MENDELIAN GENETICS

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GREGOR JOHANN MENDEL

Austrian monkStudied the inheritance of traits in pea plantsDeveloped the laws of inheritanceMendel's work was not recognized until the turn of the 20th centuryBetween 1856 and 1863, Mendel cultivated and tested some 28,000 pea plantsHe found that the plants' offspring retained traits of the parentsCalled the “Father of Genetics"

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SITE OF MENDEL’S EXPERIMENTAL GARDEN IN THE CZECH REPUBLIC

NOTE THE RATIO OF FLOWER COLORS…

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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

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MENDEL’S PEA PLANT EXPERIMENTS

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MENDEL’S EXPERIMENTAL METHODS

Mendel hand-pollinated flowers using a paintbrush

He could snip the stamens to prevent self-pollinationCovered each flower with a cloth bag

He traced 8 traits through the several generationsUltimately he produced pure strains for each

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MENDEL’S FLOWERS

Once the pure strains were made; Mendel then began systematically crossing plants, this time however, he let the plants self-pollinate. Why?

He observed the offspring produced for each trait, in each generation, for any patterns that appeared.

Mendel’s questions: If traits are known, can the inheritance of

the traits in offspring be predicted? What is the ‘mechanism’ for inheritance?

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GENERATION “GAP”

Parental P1 Generation = the parental generation in a breeding experiment, the pure strain

F1 generation = the first-generation offspring in a breeding experiment. (1st filial generation)

F2 generation = the second-generation offspring in a breeding experiment. (2nd filial generation)

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WHY no white? What happened to the

white trait?

White trait reappeared in this generation,

so there must be some ‘factor’ in the purple flower

that masked it

MENDEL’S CONCLUSIONS

Dominant – trait that was expressed if present; represented by a capital letter (R)

Recessive – trait that is expresses only if the dominant is not present; represented by a lowercase letter (r)

Factors: sequence of DNA that codes for a trait, today we know those are the GENES

Alleles - two forms of a gene (dominant & recessive) Ex: Trait=Flower Color

Alleles=Purple/White

Regardless of the trait, results were the same and could be predicted in future generations…Mendel developed his first law of genetics; the Law of Dominance

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LAW OF DOMINANCE

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In a cross of parents(P) that are pure for contrasting traits, only one form of the trait will appear in the next generation(F1).

All the offspring will be heterozygous and express only the dominant trait.

RR x rr yields all Rr (round seeds)

EIGHT PEA PLANT TRAITS GENES ALLELES 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)

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LET’S SEE HOW IT WORKS, BY ADDING THE ALLELE NOTATIONS…

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RR rr

Rr

RR

Rr x Rr

RrrrRr

PREDICTABLE GENERATIONS

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Cross 2 Pure

PlantsTT x tt

Results in all

HybridsTt

Cross 2 Hybrids

get3 Tall & 1

ShortTT,Tt,Tt,tt

YOU TRY ONE…LET’S USE SEED COLOR

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Gg

Gg

Gg

Gg

GG ggP generation

F1

GG

GG

Gg

Gg

Gg

Gg

gg

ggF2

All Gg, yellow

¾ yellow¼ GG, 2/4 Gg

¼ green¼ gg

GENOTYPES

Genotype - allele combination for a trait

Homozygous – allele combination involving two of the same alleles; can be either Homozygous dominant: RR Homozygous recessive: rr

Heterozygous – allele combination of one dominant & one recessive allele  Heterozygous: Rr

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PHENOTYPE Phenotype - the physical features

resulting from the genotype; am I…

Phenotype also refers to your chemical features as well; enzymes and functional proteins are also determined by genes and allele combinations 19

PURPLE or, am I WHITE

GENOTYPE & PHENOTYPE IN SEED COLOR

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Genotype of alleles: R = yellow seed r = green seed

In the F2

The possible combinations are:Genotypes RR Rr rr

Phenotypes YELLOW YELLOW GREEN

BUNNY RABBIT

PUNNETT SQUARES Diagram that is used to predict an

outcome of a particular cross or breeding experiment

Illustrate Mendel’s second law Law of Segregation

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MONOHYBRID CROSSES

CROSSES INVOLVING 1 GENE: 2 ALLELES

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P1 MONOHYBRID CROSS

Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Wrinkled

seeds RR x rr

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R

R

rr

Rr

RrRr

RrGenotype: RrRatio: 1:1 or 100%

Phenotype: RoundRatio: 1:1 or 100%

F1 MONOHYBRID CROSS

Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Round seeds

Rr x Rr

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R

r

rR

RR

rrRr

Rr

Genotype: RR, Rr, rr

Ratio: 1:2:1

Phenotypes: Round & wrinkled

Ratio: 3:1

P1 MONOHYBRID CROSS REVIEW

Homozygous dominant x Homozygous recessive

Offspring all Heterozygous (hybrids)

Offspring called F1 generation Genotypic & Phenotypic ratio

is ALL ALIKE

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F1 MONOHYBRID CROSS REVIEW

Heterozygous x heterozygous Offspring:

25% Homozygous dominant RR50% Heterozygous Rr25% Homozygous Recessive rr

Offspring called F2 generation Genotypic ratio is 1:2:1 Phenotypic Ratio is 3:1

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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

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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.

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DIHYBRID CROSS

A breeding experiment that tracks the inheritance of two traits.

Mendel’s “Law of Independent Assortment”

a. Each pair of alleles segregates independently during gamete formation

b. Formula: 2n (n = # of heterozygotes)

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QUESTION:HOW MANY GAMETES WILL BE PRODUCED FOR THE FOLLOWING ALLELE ARRANGEMENTS?

Remember: 2n (n = # of heterozygotes)

1. RrYy

2. AaBbCCDd

3. MmNnOoPPQQRrssTtQq

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ANSWER:

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1. RrYy: 2n = 22 = 4 gametesRY Ry rY ry

2. AaBbCCDd: 2n = 23 = 8 gametesABCD ABCd AbCD AbCdaBCD aBCd abCD abCD

3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64 gametes

DIHYBRID CROSS

Traits: Seed shape & Seed color Alleles: R round Y yellow

r wrinkled y green To help determine allele combinations

use the math order of operations FOIL First-Outside-Inside-Last

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RrYy x RrYy

RY Ry rY ry RY Ry rY ry

All possible allele combinations

DIHYBRID CROSS

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RY Ry rY ry

RY

Ry

rY

ry

Rryy RRYy RrYY RrYy

RRYy Rryy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

DIHYBRID CROSS

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RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

Round/Yellow: 9RRYY 1RRYy 2RrYY 2RrYy 4

Round/green: 3RRyy 1Rryy 2

wrinkled/Yellow: 3rrYY 1rrYy 2

wrinkled/green: 1Rryy 1

9:3:3:1 phenotypic ratio

RY Ry rY ry

RY

Ry

rY

ry

DIHYBRID CROSS

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Round/Yellow: 9Round/green: 3wrinkled/Yellow: 3wrinkled/green: 1

9:3:3:1

GENETIC PRACTICE PROBLEMS

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BREED THE P1 GENERATION

tall (TT) x dwarf (tt) pea plants

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T

T

t t

SOLUTION:

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T

T

t t

Tt

Tt

Tt

Tt All Tt = tall(heterozygous tall)

produces theF1 generation

tall (TT) vs. dwarf (tt) pea plants

BREED THE F1 GENERATION

tall (Tt) vs. tall (Tt) pea plants

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T

t

T t

SOLUTION:copyright cmassengale

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TT

Tt

Tt

tt

T

t

T tproduces theF2 generation

1/4 (25%) = TT1/2 (50%) = Tt1/4 (25%) = tt1:2:1 genotype 3:1 phenotype

tall (Tt) x tall (Tt) pea plants