Ch 14 Gregor Mendel and Inheritance Study

Began research late 1850’s

St. Thomas (Augustinian) monastery, Brno, Yugoslavia

• What genetic principles account for the passing of traits from parents to offspring?

•Mendel’s pea breeding data• supported the particulate mechanism

1. “blending” hypothesis–genetic material from the two parents blends together (like blue and yellow paint blend to make green)

2. “particulate” hypothesis–parents pass on discrete heritable units (genes)

Scientific approach

Asked question: how are traits inherited?

Used convenient model system: garden peas

-controlled breeding

-a variety of either/or visible traits

-quantitative data

Derived conclusions based on the data

For example

To examine inheritance of flower color

Cross-fertilized (hybridized):

true breeding purple x true breeding white

Measurable character: phenotype

LE 14-2

Removed stamensfrom purple flower

Transferred sperm-bearing pollen fromstamens of whiteflower to egg-bearing carpel ofpurple flower

Carpel Stamens

Parentalgeneration(P)

Pollinated carpelmatured into pod

Planted seedsfrom pod

Examinedoffspring:all purpleflowers

Firstgenerationoffspring(F1)

Materials & Methods

100% purplephenotype

“True-breeding”

plants that produce offspring of the same variety when they self-pollinate

• True-breeding parents– P (generation)

• Hybrid offspring (cross between 2 different true-breeding parents)

– F1 (generation)

• Progeny of F1 self-pollination– F2 (generation)

Nomenclature

LE 14-3

P Generation

(true-breedingparents)

F1 Generation

(hybrids)

F2 Generation

Purpleflowers

Whiteflowers

All plants hadpurple flowers

Results fromtwo crosses

F1 x F1

F2

P1 x P2

• Only purple flowers in the F1 hybrids– Purple: dominant trait– White: recessive trait

• Same pattern of dominant inheritance- six other pea plant phenotypes, each represented by two

traits

Mendel’s “heritable factors” (= genes)

(though he did not know the existence of DNA)

Conclusions

Mendel’s Model

• Inheritance patterns obeyed rules of probability

1. Assumes each trait has two variants

Current knowledge: phenotype is controlled by genes (genotype)

paternal and maternal alleles (genes)=two variantspositioned at chromosomal locus

LE 14-4

Allele for purple flowers

Homologouspair ofchromosomes

Allele for white flowers

Locus for flower-color gene

2. Offspring inherit two alleles, one from each parent

P= purple (dominant allele) p= white (recessive allele)

Homozygous if PP or pp (noun; homozygote)

Heterozygous if Pp (noun: heterozygote)

PP=_____________________________

pp=___________________________

3. One of the alleles determines phenotype

4. Law of segregationtwo alleles for any given phenotype separate duringgamete formation.

Thus, offspring inherit one allele from each parent.

During anaphase I

When do alleles first separate from each other?

Convenient quantitative way to predict outcome of crosses

Punnett Square

1. Name alleles with single letter2. Write down genotype of each parent3. Fill in boxes with all possible combinations of alleles

according to parental genotypes

On Board

What is the frequency of genotypes of the offspring of the parental cross?

What is the frequency of phenotypes of the offspring of a parental cross?

What is the frequency of genotypes of the F1 cross progeny?

What is the frequency of phenotypes of the F1 cross progeny?

50% heterozygous; 25% dominant and recessive homozygous

75% purple, 25% white (3:1)

100% heterozygous

100% purple

LE 14-5_2

Appearance:

P Generation

Genetic makeup:

Gametes

F1 Generation

Appearance:Genetic makeup:

Gametes:

F2 Generation

Purple flowersPp

P p1 21 2

P p

F1 sperm

F1 eggsPP Pp

Pp pp

P

p

3 : 1

PurpleflowersPP

Whiteflowerspp

P p

LE 14-6Phenotype

Purple

Purple3

Purple

Genotype

PP(homozygous

Pp(heterozygous

Pp(heterozygous

pp(homozygous

1

2

1

Ratio 1:2:1

White

Ratio 3:1

1

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

Genotype possibilities (purple flower color)?

PP or Pp

Test cross

Hybridize to individual with homozygous recessive individual (pp)

Do Punnett for each. Outcome?

LE 14-7

Dominant phenotype,unknown genotype:

PP or Pp?

If PP,then all offspring

purple:

p p

P

P

Pp Pp

Pp Pp

If Pp,then 1

2 offspring purpleand 1

2 offspring white:

p p

P

Ppp pp

Pp Pp

Recessive phenotype,known genotype:

pp

Monohybrid vs dihybrid

Heterozygous for 1 character Heterozygous for 2 characters

Set up a cross betweenheterozygous purple flowered pea plants

Pp x Pp

Do a Punnett Square: determine genotypic and phenotypic ratios.

What are patterns of inheritance 2 characters?

Given: true breeding Pfor pea color & shape

Dominant charactersYellowRound

Recessive charactersgreenwrinkled

Set up Parental Cross

Set up F1 cross

What is phenotypic frequency of the offspring?

LE 14-8

P Generation

F1 Generation

YYRR

Gametes YR yr

yyrr

YyRr

Hypothesis ofdependentassortment

Hypothesis of independent assortment

SpermEggs

YR

Yr

yrYR

YR

yr

Eggs

YYRR YyRr

YyRr yyrr yR

yrPhenotypic ratio 3:1

F2 Generation(predictedoffspring)

YYRR YYRr YyRR YyRr

YYRr YYrr YyRr Yyrr

YyRR YyRr yyRR yyRr

YyRr Yyrr yyRr yyrr

Phenotypic ratio 9:3:3:1

YR Yr yR yr

Sperm

12

14

14

14

14

1 43

4

12

12

12

14

916

316

316

316

14

14

14

• Observation of a dihybrid cross - Each pair of alleles segregates independently of

other pairs of alleles during gamete formation

- Mendel proposed the law of independent assortment

•Note: Applies only to genes on different, nonhomologous chromosomes

Genes located near each other on same chromosome tend to be inherited together: linkage

(Fast forward) Established at metaphase I & II

Autosomes (22 pr)

Sex chromosomes

Inheritance of Sex-Linked Genes

Sex chromosomesCarry genes that determine sexual phenotype & other traits

Sex-linked genes:Any gene on sex chromosome (usually refers to X-chromosome because of bigger size, more

genes)

LE 15-10

Sperm

Ova

Sperm

Ova

Sperm

Ova

How to show inheritance of sex-linked traits:follow X and Y chromosomes in a cross

• Sex-linked recessive disorders

–Color blindness–Duchenne muscular dystrophy–Hemophilia

Present hemophilia problem

Mother carrier/ father normal

We’re celebripeas!Questions?