Biology 1111

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Lecture 20

• Biology 1111
• Chapter 9 Genetics

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Todays Learning Objectives

• Be able to
• Perform blood type Punnett squares
• Explain the polygenic inheritence model
• List 3 sex-linked diseases
• Explain
• Why males more likely to be color blind

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Dihybrid Cross

• Considers 2 genes
• Each is independent of the other!
• Gametes still carry only single copy of each gene
• Techniques for Punnett Squares basically same

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Dihybrid Cross Sample Question

• On a half sheet
• Show the complete set of Punnett squares for this
  cross
• Dihybrid cross of homozygous Round/Yellow with
  Green/Wrinkled
• Yellow Y (capital)
• Round R (capital)

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Correct Answer

• I dont like books technique
• Weird square
• Skips F1 square

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Two Independent Genes and Parakeet Color

• In this case, yellow blue make green

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Two Independent Genes and Retrievers

• Again, 9331 ratio
• Be able to do these Punnett Squares

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Dihybrid Cross

• Considers 2 genes
• Each is independent of the other!
• Gametes still carry only single copy of each gene
• Techniques for Punnett Squares basically same

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Dihybrid Cross Sample Question

• On a half sheet
• Show the complete set of Punnett squares for this
  cross
• Dihybrid cross of homozygous Round/Yellow with
  Green/Wrinkled
• Yellow Y (capital)
• Round R (capital)

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Correct Answer
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Two Independent Genes and Parakeet Color

• In this case, yellow blue make green

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Two Independent Genes and Retrievers

• Again, 9331 ratio
• Be able to do these Punnett Squares

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7th Inning Stretch
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Single Gene Human Traits

• Freckles vs. none
• Widows peak vs. straight hairline
• Free vs. attached earlobe

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One, Two, now Many Genes

• Monohybrid one
• Dihybrid two
• Polygenic many
• Many human traits are complex
• Controlled by several genes

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Polygenic Inheritance

• Example skin pigmentation
• Skin color clearly inherited
• Many, many phenotypes possible
• Not a light switch, but a dimmer
• Clearly, single-gene model not sufficient

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Polygenic Inheritance (contd)

• Book hypothetically uses 3 genes to demonstrate
• A dark, a light
• B dark, b light
• C dark, c light
• Overall effect of genes is additive
• Punnett square done same way, but F2 generation
  more complex
• 8 possible sperms, 8 possible eggs

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Note on Skin Pigmentation

• May be controlled by several genes
• But, tanning (environmental exposure) can
  contribute
• An individuals ability to cope w/sun genetic
• Therefore, genetics explains only part of skin
  color
• Is height completely genetic?

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To Sum Up

• Polygenic inheritance
• Might explain traits w/ continuous (dimmer
  switch) states
• Makes genetic research more difficult

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Chromosomal Basis of Inheritance

• 1866 Mendel published pea results
• Didnt know about DNA, chromosomes
• Early 1900s, biologist begin to see parallels in
  chromosome behavior Mendelian genetics

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Sex Chromosomes

• X is large (carries lots of information)
• Y is small (carries little information)
• XX female
• XY male

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Sex-linked Recessives

• Some human genetic disorders are more common for
  males
• Why?

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Sex-linked Recessives

• Some human genetic disorders are more common for
  males
• Why?
• Males have only single copy of X
• If the one X inherited has disorder allele, then
  male is afflicted
• Females have two X chromosomes
• To have a sex-linked disorder, must inherit 2
  copies of bad allele

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Examples of Sex-Linked Disorders

• Red-green colorblindness
• Hemophilia
• Duchenne muscular dystrophy

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Royal Family of Russia