Mendel’s Genetics

• Mendel’s research provided the groundwork for our understanding of inheritance of traits.

Real World Genetics

• MOST traits are not inherited in such a simple manner as shown by Mendel. For example: – Many traits are controlled by more than one gene– Many alleles are neither completely recessive nor

dominant• We now know a gene is actually a small

section of a DNA molecule.

Exceptions to Mendel

• Incomplete Dominance• Codominance• Multiple Alleles for a Trait• Multiple Genes for a Trait

Jackalope = cross between

a jack rabbit and an

antelope

Exceptions to Mendel

• Incomplete dominance/Co-dominance – neither allele is dominant. Neither allele is recessive– For example: Red + White sometimes makes …

Incomplete Dominance/Co-Dominance

• Genetics of four-o-clock flowers:– R = red allele– W = white alleleRR = red flower, WW = white flower, RW = pink flower

Incomplete Dominance• Draw a Punnett Square showing a cross between a red flowering four

o’clock and a white flowering four o’clock.

• What are the phenotypes and genotypes of the offspring?

Incomplete Dominance

• Phenotype: All of the offspring are pink

• Genotype: All of the offspring are RW

• Draw a Punnett Square showing a cross between two pink flowering four o’clock.

• What are the phenotypes and genotypes of the offspring?

Incomplete Dominance

• Have the traits for red and white(R and W) disappeared?

• Now show a cross between a pink and white flower.

R W

R

W

RR RW

RW WW

Incomplete Dominance

• What are the genotypes of the cross?

• What is the phenotype ratio?

• 1 pink: 1 white

R W

W

W

RWWW

RW WW

Incomplete Dominance

• Each allele in the example provides instructions to a cell for making an enzyme.

• The R allele produces an enzyme necessary for the production of red pigment.

• The W allele produces an enzyme necessary for the production of white pigment.

• Both alleles are active in incomplete dominance.

Multiple alleles for one trait

• More than two alleles exist for a trait--however, each individual still only has two alleles.

Multiple alleles for one trait

• Example: Hair Color1. Some alleles, such as brown-ebony, produce that make a large quantity

of the brown pigment.

2. Some alleles, such as brown-Swedish-blonde, produce enzymes that make only a small amount of the brown pigment.

3. In hair color, alleles with more activity are dominant to alleles with less activity. More activity —› more pigment —› hair color phenotype.

http://www.indiana.edu/~oso/lessons/Genetics/RealColors.html

Multiple Alleles for a Trait

• Example: Human Blood Groups– Before the 1900’s people

thought all blood was the same. Fatalities resulted from

• Transfusions of animal blood into humans!

• Transfusions of some human blood to other humans

Multiple Alleles for a Trait

• It was then discovered there are different types of red blood cells.

• Each type contains a different antigen.• Antigen = a substance (a protein or sugar) that

triggers the production of antibodies. • Antibodies are part of the body’s defense for

fighting off infection

Multiple Alleles for a Trait•Human Blood Groups – Three alleles

• A = contains one type of antigen• B = contains a different type of antigen• O = does not contain an antigen

Multiple Alleles for a Trait

• What are all of the possible genotypes using these three alleles (A, B, O)?

• (Hint:  Remember each person inherits two alleles for a trait!!)

AA AO BB BO AB OO

Multiple Alleles – Blood Types

Genotypes Phenotypes• AA Blood

Type A • AO Blood

Type A• BB Blood

Type B• BO Blood

Type B• AB Blood

Type AB• OO Blood

Type O

Multiple Alleles – Blood Types

• Which blood type shows codominance? _________

• Which blood type is the universal donor? _________

• Which blood type is the universal recipient?_________

AB

AB

O

Multiple Alleles – Blood Types

• Blood types in the United States are more or less common based on race and ethnicity.

• However, Type O is most common among all groups and Type AB is least common.

Multiple Alleles – Blood Types

• Complete Reading on Blood Types• Discuss articles with your group• Answer questions

Exceptions to Mendel

• Multiple genes for a single trait: – Example: height. Height is made up of the length

of different sections of your body added together. Each section is controlled by a different gene.

– Example: Eye color in humans. Controlled by interactions of at least 3 different genes.

Multiple Genes for a Single Trait

• Skin color: – Scientists now believe that 3 genes control skin color.– You get 3 alleles from your mother and 3 alleles from your

father for 6 alleles. – If all 6 of the alleles are for dark skin, you will have the

darkest possible skin. – If you have 5 dark alleles and one light, you will have very

dark skin.– If you have all 6 light alleles then you will have the lightest

skin possible. – http://www.dep.anl.gov/

Exceptions to Mendel

Complete Worksheets in Groups