NonMendelian Genetics

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Non-Mendelian Genetics

• Some (many) patterns of inheritance do not follow
  Mendels observed patterns, hair texture for
  example.

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Review/recall
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This represents a ___ cross. The purple circles
represent ___
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Linkage

• What if round seeds were always green? What
  could one infer?

Perhaps seed shape and color are controlled by
the same gene
Or, they could be controlled by two genes
positioned very close to each other on the same
chromosome. This is called a linkage group.
They tend to be inherited together, because when
crossing over occurs, the DNA segments that are
exchanged, are long enough to contain both the
genes. Thus they appear to be the same gene.
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Linkage

• So if one notices two traits occurring together,
  what can one infer regarding the location of the
  genes that control the trait?
• This helps guide geneticists to the appropriate
  chromosome to look for abnormalities perhaps or
  gene variations.

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Independent segregation is not possible with this
pair of traits
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• Mutations in a gene result in alleles
• This ultimately leads to a change in the amino
  acid sequence and, hence, activity of the protein
• Recall your genetic disease research. Mutations
  in gene created an allele, a variation of a gene
  for the given trait such as in Sickle cell or
  hemophilia or CF.
• Natural selection may favor one allele over
  another

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Why Some Traits Dont Show Mendelian Inheritance

• Mendelian segregation of alleles can be disguised
  by a variety of factors
• Continuous variation or polygenic
• Pleiotropic effects
• Incomplete dominance
• Environmental effects
• Codominance

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Continuous Variation

• Most traits are polygenic
• They result from the action of more than one gene
• These genes contribute in a cumulative way to the
  phenotype
• The result is a gradation in phenotypes or
  continuous variation

Extremes are much rarer than the intermediate
values
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Polygenic traits
Skin color is determined most likely by two or
three pairs of genes, interacting
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Recall

• Genes code for enzymes. Enzymes control and
  coordinate chemical reactions in the body, and
  thus various needed products, some of which
  become reactants in subsequent reactions.
• If a single enzyme is missing or not operating
  properly, early in a sequence of reactions, are
  the consequences likely to be limited to one or
  two, or possibly several?

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Pleiotropic Effects

• Alleles that have more than one phenotypic effect
  are said to be pleiotropic

• Cystic fibrosis

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Pleiotropic Effects PKU
The gray represents the normal pathway. The blue
shows the direction of the pathway in the case of
a gene mutation
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Incomplete Dominance

• Some pairs of alleles exhibit incomplete
  dominance
• They produce a heterozygote phenotype that is
  intermediate between that of the homozygotes
• Example
• Flower color in the Japanese four oclock

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Environmental Effects
Color resembles snowy background in winter

• The expression of some genes is influenced by
  environmental factors, such as temperature

Color resembles tundra background in summer

• Some alleles are heat-sensitive
• Arctic foxes make fur pigment only when the
  weather is warm

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Codominance
Unlike incomplete dominance, both alleles are
expressed

• Some pairs of alleles exhibit codominance
• They produce a heterozygote phenotype that is a
  combination of that of the two homozygotes
• Example
• Roan color in horses

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Apply the concepts

• Human blood types A, B, AB and O.
• These are
• Alleles
• Genotypes
• Phenotypes
• loci

Does blood type seem to follow Mendelian
genetics? How can you tell?
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• The gene (termed I) that determines the ABO blood
  group in humans has more than one dominant allele
• The encoded enzyme adds sugar molecules to lipids
  on the surface of red blood cells
• IA adds galactosamine
• IB adds galactose
• i adds neither sugar
• The IA and IB alleles are codominant
• The i allele is recessive to both
• The different combinations of the three alleles
  produces four different phenotypes

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Multiple alleles controlling the ABO blood groups
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Human Chromosomes

• Human somatic cells have 23 pairs of chromosomes
• 22 pairs of autosomes
• 1 pair of sex chromosomes
• XX in females
• XY in males The XY pair is NOT homologous.

http//www.meds.com/leukemia/guide/guide_chapter2.
html
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Nondisjunction Involving Sex Chromosomes

• Nondisjunction of the Y chromosome
• Yields YY gametes and ultimately XYY zygotes
• Frequency of XYY is 1 in 1,000 males
• In general, these are phenotypically normal

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Nondisjunction of the X chromosome
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Hemophilia A Sex-Linked Trait

• Hemophilia is an inherited condition in which the
  blood clots slowly or not at all
• Two genes that encode blood-clotting proteins
  reside on the X chromosome
• Hemophilia is an X-linked recessive disorder
• Males develop hemophilia if they inherit one
  mutant allele from their mother
• For females to develop hemophilia, they have to
  inherit two mutant alleles, one from each parent

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Hemophilia inheritance

• Controlled by a recessive gene carried on the X
  chromosome. Let h represent the hemophilia gene
  Xh
• Cross a carrier female with a normal male
• Female sex chromosome pair is XX, but how do we
  indicate she is a carrier?

XH Xh
Male sex chromosome pair is XY, but how do we
indicate he is normal?
Do a Punnett square. What is the probability
they will have a child with hemophilia? Male or
Female?
XH Y
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• Royal hemophilia
• Started by a mutant allele in Queen Victoria of
  England
• Three of her nine children received the defective
  allele
• They transferred it by marriage to other royal
  families

Queen Victoria
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• In all, 10 of Victorias male descendants had
  hemophilia

Escaped the disorder
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The Role of Mutationsin Human Heredity

• Mutations are accidental changes in genes
• Per gene, they are rare and random
• Some mutations are harmless, some are fatal and
  some mutations can cause genetic disorders
• The inheritance of these disorders, as well as
  harmless traits, is studied by looking at
  pedigrees
• Family trees that identify individuals with the
  disease/trait

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Tay-Sachs Disease Recessive Trait

• Tay-Sachs disease is an autosomal recessive trait
  in which the enzyme hexosaminidase A is defective
• Affected individuals cannot break down specific
  lipids
• These lipids accumulate in brain cells
• Children die by five years of age
• The disease is very rare in human populations
• However, it has high incidence in Ashkenazi Jews

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Tay-Sachs Disease Recessive Trait
Enough enzyme to prevent CNS deterioration
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Huntingtons Disease Dominant Trait

• Huntingtons disease is an autosomal dominant
  trait that causes progressive deterioration of
  brain cells

• It is a fatal disease
• However, it persists in human populations because
  it has a late onset

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Review

• T or F. Mendelian genetics are fairly
  predictable.

• T or F. There is a simple, precise and accurate
  way to predict all the traits an offspring will
  inherit if the parents are known.

• T or F. Genes code for proteins.

• T of F. Mutations in genes are always an
  undesirable thing.