To Mendel and BEYOND!!

To Mendel and BEYOND!!• Mendel was fortunate to use pea plants because

each trait that he observed was not only controlled by one set of genes, but there were also two alleles – dominant vs. recessive.

• Many traits are not that simple!!• Other patterns of inheritance include:

– Incomplete dominance–Codominance–Multiple Alleles–Polygenic traits

Incomplete Dominance

• Pattern of inheritance where the dominant trait does not completely “mask” the recessive trait resulting in a blending of the two traits

• Example: Snapdragons and Four O’Clock flowers– Red x White Pink

Snapdragons & Four O’Clocks

Incomplete Dominance in Snapdragons

Alleles: - Red = R - White = W

Genotypes: - RR - WWPhenotypes: - Red - White

Genotypes: - RWPhenotypes: - Pink

RR WW

RW RW

RR RW RW WW

RR X WW

100% RW

Therefore, 100% PINK!

RW RW

RW RW

R R

W

W

Why did the F1 generation only have pink flowers?

What happened to the red flowers and the white flowers?

RW

RWRR

Heterozygous Crosses• What would be the results if the crosses made

were between two heterozygous plants?

RW x RW

• The results are as follows: – Genotype: 1 RR: 2 RW: 1 WW– Phenotype: 1 red: 2 pink: 1 white

WW

R

W

RW

Codominance

• Pattern of inheritance where both alleles are dominant and both are expressed independently.

• Example: Cows– NOT PINK CATTLE . . . Both red and white fur

are present . . . known as roan.– White cow x red bull roan

Working with Codominance• Red Allele: FR

• White Allele: FW

• Red Bull’s Genotype: FRFR

• White Cow’s Genotype: FWFW

The Cross: FRFR x FWFW

• Roan Genotype: FRFW

FR FR

FR FR

FR FR

FW FW

FW FW

FW

FW

Working with Codominance

• If the cross were between two roan cows:FRFW x FRFW

• Show a Punnett Square:

FR

FR FR FR

FW

FW

FW FWFW

FW

FR

FR

• Genotypic ratio:• 1 FRFR:2FRFW:1FWFW

• Phenotypic ratio:• 1 red:2roan:1white

X

Multiple Alleles

• Mendel worked with traits that had only two alleles:– Pea Flower Color: Purple (P) vs. white (p)– Pea Plant Height: Tall (T) vs. short (t)– Pea Color: Yellow (Y) vs. green (y)

• Many traits are controlled by more than just two alleles; therefore they are called multiple alleles.

Examples of Multiple Alleles

• Rabbit fur:

In rabbits, coat color is controlled by multiple alleles. Full color (C), white (c), light-gray or chinchilla (cch) and white with black points or a Himalayan (ch). Full color is dominant to all the other alleles. Chinchilla is dominant to Himalayan and white.

ABO Blood Typing

• ABO blood grouping in people is classic example of multiple alleles in a SINGLE gene

• Four Blood Types:– A– B– AB– O

Why is it Multiple Alleles?• IF it was just simple, there would be only be

three genotypes: II (homozygous dominant), Ii (heterozygous), or ii (homozygous recessive).

• Blood typing is an example of multiple alleles because it has more than two alleles

• There’s more than just dominant (I) and recessive (i)

• However, there are three different alleles:– IA (dominant)– IB (dominant)– i (recessive)

Two forms of the dominant allele!

Genotypes of Blood Types

• Type A– IAIA or IAi

• Type B– IBIB or IBi

• Type O– ii

• Type AB– IAIB

Each allele codes for a specific ANTIGEN that’s found on the surface of the red blood cells:

IA = antigen A (protein A)

IB = antigen B (protein B)

i = no antigen

The antibodies in the blood “attack” the antigens making mixed red blood cells to clump.

Summary of Blood Types

Blood Type Patterns

• Type A can receive A or O• Type B can receive B or O• Type O can only receive type O• Type AB can receive from any

blood type (universal recipient)• Type O can give blood to any

blood type (universal donor)• Type AB can only give to other AB

Polygenic Traits

• Traits that are controlled by two or more genes

• Not to be confused with genes with multiple alleles!! (One set vs. many sets)

• In humans: skin color, eye color, height• Ex: gene A; gene B; gene D; gene E• Each gene would have two alleles:

dominant and recessive (A & a; B & b . . . Etc)

Interpreting Polygenic Genotypes

• Skin Color:• Dominant allele = pigment• Recessive allele = no pigment• A, B, D, E = pigment• a, b, d, e = no pigment• AABBDDEE = 8 dominant: 0 recessive• aabbddee = 0 dominant: 8 recessive• Which is darkest? Which is lightest?

Interpreting Polygenic Genotypes

• How many variations are there for AABBDDEE or aabbddee?

• Only one way to get 8 dominant or 8 recessive alleles

• Consider genotype: AaBbDdEe• How many dominant alleles? How many

recessive?• What can you infer about the skin color?

Interpreting Polygenic Genotypes

• Is there another way to get 4 dominant alleles and 4 recessive alleles within the genotype?

• Examples: AAbbDdEe and aaBBDdEe• Each example above have 4 dominant

alleles and 4 recessive; therefore, their phenotypes are the same as AaBbDdEe!

Interpreting Polygenic Genotypes

• The intermediate combination occurs most frequently; therefore, most in a population show that particular phenotype and the two extremes show up the least

• This pattern is also seen in height!

Polygenic Traits – Skin Color

Polygenic Traits in Humans

Heterochromia

Famous People with Heterochromia

Kate Bosworth

Mila Kunis

Max Scherzer

Heterochromia in Pets

Albinism

Albino Animals

X-linked Disorders

(also known as sex-linked)

X-linked

• Genes located on the X-chromosome• Women = two X chromosomes• Men = only one X chromosome• Disorder occurs more often in males

than females• Examples: Color blindness,

hemophilia, muscular dystrophy

Test for Color Blindness

Ishihara Test

Hemophilia

Color Blindness