NOTES FOR BIOLOGY 1201

Section 001

Spring 2005

DR. STEVEN POMARICO

Heredity: The Story of Gregor Mendel - 9.1.1

>>>>>>The father of genetics: Johann (Gregor) Mendel

Mendel was a monk who was trained in science at the University of Vienna.

The realization that both parents contributed to the characteristics of the offspring preceded the work of Mendel. The favor explanation of how this occurred was the blending theory of heredity.

Modern genetics began with Mendel, and lead to the replacement of the blending theory with the particulate theory of heredity.

Among Mendel’s teachers were:

-a physicist named Doppler who taught Mendel to use a quantitative experimental

approach in studying natural phenomena.

-a botanist named Unger who interested Mendel in the inheritable variation in plants

These influences helped lead Mendel to the study of the variation of garden peas.

Why peas??

-Many different characters and traits

---Characters

---Traits

For Mendel’s pea plants the characters and traits he examined were

1. Flower color (purple or white)

2. Flower position (axial or terminal)

3. Seed color (yellow or green)

4. Seed shape (round or wrinkled)

5. Pod shape (inflated or constricted)

6. Pod color (green or yellow)

7. Stem length (tall or dwarf)

Mendel’s Findings: A First Look at Phenotypic Ratios - 9.1.2

Mendel started his experiments with true-breeding plant varieties which he hybridized (cross-pollinated)

---True-breeding

Terminology for breeding (or genetics) experiments

-P generation

-F₁ generation

-F₂ generation

Mendel’s Conclusion: Alternate Alleles and Dominance - 9.1.3

Mendel observed the transmission of the selected traits over these three generations and arrived at two principles of heredity:

The law of segregation

The law of independent assortment

>>>>>According to the law of segregation, two alleles for a character are packaged into separate gametes.

---Alleles

In a monohybrid cross, one of Mendel’s typical experiments, he examine the way traits (purple flowers or white flowers) were expressed over three generations (P, F₁, and F₂)

He found that in the F₁ generation these character traits did not blend as predicted. Instead the white color was lost.

Mendel hypothesized that the loss of the white color meant the plants had lost that allele and when he crossed the F₁ plants he would get only purple-flowered plants.

He performed the crosses and the F₂ generation had purple-flowered plants and white-flowered plants in a 3:1 ratio.

REJECT THE HYPOTHESIS!!

Mendel’s Conclusions: Segregation and Recombination - 9.1.4

The white allele was not lost but was masked by the presence of the purple allele.

Mendel had found that the trait for purple flowers was a dominant trait and the trait for white flowers was a recessive trait.

Mendel repeated these experiments with the six other characters and found similar 3:1 ratios in the F₂ generations. This led him to the following four-part hypothesis:

1. Alternative forms of genes are responsible for variations in inherited characters.

2. For each character, an organism inherits two alleles, one from each parent.

3. If the two alleles differ, one is fully expressed (dominant allele); the other is completely masked (recessive allele).

4. The two alleles for each character trait segregate during gamete

production.

---Dominant allele

---Recessive allele

---Mendel’s law of segregation

-predicts the 3:1 ratio observed in the F₂ generation.

-Punnett square

---Homozygous

---Heterozygous

Genotype versus Phenotype

---Phenotype

---Genotype

Determining Heterozygosity: Test Crosses and Back Crosses - 9.2.1

Because the genotype is not always apparent from the phenotype a testcross can be use to determine the genotype.

— testcross

Segregation and Independent Assortment - 9.3.1

>>>>>>According to the law of independent assortment, each pair of alleles segregates into gametes independently

The parental crosses that Mendel did in his earlier experiments involved parental varieties that differed in a single trait (i.e., monohybrid crosses)

In later experiments Mendel crossed parental varieties that differed at two characters (i.e., dihybrid crosses)

By using plants that were true-breeding (homozygous) for two different characters, Mendel tested how the traits for these characters segregated.

He hypothesized that the two characteristics would either segregate together (Null hypothesis), or they would segregate independently (alternative hypothesis).

If they segregated together then the F₂ will show a 3:1 phenotype

If they segregated independently then the F₂ will have a 9:3:3:1 phenotypic ratio.

Results showed a 9:3:3:1 ratio THE NULL HYPOTHESIS IS REJECTED!!

Independent Assortment: An Explanation - 9.3.2

This result led Mendel to the law of independent assortment.

---Mendel’s law of independent assortment

>>>>>Mendelian inheritance reflects the rules of probability

- Probability of an event occurring ranges from 0 to 1.

-The probabilities of all outcomes for a single event must add up to 1

-Each successive event is independent of previous events.

Laws of Probability: Rule of Multiplication - 9.4.1

The Multiplicative Law: Some Extensions - 9.4.2

Laws of Probability: The Additive Rule - 9.4.3

>>>Two basic rules of probability are used in solving genetic problems:

1. The rule of multiplication

Example: Probability that 2 coin tosses will both produce heads is

1/2 x 1/2 = 1/4.

2. The rule of addition

Example: The probability that 2 coin tosses will produce 1 head and 1 tail.

Two ways this can happen

(1) 1st head then tail,

(2) 1st tail then head.

The probability for each way is 1/4 (because there are four possible outcomes),

thus the total probability is 1/4 + 1/4 = 1/2.

Probability provides statistical framework against which results can be compared.

Gene segregation is random; the genotype of an individual plant cannot be predicted. Large samples tend to be consistent with predicted frequencies.

>>>>>Significance of Mendel's research:

Inheritance of traits is due to transmission of discrete factors (genes) that are passed from generation to generation according to rules of probability.

Same principle applies to all sexually reproducing organisms.

Mendel worked with traits that have dominant and recessive alleles, and follow the rule of independent assortment - the simplest scenario by which genetic information is sexually transmitted.

What is a Dominant Gene? Intermediate Inheritance - 9.5.1

>>>>>>The relationship between genotype and phenotype is rarely simple

INTERMEDIATE INHERITANCE or incomplete dominance

---Incomplete dominance

>sounds like blending theory but it’s not

-recessive trait can still emerge in the F₂

-genotype is apparent from phenotype

>>Complete dominance to codominance

The dominance/recessiveness relationships vary over a continuum.

COMPLETE <====== INCOMPLETE ======> CODOMINANCE

DOMINANCE DOMINANCE

---Codominance

Codominance and Multiple Alleles: ABO Blood Groups - 9.5.2

>>Multiple alleles

A gene may have more than two allele forms.

ABO blood group is an example of multiple alleles

BLOOD TYPE	POSSIBLE GENOTYPES	ANTIGENS PRESENT ON RBC	ANTIBODIES PRESENT IN SERUM
A	I^AI^A or I^Ai	A	anti-B
B	I^BI^B or I^B i	B	anti-A
AB	I^AI^B	AB	none
O	ii	O	anti-A and anti-B

Epistasis: One Gene Affecting Another - 9.6.1

>>>Epistasis -one gene that changes the expression of another gene.

---Epistasis

-Mouse fur coloration

Polygenic Inheritance - 9.7.1

>>>Polygenic inheritance of quantitative characters

-multiple loci involved in a single trait.

-trait varies in a continue

---Polygenic inheritance

-additive effect of two or more genes on a single phenotypic characteristic

-contrasted with pleiotropy and epistasis

-Skin pigmentation

Pleiotropy: Multiple Phenotypic Effects - 9.7.2

>>>Pleiotropy - genes that affect many traits

---Pleiotropy

-Sickle-cell anemia

>>>Environmental impact on phenotype

A single genotype may product a range of phenotypes in response to environmental factors.

The phenotypic range of the norm of reaction for the genotype.

---Norm of reaction