Chapter 13 MENDEL AND THE GENE Why do we look like family members or not?

Chapter 13 MENDEL AND THE GENEWhy do we look like family members or not?

History

• It started with farmers and botanists•Knight used pure breeding peas, one

variety with purple flowers, one variety with white flowers.▫ Crossing the two varieties he found that the

offspring all had purple flowers.▫ When he crossed the offspring, some had purple

flowers, some had white flowers.▫ Conclusion: some traits have a stronger tendency

to appear than others. No Numbers

History

•Mendel, an Austrian monk, repeated Knight’s experiments:▫Also used true-breeding peas and studied 7

different traits, fig 13.2.▫Cross-fertilized peas showing two

variations of the same trait, ex. round peas vs. wrinkled peas.

Figure 13-2

Trait Phenotypes

Seed shape

Seed color

Pod shape

Pod color

Flower color

Flower and   pod position

Stem length

Tall Dwarf

Terminal (at tip)Axial (on stem)

Purple White

YellowGreen

Inflated Constricted

GreenYellow

Round Wrinkled

Figure 13-1

Self-pollination

Female organ(receives pollen)

Eggs

Male organs(produce pollengrains, whichproduce malegametes)

Cross-pollination

CROSS-POLLINATION

3. Transfer pollen to thefemale organs of theindividual whose maleorgans have beenremoved.

2. Collect pollen from adifferent individual.

1. Remove male organsfrom one individual.

SELF-POLLINATION

Mendel Studied a Single Trait

•Mendel cross-fertilized two plants, one with white flowers with one with purple flowers. The hybrids, the F1 generation, all had purple flowers.

•Studying one trait through cross-fertilization is termed a monohybrid cross.

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Pollen transferred from white flower to stigma of purple flower

Anthersremoved

All purple flowers result

Mendel Studied a Single Trait

•Mendel’s experiments cont’d▫Mendel allowed F1 generation plants to

self- fertilize.▫Their offspring, the F2 generation,

expressed (demonstrated) both purple and white flowers. The ratio of plants with purple to white flowers was always 3:1.

▫Where did these white flowered plants come from?

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P (parental)generation

Cross-fertilize

F1 generation

Purple White

3

Self-fertilize

White

Purple

Purple

Purple

: 1

F2 generation

Mendel Studied a Single Trait

•Mendel cont’d▫The F1 generation plants all resembled

only parent plant; i.e. one variation of the trait is dominant.

▫The F2 generation showed plants with both variations of the character, purple and white. The variation of the trait that was only seen in the F2 generation (white flowers) is recessive.

Mendel Studied a Single Trait

•Mendle cont’d▫ The F2 generations were allowed to self-fertilize.

Looking at the F3 generation, Mendel discovered that the F2 generation actually consisted of 3 different types of plants: Pure breeding purple Not pure breeding purple (produced both purple and

white flowered plants. Pure breeding white. The ratio was actually 1:2:1.

Mendel Studied a Single Trait

•Conclusions (cross involving 1 trait)

Genes and Mendel’s Findings•Traits are carried by genes.•An individual has 2 genes or alleles for

each trait, 1 on each homologous chromosome.

•Meiosis results in separation of the homologous chromosomes and the alleles so that each is carried by a different gamete.

Genes and Mendel’s Findings

•An individual with 2 identical alleles is said to be homozygous, while an individual with 2 different alleles is said to be heterozygous.

•The genetic make-up of an individual is its genotype. The appearance or expression of the genotype is called its phenotype.

Genes and Mendel’s Findings•Mendel’s results can be predicted using

Punnett squares.▫Dominant genes are represented by

uppercase letters, ex. round peas (R) . Expressed when there is 1 or 2 dominant alleles present.

▫Recessive genes are represented by lowercase letters, ex. wrinkled peas (r). Only expressed when there are 2 recessive alleles present.

Figure 13-4

Homozygousmother

Meiosis

Offspring genotypes: All Rr (heterozygous)

Homozygousfather

Mal

e g

amet

es

Female gametes

Meiosis

Offspring phenotypes: All round seeds

A cross between two homozygotes

A cross between two heterozygotes

Female gametes

Heterozygousmother

Heterozygousfather

Offspring genotypes: 1/4 RR : 1/2 Rr : 1/4 rr

Mal

e g

amet

es

Offspring phenotypes: 3/4 round : 1/4 wrinkled

Genes and Mendel’s Findings

•Mendels’ Principle of Segregation, fig 13.7:

Figure 13-7

Meiosis IAlleles segregate

Recessive allelefor seed shape

Ga

me

tes

Chromosomes replicate

Rr parent

Dominant allelefor seed shape

Meiosis II

Principle of segregation: Each gamete carries only one allele for seed shape, because the alleles have segregated during meiosis.

Mendel Studied 2 Traits

•Mendel then looked at two traits simultaneously – dihybrid cross. Ex. plants that produced round (R), yellow (Y) peas and plants that produced wrinkled (r), green (y) peas.

•The pure breeding parents’ genotypes were RRYY and rryy, fig 13.5.

•What is the genotype and phenotype of the F1 generation? The F2 generation?

Figure 13-5a     Hypothesis of independent assortment: Alleles of different genes don’t stay together when gametes form.

Mal

e g

amet

esMale parent

F1 PUNNET SQUARE

F1 offspring all RrYy

F2 female parent

Female parent

Female gametes

Mal

e g

amet

es

F2 offspring genotypes: 9/16 R–Y– : 3/16 R–yy : 3/16 rrY– : 1/16 rryy

F2 offspring phenotypes: 9/16 : 3/16 : 3/16 : 1/16

F2 maleparent

Female gametesF2 PUNNET SQUARE

Alleles at R gene and Y gene go to gametes independently of each other

Genes and Mendel’s Findings

•Mendel’s Principle of Independent Assortment:, fig 13.8.

Figure 13-8

Meiosis I

Replicated chromosomesprior to meiosis

Gam

etes

Alleles for seed shape

Meiosis II

Principle of independent assortment: The genes for seed shape and seed colorassort independently, because they are located on different chromosomes.

Meiosis II

Meiosis I

Alleles for seed color

1/4 RY 1/4 ry 1/4 Ry 1/4 rY

Chromosomes can line up in two ways during meiosis I

R

Ry yr

YY

R R

R R

R R

r r

r r

r r

r

Y Y

Y Y

Y Y

y y

y y

y yy y

y y

y y

R R

R R

R R

r r

r r

r r

Y Y

Y Y

YY

Peas are Easy

Most phenotypes (expression of genes) are the result of the action of more than one gene.

Continuous variation:

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20

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of

ind

ivid

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Peas are Easy

Pleiotropic effects: An individual gene may have effects on many traits. • Example:

• A dominant gene causes yellow hair in mice.

• If the mouse is homozygous for the gene it dies = a lethal defect. (In this case the gene was acting as if it was a recessive gene).

• Other examples: Marfan’s syndrome.

Peas are Easy

Incomplete dominance,fig 13.17.

Figure 13-17Flower color is variable in four-o’clocks.

Incomplete dominance in flower color

Parentalgeneration

F2 generation

F1 generation

Self-fertilization

Purple Lavender White

Peas Are Easy•Co dominance: Some phenotypes

represent both alleles, ex. blood types.•ABO Blood groups - CoDominance

▫2 dominant alleles, A and B, one recessive allele, i.

▫Alleles code for different RBC membrane proteins. These protein act as antigens (can cause an immune response).

▫Immune response = antibodies.

Peas Are Easy•ABO Blood Groups, cont’d

▫ Type A blood type has IA,IA or IA,i alleles▫ Type B blood type has IB, IB or IB,i alleles▫ Type AB blood type has IA, IB alleles▫ Type O blood type has i, i alleles (recessive form, no

antigens on their RBCs).•Rh blood group: the Rh factor consists of

8 different antigens. A person that has even one of these antigens is Rh+ while those having none of the antigens is Rh-.

Peas are Easy

Environmental effects:

Human Genetics

Random mutation of genes occurs constantly, but most do not produce changes in phenotype or disease symptoms.

Most gene disorders are rare, i.e. the frequency of occurrence of a defective allele is low. Exceptions: “closed societies”, ex. Tay Sachs, Sickle Cell

Human Genetics

Most gene disorders are recessive and only expressed when both alleles are recessive forms of the gene. Exceptions: Huntington disease is caused by a dominant gene.

Figure 13-21

Carrier male Carrier female

Affectedmale

Affectedfemale

I

II

III

IV

Eac

h r

ow

rep

rese

nts

a g

ener

atio

n

Carriers (heterozygotes) are indicated with half-filled symbols

Pedigree of a family with an autosomal recessive disease

Figure 13-22

Affected femaleI

II

III

IV

Unaffected male

If a child shows the trait, then one of the parents shows the trait as well

Pedigree of a family with Huntington’s Disease

Patterns of Inheritance in Humans

Controlled mating is not practical. Solution: Pedigrees – constructed from the

progeny of matings over many generations. Ex. – hemophilia in family of Queen Victoria. • The defective gene is recessive and occurs on the X

chromosome. Heterozygous females are carriers.

• Because the male Y chromosome does not express many of its genes, the defective gene is expressed in males, i.e. it is sex-linked.

A Pedigree of an X-Linked Recessive Disease

Queen Victoria Prince Albert

Female carrier of hemophilia allele

I

II

III

IV

Affected male

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George III

EdwardDuke of Kent

Louis IGrand Duke of HesseI

KingEdward VII

Duke ofWindsor

QueenElizabeth II

PrincePhilip

Margaret

PrincessDiana

PrinceCharles

Anne Andrew Edward

William Henry

KingGeorge VI

KingGeorge V

Earl ofMountbatten

ViscountTremation

Alfonso Jamie GonzaloPrinceSigismond

PrussianRoyalHouse

British Royal House

Spanish Royal House

RussianRoyalHouse

Henry Anastasia Alexis? ?

? ?

? ?

?

Waldemar

Queen VictoriaPrince Albert

FrederickIII

I

II

III

IV

V

VI

VII

Victoria Alice Alfred Arthur Leopold Beatrice PrinceHenry

HelenaDuke ofHesse

No hemophilia No hemophilia

GermanRoyalHouse

Juan

King JuanCarlos

No evidenceof hemophilia

No evidenceof hemophilia

Irene CzarNicholas II

CzarinaAlexandra

Earl ofAthlone

PrincessAlice

QueenEugenie

AlfonsoKing ofSpain

Maurice Leopold

Gen

erat

ion

Patterns of Inheritance in Humans

Some genetic disorders arise from the mutation of a single base on the DNA. This can alter one amino acid of a single protein and have lethal effects. Ex. Sickle cell anemia.

Gene Therapy Replacing a defective gene with a functional

gene. In the past, this type of therapy has worked in some isolated instances.

Problems: • The functional gene is carried as part of the DNA of an

adenovirus (cold virus), the vector. • The virus can cause a strong immune response

causing 1) the destruction of the virus and the gene destroyed or 2) death of the patient.

• The gene may also be incorporated into the patient’s DNA at random and cause lethal mutations.

Gene Therapy

Gene therapy was banned for several years but a new vector AAV (paravirus with only 2 genes of its own) is showing promise.

Animal trials have shown positive results with few problems. Clinical trials are underway for cystic fibrosis, etc.