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Chapter 11, Section 2
Incomplete Dominance
Neither allele seems dominant
Result is an intermediate phenotype
When the offspring of two homozygous
parents show intermediate phenotype
Example: A red flower and a white flower are
crossed. The result is a pink flower. Nothing
is dominant.
Multiple Alleles
A trait that is controlled by more than two
alleles is controlled by this.
Produce more than three phenotypes of that
Blood type produces four phenotypes (A
blood, B blood, AB blood, and O blood)
Alleles are A, B, and O.
O is recessive to both A and B
A and B are both dominant
Possible genotypes: AA, AO, BB, BO, AB, OO
Polygenic Inheritance
Occurs when a group of gene pairs acts
together to produce a trait.
More than one gene contributes to the phenotype
Many alleles, examples: eye color, skin color,
height, weight, intelligence, behavior
Traits vary over a wide range
Environment plays an important role in the
expression of traits controlled by this
Impact of Environment
Your environment plays a role in how some of
your genes are expressed or whether they are
expressed at all.
Environmental influences can be external or
You might be able to influence their expression
by the decisions you make. We will discuss
Errors in the DNA when it is copied inside of a
cell—changes in genes
Not all mutations are harmful
Possible causes: X-rays, sunlight, chemicals
Problems can also occur if the incorrect
number of chromosomes is inherited because
of mistakes in the process of meiosis.
Example—Down’s syndrome results when
three copies of chromosome 21 are produced
Recessive Genetic Disorders
Disorders caused by recessive genes
Cystic fibrosis—thick mucus builds up in the
lungs and it is hard to breathe, also hard to
digest food because mucus reduces the flow
of substances necessary for digestion
Sex Determination
Dad determines the sex of the child
Alleles for females: XX
Alleles for males: XY
Each parent gives one allele and a mom can
only give an X.
Sex-Linked Disorders
Sex-linked gene—an allele inherited on a sex
Example: color blindness caused by a trait on
the X chromosome
Males are colorblind if their X has the trait
Females are colorblind if BOTH of their X’s
has the trait.