the F 2 generation. Based on these data, he proposed the Law of Independent Assortment, which states that when gametes form, each pair of hereditary factors (alleles) segregates independently of the other pairs. 45

Laws Of Probability Govern Mendelian Inheritance • Mendel’s laws of segregation and independent assortment

Laws Of Probability Govern Mendelian Inheritance • Mendel’s laws of segregation and independent assortment reflect the rules of probability – The multiplication rule § States that the probability that two or more independent events will occur together is the product of their individual probabilities – The rule of addition § States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities 46

Laws Of Probability - Multiplication Rule • • • The probability of two or

Laws Of Probability - Multiplication Rule • • • The probability of two or more independent events occurring together is the product of the probabilities that each event will occur by itself Following the self-hybridization of a heterozygous purple pea plants (Pp), what is the probability that a given offspring will be homozygous for the production of white flowers (pp)? Probability that a pollen seed will carry p: ½ Probability that an egg will carry p: ½ Probability that the offspring will be pp: 1/2 X 1/2 = 1/4 47

Laws Of Probability - Addition Rule • • The probability of either of two

Laws Of Probability - Addition Rule • • The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities Following the self-hybridization of a heterozygous purple pea plant (Pp), what is the probability that a given offspring will be purple? Probability of maternal P uniting with paternal P: 1/4 Probability of maternal p uniting with paternal P: 1/4 Probability of maternal P uniting with paternal p: 1/4 Probability that the offspring will be purple: 1/4 + 1/4 = 3/4 48

Probability In A Monohybrid Cross • Can be determined using these rules Rr Rr

Probability In A Monohybrid Cross • Can be determined using these rules Rr Rr Segregation of alleles into eggs Segregation of alleles into sperm Sperm 1⁄ R 2 R 1⁄ R 2 1⁄ Eggs 1⁄ 2 R r r 1⁄ R 1⁄ 4 r 1⁄ 4 R r 2 r 4 r 1⁄ 4 49

Mendel’s conclusions • • Genes are distinct entities that remain unchanged during crosses Each

Mendel’s conclusions • • Genes are distinct entities that remain unchanged during crosses Each plant has two alleles of a gene Alleles segregated into gametes in equal proportions, each gamete got only one allele During gamete fusion, the number of alleles was restored to two 50

Summary of Mendel’s Principles • Mendel’s Principle of Uniformity in F 1: – –

Summary of Mendel’s Principles • Mendel’s Principle of Uniformity in F 1: – – F 1 offspring of a monohybrid cross of true-breeding strains resemble only one of the parents. Why? Smooth seeds (allele S) are completely dominant to wrinkled seeds (alleles). 51

 • Mendel’s Law of Segregation: – – Recessive characters masked in the F

• Mendel’s Law of Segregation: – – Recessive characters masked in the F 1 progeny of two true-breeding strains, reappear in a specific proportion of the F 2 progeny. Two members of a gene pair segregate (separate) from each other during the formation of gametes. 52

 • Mendel’s Law of Independent Assortment: – – Alleles for different traits assort

• Mendel’s Law of Independent Assortment: – – Alleles for different traits assort independently of one another. Genes on different chromosomes behave independently in gamete production. 53

Monohybrid cross • Joe has a white cat named Sam. When Joe crosses Sam

Monohybrid cross • Joe has a white cat named Sam. When Joe crosses Sam with a black cat, he obtains 1/2 white kittens and 1/2 black kittens. When the black kittens are interbred, all the kittens that they produce are black. On the basis of these results, would you conclude that white or black coat color in cats is a recessive trait? Explain your reasoning. 54

 • • In watermelons, bitter fruit (B) is dominant over sweet fruit (b),

• • In watermelons, bitter fruit (B) is dominant over sweet fruit (b), and yellow spots (S) are dominant over no spots (s). The genes for these two characteristics assort independently. A homozygous plant that has bitter fruit and yellow spots is crossed with a homozygous plant that has sweet fruit and no spots. The F 1 are intercrossed to produce the F 2. a. What are the phenotypic ratios in the F 2? b. If an F 1 plant is backcrossed with the bitter, yellow-spotted parent, what phenotypes and proportions are expected in the offspring? c. If an F 1 plant is backcrossed with the sweet, non spotted parent, what phenotypes and proportions are expected in the offspring? 55