BEYOND MENDEL

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BEYOND MENDELS LAWS

• Mendel studied traits that were purely dominant
  or recessive. Unfortunately, in nature, there
  exists more than simply two choices for some
  traits.
• Snapdragons can be white, red, pink, yellow, or
  orange.

• Incomplete dominance
• Codominance
• Multiple alleles
• Polygenic inheritance
• Linked genes
• Sex-linked inheritance

Lesson 2.
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INCOMPLETE DOMINANCE

• Neither trait is dominant or recessive.
• A heterozygous individual is a blend of the two
  traits.
• Ex. Snapdragons
• Instead of using R and r, we use FR and FW to
  show
• incomplete dominance.
• Red FR FR
• Pink FR FW
• White FW FW

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INCOMPLETE DOMINANCE

• Cross a Red Flower with a White flower
• If we crossed two plants from F1, we would get

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CO-DOMINANCE

• BOTH alleles for a trait are dominant, a
  heterozygous individual expresses both traits.
• Ex. Black rooster FBFB x white hen FWFW.
• The F1 are all barred (FBFw
• black and white feathers)

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EXAMPLE

• In four oclock plants, red flowers are
  incompletely dominant over white flowers.
  Heterozygous flowers are pink. If a red flower
  is crossed with a white flower what is the colour
  of
• The F1 generation?
• The F1 generation crossed with the red parent?
• The F1 generation crossed with the white parent?

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EXAMPLE

• A cross between a yellow snake a blue snake
  produces offspring that are all green 
• What are the genotypes of the parent generation
  (P)?
• What are the genotypes of the F1 generation?
• What would be the phenotypic ratios of offspring
  produced by two green snakes?

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What is Sickle Cell Anemia (SCA)?

• First described in Chicago in 1910 by James
  Herrick as an inherited condition that results in
  a decrease in the ability of red blood cells to
  carry oxygen throughout the body

• Sickle red blood cells become hard and
  irregularly shaped (resembling a sickle)
• Become clogged in the small blood vessels and
  therefore do not deliver oxygen to the tissues.
• Lack of tissue oxygenation can cause excruciating
  pain, damage to body organs and even death.

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Some Genetic History

• The error in the hemoglobin gene results from a
  genetic mutation that occurred many thousands of
  years ago in people in parts of Africa, the
  Mediterranean basin, the Middle East, and India.
• A deadly form of malaria was very common at that
  time
• Malaria epidemics caused the death of many
• In areas where malaria was a problem, children
  who inherited one sickle hemoglobin gene and who,
  therefore, carried the sickle cell trait - had a
  survival advantage.
• Unlike the children who had normal hemoglobin
  genes, they survived the malaria epidemics they
  grew up, had their own children, and passed on
  the gene- for sickle hemoglobin.

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• Genetics
• 2 copies of the gene for Hb (each parent)
• HbS Recessive
• SSickle
• ANormal

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Multiple Alleles Human Blood Groups

• In humans a single gene controls a persons ABO
  blood type
• This gene determines what type of an antigen
  protein is attached to membrane of a red blood
  cell
• Three alleles A,B and O
• Combination of three alleles make 4 different
  blood types ___________

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BLOOD TYPES

• A human blood group or type is a group of red
  blood cells that have the same antigen protein
  markers on the cell surface
• There are four human blood groups
• A, B, AB, O

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BLOOD TYPES
Blood Type Amongst Canadians Can Receive From Can Donate To
A 42 A, O A, AB
B 9 B, O B, AB
AB 3 A, B, AB, O AB
O 46 O A, B, AB, O

• AB Universal recipient (can receive from
  anyone)
• O Universal donor (can donate to anyone)
• Each blood type has and versions (Rh factor)
• Universal recipient (can receive from anyone)
• Universal donor (can donate to anyone)

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EXAMPLE

• If a woman has AB blood, and a man has type A
  blood, what are the possible blood types of their
  offspring?

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• Sickle Cell Trait
• Sickle haemoglobin (S) Normal haemoglobin (A)
  in RBC
• Adequate amount of normal Hb (A) in red blood
  cells
• RBC remain flexible
• Carrier
• Do Not have the symptoms of the sickle cell
• Heterozygous advantage (HbA HbS)

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Polygenic Inheritance

• Display continuous variation, in which phenotypes
  vary gradually from one extreme to another
• Height, skin colour, eye colour
• Variation is controlled by more than one gene
  polygenic trait

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Polygenic inheritance explains this
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Or this
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Inheritance of linked genes
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Lack of independent assortment with linked genes
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Only way for Pl and pL is crossing over 3 of
the time
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SEX-LINKED INHERITANCE

• Using fruit flies as
• test subjects, Thomas
• Morgan studied eye
• colour using simple
• monohybrid crosses.
• Red eyes (R) are
• dominant over white
• eyes (r).

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SEX-LINKED INHERITANCE

• When he crossed
• purebred white-eyed
• males with red-eyed
• females, he was unable to
• produce a female with
• white eyes.
• He concluded that the
• gene must be located on
• the X chromosome.

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SEX-LINKED INHERITANCE

• Some traits are located on the sex chromosomes,
  so the inheritance of these traits depends on the
  sex of the parent carrying the trait.

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SEX-LINKED INHERITANCE

• Most known sex-linked traits are X-linked
  (carried on the X chromosome). This is probably
  because the X chromosome is much larger than the
  Y chromosome

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SEX-LINKED DISORDERS

• Some sex-linked traits are associated with
  disorders.
• Most are found on the X chromosome, Y-linked
  disorders are rare.
• Males are at a much greater risk for inheriting
  sex-disorders because they only inherit one X, so
  if the X has the allele for the disorder, they
  will suffer from the disorder.
• Recessive lethal X-linked traits result in death.

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EXAMPLES OF SEX-LINKED TRAITS and DISORDERS

• Red-green colour vision deficiency (CVD) ,
  hemophilia, X-linked severe combined
  immunodeficiency (SCID)

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Hemophilia

• Condition that affects bodys ability to produce
  proteins involved in blood clotting
• X-linked recessive
• Royal Disease

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Symptoms

• Uncontrolled bleeding bleeds
• Most bleeding is internal and often into joint
  spaces
• Prior to 1960, life expectancy was 11 years.
• Today, life expectancy only 10 years shorter than
  an unaffected male

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SEX-LINKED INHERITANCE

• Punnett squares are used to predict the outcome
  of sex-linked inheritance.
• Assume the trait is X-linked unless told
  otherwise!
• Most disorders are recessive, some are dominant,
  the question will tell you.
• A carrier is a female who is heterozygous for
  the trait.

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EXAMPLE

• Hemophilia is a recessive X-linked trait. What
  is the probability of a couple having a
  hemophiliac child if the man does not have
  hemophilia and the woman is a carrier?

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Determining sex-link patterns in a pedigree

• This pedigree shows the inheritance of red-green
  CVD in a family. Identify the genotype of each
  family member.
• How does the inheritance pattern support X-linked
  inheritance?
• Hint Start with a legend

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Barr Bodies Inactive X Chromosomes

• In females every cell only has one functioning X
  chromosome
• Early in embryo development one X chromosome is
  condensed tightly into a Barr body
• Occurs randomly in each cell
• Explains calico and tortoiseshell cats (50 of
  cells have active X with allele for black, and
  50 have and active X allele for orange)
• All calicos are female, male calicos are sterile.
  Why?