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).

Gregor Mendel Father of Modern Genetics.

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.

Another Sample Cross

P1 Tall X short (TT x tt)

F1 all Tall (Tt)

F2 3 tall to 1 short

(1 TT: 2 Tt: 1 tt)

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 Hypothesis3. 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.

Mendel's Hypothesis

4. The two alleles for each trait separate during gamete formation. This now called: Mendel's Law of Segregation

Law of Segregation

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 PhenotypeTT 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.

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: 3:175% chance of the dominant25% chance of the recessive

Rule of Multiplication

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

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.(blending)

Ex. Red X White snapdragons F1 = all pink F2 = 1 red: 2 pink: 1 white

Result No hidden Recessive. 3 phenotypes and

3 genotypes 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

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.

Epistasis in Mice

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.

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.

Genetic Screening

Risk assessment for an individual inheriting a trait.

Uses probability to calculate the risk.

General FormalR = F X M X D

R = riskF = 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 every 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.

Ex. Heart Disease

Genetic Diet Exercise Bacterial Infection