Mendel and the Gene Mendel and the Gene IdeaIdea

CHAPTER 14

What genetic principles account

for the transmission of traits from

parents to offspring?

• One possible explanation of heredity is a “blending” hypothesis– The idea that genetic material

contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green

• An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea– Parents pass on discrete heritable

units, genes

•Gregor Mendel– Documented a particulate

mechanism of inheritance through his experiments with garden peas

– DNA wasn’t known yet

• Mendel used the scientific approach to identify two laws of inheritance

• Mendel discovered the basic principles of heredity By breeding garden peas in carefully planned experiments

Mendel’s Experimental,

Quantitative Approach• Mendel chose to work with peas

because– They are available in many varieties– He could strictly control which plants

mated with which– Self pollinating (able to start with pure

plants)– Lots of offspring

CROSSING PEA PLANTS1

5

4

3

2

Removed stamensfrom purple flower

Transferred sperm-bearing pollen fromstamens of white flower to egg-bearing carpel of purple flower

Parentalgeneration(P)

Pollinated carpelmatured into pod

Carpel(female)

Stamens(male)

Planted seedsfrom pod

Examinedoffspring:all purpleflowers

Firstgenerationoffspring(F1)

APPLICATION By crossing (mating) two true-breedingvarieties of an organism, scientists can study patterns ofinheritance. In this example, Mendel crossed pea plantsthat varied in flower color.

TECHNIQUETECHNIQUE

When pollen from a white flower fertilizeseggs of a purple flower, the first-generation hybrids all have purpleflowers. The result is the same for the reciprocal cross, the transferof pollen from purple flowers to white flowers.

TECHNIQUERESULTS

SOME GENETIC VOCABULARY

• Character: a heritable feature, such as flower color

• Trait: a variant of a character, such as purple or white flowers

More Genetic Vocabulary

• An organism that is homozygous for a particular gene– Has a pair of identical alleles for

that gene (RR or rr)– Exhibits true-breeding (pure)

• An organism that is heterozygous for a particular gene– Has a pair of alleles that are

different for that gene (Rr)– Hybrid

•An organism’s phenotype– Is its physical appearance

•An organism’s genotype– Is its genetic makeup

Phenotype Versus Genotype

3

1 1

2

1

Phenotype

Purple

Purple

Purple

White

Genotype

PP(homozygous)

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

Ratio 3:1 Ratio 1:2:1

• Mendel chose to track– Only those characters that

varied in an “either-or” manner

• Mendel also made sure that– He started his experiments with

varieties that were “true-breeding”

• In a typical breeding experiment– Mendel mated two contrasting,

true-breeding varieties, a process called hybridization

• The true-breeding parents

– Are called the P1 generation

– Cross RR x rr yielded all Rr (hybrids)

• The hybrid offspring of the P1 generation

– Are called the F1 generation

• When F1 individuals self-pollinate

– The F2 generation is produced

– Hybrid x Hybrid Rr x Rr– Offspring 25% RR, 50% Rr, 25% rr– 3:1 phenotypic ratio– 1:2:1 genotypic ratio

The Law of Segregation

• Mendel derived the law of segregation– By following a single trait

• The F1 offspring produced in this cross– Were monohybrids,

heterozygous for one character

The Law of Segregation

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

• Alternative Versions Of Genes– Account for variations in inherited

characters, which are now called alleles– Capital letter (dominant allele)– Lowercase (recessive)

Allele for purple flowers

Locus for flower-color geneHomologouspair ofchromosomes

Allele for white flowers

• For each character

– An organism inherits two alleles, one from each parent

– A genetic locus is actually represented twice

The Testcross or F2

• Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype

• Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait

• RR x Rr or rr x Rr • 1:1 ratio

THE TESTCROSS

Dominant phenotype,unknown genotype:

PP or Pp?

Recessive phenotype,known genotype:

pp

If PP,then all offspring

purple:

If Pp,then 1⁄2 offspring purpleand 1⁄2 offspring white:

p p

P

P

Pp Pp

PpPp

pp pp

PpPpP

p

p p

APPLICATION An organism that exhibits a dominant trait,such as purple flowers in pea plants, can be either homozygous forthe dominant allele or heterozygous. To determine the organism’sgenotype, geneticists can perform a testcross.

TECHNIQUE In a testcross, the individual with theunknown genotype is crossed with a homozygous individualexpressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent.

RESULTS

The Law of Independent Assortment

• Mendel derived the law of independent assortment– By following a two traits

• The F1 offspring produced in this cross– Were dihybrids, heterozygous

for both characters

YYRRP Generation

Gametes YR yr

yyrr

YyRrHypothesis ofdependentassortment

Hypothesis ofindependent

assortment

F2 Generation(predictedoffspring)

1⁄2 YR

YR

yr

1 ⁄2

1 ⁄2

1⁄2 yr

YYRR YyRr

yyrrYyRr

3 ⁄4 1 ⁄4

Sperm

Eggs

Phenotypic ratio 3:1

YR1 ⁄4

Yr1 ⁄4

yR1 ⁄4

yr1 ⁄4

9 ⁄163 ⁄16

3 ⁄161 ⁄16

YYRR YYRr YyRR YyRr

YyrrYyRrYYrrYYrr

YyRR YyRr yyRR yyRr

yyrryyRrYyrrYyRr

Phenotypic ratio 9:3:3:1

315 108 101 32 Phenotypic ratio approximately 9:3:3:1

F1 Generation

EggsYR Yr yR yr1 ⁄4 1 ⁄4 1 ⁄4 1 ⁄4

Sperm

RESULTS

CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other.

EXPERIMENT Two true-breeding pea plants—one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant.

• A dihybrid cross– Illustrates the inheritance of two characters

• Produces four phenotypes in the F2 generation

• Using the information from a dihybrid cross, Mendel developed the law of independent assortment– Each pair of alleles

segregates independently during gamete formation

LAWS OF PROBABILITYLAWS OF PROBABILITY

The Laws Of Probability Govern Mendelian Inheritance

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

The Multiplication and Addition Rules Applied to

Monohybrid Crosses• The Multiplication Rule

– States that the probability that two or more independent events will occur together is the product of their individual probabilities

• Probability of two independent events A,B:

• P(A and B) = P(A)*P(B)

• Probability in a monohybrid cross– Can be determined using this rule

Rr

Segregation ofalleles into eggs

Rr

Segregation ofalleles into sperm

R r

rR

RR

R1⁄2

1⁄2 1⁄2

1⁄41⁄4

1⁄4 1⁄4

1⁄2 rr

R rr

Sperm

Eggs

•The Rule Of Addition–States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities

Solving Complex Genetics Problems with the Rules

of Probability

• We can apply the rules of probability– To predict the outcome of crosses

involving multiple characters

• A dihybrid or other multicharacter cross– Is equivalent to two or more

independent monohybrid crosses occurring simultaneously

• In calculating the chances for various genotypes from such crosses– Each character first is considered

separately and then the individual probabilities are multiplied together

Probability Problem

• If H horns is dominant to h hornless, and T fancy tail in dragons is dominant to t plain tail, what is the probability that results from a cross of Dragon #1 HHtt x HhTt Dragon #2:– Dragon #1 always contributes H &

Dragon #2 contributes H half the time and h the other half SO the probability of having horns (HH x

Hh) is equal to 1 and no horns is

equal to 0

Probability Problem cont.)

Dragon #1 HHtt x HhTt Dragon #2

• Dragon #1 always contributes a t while Dragon #2 contributes a T half the time and a t half the time

• (tt x Tt)• SO the probability of having a

fancy tail is ½ and the probability of having a plain tail is ½

Probability Problem cont.)

SO what’s the probability of having a baby dragon with:– Horns and fancy tail? 1 x ½ = ½– Horns and a plain tail? 1 x ½ = ½– Hornless and fancy tail? 0 x ½ = 0– Hornless and plain tail? 0 x ½ = 0

OTHER FACTORS AFFECTING OTHER FACTORS AFFECTING INHERITANCEINHERITANCE

• Inheritance patterns are often more complex than predicted by simple Mendelian genetics

• The relationship between genotype and phenotype is rarely simple

• The inheritance of characters by a single gene may deviate from simple Mendelian patterns

The Spectrum of Dominance

• Complete dominance– Occurs when the phenotypes of the

heterozygote and dominant homozygote are identical

• Codominance– Two dominant alleles affect the

phenotype in separate, distinguishable ways

– Human ABO blood type is an example of codominance

• The ABO blood group in humans– Is determined by multiple alleles– A and B are dominant to 0

• Incomplete Dominance– The phenotype of F1 hybrids is

somewhere between the phenotypes of the two parental varieties

P Generation

F1 Generation

F2 Generation

RedCRCR

Gametes CR CW

WhiteCWCW

PinkCRCW

Sperm

CR

CR

CR

Cw

CR

CRGametes1⁄2 1⁄2

1⁄2

1⁄2

1⁄2

Eggs1⁄2

CR CR CR CW

CW CWCR CW

Pleiotropy• In Pleiotropy

– A gene has multiple phenotypic effects– Frizzle gene in chickens causes feathers to curl outward,

abnormal body temperature, and greater digestive capacity

Extending Mendelian Genetics for Two or More Genes

• Some traits– May be determined by two or

more genes

• In Epistasis– A gene at one locus alters the

phenotypic expression of a gene at a second locus

•Color is recessive to no color at one allelic pair• This recessive allele must be expressed before the specific color allele at a second locus is expressed.• At the first gene, white colored squash is dominant to colored squash, and the gene symbols are W=white and w=colored

Example of Epistasis - Fruit Color in Squash

Fruit Color in Squash cont.)

• At the second gene, yellow is dominant to green, and the symbols used are Y=yellow, y=green

• The presence of the dominant W allele masks the effect of either the Y or y alleles

• W_Y_ & W_yy give white (12)• wwY_ is yellow (3)• wwyy is green (1)• 12:3:1 ratio

• Another Example Of Epistasis

BC bC Bc bc1⁄41⁄41⁄41⁄4

BC

bC

Bc

bc

1⁄4

1⁄4

1⁄4

1⁄4

BBCc BbCc BBcc Bbcc

Bbcc bbccbbCcBbCc

BbCC bbCC BbCc bbCc

BBCC BbCC BBCc BbCc

9⁄163⁄16

4⁄16

BbCc BbCc

Sperm

Eggs

AaBbCc AaBbCc

aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC

20⁄64

15⁄64

6⁄64

1⁄64

Fra

ctio

n o

f pro

geny

• Quantitative variation usually indicates Polygenic Inheritance– An additive effect of two or more genes on a

single phenotype– Skin color– Eye Color– Hair color

Nature and Nurture: The Environmental Impact on

Phenotype• Another departure from simple

Mendelian genetics arises– When the phenotype for a character

depends on environment as well as on genotype

– May inherit genes for height, but not receive the needed nutrition to grow tall

– Heart disease & cancer

• The Norm Of Reaction– Is the phenotypic range of a particular

genotype that is influenced by the environment (iron in soil affects color)

GENETIC PATTERNS IN GENETIC PATTERNS IN HUMANSHUMANS

Pedigree Analysis

• A pedigree– Is a family tree that describes the

interrelationships of parents and children across generations

– Male– Female

• Inheritance patterns of particular traits Can be traced and described using pedigrees

Ww ww ww Ww

wwWwWwwwwwWw

WWor

Ww

ww

First generation(grandparents)

Second generation(parents plus aunts

and uncles)

Thirdgeneration

(two sisters)

Ff Ff ff Ff

ffFfFfffFfFF or Ff

ff FForFf

Widow’s peak No Widow’s peak Attached earlobe Free earlobe

(a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)

• Pedigrees– Can also be used to make

predictions about future offspring

• Recessively inherited disorders– Show up only in individuals

homozygous for the allele (albino)

• Carriers– Are heterozygous individuals

who carry the recessive allele but are phenotypically normal

– Indicated by ½ shaded circle

Cystic Fibrosis• Cystic fibrosis (CF)

– Recessively inherited, defective gene

– Carried by millions of people– Must inherit 2 genes (one from

each parent)

• Symptoms of cystic fibrosis include– Mucus buildup in the some internal

organs– Abnormal absorption of nutrients

in the small intestine

Sickle-Cell Disease• Sickle-cell disease

– Affects one out of 400 African-Americans

– Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells

• Symptoms include– Physical weakness, pain, organ damage, and even paralysis

Mating of Close Relatives• Matings between relatives

– Can increase the probability of the appearance of a genetic disease

– Hemophilia (free bleeders) once found in royal families that intermarried (female carriers)

– Are called consanguineous matings

– Prohibited in the United States

Dominantly Inherited Genetic Disorders

• One example is achondroplasia– A form of dwarfism that is lethal when

homozygous for the dominant allele

Dominantly Inherited Genetic Disorders

• Huntington’s disease– Is a degenerative

disease of the nervous system

– Has no obvious phenotypic effects until about 35 to 40 years of age (adult onset)

Genetic Testing and Counseling

• Genetic counselors– Can provide information to

prospective parents concerned about a family history for a specific disease

– Genetic counselors help couples determine the odds that their children will have genetic disorders

Tests for Identifying Carriers

• For a growing number of diseases– Tests (1000’s) are available that

identify carriers and help define the odds more accurately

– Newborn screening (PKU)– Carrier screening (Tay-Sach)– Adult onset screeening

(Huntington’s) – Estimating risk screening

(Alzheimer’s)

Fetal Testing• In amniocentesis

– The liquid that bathes the fetus is removed and tested

• In chorionic villus sampling (CVS)– A sample of the placenta is

removed and tested

Fetal Testing(a) Amniocentesis

Amnioticfluidwithdrawn

Fetus

Placenta Uterus Cervix

Centrifugation

A sample ofamniotic fluid canbe taken starting atthe 14th to 16thweek of pregnancy.

(b) Chorionic villus sampling (CVS)

FluidFetalcells

Biochemical tests can bePerformed immediately onthe amniotic fluid or lateron the cultured cells.

Fetal cells must be culturedfor several weeks to obtainsufficient numbers forkaryotyping.

Severalweeks

Biochemicaltests

Severalhours

Fetalcells

Placenta Chorionic viIIi

A sample of chorionic villustissue can be taken as earlyas the 8th to 10th week ofpregnancy.

Suction tubeInserted throughcervix

Fetus

Karyotyping and biochemicaltests can be performed onthe fetal cells immediately,providing results within a dayor so.

Karyotyping

Newborn Screening

• Some genetic disorders can be detected at birth– By simple tests that are now

routinely performed in most hospitals in the United States

– Phenylketonuria (PKU)