Chapter 14

Mendel and the Gene Idea

Inheritance

• The passing of traits from

parents to offspring.

• Humans have known about

inheritance for thousands of

years.

Genetics

• The scientific study of the

inheritance.

• Genetics is a relatively “new”

science (about 150 years).

Genetic Theories

1. Blending Theory -

traits were like paints and

mixed evenly from both

parents.

2. Incubation Theory -

only one parent controlled

the traits of the children.

Ex: Spermists and Ovists

3. Particulate Model -

parents pass on traits as

discrete units that retain their

identities in the offspring.

Gregor Mendel

• Father of Modern Genetics.

• Mendel’s paper published in

1866, but was not recognized

by Science until the early

1900’s.

Reasons for Mendel's Success

• Used an experimental

approach.

• Applied mathematics to the

study of natural phenomena.

• Kept good records.

• Mendel was a pea

picker.

• He used peas as

his study organism.

Why Use Peas?

• Short life span.

• Bisexual.

• Many traits known.

• Cross- and self-pollinating.

• (You can eat the failures).

Cross-pollination

• Two parents.

• Results in hybrid offspring

where the offspring may be

different than the parents.

Self-pollination

• One flower as both parents.

• Natural event in peas.

• Results in pure-bred offspring

where the offspring are

identical to the parents.

Mendel's Work

• Used seven characters, each

with two expressions or traits.

• Example:

• Character - height • Traits - tall or short.

Monohybrid or Mendelian

Crosses

• Crosses that work with a

single character at a time.

Example - Tall X short

P Generation

• The Parental generation or the

first two individuals used in a

cross.

Example - Tall X short

• Mendel used reciprocal

crosses, where the parents

alternated for the trait.

Offspring

• F1 - first filial generation.

• F2 - second filial generation,

bred by crossing two F1 plants

together or allowing a F1 to

self-pollinate.

Results - Summary

• In all crosses, the F1

generation showed only one

of the traits regardless of

which was male or female. • The other trait reappeared in

the F2 at ~25% (3:1 ratio).

Mendel's Hypothesis

1. Genes can have alternate

versions called alleles.

2. Each offspring inherits two

alleles, one from each parent.

Mendel's Hypothesis

3. If the two alleles differ, the

dominant allele is expressed.

The recessive allele remains

hidden unless the dominant

allele is absent.

Comment - do not use the

terms “strongest” to describe

the dominant allele.

Allele for purple flowers

Homologous

pair of chromosomes

Locus for flower-color gene

Allele for white flowers

Allele

Purple

White

Mendel's Hypothesis

4. The two alleles for each trait

separate during gamete

formation and end up in

different games.

This now called:

Mendel's Law of Segregation

Law of Segregation

Mendel’s Experiments

• Showed that the Particulate

Model best fit the results.

Vocabulary

• Phenotype - the physical appearance of the organism.

• Genotype - the genetic makeup of the organism, usually shown in a code. • T = tall

• t = short

Helpful Vocabulary

• Homozygous - When the two

alleles are the same (TT/tt).

• Heterozygous- When the two

alleles are different (Tt).

6 Mendelian Crosses are Possible

Cross Genotype Phenotype

TT X tt all Tt all Dom

Tt X Tt 1TT:2Tt:1tt 3 Dom: 1 Res

TT X TT all TT all Dom

tt X tt all tt all Res

TT X Tt 1TT:1Tt all Dom

Tt X tt 1Tt:1tt 1 Dom: 1 Res

Test Cross

• Cross of a suspected

heterozygote with a

homozygous recessive.

• Ex: T_ X tt

If TT - all dominant

If Tt - 1 Dominant: 1 Recessive

Dihybrid Cross

• Cross with two genetic traits.

• Need 4 letters to code for the

cross. • Ex: TtRr

• Each Gamete - Must get 1

letter for each trait. • Ex. TR, Tr, etc.

Number of Kinds of Gametes

• Critical to calculating the

results of higher level crosses.

• Look for the number of

heterozygous traits.

Equation

The formula 2n can be used,

where “n” = the number of

heterozygous traits.

Ex: TtRr, n=2

22 or 4 different kinds of

gametes are possible.

TR, tR, Tr, tr

Dihybrid Cross

TtRr X TtRr

Each parent can produce 4

types of gametes.

TR, Tr, tR, tr

Cross is a 4 X 4 with 16

possible offspring.

Results

• 9 Tall, Red flowered

• 3 Tall, white flowered

• 3 short, Red flowered

• 1 short, white flowered

Or: 9:3:3:1

Law of Independent Assortment

• The inheritance of 1st genetic

trait is NOT dependent on the

inheritance of the 2nd trait.

• Inheritance of height is

independent of the inheritance

of flower color.

Comment

• Ratio of Tall to short is 3:1

• Ratio of Red to white is 3:1

• The cross is really a product

of the ratio of each trait

multiplied together.

(3:1) X (3:1)

Probability

• Genetics is a specific

application of the rules of

probability.

• Probability - the chance that

an event will occur out of the

total number of possible

events.

Genetic Ratios

• The monohybrid “ratios” are

actually the “probabilities” of

the results of random

fertilization.

Ex: 3:1

75% chance of the dominant

25% chance of the recessive

Rule of Multiplication or

Product Rule

• The probability that two alleles

will come together at

fertilization, is equal to the

product of their separate

probabilities.

Example: TtRr X TtRr

• The probability of getting a tall offspring is ¾.

• The probability of getting a red offspring is ¾.

• The probability of getting a tall red offspring is ¾ x ¾ = 9/16

Comment

• Use the Product Rule to

calculate the results of

complex crosses rather than

work out the Punnett Squares.

• Ex: TtrrGG X TtRrgg

Solution

“T’s” = Tt X Tt = 3:1

“R’s” = rr X Rr = 1:1

“G’s” = GG x gg = 1:0

Product is:

(3:1) X (1:1) X (1:0 ) = 3:3:1:1

Dominance vs Phenotype

• A dominant allele does not

subdue a recessive allele;

alleles don’t interact.

• Alleles are simply variations in

a gene’s nucleotide sequence.

Variations on Mendel

1. Incomplete Dominance

2. Codominance

3. Multiple Alleles

4. Epistasis

5. Polygenic Inheritance

Incomplete Dominance

• When the F1 hybrids show a phenotype somewhere between the phenotypes of the two parents.

• Often a “dose” effect

Ex. Red X White snapdragons

F1 = all pink

F2 = 1 red: 2 pink: 1 white

Result

• No hidden Recessive

• 3 phenotypes and

3 genotypes

(Hint! – often a “dose”

effect) • Red = CR CR

• Pink = CRCW

• White = CWCW

Another example

Codominance

• Both alleles are expressed

equally in the phenotype.

• Ex. MN blood group • MM

• MN

• NN

Result

• No hidden Recessive

• 3 phenotypes and

3 genotypes

(but not a “dose” effect)

Multiple Alleles

• When there are more than 2

alleles for a trait

• Ex. ABO blood group • IA - A type antigen

• IB - B type antigen

• i - no antigen

Result

• Multiple genotypes and

phenotypes.

• Very common event in many

traits.

Alleles and Blood Types

Type Genotypes

A IA IA or IAi B IB IB or IBi

AB IAIB

O ii

Comment

• Rh blood factor is a separate

factor from the ABO blood

group.

• Rh+ = dominant

• Rh- = recessive

• A+ blood = dihybrid trait

Epistasis

• When 1 gene locus alters the

expression of a second locus.

• Ex:

• 1st gene: C = color, c = albino

• 2nd gene: B = Brown, b = black

Gerbils

In Gerbils

CcBb X CcBb

Brown X Brown

F1 = 9 brown (C_B_)

3 black (C_bb)

4 albino (cc__)

Result

• Ratios often altered from the

expected.

• One trait may act as a

recessive because it is

“hidden” by the second trait.

Polygenic Inheritance

• Factors that are expressed as

continuous variation.

• Lack clear boundaries

between the phenotype

classes.

• Ex: skin color, height

Genetic Basis

• Several genes govern the

inheritance of the trait.

• Ex: Skin color is likely

controlled by at least 4 genes.

Each dominant gives a darker

skin.

Result

• Mendelian ratios fail.

• Traits tend to "run" in

families.

• Offspring often intermediate

between the parental types.

• Trait shows a “bell-curve” or

continuous variation.

Genetic Studies in Humans

• Often done by Pedigree

charts.

• Why? • Can’t do controlled breeding studies in

humans.

• Small number of offspring.

• Long life span.

Pedigree Chart Symbols

Male

Female

Person with trait

Sample Pedigree

Dominant Trait Recessive Trait

Human Recessive Disorders

• Several thousand known: • Albinism

• Sickle Cell Anemia

• Tay-Sachs Disease

• Cystic Fibrosis

• PKU

• Galactosemia

Sickle-cell Disease

• Most common inherited disease

among African-Americans.

• Single amino acid substitution

results in malformed

hemoglobin.

• Reduced O2 carrying capacity.

• Codominant inheritance.

Tay-Sachs

• Eastern European Jews.

• Brain cells unable to

metabolize type of lipid,

accumulation of causes brain

damage.

• Death in infancy or early

childhood.

Dominance vs Phenotype

• For any character,

dominance/recessiveness

relationships of alleles depend

on the level at which we

examine the phenotype.

Example -Tay-Sachs

• Disease is fatal; a dysfunctional

enzyme causes an

accumulation of lipids in the

brain.

• At the organismal level, the

allele is recessive.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Tay-Sachs

• At the biochemical level, the

phenotype (i.e., the enzyme

activity level) is incompletely

dominant.

• At the molecular level, the

alleles are codominant.

Cystic Fibrosis

• Most common lethal genetic

disease in the U.S.

• Most frequent in Caucasian

populations (1/20 a carrier).

• Produces defective chloride

channels in membranes.

Recessive Pattern

• Usually rare.

• Skips generations.

• Occurrence increases with

consaguineous matings.

• Often an enzyme defect.

Human Dominant Disorders

• Less common then

recessives.

• Ex: • Huntington’s disease

• Achondroplasia

• Familial Hypercholsterolemia

Inheritance Pattern

• Each affected individual had one affected parent.

• Doesn’t skip generations.

• Homozygous cases show worse phenotype symptoms.

• May have post-maturity onset of symptoms.

Homework

• Read Chapter 14 (Hillis – 8)

• Chapter 14 – Mon. 1/28

Genetic Screening

• Risk assessment for an

individual inheriting a trait.

• Uses probability to calculate

the risk.

General Formula

R = F X M X D

R = risk

F = probability that the female carries the gene.

M = probability that the male carries the gene.

D = Disease risk under best conditions.

Example

• Wife has an albino parent.

• Husband has no albinism in

his pedigree.

• Risk for an albino child?

Risk Calculation

• Wife = probability is 1.0 that she has the allele.

• Husband = with no family record, probability is near 0.

• Disease = this is a recessive trait, so risk is Aa X Aa = .25

• R = 1 X 0 X .25

• R = 0

Risk Calculation

• Assume husband is a carrier,

then the risk is:

R = 1 X 1 X .25

R = .25

There is a .25 chance that any

child will be albino.

Common Mistake

• If risk is .25, then as long as

we don’t have 4 kids, we won’t

get any with the trait.

• Risk is .25 for each child.

It is not dependent on what

happens to other children.

Carrier Recognition

• Fetal Testing • Amniocentesis

• Chorionic villi sampling

• Newborn Screening

Fetal Testing

• Biochemical Tests

• Chromosome Analysis

Amniocentesis

• Administered between 11 - 14 weeks.

• Extract amnionic fluid = cells and fluid.

• Biochemical tests and karyotype.

• Requires culture time for cells.

Chorionic Villi Sampling

• Administered between 8 - 10 weeks.

• Extract tissue from chorion (placenta).

• Slightly greater risk but no culture time required.

Newborn Screening

• Blood tests for recessive

conditions that can have the

phenotypes treated to avoid

damage. Genotypes are NOT

changed.

• Ex. PKU

Newborn Screening

• Required by law in all states.

• Tests 1- 6 conditions.

• Required of “home” births too.

Multifactorial Diseases

• Where Genetic and

Environment Factors interact

to cause the Disease.

• Becoming more widely

recognized in medicine.

Ex. Heart Disease

• Genetic

• Diet

• Exercise

• Bacterial Infection

Genes & Environment

Summary

• Know the Mendelian crosses

and their patterns.

• Be able to work simple genetic

problems (practice).

• Watch genetic vocabulary.

• Be able to read pedigree

charts.

Summary

• Be able to recognize and work

with some of the “common”

human trait examples.