Biology Genetics Part 2

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Biology Genetics Part 2

• Objectives a) Review Mendelian Genetics
• b) Types of cells/Types of
  Chromosomes
• c) Sex linked traits
• d) Sex Influenced
  traits
• e) Polygenics
• f) Epistasis

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Cells

• We have 2 types of cells
• 1. Somatic Cells (2n) body cells
• 2. Sex Cells (n) sperm and egg

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Chromosomes

• We have 2 types of Chromosomes
• 1. Autosomal chromosomes pairs 1 through 22 in
  humans (non-sex chromososmes)
• 2. Sex chromosomes xx or xy
• What about a fruit fly with a 2n 8?

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Sex-linked Traits
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Definition

• Sex-linked traits are traits that are controlled
  by genes on the sex chromosomes
• The X and Y chromosomes

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Are they controlled by both sex chromosomes?

• Most sex-linked traits are controlled by genes on
  the X chromosome.
• This is because an X chromosome is much larger
  than a Y chromosome.
• A few traits are suspected to be controlled by
  genes on the Y chromosome, however there is less
  research about Y-linked traits.

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Different Forms of Sex-linked Inheritance

• There are three different forms of sex-linked
  inheritance that we will be examining
• X-linked recessive inheritance
• X-linked dominant inheritance
• Y-linked inheritance

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X-linked Recessive Inheritance

• X-linked recessive traits are traits resulting
  from a recessive allele on the X chromosome.
• There are over 100 different human conditions
  that are caused by recessive alleles found on the
  X chromosomes.
• X-linked recessive alleles are represented by a X
  , superscript lower case letter or a plus/minus
• (/-)

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X-linked Recessive Inheritance

• These traits tend to show up in males more than
  females.
• Why?

Because males only carry one X chromosome! (
they have one x chromosome, and one Y chromosome
that make up their sex chromosomes).
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X-linked Recessive Inheritance

• Example 1 Colorblindness
• The allele that controls colorblindness is found
  on the X chromosome and is recessive .
• What genotype must a female have to be diagnosed
  as colorblind?

Homozygous recessive (X-X-)
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Are you colorblind?
Normal Color VisionA 29, B 45, C --, D 26
Red-Green Color-BlindA 70, B --, C 5, D --
3. Red Color-blindA 70, B --, C 5, D 6 4.
Green Color-BlindA 70, B --, C 5, D 2
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Try this problem!

• What is the probability that the sons of a
  homozygous recessive mother would be colorblind?
• Represent colorblindness with a -

Mothers Genotype X- X - Each son would receive
an X chromosome from their mother which
means Each son would have an X chromosome with a
recessive allele for colorblindness and would
express the trait!
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X-linked Recessive Inheritance

• Example 2 Hemophilia Hemophilia
• Hemophilia is a serious ailment in which the
  blood lacks a clotting factor, and therefore when
  an individual is injured, they cannot stop
  bleeding.
• Hemophilia is also controlled by recessive
  alleles on the X chromosome.

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Try this problem

• A woman who is heterozygous (a carrier) for
  hemophilia marries a normal man
• a. What are the genotypes of the parents?
• b. Make a Punnett square for the above cross.
• c. What is the probability that a male offspring
  will have hemophilia? __________
• d. What is the probability of having a
  hemophiliac female offspring? _________

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X-linked Dominant Inheritance

• X-linked dominant traits are traits that result
  from the presence of a dominant allele on the X
  chromosome.
• Unlike X-linked recessive traits, females and
  males both require only ONE dominant allele in
  order to express the trait.
• X-linked dominant traits are represented by an X,
  superscript capital letter or a ()

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X-linked Dominant Inheritance

• Example Faulty Tooth Enamel and Dental
  Discoloration
• Individuals who have an X chromosome that carries
  a dominant allele for this trait will have dental
  discoloration.

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Try these problems!

• Would a heterozygous woman for dental
  discoloration display the trait?

YES! Because this is a X-linked dominant trait,
you only need to carry one dominant allele in
order to express the trait.
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Try these problems!

• What percentage of the children from a
  heterozygous mother and an affected father would
  have dental discoloration?
• Represent dental discoloration with a D

All of the daughters will display the trait as
they each carry at least one dominant allele. One
son will demonstrate the trait while the other
wont.
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Y-linked Inheritance

• Y-linked traits are controlled by alleles on the
  Y chromosome
• Another word for Y-linked traits is holandric
  traits as they are wholly male.

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Y-linked Inheritance

• Do the words homozygous or heterozygous apply to
  Y-linked traits?
• Are females affected by Y-linked traits?

No, because there is only ever one Y chromosome
present at a time!
No, because females dont carry Y chromosomes!
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What is this?
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Y-linked Inheritance

• An example of a Y-linked trait is . . . . .
  HAIRY EARS!

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Y-linked Inheritance

• All of the sons of an affected male will display
  this Y-linked trait.

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A final example of Sex-linked Traits

• Eye color in fruit flies ( Drosophilia
  melanogaster )
• Eye color is controlled by the X chromosome.
• Red eyes are dominant to white.
• White eyes are most common in males.
• Females only display white eyes if they are
  homozygous recessive for the trait.

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Fruit Flies Continued

• What type of X-linked inheritance is this?
• Try your new knowledge out on the handout you are
  about to receive!

X-linked recessive Inheritance because the males
show the trait more often that the females.
Also, the females must have two X chromosomes,
both of which carry the recessive alleles for
white eye color in order to have white eyes!
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Lets Try These

• Problems
• Practice Problems