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Chapter 11: Complex Inheritance & Human Heredity

Ch. 11.1 Basic Patterns of Human Inheritance

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Ch. 11.2 Complex Patterns of Inheritance

Beyond Dominant and Recessive Alleles

Despite Mendel's work, not all genes show simple patterns of inheritance
In most organisms, genetics is more complicated because the majority of genes have more than two alleles

Incomplete Dominance

*Some alleles are neither dominant nor recessive
Incomplete dominance- when one allele is not completely dominant over another
The heterozygous phenotype lies somewhere between the two homozygous phenotypes
Snapdragon flower: red (RR) and white (WW) give pink (RW)


Codominance- the phenotypes produced by both alleles are clearly expressed

Sickle-cell Disease

Sickle-cell disease affects red blood cells and their ability to transport oxygen. Changes in hemoglobin, which is the protein in red blood cells, cause red blood cells to to change to a sickle shape. Individuals that are heterozygous produce normal and sickle cells. Individuals with sickle-cell disease have a higher resistance to malaria and so pass it on at higher rates in areas of malaria.

Another example, "Erminette" chicken in which black and white feathers are both produced.

Multiple Alleles

*Many genes exist in several different forms and are therefore said to have multiple alleles
Multiple alleles- a gene with more than two alleles
An individual has only 2 copies of each gene
Rabbit fur color has 4 alleles that follow simple dominance; human blood type A (A antigen), B (B antigen), AB (A&B antigen) and O (neither AorB antigen)


Sex Determination

Humans have 23 pairs of chromosomes in our cells. Sex chromosomes- determines an individual's gender. Male XY, female XX. Other 22 pairs of chromosomes are autosomes.

Sex-Linked Traits

Sex-linked traits- traits controlled by genes located on the X chromosome; also called X-linked traits. Males are affected by recessive X-linked traits more often than females since they have only one X chromosome. Females other X chromosome may mask a recessive X-linked trait.

Red-Green Color Blindness

The trait for red-green color blindness is a recessive X-linked trait. About 8% of males in the U.S. have red-green color blindness but it is very rare in females.

Polygenic Traits

*Many traits are produced by the interaction of several genes
Polygenic traits- traits controlled by two or more genes.  
Examples: Height, Eye Color, Fingerprint pattern, and Human skin color (controlled by more than 4 different genes).

Environment Influences

Phenotype of an organism is only partly determined by it's genotype
Environmental conditions can affect gene expression and influence genetically determined traits


Siamese cats: dark fur in areas that are cooler in temperature. The gene that codes for production of color pigment only functions under cooler conditions.

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Ch. 11.3 Chromosomes & Human Heredity


Cell division in which sister chromatids fail to separate properly is nondisjunction. When nondisjunction occurs it produces gametes without the correct number of chromosomes. When the gamete fertilizes with another gamete the resulting offspring will have extra chromosomes or only one copy of a particular chromosome.

  • Trisomy- a set of three chromosomes.
  • Monosomy- only one set of chromosomes.

In humans, alterations of chromosome numbers are associated with serious human disorders, often fatal.

Down Syndrome

The result of an extra chromosome 21, often called trisomy 21. Characteristics are: distinctive facial features, short stature, heart defects, and mental disability. The frequency of Down syndrome increases with the age of the mother.

Sex Chromosomes

Nondisjunction can occur in sex chromosomes as well. Turner's syndrome results from fertilization with a gamete that had no sex chromosome.

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Page last updated August 16, 2017.