Chapter 14:

Mendel and the Gene Idea

3. Human Genetics

2. Beyond Mendelian Genetics

1. The Experiments of Gregor Mendel

1. The Experiments of

Gregor Mendel

Chapter Reading – pp. 268-276

TECHNIQUE

RESULTS

Parentalgeneration(P) Stamens

Carpel

1

2

3

4

Firstfilialgener-

ationoffspring(F1)

5

Gregor Mendel

Deduced the basics of

inheritance in the 19th

century by analyzing

pea plant crosses:

• examined

several

characters

exhibiting

dominant &

recessive

traits or

phenotypes

EXPERIMENT

P Generation

(true-breeding

parents) Purpleflowers

Whiteflowers

F1 Generation

(hybrids)All plants hadpurple flowers

F2 Generation

705 purple-floweredplants

224 white-floweredplants

Mendel’s

Crosses

1) Cross plants

that breed true

for character

of interest

2) Cross the

resulting

hybrids

(a monohybrid

cross)

Results for

Various

Characters

For each character

Mendel saw the same

basic pattern:

• a 3:1 ratio of dominant

to recessive traits

following a

monohybrid cross

Phenotype

Purple

Purple3

Purple

Genotype

1 White

Ratio 3:1

(homozygous)

(homozygous)

(heterozygous)

(heterozygous)

PP

Pp

Pp

pp

Ratio 1:2:1

1

1

2

Phenotype vs Genotype

P Generation

Appearance:Genetic makeup:

Gametes:

Purple flowers White flowersPP

P

pp

p

F1 Generation

Gametes:

Genetic makeup:Appearance: Purple flowers

Pp

P p1/21/2

F2 Generation

Sperm

Eggs

P

P

PP Pp

p

p

Pp pp

3 1

Mendel’s Law of

Segregation

Each gamete ends up with

only one of the two copies

of each gene in the parent.

• e.g., either the purple flower

allele or the white flower allele

• 50% of gametes contain one

copy, 50% contain the other

This is due to the segregation of

chromosomes during meiosis.

TECHNIQUE

RESULTS

Dominant phenotype,unknown genotype:

PP or Pp?

Predictions

Recessive phenotype,known genotype:

pp

If PP If Ppor

Sperm Sperm

p p p p

P

P

P

p

Eggs Eggs

Pp

Pp Pp

Pp

Pp Pp

pp pp

or

All offspring purple 1/2 offspring purple and1/2 offspring white

Test Crosses

• cross the unknown with

a homozygous recessive

individual

Test crosses are used

to reveal an unknown

genotype:

• if all offspring display

dominant phenotype:

homozygous dominant (PP)

• if ~½ each of recessive and

dominant phenotypes:

heterozygous (Pp)

P? x pp

Allele for purple flowers

Homologous

pair ofchromosomes

Locus for flower-color gene

Allele for white flowers

Genetic Alleles & Genetic Loci

All genes have a specific chromosomal location

referred to as its locus.

Genes can have different versions known as alleles.

• e.g., purple flower vs white flower alleles

• homologous chromosomes have the same genetic loci

Important Genetic Terms

Character• physical characteristic (e.g., eye color)

• individual’s observable traits (e.g., brown eyes)

Phenotype

• the alleles an individual has for a given gene

(e.g., AA or Aa or aa)

Genotype

• different forms or versions of the same geneAllele

Dominant allele (designated by capital letter: A)

• 1 copy of the allele determines phenotype (A–)

Recessive allele (designated by lower-case letter: a)

• affects phenotype only when homozygous (aa)

• the 2 alleles for a gene are identical (AA or aa)

Homozygous

• the 2 alleles for a gene are different (Aa)

Heterozygous

Homozygous dominant = AA

Homozygous recessive = aa

Heterozygous = Aa

Rr Rr

Segregation ofalleles into eggs

Sperm

R

R

R R

R

R r

rr

r

r

r1/2

1/2

1/2

1/2

Segregation ofalleles into sperm

Eggs

1/41/4

1/41/4

Genetics & Probability

All genetic

inheritance is

based on the

probability of

inheriting specific

genetic alleles

from each parent.

The Nature of Probability

The probability of multiple events happening

simultaneously is the product of the probabilities of

each single event.

The probability of one event OR another is the sum

of the probabilities of each event.

EXPERIMENT

RESULTS

P Generation

F1 Generation

Predictions

Gametes

Hypothesis of

dependent

assortment

YYRR yyrr

YR yr

YyRr

Hypothesis of

independent

assortment

orPredicted

offspring of

F2 generation

Sperm

Sperm

YR

YR

yr

yr

Yr

YR

yR

Yr

yR

yr

YRYYRR

YYRR YyRr

YyRr

YyRr

YyRr

YyRr

YyRr

YYRr

YYRr

YyRR

YyRR

YYrr Yyrr

Yyrr

yyRR yyRr

yyRr yyrr

yyrr

Phenotypic ratio 3:1

EggsEggs

Phenotypic ratio 9:3:3:1

1/2 1/2

1/2

1/2

1/4

yr

1/41/4

1/4 1/4

1/4

1/4

1/4

1/43/4

9/163/16

3/161/16

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

Multiple Gene InheritanceIn a dihybrid cross,

the probability of

each combined

phenotype is the

product of each

individual

phenotype.

Expected Ratios:

9 Y–R–

3 Y–rr

3 yyR–

1 yyrr

Mendel’s Law of Independent

Assortment

“Each pair of genetic alleles segregates

independently of all other pairs of genetic

alleles during gamete formation (meiosis).”

• the distribution of the alleles of a given gene

into gametes has no connection to the

distribution of alleles for other genes

• applies only to genes on different chromosomes

2. Beyond Mendelian Genetics

Chapter Reading – pp. 276-281

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes1/2

1/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR CRCW

CRCW CWCW

1/21/2

1/2

1/2

Incomplete

Dominance

Dominant allele is

expressed to lesser

degree when only 1

copy is present

(heterozygous):

• heterozygotes have

an intermediate

phenotype (e.g., pink

vs red or white)

IA

IB

i

A

B

none

(a) The three alleles for the ABO blood groupsand their associated carbohydrates

Allele Carbohydrate

GenotypeRed blood cell

appearancePhenotype

(blood group)

IAIA or IA i A

BIBIB or IB i

IAIB AB

ii O

(b) Blood group genotypes and phenotypes

Codominance

• “A” & “B”

alleles are

codominant

(both alleles

expressed)

• “O” allele is

recessive

Two different alleles

when paired

together are

expressed equally(e.g., ABO blood type)

BbCc BbCc

Sperm

Eggs

BC bC Bc bc

BC

bC

Bc

bc

BBCC

1/41/4

1/41/4

1/4

1/4

1/4

1/4

BbCC BBCc BbCc

BbCC bbCC BbCc bbCc

BBCc BbCc

BbCc bbCc

BBcc Bbcc

Bbcc bbcc

9 : 3 : 4

Epistasis

The expression of

a genetic allele

from one locus

alters the

expression of

alleles at another

locus.

• in this example the

homozygous

recessive genotype

for one gene

overrides the

expression of the

other gene

Eggs

Sperm

Phenotypes:

Number ofdark-skin alleles: 0 1 2 3 4 5 6

1/646/64

15/6420/64

15/646/64

1/64

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/81/8

1/81/8

1/81/8

1/81/8

AaBbCc AaBbCc

Polygenic

Inheritance

Many phenotypes

are due to the

effects of multiple

genes (such as

skin color).

• in this example,

3 different genes

influence one

phenotype

resulting in 64

different variants

3. Human Genetics

Chapter Reading – pp. 282-285

Key

Male

Female

Affectedmale

Affectedfemale

Mating

Offspring, inbirth order(first-born on left)

1st generation(grandparents)

2nd generation(parents, aunts,and uncles)

3rd generation(two sisters)

Widow’s peak No widow’s peak

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

Ww ww

Ww Wwww ww

ww

wwWw

Ww

wwWW

Wwor

Pedigrees

Diagrams

illustrating

family history

that allow

inheritance

patterns to be

deduced.

We can’t do

experimental

crosses on

humans, we

can only

analyze

existing family

pedigrees.

Attached earlobe

1st generation(grandparents)

2nd generation(parents, aunts,and uncles)

3rd generation(two sisters)

Free earlobe

(b) Is an attached earlobe a dominant or recessive trait?

Ff Ff

Ff Ff Ff

ff Ff

ff ff ff

ff

FF or

orFF

Ff

A Recessive Human Trait

Parents

Normal Normal

Sperm

Eggs

NormalNormal(carrier)

Normal(carrier) Albino

Aa Aa

A

AAA

Aa

a

Aaaa

a

Albinism is also Recessive

Eggs

Parents

Dwarf Normal

Normal

Normal

Dwarf

Dwarf

Sperm

Dd dd

dD

Dd dd

ddDd

d

d

Achondroplasia is Dominant

Key Terms for Chapter 14

• epistasis polygenic inheritance

• dominant, recessive, codominant

• test cross, Punnet square

• trait, phenotype, genotype, allele, locus

• true-breeding, monohybrid, dihybrid

• homozygous, heterozygous

• pedigree

Relevant

Chapter

Questions 1-14