Sex-Linked Traits

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Sex-Linked Traits
2
Homework

• Test Monday 4/5

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Objectives

• Be able to construct and interpret a monohybrid
  cross for a sex-linked gene
• Distinguish between autosomal and sex chromosomes
• Be able to recognize sex-linkage in a pedigree

4
Chromosomes

• There are two broad categories of chromosome
• Sex Chromosomes A chromosome that men and women
  have different amounts of. (X and Y in humans.)
• Autosomal Chromosomes All the other chromosomes
  BESIDES the sex chromosomes. (Chromosomes 1-22
  in humans)

5
Chromosomes

• So far, weve only studied autosomal traits,
  because a gene on a sex chromosome is a bit more
  complicated.
• Why might it be harder to figure out genotypes
  when the gene is on a sex chromosome?

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Sex-Linkage

• Sex-Linked Traits The gene for the trait is
  located on either the X or Y chromosome.
• When writing alleles that are sex-linked, we use
  a convention like that more complicated one from
  back at the beginning we write the X or Y
  normally, and make the allele itself a
  superscript.
• This can produce some interesting effects. For
  instance, say theres a disease. Well use the
  alleles A and a.

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Sex-Linkage

• If the A gene is on the X chromosome, then
  genotypes can have one of these alleles XA, Xa,
  and Y.
• If theres no allele, the Y becomes sort of like
  the free square in the middle of a bingo board.
  It doesnt affect the phenotype at all.
• So a man who is XAY will have the dominant
  version of the trait, and a man who is XaY will
  have the recessive version of the trait. He
  doesnt have two little-as, true, but theres
  also no dominant allele to drown out the
  recessive allele.

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Sex-Linkage

• Meanwhile, if the gene is on the Y chromosome, we
  could have the alleles X, YA, or Ya.
• Would each of these people have the dominant
  phenotype or the recessive?
• XX
• XYA
• XYa

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Sex-Linkage

• Meanwhile, if the gene is on the Y chromosome, we
  could have the alleles X, YA, or Ya.
• Would each of these people have the dominant
  phenotype or the recessive?
• XX - Will not have the trait, whatever it is.
• XYA - Dominant phenotype
• XYa - Recessive phenotype

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Sex-Linkage

• How about phenotypes for these people with a gene
  on the X-chromosome?
• XBXB
• XBXb
• XbXb
• XBY
• XbY

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Sex-Linkage

• How about phenotypes for these people with a gene
  on the X-chromosome?
• XBXB - Dominant
• XBXb - Dominant
• XbXb - Recessive
• XBY - Dominant
• XbY - Recessive

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Sex-Linkage

• Punnett Squares for sex-linked traits work like
  normal, except that you use the superscripts.
• A Punnett Square for a cross of XBXb with XBY
  looks like this

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Sex-Linkage

• Punnett Squares for sex-linked traits work like
  normal, except that you use the superscripts.
• A Punnett Square for a cross of XBXb with XBY
  looks like this
• 50
  girl w/ dominant phenotype
• 
  25 boy w/ dominant phenotype
• 
  25 boy w/ recessive phenotype

XB Xb
XB XB XB XB Xb
Y XBY XbY
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Sex-Linkage

• With your partner, practice with these crosses
• XDXD x XdY
• XdXd x XDY
• XX x XYE

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Pedigrees

• On a pedigree for a Y-linked trait, fathers with
  the trait always give it to their sons.

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Pedigrees

• For a recessive X-linked trait, it will skip
  generations and usually only men will have the
  trait.

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Pedigrees

• X-linked dominant traits are very hard to tell
  apart from simple dominance. The only helpful
  hint is that fathers with the trait always give
  it to their daughters.