Linked and Sexlinked genes

1
Linked and Sex-linked genes

• Read 142-145
• Bring your science fair data
• NEXT CLASS to work on.

2
Linked genes

• When looking back at dihybrid crosses, we have
  only considering each gene separate, giving us a
  9331 ratio.
• When we study genes on the same chromosome, we
  refer to them as linked genes and note that the
  ratio is only 31
• Notice how a linked-gene punnett is different
  from a dihybrid.

3
What is a sex-linked trait?

• Sex-linked traits are due to genes located on sex
  chromosomes.
• As we know, male sex chromosomes are XY while
  female sex chromosomes are XX.
• The X chromosome contains over 1000 genes while
  the Y chromosome contains as few as 26.
  Therefore, many sex-linked traits are discussed
  in terms of the X-chromosomes.

4
Carriers

• Because females have two copies of the X
  chromosome, it is possible to have certain traits
  hidden by a dominant copy.
• However, because males only have one X
  chromosome, the observable phenotype is obvious
  and identifies the genotype.

5
Carriers

• When a female contains a recessive allele that is
  hidden by the dominant allele, we call them
  carriers.
• A carrier maintains the ability to pass on a
  trait even if they do not express/show it.

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Examples of X-linked genes

• Other than determining sex, genes on the X
  chromosome are responsible for traits. Some
  examples are
• Hemophilia
• Red-green color blindness
• High blood pressure
• Muscular dystrophy
• Fragile-X syndrome

7
Punnett Practice

• Determine the probability of the offspring being
  a carrier when
• The woman is a carrier and her partner does not
  have the trait. -- What do the boys have of
  receiving the trait? What of the girls?
• If the man has an X-recessive trait and his
  partner does not have it (nor is a carrier).
  What of boys will have it? What of girls will
  have it?

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Recessive Lethals

• In some rare instances, recessive traits are
  lethal-- meaning the organism is born very weak
  and sickly or dies not long after birth.
• When looking at non-sex linked chromosomes, we
  can apply standard punnett square probability.
• However, when looking at sex linked chromosomes
  we see that males have a much higher tendency for
  recessive lethals. Why?

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Practice All Punnetts

• Find the genotype ratios for a homozygous
  dominant x heterozygous
• Find the phenotype ratios for a heterozygous x
  heterozygous
• What of male offspring will be hemophilliacs if
  the mother is a carrier and the father is a
  hemophilliac?
• Determine the parent if a unknown dominant parent
  produces 100 heterzygous offspring?
• What are the chances of a)male and b)female
  offspring to be infected (but not carriers) if a
  mother is a carrier and the father is not
  infected?
• What is the chance of a recessive lethal when
  crossing a heterozygous with a homozygous
  dominant?

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• http//www2.edc.org/weblabs/Punnett/punnettsquares
  .html