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 Theories1. 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.

Another Sample CrossP 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 Hypothesis1. 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.

Mendel's Hypothesis4. The two alleles for each trait separate during

gamete formation. 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 PossibleCross 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.

Number of Kinds of Gametes Critical to calculating the results of higher

level crosses. Look for the number of heterozygous traits.

EquationThe 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 CrossTtRr 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

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

Variations on Mendel1. 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.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 TypesType 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 GerbilsCcBb 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

Problem

Wife is type A Husband is type AB Child is type O

Question - Is this possible?

Bombay Effect Epistatic Gene on ABO group. Alters the expected ABO outcome. H = dominant, normal ABO h = recessive, no A,B, reads as type O blood.

Genotypes

Wife: type A (IA IA , Hh) Husband: type AB (IAIB, Hh) Child: type O (IA IA , hh)

Therefore, the child is the offspring of the wife and her husband.

Bombay - Detection When ABO blood type inheritance patterns

are altered from expected.

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 SymbolsMale

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 Formal

R = F X M X DR = 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

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.