Chapter 14 Mendel and the

Gene Idea

Genetics

The scientific study of 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 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 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

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).

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 #1

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 (Tall:Short)

“R’s” = rr X Rr = 1:1 (Red:White)

“G’s” = GG x gg = 1:0 (Yellow:green)

Product is:

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

Tall, Red, Green peas (3x1x0)

Variations on Mendel

1. Incomplete Dominance

2. Codominance

3. Multiple Alleles

4. Sex-Linked

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 Red = CR CR

Pink = CRCW

White = CWCW

Codominance

Both alleles are expressed equally in the phenotype.

Ex. Sickle Cell Anemia AA=Normal blood cells AA’=Some normal some sickle A’A’= All Sickle shaped

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

Linked genes

There are many genes, but only a few chromosomes.

Therefore, each chromosome must carry a number of genes together as a “package”.

Linked Genes

Traits that are located on the same chromosome.

Result: Failure of Mendel's Law of

Independent Assortment. Ratios mimic monohybrid

crosses.

Crossing-Over

Breaks up linkages and creates new ones.

Recombinant offspring formed that doesn't match the parental types.

If Genes are Linked:

Independent Assortment of traits fails.

Linkage may be “strong” or “weak”.

Linkage Strength

Degree of strength related to how close the traits are on the chromosome. Weak - farther apart Strong - closer together

End of part 1

Chromosomal Basis of Sex in Humans

X chromosome - medium sized chromosome with a large number of traits.

Y chromosome - much smaller chromosome with only a few traits.

Human Chromosome Sex

Males - XYFemales - XX

Comment - The X and Y chromosomes are a homologous pair, but only for a small region at one tip.

Sex Linkage

Inheritance of traits on the sex chromosomes.

X- Linkage (common) Y- Linkage (very rare if exists

at all)

Males Hemizygous - 1 copy of X

chromosome. Show ALL X traits

(dominant or recessive). More likely to show X

recessive gene problems than females.

X-linked Disorders

Color blindness Duchenne's Muscular

Dystrophy Hemophilia (types a and b) Immune system defects

X-linked Patterns Trait is usually passed from a

carrier mother to 1/2 of sons. Affected father has no

affected children, but passes the trait on to all daughters who will be carriers for the trait.

Can Females be color-blind?

Yes, if their mother was a carrier and their father is affected.

Sex Limited Traits

Traits that are only expressed in one sex.

Ex – prostate

Sex Influenced Traits Traits whose expression

differs because of the hormones of the sex.

These are NOT on the sex chromosomes.

Ex. – beards, mammary gland development, baldness

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.

Recessive Pattern

Usually rare. Skips generations. Occurrence increases with

inbred matings.

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.

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 any child will be albino.