Blood

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Non-Mendelian Inheritance
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Complex Patterns of Inheritance

• Many things can happen to Mendels factors during
  the process of meiosis
• Mutations
• cross-over between homologues
• non-disjunction.

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Complex Patterns of Inheritance (contd)

• There are also exceptions to the postulate that
  factors occur in pairs and the law of dominance,
  as well as the law of independent assortmentit
  is now knows many genes are linked on the same
  chromosome.
• Linked traits are genes that are located on the
  same chromosome.

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Increasing Genetic Variability

• Recall Cross-over in Meiosis
• During Meiosis I, the chromatids in a tetrad pair
  are so tightly aligned together that the
  non-sister chromatids from homologous chromosomes
  actually exchange genetic material in a process
  known as crossing over.

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• This further shuffles the ancestral genes so that
  a single chromosome in a gamete may have genes
  from both the maternal and paternal ancestors.
• Crossing over can occur at any location on a
  chromosome, and it can occur at several locations
  at the same time. 
• It is estimated that during meiosis in humans,
  there is an average of two to three crossovers
  for each pair of homologous chromosomes.

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• The chromosome pieces further away from the
  centromere cross over most frequently the
  frequency diminishes as the centromere is
  approached.
• There can also be multiple cross-overs.
• They may occur because the genetic code on each
  section of the chromatid is similar.

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Sex-Linked Traits (p 318-320)

• Some patterns of inheritance seem to weigh more
  heavily in males
• I.e. more males than females have hemophilia,
  red-green colour blindness, Duchenne muscular
  dystrophy and others.

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How this was discovered

• In 1910 T. H. Morgan studied the Drosophila fly
  and found a mutant male fly, which expressed the
  trait of white eyes instead of the normal red
  eyes.
• This trait was very unusual in that species and
  Morgan wanted to see if the trait would be passed
  on to its offspring.

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• He experimented to find if this strange trait
  would be inherited according to Mendel's
  research.
• First he crossed the mutant male fly with a
  normal female with red eyes, to observe whether
  the white or red eyes were dominant.

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• The F1 generation all had red eyes, which made
  Morgan conclude that red eyes were dominant over
  white. (See Fig 12.11 p 319)
• He continued the steps of Mendel's experiment by
  crossing two flies from the F1 generation with
  each other.
• Out of 4252 flies in his F2 generation, 782 had
  white eyes but surprisingly all the flies with
  white eyes were also male.

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• This strange observation puzzled Morgan to wonder
  why there weren't any females with white eyes.
• He then crossed flies from the F1 generation with
  the original male fly with white eyes.
• This cross resulted in white-eyed and red-eyed
  males and females, making a 1111 ratio.

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• We see this pattern in humansin hemophilia, and
  red-green colourblindness.
• Why? How does it specifically affect males more
  often than females?

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Sex-Linked Trait

• If a gene is found only on the X chromosome and
  not the Y chromosome, it is said to be a
  sex-linked trait.
• Because the gene controlling the trait is located
  on the sex chromosome, sex linkage is linked to
  the gender of the individual.

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• Usually such genes are found on the X chromosome.
  The Y chromosome may be missing such genes (See
  Diagram above.).

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• The result is that females will have two copies
  of the sex-linked gene while males will only have
  one copy of this gene.
• If the gene is recessive, then males only need
  one such recessive gene to have a sex-linked
  trait rather than the customary two recessive
  genes for traits that are not sex-linked.
• This is why males exhibit some traits more
  frequently than females.

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Showing sex-linked Punnet squares, Red-Green
Colourblindness

• Because the allele is linked to the X chromosome,
  we show it as a superscript on the X
• Eg. XC normal vision gene
• Xc colour blind gene

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• Children who inherit this trait have difficulties
  with green hues, usually seeing them towards a
  red spectrum.
• Reds tend to be seen darker, and in low light
  colour differentiation is difficultboth
  appearing black to the person.

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• Eg. In humans, red-green colourblindness is a
  recessive trait located on the X chromosome.
• The Y chromosome does not carry this trait at
  all.
• A normal-vision mother whose father was colour
  blind has a child with a normal-vision man.
• What is the probability the couple will have a
  child with colour-blindness?
• What is the probability a son born to them will
  be colour blind?

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XCXc x XCY
XC Y
XC Xc


XC XC
XCY
XcY
XC Xc

• Genotypic Ratios
• Phenotypic Ratios
• Notice, Y doesnt have this gene!

1111
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Hemophilia

• Hemophilia is a term that covers a wide variety
  of clotting disorderssome clotting factor(s) are
  missing or are defective, resulting sometimes in
  uncontrolled bleeding.

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• Hemophilia is a recessive sex-linked trait on the
  X chromosome.
• What are the chances a couple will have a
  daughter with hemophilia if the mother is a
  probable carrier and the dad has a form of
  hemophilia?
• What is the chance they would have a child with
  hemophilia?

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XHXh x XhY
Xh Y


XH Xh

• Genotypic Ratios
• Phenotypic Ratios
• Notice, Y doesnt have this gene!

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SEX-INFLUENCED TRAIT

• These are not true sex-linked because it is not
  on the X nor Y chromosomes, but because of
  hormones or other such differences between
  genders, these traits show up and look like
  sex-linked traits.
• For example, in male-pattern-baldness is a
  dominant trait in males but recessive in females.
  

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Male Pattern Baldness

• For example, male-pattern-baldness is a dominant
  trait in males but recessive in females.
• Male heterozygotes will go bald.
• Female heterozygotes will not go bald.
• A female would need to inherit the trait from
  both parents to lose her hair.

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• We would do a standard punnett square for a
  dihybrid cross
• A female with a lot of hair, is a carrier for
  baldness has children with a man who is losing
  his hair, yet is a carrier (dad was not bald).
• What will be the phenotypes and genotypes of
  their children?

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XXBb x XYBb
XB Yb


XB Xb

• Genotypic Ratios
• Phenotypic Ratios
• REMEMBER the Bb has different meanings depending
  on gender!!!

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TYPES OF DOMINANCE EXCEPTIONS to MENDELs Law
of Dominance
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Incomplete Dominance

• In Mendels law of dominance, heterozygotes
  exhibited the dominant phenotype.
• When two alleles of a gene appear to be blended
  in the phenotype, the alleles are said to show
  incomplete dominance they dont look like
  either parent.

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An example is snapdragon colour

• Red snapdragons crossed with white snap dragons
  yield pink snap dragonsa complete BLENDING of
  the alleles.
• In the example, note how the allele is shown.

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• Another way the alleles are shown is to use RR
  for the incompletely dominant red colour, and
  RR for the white colour. THUS pink flowers
  would be RR
• If two pink snap dragons are crossed, what will
  be the expected phenotypes and genotypes?

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__________    X   _________
? ?


? ?
Genotypic Ratios Phenotypic Ratios
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Curly Hair

• In humans, curly hair is incompletely dominant to
  straight hair.
• Children who are heterozygous will have wavy
  hair.
• What will be the phenotypes and genotypes of the
  children from a female with curly hair and man
  with wavy hair?
• C represents curly hair OR HC
• C represents straight hair or HS

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__________    X   _________
? ?


? ?
Genotypic Ratios Phenotypic Ratios
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Co-Dominance Inheritance

• It may seem as if incomplete dominance and
  co-dominance are the same, but they are not.
• When two alleles of a gene are clearly expressed
  in the phenotype, the alleles are said to be
  co-dominant.
• This results in two distinct and detectable gene
  products

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Blended At a distance, the cattle appear roan
coloured and mottled

• The individual hairs are either red or they are
  white.

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• In doing the PUNNET square we use both capital
  letters for the traitR for red and W for white.
• Eg. A roan bull (RW) mates with a white cow.
• What will be the phenotypes and genotypes of
  their offspring?

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RW     X   WW

• Genotypic Ratios
• Phenotypic Ratios
• 50 will be roan and 50 will be white

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Appaloosa Horses

• In horses, gray horses (GG) are codominant to
  white horses (WW). 
• The heterozygous horses(GW) is an appaloosa horse
  (a white horse with gray spots on the rump and
  loins).
• Cross a white horse with an appaloosa horse

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__________    X   _________
? ?


? ?
Genotypic Ratios Phenotypic Ratios
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Blood Typing

• Blood is typed according to what type(s) of
  antigen (a cellular product that induces antibody
  formation in a foreign host) are found on the
  surface of the red blood cells.
• Blood type is determined by reacting the blood
  with antibody against the antigens.
• Typical blood types are the ABO blood-groups.
  (Text P 325 978)

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Type AB Blood

• The AB blood type in humans is the result of an
  individual carrying both the IA and the IB
  alleles.

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MN Blood Group system in Humans

• In humans, our M and N blood groups are
  co-dominant.
• Our blood cells exhibit both antigens but on
  separate blood cells.
• The MN blood group system is under the control of
  an autosomal locus found on chromosome 4, with
  two alleles designated LM and LN.

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• The blood type is due to a glycoprotein present
  on the surface of red blood cells.

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The table below is an old one but it shows the
frequencies of MM blood, MN blood, and NN blood.
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Blood Type and Geographic Location

• HowStuffWorks Videos "Why Tell Me Why Different
  Blood Types"

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INCREASING THE GENE-POOL
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MULTIPLE ALLELE INHERITANCE

• Mendel never knew that some traits occur in more
  than pairs.
• This is called multiple allele inheritance
  because more than two alleles are possible for
  one traitbut only two alleles are inherited and
  involved.
• In humans, our major blood type system is a
  classic example.

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ABO Blood Grouping

• We commonly call it the ABO system.
• As a multiple allele, we write it like we did for
  co-dominancethat is because both the A and B are
  equally strong.
• The exception is the type O which is a recessive
  condition

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• IA - for type A antigens
• IB - for type B antigens
• i - for the recessive O condition (which
  produces neither the A nor B antigens and does
  not interfere with type A or B blood).

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• The following allele combinations are possible
• IA IA IA i type A blood (notice A is
  dominant to O)
• IB IB IB i type B blood
• IA IB type AB bloodboth antigens are
  present
• ii type O bloodno A nor B antigens are
  present

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• A woman is homozygous for type B blood (IBIB)has
  a child with a man who is heterozygous for type A
  blood.
• This means he is IAi.
• What will be the genotypes and phenotypes of
  their children?

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IAi     X   IBIB
IA i


IB IB
Genotypic Ratios Phenotypic Ratios
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• What is the probability a couple whose blood
  types are AB and O will have a child with type A
  blood?

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__________    X   _________
? ?


? ?
Genotypic Ratios Phenotypic Ratios
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POLYGENIC INHERITANCE

• Many characters cannot vary in a population
  across a continuum (gradient).
• For example, skin color in humans is a
  quantitative character this means the character
  is controlled by more than one gene at the same
  time (polygenic inheritance) that is, the trait
  depends on several chromosomal locations at the
  same time.

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• This is different from multiple allele
  inheritance where only TWO alleles are passed on
  but in the population there are several types of
  alleles for one traittry not to confuse these!.

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