genetics complex inheritance sex linkage x inactivation
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Genetics: Complex Inheritance, Sex Linkage, X-Inactivation

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Genetics: Complex Inheritance, Sex Linkage, X-Inactivation. AP Biology Unit 3. Incomplete Dominance. Heterozygous phenotype is a blend of the 2 homozygous phenotypes Ex. Red flower crossed with white flower  heterozygous flower is pink. Epistasis.

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incomplete dominance
Incomplete Dominance
  • Heterozygous phenotype is a blend of the 2 homozygous phenotypes
  • Ex. Red flower crossed with white flower  heterozygous flower is pink
  • When one gene product affects the expression of another gene.
  • B and b are fur color alleles
  • bb = brown fur
  • Fur color will only be expressed if the C gene is also present
polygenic inheritance
Polygenic Inheritance
  • Two or more genes work together to create a single phenotype
  • Eye color, skin color are good examples
  • Opposite is pleiotropy (where one gene affects several different phenotypes) ex. PKU
Nature vs. Nurture
  • Virtually all human diseases have some genetic component
  • Used to trace the genotypes for a particular trait in a family
  • Can help determine the probability that future offspring will have a trait.
sample pedigrees
Sample Pedigrees

Widow’s Peak = Dominant Trait

W = widow’s peak

  • What is the grandfather’s genotype?
  • Ww
sample pedigrees1
Sample Pedigrees
  • Attached earlobes = Recessive Trait

F = unattached

f = attached

  • What is the genotype of these grandparents?
  • Both Ff
sex linkage
Sex Linkage
  • When a trait is carried on the X or Y chromosomes, it is called a sex-linked trait
  • Don’t confuse this with linked genes = when 2 genes are on the same chromosome








sex linked genes and punnett squares
Sex-linked genes and Punnett Squares
  • You have to include the X and Y chromosomes in the Punnett Square
  • Superscripts on the X and Y denote which allele is present
    • XA, Xa
red green colorblindness
Red-Green Colorblindness
  • Gene that controls this (opsin gene) is on the X chromosome
  • Colorblindness is caused by a recessive allele (mutation in the opsin gene)
  • Who is more likely to be color blind– men or women?
    • Men: only 1 X chromosome – if they have the recessive allele they don’t have another X to make up for it.
sex limited trait
Sex-limited Trait
  • Trait whose expression depends on the sex of the individual
  • Not found on the X or Y chromosome- NOT the same thing as sex-linked trait.
  • Ex. Milk production in females, pattern baldness in males (triggered by hormones)
x inactivation in female mammals
X-inactivation in female mammals
  • In females, one of the two X chromosomes in each cell becomes inactive during embryonic development
  • Why would one X chromosome inactivate itself in females?
    • Cells of females and males would have same effective dose of genes on the X chromosome
  • Inactive X chromosomes are called Barr bodies
example in humans
Example in humans
  • Anhidrotic dysplasia
  • X linked mutation prevents the development of sweat glands
  • A woman who is heterozygous will have patches of normal skin and patches of skin without sweat glands
  • Difficulty controlling body temperature
practice problem 5
Practice Problem #5
  • A normal (not colorblind male) marries a woman who is a carrier for the colorblindness allele
  • What are the chances their son will be color blind?
  • What are the chances their daughter will be colorblind?
answer 5
Answer #5
  • The man is XBY, the woman is XBXb.
  • 50% chance that son will be colorblind
  • 0% chance that daughter will be colorblind, but she could be a carrier.
practice problem 6


Practice Problem #6
  • Is this trait on the X or Y chromosome?
  • Is it dominant or recessive?
  • If individual A marries an unaffected male, what are the % chances her children will have the trait?
  • If individual B marries an unaffected woman, what are the chances their children will have the trait?
answer 6
Answer #6
  • Trait must be on the X chromosome (X*). only men would have it if on Y chromosome.
  • Dominant trait– determine the genotypes of males  look at their mother’s to help determine recessive or dominant.
  • Individual A must be X*X (since her mother was unaffected). 50% chance children will have trait (girls or boys).
  • Individual B must be X*Y. All daughters will have trait, none of sons will.