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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/