Mendelian Genetics: Consider this….

8 million possible

chromosome

combinations in each

egg, and each

sperm…

= >70 trillion

possibilities!

How are we able to predict ANYTHING

about inheritance??

With all these possibilities, how can we

predict anything about inheritance?

Gregor Mendel

•1857- this monk

(with extensive

training in physics

and botany) begins

studying genetics

•Current dogma is

“Blended

Inheritance”

Mendel’s

Technique:

Studies peas-

•Typically Self-

Fertilizing

•Multiple distinct

CHARACTERS, with easy

to identify TRAITS

•Several TRUE-

BREEDING varieties

available

What

Mendel

Observes:

What does

this data

suggest about

“blended

inheritance”?

1. Alternate versions of different genes

(alleles) account for variations in

inherited characteristics.

2. For each character, an organism inherits

two alleles, one from each parent.

Mendel consistently

observes this pattern

of 3:1 ratios, and

develops a 4 part

HYPOTHESIS:

3. If the two alleles differ, then one, the DOMINANT allele, controls the phenotype; the other, the RECESSEVE allele, has no noticeable effect on the organism’s appearance.

4. The two alleles from each character segregate during gamete production (Mendel’s Law of Segregation)

Mendel consistently

observes this pattern

of 3:1 ratios, and

develops a 4 part

HYPOTHESIS:

For many traits, we can predict the

genotypic frequencies of the offspring of

two individuals using a PUNNETT SQUARE:

Note that you

could also do

this

mathematically!

The PUNNETT

Square

constructed for

Mendel’s

experiments

predicts a 3:1

ratio.

Does this data

support Mendel’s

hypotheses?

Understanding the predicted results

of a PUNNETT SQUARE, allows for a

TESTCROSS

What’s my

phenotype?

My

genotype?

Understanding

the predicted

results of a

PUNNETT SQUARE, allows

for a

TESTCROSS!

You Try!

Part 1,

Question 2

All the examples considered

so so far are MONOHYBRIDS

Will Mendel’s principles hold for the analysis of

MULTIPLE CHARACTERS (DIHYBRID CROSSES)?

Testing the Inheritance Patterns of Multiple Characters:

Complication #1: (Mendel was lucky!)

INCOMPLETE DOMINANCE

Heterozygotes have a

unique phenotype,

between that of the

homozygous dominant

or recessive parents.

Note: This is not

blended inheritance!

Complication #1: (Mendel was lucky!)

INCOMPLETE DOMINANCE

Complication

#2:

PLEIOTROPY

(multiple

effects)

Example:

Sickle-Cell

Disease

Complication

#3:

EPISTASIS (genetic

interaciton)

Example:

The “color gene”,

C, allows pigment

to be deposited in

hair. When

lacking, a mouse

is albino,

regardless of its

genotype at the

other locus.

Epistasis and Lab Pups

Black is dominant to Brown, so

Heterozygotes (Bb) are black. The

delivery gene is also dominant, so EE or

Ee individuals both express their

pigments. Only ee individuals are

yellow.

Coat color in labradors is

determined by 2 genes, a

pigment gene (B), and a

pigment delivery gene (E).

If I cross a Brown Lab (bbEe)

with a Black Lab (BbEe),

can I expect any yellow

puppies?

If so, what proportion of the pups

would I expect to be yellow?

Bonus Question:

Epistasis and Lab Pups

Complication #4: Codominance + Multiple Alleles

Example: Human Blood Types

Complication #4: Codominance + Multiple Alleles

Example: Human Blood Types

Example: Paternity Testing

Scenario : Suppose mother is Type A, baby is

Type B.

Consider these three putative fathers: can any

be the actual father? You Try! (Question 3)

#1 (Type A): Yes or No?

#2 (Type B): Yes or No?

#3 (Type O): Yes or No?

This usually

indicates

POLYGENIC

INHERITANCE,

where two or more

genes create a

single phenotypic

character

Other Issues: Individuals

may display a range of

small differences in traits,

known as CONTINUOUS

VARIATION

Many factors,

both genetic

and

environmental,

influence the

phenotype.

Other Issues: Environmental Effects on

Phenotype

Adding

Chromosomes

to the

Inheritance

Story!

Sex

Chromosomes

& Autosomes

Thomas Hunt Morgan’s

Contributions: Fruit Fly

Genetics•Single mating produces

100+ offspring

•A new generation can

be bred every two weeks

•Only four pairs of

chromosomes- 3 pairs of

autosomes, 1 pair sex

chromosomes (XX and XY)

Unlike Mendel,

Morgan does not

have access to true-

breeding strains.

He breeds flies for a

year, looking for

distinct varieties.

He discovers a male

fly with white eyes,

instead of red.

In Drosophila,

red eyes = Wild type

(the most common

phenotype in a natural

population)

white eyes = a Mutant

Phenotype.

Morgan’s Results:

First Experiment:

Morgan crosses a

red-eyed female with

a white-eyed male.

ALL the offspring

have red eyes.

How would Mendel

explain these results??

What would Mendel do

next??

Next Experiment:

Morgan crosses two

of the red-eyed F1

flies with each other.

What should he see

if Mendel is

correct??

Morgan’s Results:

He DOES find a

3:1 ratio, but

ALL the white-

eyed flies are

male!!

Was Mendel

wrong?? What

happened?!?

Morgan’s Results:

Morgan Discovers Sex-Linked

Genes!

The Transmission of SEX-LINKED Traits

“A” = dominant allele carried on the X

chromosome;

“a” = the recessive allele.

Note that both males and females are

affected by sex-linked disorders!

Practice Question: Sex-Linked Traits

If you see the

number 74, then you

do not have red-green color

blindness. If you see

the number 21, you

are color blind to

some extent. A

totally color-blind

person will not be

able to see any of

the numbers.

You Try!

Question 4

& 5

More on Linked Genes:

We can use our

understanding of

Meiosis to think

about WHY

Mendel’s Results

were so

predictable!

More on Linked Genes:

So… What happens if two genes

are on the SAME chromosome?!?

So… What happens

if two genes are on

the SAME

chromosome?!?

So… What happens if two genes

are on the SAME chromosome?!?

So why isn’t this ratio

what we expect for

a dihybrid cross???

Cross-over!

Thought

Question:

Consider a chromosome containing 5

genes, A through E.

Are genes A and B (or a and b) more or

less likely to be inherited together than

genes A and D? Explain your answer.