Biology 102

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Biology 102

• Patterns of Inheritance (cont.)

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Lecture outline

• Inheritance of multiple traits
• Same chromosome (linkage)
• Sex chromosomes
• Overview
• Sex linkage
• Incomplete dominance
• Codominance
• Other variations
• Sickle cell anemia example
• Pedigrees

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Crosses with two traits

• If genes are on different chromosomes, they sort
  independently

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Genes on the same chromosome tend to be inherited
together

• They are on the same chromosome. Considered to
  be linked.

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Recombination creates new combination of linked
alleles

• The farther away two genes are on a chromosome
  from each other, the more likely they will be
  exchanged during crossing over
• The closer together two genes are on a
  chromosome, the more likely they will travel
  together

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Sex chromosomes

• Differ in appearance
• Only small parts are homologous (carrying same
  genes)
• Sex-linked genes
• Found only on one of the sex chromosomes
• Only 20 genes on Y
• Related to male characteristics
• 1500 genes on X
• Related to a variety of traits (few are
  gender-related)

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Sex determination

• Male determines the gender in mammals
• How so?
• Birds are the opposite
• Females XY, Males XX

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Sex-linked inheritance Eye color in fruit flies
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Incomplete dominance

• Snapdragon example
• Heterozygote is intermediate between the two
  homozygotes
• R codes for enzyme catalyzing production of red
  pigment
• R codes for non-functional enzyme
• Amount of enzyme determines amount of pigment

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Human eye colorPolygenic, incomplete dominance
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Codominance

• Both phenotypes expressed fully.
• Example Human blood groups
• These genes code for enzymes that stick sugars to
  cell-surface glycoproteins
• Alleles A and B code for addition of distinct
  sugars
• O does not code for a sugar
• Make antibodies to sugars not present on own
  cells

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Human Blood Types

• Freq

• Donates

• Re-ceives

• Anti-bodies

• RBCs

• Genotype

• Type

• 40

• A or AB

• A or O

• B

• AA or AO

• A

• 10

• B or AB

• B or O

• A

• BB or BO

• B

• 4

• AB

• AB, A,B, O(universal)

• Neither

• AB

• AB

• 46

• O,AB,A,B(universal)

• O

• Both

• OO

• O

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Other variations

• Environmental influences
• Expression of gene for black pigment in Himalayan
  rabbit depends upon temperature
• Most traits affected by both genes and
  environment
• Pleitropy
• Multiple effects of a single gene
• Example Sickle cell anemia
• Also great example from single error in a single
  nucleotide to whole organism effect

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Some human genetic diseases

• Sickle-cell anemia A case of incomplete
  dominance
• Go from gene to whole organism effects

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Sickle cell anemia From amino acid substitution
to whole body effects

• A single nucleotide is substituted in 18th
  position of B-chain of Hb molecule (6th amino
  acid)
• GUG instead of GAG
• Altered primary protein structure
• Valine instead of Glutamic acid
• Altered tertiary structure
• Altered quaternary protein structure
• Chains dont fit together correctly

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Sickle cell anemia From amino acid substitution
to whole body effects

• Hemoglobin molecules stick together
• Red blood cell shape is altered

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Sickle cell anemia (cont.)

• Whole body effects (symptoms of sickle cell
  anemia)

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Investigating human genetic disorders

• Pedigrees Use to figure out how trait is
  inherited (dominant, recessive, sex-linked)