Chapter 16: Expect the Unexpected: Non-Mendelian Patterns of Inheritance summary

Types of inheritance complicated by other factors

  • Incomplete dominance: Two alleles can join to form a phenotype that is between recessive phenotype and the dominant phenotype.
  • Codominance: The effects of two alleles contribute equally to the phenotype.
  • Pleiotropy: It occurs when a single gene has several different phenotype characteristics.
  • Polygenic traits: The phenotype is affected by several different genes.
  • Linked genes: These are genes that are more close to chromosomes. They defy the independent assortment law.
  • Sex-linked traits: They are found on the sex chromosomes.

Incomplete dominance

  • During incomplete dominance, the presence of a recessive allele is partially masked by the dominant allele.
  • The color of some flowers serves a good example. The red allele is not completely dominant compared to the white allele.


  • Codominance is almost the same as incomplete dominance because heterozygotes can show the effects of two alleles.
  • In codominance, the two alleles both show up in the heterozygote.
  • A good example of codominance is the ABO marker on the human blood group.
  • Three blood group alleles exist in humans: Allele IA, IB and i.
  • The above alleles are enzyme blueprints that attach carbohydrates on red blood cell surfaces.
    • Allele IA promotes the production of an enzyme that can attach carbohydrate A to blood cells.
    • Allele IB allows the making of an enzyme that can attach carbohydrate B to blood cells.
    • Allele i is nonfunctional and cannot cause the attachment of carbohydrate A or B to blood cells.

Pleiotropic genes

  • These are genes that affect more than one trait.
  • An example is patients with sick cell anemia because they have two recessive alleles for the defective hemoglobin.
  • When the levels of oxygen in the cell decrease, the defective hemoglobin change shape causing red blood cells to change shape.

Polygenic traits

  • Polygenic traits are traits controlled by many genes.
  • Many phenotypes occur in people because of the many possible combinations of alleles in genotypes.
  • Polygenic traits in humans include skin color, eye color, and height.

Linked genes

  • Linked traits have genes for the trait on the same chromosome.
  • Genes that are close together on the same chromosome travel together during anaphase I stage in meiosis.
  • Completely linked genes are very close together on the chromosomes they are located in.

Building a chromosome map

  • The more two linked genes are far from each other on a chromosome, the higher the chances of recombination between the two genes.
  • Performing crosses and observing the recombination amount helps to show how two genes are far apart from each other on the chromosome.
  • To build a map, the following should be considered:
    • Perform mating of organisms that are homozygous for the chosen trait.
    • Mating F1 heterozygotes.
    • Count F2 offspring number with the parental phenotype.
    • Count F2 offspring number with recombinant phenotypes.
    • Calculate recombinants percentage in the F2.

Sex inheritance

  • Sex-linked traits are traits that are inherited in females and males differently.
  • An example is hemophilia disease which results from blood-clotting proteins defect.
  • Hemophilia is an X-linked recessive trait.
  • This means that the disease is propagated by an allele found on the X chromosome.

Patterns of sex-linked traits

  • X-linked dominant traits.
    • They affect both females and males.
    • The traits don’t skip generations because they are dominant.
    • The daughters from the affected father are also affected.
  • Y-linked traits.
    • They affect only men.
    • Men affected by the trait can pass it to their sons.


complete dominance
Mendel’s pea crosses always looked like one of the parental varieties

incomplete dominance
neither allele is dominant over the other and expression of both alleles occurs

incomplete dominance
does not support the blending hypothesis because the original parental phenotypes reappear in the F2 generation

incomplete dominance
Two parents that have different phenotypes create an offspring that have an intermediate blend

both parents’ phenotypes expressed equally in the offspring


incomplete dominance
RR * WW= RW (pink)

multiple allelic inheritance
Although an individual can at most carry two different alleles for a particular gene, more than two alleles often exist in the wider population

multiple allelic inheritance
ex: blood type AB

multiple allelic inheritance
neither allele is dominant over the other and expression of both alleles is observed as a distinct phenotype in the heterozygous individual

involves 2 different genes: 1 gene codes for a particular trait, but a second gene controls the expression of the first

pigment in labs

bombay allele
IBIB hh parent will have Type O blood

multifactorial inheritance
environment as well as inheritance plays a very significant role in the expression of a gene

multifactorial inheritance
hydrangea/Asian women

polygenic inheritance
Inheritance of a single trait that involves multiple genes on multiple chromosome

polygenic inheritance
human skin color
human eye color
human weight

polygenic inheritance
Traits that follow this type of inheritance follow a bell shaped curve in a population

chromosome theory of inheritance
genes occupy specific loci (positions) on chromosomes and chromosomes undergo segregation and independent assortment during meiosis

linkage map
a listing of the relative locations of genes along a chromosome, as determined by recombination frequencies

a human genetic disease cause by a sex-linked recessive allele; characterized by excessive bleeding following injury

red-green colorblindness
A category of common, sex-linked human disorders involving several genes on the X chromosome; characterized by a malfunction of light- sensitive cells in the eyes; affects mostly males but also homozygous females

duchenne muscular dystophy
a human genetic disease caused by a sex-linked recessive allele; characterized by progressive weakening and a loss of muscle tissue

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