Today:

• Onward through Mendelian Genetics and Exceptions• Adding Chromosomes to

the Story

Testing Mendel’s Law of Testing Mendel’s Law of Segregation: Segregation:

The Punnett SquareThe Punnett Square

The Punnett Square for Mendel’s Experiments:

What will the F1 Generation look like? The F2

Generation?

The Punnett Square for Mendel’s Experiments:

vs

Understanding the predicted results of a PUNNETT SQUARE, allows for a

TESTCROSS What’s my

phenotype? My

genotype?

Using Simple Mendelian Using Simple Mendelian GeneticsGenetics

Sickle Cell Disease

Sickle Cell Disease Questions:

1.Two individuals who are heterozygous at the Sickle Cell locus have four children together. One of the children is affected with the disorder. Based on this information, is the sickle cell trait dominant or recessive?

Sickle Cell Disease Questions:

2. If the affected offspring has a child with an unaffected individual (who does not carry the sickle allele), what is the probability that any given child will be unaffected? Be a carrier? Be affected?

An Aside: Unusual Gene An Aside: Unusual Gene Frequencies!?Frequencies!?

What do you

notice? What does

this suggest?

Mendelian Genetics- Example

2:

Cystic Fibrosis is also an Autosomal Recessive

Trait with Unusual Gene Frequencies1.If two carriers of the cystic fibrosis

trait have children, what is the probability that their first child will be affected?

2.It they eventually have three children, what is the probability that all three will be affected?

CalculatinCalculating g

ProbabilitProbabilitiesies

Dependent Assortment?Dependent Assortment?

Mendel’s Next Question: What happens in a dihybrid

cross?

What would the outcome look like if

it’s dependent

assortment??

What What Mendel Mendel Sees:Sees:

So is it dependen

t assortme

nt??

Mendel’s ContributionsMendel’s Contributions

Law #1: SegregationLaw #1: Segregation

Law #2: Independent Law #2: Independent AssortmentAssortment

Practice Problem: Practice Problem: Monohybrid CrossMonohybrid Cross

In pea plants, In pea plants, spherical seeds (S) spherical seeds (S) are dominant to are dominant to dented seeds (s). In dented seeds (s). In a genetic cross of a genetic cross of two plants that are two plants that are heterozygous for the heterozygous for the seed shape trait, seed shape trait, what fraction of the what fraction of the offspring should offspring should have spherical have spherical seeds? seeds?

A. None B. ¼

C. ½ D. ¾

E. All

Practice Problem: Dihybrid Practice Problem: Dihybrid CrossCross

In a dihybrid cross, In a dihybrid cross, AaBb x AaBb, AaBb x AaBb,

what fraction of what fraction of the offspring will the offspring will be homozygous be homozygous

for both for both recessive traits? recessive traits?

A. 1/16 B. 1/8

C. 3/16 D. 1/4

E. 3/4

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!Wh

y?

Complication #1: (Mendel was lucky!)

INCOMPLETE DOMINANCE

Another Exception: Codominance

In codominance, both alleles affect the phenotype in separate, distinguishable

ways.

Example:

•Human blood groups M, N, and MN

Group MN produce both antigens on the surface of blood cells

Another Exception: Codominance

Example:

Tay-Sachs disease-

Heterozygous individuals produce both functional, and dysfunctional enzymes. organismal level = recessive, biological level = codominant.

A section of the brain of a Tay Sachs child. The empty vacuoles are lysosomes that had been filled with glycolipid until extracted with alcohol in preparing the tissue.

Three Important Points about Dominant/Recessive

Traits:

1.They range from complete dominance incomplete dominance codominance. (can be a subtle distinction!)

2.They reflect mechanisms through which specific alleles are expressed in the phenotype (i.e. this is not one allele subduing another at the DNA level)

3.They’re not related to the abundance of an allele within a population!

• The Palomino horse is a hybrid exhibiting a golden color with lighter mane and tail.

• A pair of codominant alleles (D1 and D2) control the inheritance of these coat colors.

• Genotypes homozygous for the D1 allele are chestnut-colored (reddish), heterozygous genotypes are Palamino-colored, and genotypes homozygous for the D2 allele are almost white and called cremello.

Codominance Practice Question:

1. From the matings between two Palaminos, determine the expected Palamino : non-Palamino ratio among the offspring.

2.What percentage of the non-Palamino offspring in part 1 will breed true?

3. What kind of mating will produce only Palaminos?

Codominance Practice Question:

(a)From the matings between two Palaminos, determine the expected Palamino : non-Palamino ratio among the offspring.

P = D1D2 x D1D2F1 = 1/4 D1D1 + 1/2 D1D2 + 1/4 D2D2Ratio 1 Palomino : 1 non-Palomino

(b)What percentage of the non-Palamino offspring in part (a) will breed true?

all non-Palomino offspring will breed true

(c) What kind of mating will produce only Palaminos?

chestnut-colored x cremello will produce all Palominos

Further Complications: Multiple Alleles

Further Complications: Multiple Alleles

       Scenario : Suppose mother is

Type A, baby is Type B.

Consider these three putative fathers: can any be the actual father?

       #1 (Type A): Yes or No?        #2 (Type B): Yes or No?        #3 (Type O): Yes or No?

Practice Question: Paternity

testing

Further Complications: Further Complications: PleiotropyPleiotropy

Most genes have multiple phenotypic effects!Most genes have multiple phenotypic effects!

Further Complications: Further Complications: PleiotropyPleiotropy

No production of melanocytes No production of melanocytes during development causes:during development causes:

1. White fur color1. White fur color

andand

2. Inability to transmit electrical 2. Inability to transmit electrical signals to brain from hair cells in signals to brain from hair cells in the ear.the ear.

More Complications:

EPISTASIS

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 Epistasis and Lab PupsPups

Black is dominant to Brown, so Heterozygotes Black is dominant to Brown, so Heterozygotes (Bb) are black. The delivery gene is also (Bb) are black. The delivery gene is also dominant, so EE or Ee individuals both dominant, so EE or Ee individuals both express their pigments. Only ee individuals express their pigments. Only ee individuals are yellow.are yellow.

Coat color in labradors is Coat color in labradors is determined by 2 genes, a determined by 2 genes, a pigment gene (B), and a pigment gene (B), and a pigment delivery gene (E).pigment delivery gene (E).

Your Question:Your Question:

If I cross a Brown Lab (bbEe) If I cross a Brown Lab (bbEe) with a Black Lab (BbEe), can with a Black Lab (BbEe), can I expect any yellow puppies?I expect any yellow puppies?

Epistasis and Lab Epistasis and Lab PupsPups

If so, what proportion of the pups would I expect to be yellow?

There’s There’s more… more…

Polygenic Polygenic InheritancInheritanc

eeThis results in a broad norm of reaction

Many factors, both genetic

and environmental, influence the phenotype.

Other Issues: Environmental Effects on Phenotype

Next: Creating the CHROMOSOME THEORY OF

INHERITANCE

Similarities between the behavior of chromosomes and

Mendel’s “factors”:

•Chromosomes and genes are both present in paired in diploid cells

•Homologous chromosomes separate and alleles segregate during meiosis

•Fertilization restores the paired conditions for both chromosomes and genes

Similarities between the behavior of chromosomes and

Mendel’s “factors”:

In 1902 the Chromosome Theory of Inheritance was proposed. In states

that Mendelian genes have specific loci on chromosomes, and these chromosomes undergo segregation

and independent assortment.

Similarities between the behavior of chromosomes and

Mendel’s “factors”:

Correlating the results of Mendel’s

dihybrid crosses with the behavior

of chromosome

s during meiosis

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:Morgan’s Results:

First Experiment: First Experiment:

Morgan crosses a Morgan crosses a red-eyed female with red-eyed female with a white-eyed male. a white-eyed male. ALL the offspring ALL the offspring have red eyes.have red eyes.

How would Mendel explain these results??

What would Mendel do next??

Morgan’s Results:Morgan’s Results:

Next Experiment: Next Experiment:

Morgan crosses two Morgan crosses two of the red-eyed F1 of the red-eyed F1 flies with each flies with each other.other.

What should he see if Mendel is

correct??

Morgan’s Results:Morgan’s Results:

He DOES find He DOES find a 3:1 ratio, a 3:1 ratio, but ALL the but ALL the white-eyed white-eyed

flies are flies are male!!male!!

Was Mendel wrong?? What happened?!?

Morgan Discovers Sex-Morgan Discovers Sex-Linked Genes! Linked Genes! (and wins Nobel

Prize, 1933)

Sex Determination Happens in a Variety

of Ways

Sex chromosomes (especially the X chromosome) carry

genes for many other characters.

In humans, the term “sex-linked” generally refers to

genes on the X chromosome.

The Transmission of SEX-LINKED RECESSIVE Traits

In this diagram “A” represents a dominant allele carried on the X chromosome; “a” represents the recessive allele. White boxes indicate unaffected individuals, light-colored boxes are carriers, and dark-colored boxes are affected individuals. Note that both males and females are affected by sex-linked disorders!

An Aside: X Inactivation in Female Mammals

In females, one X chromosome is inactivated (at random) and

condenses into a compact Barr body along the inside of the nuclear

envelope. Most genes on this X chromosome are not expressed.

Because it is random which X chromosome forms the Barr body during development,

females are Mosaics of the two cell types.

In females, one X chromosome is inactivated (at random) and

condenses into a compact Barr body along the inside of the nuclear

envelope. Most genes on this X chromosome are not expressed.

Because it is random which X chromosome forms the Barr body during development,

females are Mosaics of the two cell types.

Practice Question: Sex-Practice Question: Sex-Linked Chromosomal Linked Chromosomal

InheritanceInheritance If you see the If you see the

number 74, then number 74, then you do you do notnot have have red-green color red-green color blindness. If you see blindness. If you see the number 21, you the number 21, you are color blind to are color blind to some extent. A some extent. A totally color-blind totally color-blind person will not be person will not be able to see any of able to see any of the numbers. the numbers.

If a color blind man has children If a color blind man has children with a “wild-type” woman, what are with a “wild-type” woman, what are the chances that a the chances that a daughterdaughter of of theirs will be colorblind?theirs will be colorblind?

What are the chances that their What are the chances that their sonson will be colorblind?will be colorblind?

Can females be colorblind? What Can females be colorblind? What would the genotype of the parents would the genotype of the parents have to be?have to be?

Practice Questions: Practice Questions: Sex-Linked Sex-Linked

Chromosomal Chromosomal InheritanceInheritance