Mendelian Genetics

Preeyanan Sanpote,University of Phayao

Heredity & Genetics

• Heredity is …..the passing on of characteristics (traits) from parents to offspring

• Genetics is….. the study of heredity

• GeneticsGenetics is the study of genes. is the study of genes.

• InheritanceInheritance is how traits (characteristics) are passed is how traits (characteristics) are passed on from generation to generation.on from generation to generation.

• ChromosomesChromosomes are made up of are made up of genesgenes, which are made , which are made up of up of DNADNA..

• Genetic material Genetic material (genes,chromosomes, DNA) is found (genes,chromosomes, DNA) is found inside the nucleus of a cell.inside the nucleus of a cell.

• Gregor Mendel Gregor Mendel is considered is considered “The Father of Genetics"“The Father of Genetics"

HistoryHistory

Father of Genetics

Mendel……Austrian monkAustrian monk • used the scientific approach to identify two laws of inheritance

• discovered the basic principles of heredity by breeding garden peas (Pisum sativum) in carefully planned experiments

• reproduce sexually, reproduced quicklyreproduced quickly• have two distinct, male and female, sex

cells called gametes• their traits are easy to isolate• there are many varieties with distinct

heritable characters • showed obvious differences in the traits showed obvious differences in the traits • Mating,cross-pollination can be controlled

Mendel used peas..(Advantages of pea plants for genetic study)

Understood that there was something that Understood that there was something that carried traits from one generation to carried traits from one generation to

the next- the next- “FACTOR“FACTOR”.”.

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

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

Mendel chose only characters that occurred in two distinct alternative forms

He also used varieties that were true- breeding (plants that produce offspring of the same variety when they self-pollinate)

Particulate InheritanceParticulate Inheritance

cross-fertilized true-breeding that were cross-fertilized true-breeding that were different in just different in just one characterone character

““Law of Law of Segregation”Segregation”

Monohybrid Monohybrid CrossesCrosses

• Mendel mated two contrasting,true-breeding varieties, a process called hybridization

• The true-breeding parents are the P generation

• The hybrid offspring of the P generation are called the F1 generation

• When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the F2 generation is produced

Experiment

StamensCarpel

TECHNIQUE

First filialgenerationoffspring

(F1)

Parentalgeneration

(P)

RESULTS

3

2

1

4

5

EXPERIMENT

P Generation(true-breeding

parents) Purpleflowers

Whiteflowers

F1 Generation(hybrids) All plants had purple flowers

Self- or cross-pollination

F2 Generation

705 purple-flowered

plants

224 whiteflowered

plants

• Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids

• Mendel called the purple flower color a dominant trait and the white flower color a recessive trait

• The factor for white flowers was not diluted or destroyed because it reappeared in the F2

generation

• Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation

• Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits

• What Mendel called a “heritable factor” is what we now call a gene

What Do What Do the Peas Look Like?the Peas Look Like?

Mendel studies seven characteristics

in the garden pea

The results of Mendel’s F1 crosses for 7

characters in Pea plants

• Mendel was the first biologist to use mathematics to explain his results quantitatively.

• Mendel predicted the concept of genes that

genes occur in pairs that one gene of each pair

is present in the gametes

F2 ratio 705 : 224

or 3.15 : 1

The theoretical or The theoretical or expected ratio expected ratio of plants of plants producing round or wrinkled seeds is producing round or wrinkled seeds is 3 round : 3 round : 1 wrinkled1 wrinkled

Mendel’s Mendel’s observed ratio observed ratio was was 2.96 : 12.96 : 1

The discrepancy is due to The discrepancy is due to statistical errorstatistical error

The The larger the samplelarger the sample the more nearly the the more nearly the results approximate to the theoretical ratioresults approximate to the theoretical ratio

Did the observed ratio Did the observed ratio match the theoretical match the theoretical

ratio?ratio?

• Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring

• During the During the formation of gametes formation of gametes (eggs or (eggs or sperm), thesperm), the two alleles two alleles responsible responsible for a trait for a trait separateseparate from each other.from each other.

• Alleles for a trait are then Alleles for a trait are then "recombined" at "recombined" at fertilizationfertilization, , producing the genotype for the producing the genotype for the traits of the offspringtraits of the offspring.

““Law of Segregation”Law of Segregation”

• An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character

• An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character

• Unlike homozygotes, heterozygotes are not true-breeding

Alleles

allele for purple flowers

Locus for flower-color gene

allele for white flowers

Pair ofhomologouschromosomes

Alleles

Gene – a unit of heredity; a section of DNA sequence encoding

a single proteinGenome – the entire set of genes in an organism

Alleles – the different forms of a characteristicthe different forms of a characteristic..

Locus – a fixed location on a strand of DNA where a gene or

one of its alleles is located.

ProbabilityProbability-- the chances/percentages that something will occur. the chances/percentages that something will occur.

Punnett Squares- Punnett Squares- show how crosses are made.show how crosses are made.

DominantDominant traitstraits- traits that are expressed.- traits that are expressed.

RecessiveRecessive traitstraits- traits that are covered up.- traits that are covered up.

Genetic TerminologyGenetic Terminology

Monohybrid cross- Monohybrid cross- a genetic cross involving a single pair of genes

(one trait);looking at only one traitlooking at only one trait

Dihybrid cross Dihybrid cross –– a genetic cross involving two pairs of genes

(two traits);looking at only two traitslooking at only two traits P - Parental generation F1 - First filial generation;offspring from a genetic cross.

F2 - Second filial generation of a genetic cross

GenotypeGenotype-- the types of genes (alleles) present. the types of genes (alleles) present.

PhenotypePhenotype--the way organism looks and behaves/what it looks the way organism looks and behaves/what it looks

like.like. HomozygousHomozygous-- two of the same alleles. two of the same alleles.

HeterozygousHeterozygous-- two different alleles. two different alleles.

Trait or CharacterTrait or Character--any characteristic that can be passed from any characteristic that can be passed from

parent to offspring parent to offspring

Trait:Trait: Seed Shape Seed Shape Alleles:Alleles: RR – – RoundRound rr – – WrinkledWrinkled

Cross:Cross: Round seeds x Wrinkled seeds Round seeds x Wrinkled seeds

RRRR rrrr

GenotypeGenotype : : Rr Rr Phenotype:Phenotype: RoundRound

Genotypic Ratio:Genotypic Ratio: all alikeall alike

Phenotypic Ratio: Phenotypic Ratio: all alikeall alike

Monohybrid CrossMonohybrid Cross

gametes

rr rr

RR RrRr RrRr

RR RrRr RrRr Homozygous dominant x Homozygous recessiveHomozygous dominant x Homozygous recessive Offspring all Heterozygous (hybrids)called Offspring all Heterozygous (hybrids)called

FF11generationgeneration Genotypic & Phenotypic ratio is Genotypic & Phenotypic ratio is all alikeall alike

gametes

RR rr

RR RRRR RrRr

rr RrRr rrrr

Genotype :Genotype : RR,Rr,rrRr,rr

Phenotype :Phenotype : Round & Round & wrinkledwrinkled

Genotypic Ratio:Genotypic Ratio: 1:2:11:2:1

Phenotypic Ratio: Phenotypic Ratio: 3:13:1

Trait:Trait: Seed Shape Seed Shape Alleles:Alleles: RR – Round– Round rr – – WrinkledWrinkled

Cross:Cross: Round seeds x Round seeds Round seeds x Round seeds RrRr x x Rr Rr

Heterozygous Heterozygous x x HeterozygousHeterozygous Offspring have 3 genotypes ,called Offspring have 3 genotypes ,called FF2 2

generationgeneration Genotypic & Phenotypic ratio Genotypic & Phenotypic ratio is not is not the samethe same

P Generation

F1 Generation

F2 Generation

phenotype:

Genotype:

Gametes:

Sperm from F1 (Pp) plant

Purple flowersPp

Purple flowers White flowersPP pp

P

P

P

P

p

p

p

p

Eggs from F1 (Pp) plant

PP

ppPp

Pp

1/21/2

3 : 1

phenotype:

Genotype:Gametes:

Phenotype

Purple

Purple

Purple

White

3

1

1

1

2

Ratio 3:1 Ratio 1:2:1

Genotype

PP(homozygous)

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

Applying the Law of Applying the Law of SegregationSegregation

Test cross

• How can we tell the genotype of an individual with the dominant phenotype ?

• Such an individual could be either homozygous dominant or heterozygous

• The answer is to carry out a testcross : breeding the mystery individual with a homozygous recessive individual

Test cross : Cross with a homozygous recessive individual.

• When you have an individual with an unknown genotype, you do a test cross.

• If any offspring display the recessive phenotype, the mystery parent must be heterozygous

Example, a plant with purple flowers can either be PP or Pp

therefore, you cross the plant with a pp (white

flowers, homozygous recessive)

P- pp

• If you get all 100% purple flowers,then the

unknown parent was PP

P p P p

P p

P p

P P

p

p

P p p p

P p

p p

P p

p

p

•If you get 50% white, 50% purple flowers,then the unknown parent was Pp

Dominant phenotype,unknown genotype:

PP or Pp?

Recessive phenotype,known genotype:

pp

PredictionsIf purple-floweredparent is PP

If purple-floweredparent is Pp

or

Sperm Sperm

Eggs Eggs

or

All offspring purple 1/2 offspring purple and 1/2 offspring white

Pp Pp

Pp Pp

Pp Pp

pp pp

p p p p

P

P

P

p

TECHNIQUE

RESULTS

cross-fertilized true-breeding that were cross-fertilized true-breeding that were different in just different in just two characterstwo characters

Dihybrid Dihybrid CrossesCrosses

““The Law of Independent The Law of Independent Assortment”Assortment”

• Using a dihybrid cross, Mendel developed the law of independent assortment

• The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation

• Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome

• Genes located near each other on the same chromosome tend to be inherited together

P Generation

F1 Generation

Predictions

Gametes

EXPERIMENT

RESULTS

YYRR yyrr

yrYR

YyRr

Hypothesis ofdependent assortment

Hypothesis ofindependent assortment

Predictedoffspring ofF2 generation Sperm

Spermor

EggsEggs

Phenotypic ratio 3:1

Phenotypic ratio 9:3:3:1

Phenotypic ratio approximately 9:3:3:1315 108 101 32

1/21/2

1/2

1/2

1/41/4

1/41/4

1/4

1/4

1/4

1/4

9/163/16

3/161/16

YR

YR

YR

YRyr

yr

yr

yr

1/43/4

Yr

Yr

yR

yR

YYRR YyRr

YyRr yyrr

YYRR YYRr YyRR YyRr

YYRr YYrr YyRr Yyrr

YyRR YyRr yyRR yyRr

YyRr Yyrr yyRr yyrr

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry

Dihybrid CrossDihybrid Cross

RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

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

9:3:3:1 phenotypic ratio

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry

Dihybrid CrossDihybrid Cross

Dihybrid test cross??

If you had a tall, purple plant,how would you

know what genotype it is?

tt pp?? ??

1. TTPP 2. TTPp3. TtPP 4. TtPp

The laws of probability govern The laws of probability govern Mendelian inheritanceMendelian inheritance

• Mendel’s laws of segregation and independent assortment reflect the rules of probability

• When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss

• In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles

Segregation ofalleles into eggs

Segregation ofalleles into sperm

Sperm

Eggs

1/2

1/2

1/21/2

1/41/4

1/41/4

Rr Rr

R

R

RR

R

R

r

r

r

r r

r

Inheritance patterns are often more complex

than predicted by simple Mendelian genetics

• The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied

• Many heritable characters are not determined by only one gene with two alleles

• However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

Extending Mendelian Genetics for a Single Gene

• Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations :

– When alleles are not completely dominant

– When a gene has more than two alleles

– When a gene produces multiple phenotypes

Incomplete DominanceIncomplete Dominance

F1 hybrids F1 hybrids have an appearance somewhat in in betweenbetween the phenotypes phenotypes of the two parental varieties.

Example :Example : snapdragons (flower)snapdragons (flower)red (RR) x white (rr)

RR = red flowerRR = red flower

rr = white flower

R

R

r r

P Generation

F1 Generation

1/21/2

Red White

Gametes

Pink

Gametes

CWCWCRCR

CR CW

CRCW

CR CW

Incomplete Dominance

P Generation

F1 Generation

F2 Generation

1/21/2

1/21/2

1/2

1/2

Red White

Gametes

Pink

Gametes

Sperm

Eggs

CWCWCRCR

CR CW

CRCW

CR CW

CWCR

CR

CW

CRCR CRCW

CRCW CWCW

Summary of Mendel’s lawsSummary of Mendel’s laws

LAWLAW PARENT PARENT CROSS CROSS OFFSPRINGOFFSPRING

DOMINANCEDOMINANCE TT x tt TT x tt tall x shorttall x short

100 %100 % Tt Tt tall tall

Tt x Tt Tt x Tt tall x talltall x tall

75% or (3/4)75% or (3/4) tall tall 25% or (1/4)25% or (1/4) shortshort

INDEPENDENT INDEPENDENT ASSORTMENTASSORTMENT

RrGg x RrGgRrGg x RrGground & round & greengreen x x round & round & greengreen

9/169/16 round seeds & green pods round seeds & green pods

3/163/16 round seeds & yellow pods round seeds & yellow pods

3/163/16 wrinkled seeds & green wrinkled seeds & green

pods pods

1/161/16 wrinkled seeds & yellow wrinkled seeds & yellow

podspods

QuestionsQuestions

• What did Mendel cross?

• What are traits?• What are gametes?• What is fertilization?• What is heredity?• What is genetics?• What Did Mendel Find?What Did Mendel Find?

• The two alleles for a trait must separate when gametes are formed

• A parent randomly passes only one allele for each trait to each offspring

• The genes for different traits are inherited independently of each other.

Law of Independent Law of Independent AssortmentAssortment

Law of SegregationLaw of Segregation

• How many alleles are there for each trait?• What is an allele?• How many alleles does a parent pass on to

each offspring for each trait

Questions..???Questions..???

Dominantly Inherited Disorders

Achondroplasia, a form of dwarfism with an incidence of one case among every 10,000 people. Heterozygous individuals have the dwarf phenotype.

Huntington’s disease, a degenerative disease of the nervous system, is caused by a lethal dominant allele that has no obvious phenotypic effect until the individual is about 35 to 45 years old.

Key

Male Female Affectedmale

Affected female

1stgeneration

2ndgeneration

3rdgeneration

Is a widow’s peak a dominant or recessive trait?

Widow’speak

No widow’speak

WWor

Ww

Ww ww ww Ww

Ww Ww Wwww ww ww

ww

1stgeneration

2ndgeneration

3rdgeneration

Is an attached earlobe

a dominant or recessive?

Attachedearlobe

Freeearlobe

FForFf

Ff Ff Ff

Ff Ff

ff

ffffffFF or Ff

ff

Widow’speak

No widow’speak

Attachedearlobe

Parents

NormalAa

Sperm

Eggs

NormalAa

AANormal

AaNormal(carrier)

AaNormal(carrier)

aaAlbino

A

A

a

a

Albinism

Parents

DwarfDd

Sperm

EggsDd

Dwarfdd

Normal

DdDwarf

ddNormal

D

d

d

d

Normaldd

Dwarfism

• Chromosomes carry hereditary (genes)• Chromosomes (and genes) occur in pairs• New combinations of genes occur in sexual

reproduction• Monohybrid vs. Dihybrid crosses• Mendel’s Principles:

Dominance: one allele masks anotherSegregation: genes become separated in gamete formationIndependent Assortment: Members of one gene pair segregate independently from other gene pairs during gamete formation

Summary of Genetics

Preeyanan Preeyanan Sanpote Sanpote

School of Science

University of Phayao