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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Genetics: Complex Inheritance, Sex Linkage, X-Inactivation

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Genetics complex inheritance sex linkage x inactivation

Genetics:Complex Inheritance, Sex Linkage, X-Inactivation

AP Biology

Unit 3

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

Genetics complex inheritance sex linkage x inactivation

  • 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 calico tortoiseshell cats

Example: Calico (Tortoiseshell) cats

  • animation

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.

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