MENDELIAN GENETICS

1. Mendel’s work 2. Monohybrid inheritance and principal of segregation 3. Dihybrid inheritance and the principal of independent

assortment 4. Test cross

INTRODUCTION

• Explaining the mechanism of inheritance • The mechanism relates to the numbers of

characteristic of inheritance • The simple characteristic leads to the simpler

crossing over mechanism and ration • This is followed by excluding the mutation

effects that will be discussed later (chapter 4)

GREGOR MENDEL

• Study in University of Vienna

• His parents has a small farm in Austria

GREGOR MENDEL (cont)

• 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

GREGOR MENDEL (cont)

• 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"

MENDEL’S PEA PLANT TRAITS

GREGOR MENDEL (cont)

• Mendel stated that physical traits are inherited as “particles”

• Mendel did not know that the “particles” were actually Chromosomes & DNA

GENETIC TERMINOLOGIES

• Character – heritable feature that varies among individuals

• Trait – each variant for the character • True-breeding – Plants homozygous for a

characteristic are true-breeding (Self-pollinate)• Hybridization – mating or crossing over of two true-

breeding varieties • P generation – parental generation/parent• F1 generation – first filial generation (son) • F2 generation – second filial generation

GENETIC TERMINOLOGIES (cont)

• Allele- alternate version of a gene• Homozygote – pair of identical alleles for a character• Heterozygote – two different alleles for a character (Bb)• Dominate allele – expressed in the heterozygote• Recessive allele – not expressed in the heterozygote• Homozygous dominant- BB• Homozygous recessive - bb• Genotype – genetic makeup• Phenotype – appearance of an organism

TYPES OF GENETIC CROSS

1. Monohybrid cross - cross involving a single traite.g. flower colour

2. Dihybrid cross - cross involving two traits e.g. flower colour & plant height

PUNNET SQUARE

• Diagrammatic device for predicting the allele composition of offspring from a cross between individuals of known genetic makeup.

• 3 steps / generation = P gen, F1 gen, F2 gen• Heterozygous allele - ?• Homozygous allele - ?• Phenotype - ?• Genotype - ?

PUNNET SQUARE (cont)

• Can be used for monohybrid and also dihybrid cross.

LAW OF INHERITANCE

Gregor Mendel introduce 2 laws • Law of Segregation • Law of Independent Assortment

LAW OF SEGREGATION

• Inherit only ONE characteristic @ Monohybrid• Producing 3:1 of phenotypic inheritance • Mendel use a large group of sample size to

explain this law• Leads to a development of a model known as

Mendel’s Model

MENDEL MODEL

Four concepts in law of segregation1. Alternative versions of genes account for variations in

inherited characteristics 2. For each character, an organism inherit two alleles, one

from each parent3. If the two alleles at a locus differ, then one, the dominant

allele, determines the organism’s appearance; the other, the recessive allele, has NO noticeable effect in the organism’s appearance

4. The two alleles for a heritable character segregate (separate) during gamete formation and end up in different gametes

1 . ALTERNATIVE VERSIONS OF GENES ACCOUNT FOR VARIATIONS IN INHERITED CHARACTERISTICS

• Have 2 choices of alleles• Existing in two version • Depending on the

phenotypic or characteristic derive in the genetic make up

• Eg. Purple flower and white flower

2. FOR EACH CHARACTER, AN ORGANISM INHERIT TWO ALLELES, ONE FROM EACH PARENT

• Each somatic cell in a diploid organism has two sets of chromosome

• Genetic locus represent twice in diploid cell, once in homolog of a specific pair of chromosome

3. IF THE TWO ALLELES AT A LOCUS DIFFER, THEN ONE, THE DOMINANT ALLELE, DETERMINES THE ORGANISM’S APPEARANCE; THE OTHER, THE RECESSIVE ALLELE, HAS NO NOTICEABLE EFFECT IN THE ORGANISM’S APPEARANCE

• The plant have more purple colour due to its dominant allele, vice versa

4. THE TWO ALLELES FOR A HERITABLE CHARACTER SEGREGATE (SEPARATE) DURING GAMETE FORMATION AND END UP IN

DIFFERENT GAMETES

• An egg or sperm gets only one of the two alleles that are present in the somatic cell of the organism making the gamete

• The correspond depending on the types of reproduction between meiosis and mitosis

• Further discussion after test cross

Example of MONOHYBRID CROSS

P1 Monohybrid Cross

Trait: Seed ShapeAlleles: R – Round r – WrinkledCross: Round seeds x Wrinkled seeds

RR x rr

R

R

rr

Rr

RrRr

Rr

Genotype: Rr

Phenotype: Round

GenotypicRatio: All alike

PhenotypicRatio: All alike

P1 Monohybrid Cross Review

• Homozygous dominant x Homozygous recessive

• Offspring all Heterozygous (hybrids)• Offspring called F1 generation• Genotypic & Phenotypic ratio is ALL ALIKE

F1 Monohybrid Cross• Trait: Seed Shape• Alleles: R – Round r – Wrinkled• Cross: Round seeds x Round seeds• Rr x Rr

R

r

rR

RR

rrRr

Rr

Genotype: RR, Rr, rr

Phenotype: Round & wrinkled

G.Ratio: 1:2:1

P.Ratio: 3:1

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

HOW DOES THE PEAS LOOK LIKE?

• Genotypic Ratio &

•Phenotypic Ratio

TEST YOURSELF!

1. Between blue flower, BB and yellow, yy2. Between small leaf, ff and big leaf, Ff

LAW OF INDEPENDENT ASSORTMENT

• TWO characteristics at the same time @ Dihybrid cross

• Eg. Leaf colour and leaf size • Using both dominant and recessive alleles in

each of the characteristics.

INDEPENDENT ASSORTMENT in CHROMOSOME

• Mendel performed dihybrid crosses in plants that were true-breeding for TWO traits.

• E.g a plant with green pod colour and yellow seed, cross-pollinated with a plant that had yellow pod colour and green seeds.

• Green pod colour = GG • Yellow seed colour = YY• Yellow pod colour = gg• Green seed colour = yy• The resulting F1 generation were all heterozygous for green

pod colour and yellow seeds (GgYy)

DIHYBRID CROSS

• Involves two pairs of contrasting traits

DIHYBRID CROSS

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

Phenotypic ratio 9:3:3:1

copyright cmassengale

DIHYBRID CROSS

• Traits: Seed shape & Seed colour• Alleles: R round

r wrinkled Y yellow y green

RrYy x RrYy

RY Ry rY ry RY Ry rY ry

All possible gamete combinations

DIHYBRID CROSS

RY Ry rY ry

RY

Ry

rY

ry

DIHYBRID CROSS

RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

Round/Yellow: 9

Round/green: 3

wrinkled/Yellow: 3

wrinkled/green: 1

9:3:3:1 phenotypicratio

RY Ry rY ry

RY

Ry

rY

ry

HYPOTHESIS/CONCLUSION

• The alleles of seed colour and seed shape sort into gametes independently of each other.

• Phenotypic ratio for IA = 9:3:3:1

TEST CROSS

• To determine if an individual exhibiting a dominant trait is homozygous or heterozygous for that trait.

• If all offspring display the dominant phenotype, the individual in question is homozygous dominant; if the offspring display both dominant and recessive phenotypes, then the individual is heterozygous

TEST CROSS (cont)

• In some sources, the ‘test cross’ is defined as being a type of backcross between the recessive homozygote and F1 generation.

• F1 progeny are mated back to one of their parents (or to individual with a genotype identical to the parent)

• Backcross is often used synonymously with testcross.

TEST CROSS

A mating between an individual of unknown genotype and a homozygous recessive individual.• Example: bbC__ x bbcc

BB = brown eyesBb = brown eyesbb = blue eyes

CC = curly hairCc = curly haircc = straight hair

bC b___

bc

TEST CROSS

Possible results:

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bC b___

bc bbCc bbCc

C bC b___

bc bbCc bbccor

c

copyright cmassengale

If the plant being tested is homozygous

If the plant being tested is heterozygous

• G?W? X ggww– (G=yellow; g=green; W=round; w=wrinkled)– What will the expected phenotypic ratios be for

the above testcross?

SUMMARY of MENDEL’S LAWLAW PARENT CROSS OFFSPRING

DOMINANCE / True-breeding TT x tt

tall x short100% Tt tall

SEGREGATION Tt x Tt tall x tall

75% tall 25% short

INDEPENDENT ASSORTMENT

RrGg x RrGg round & green x round & green

9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods