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Chapter 5
Heredity
Section 2
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Genetics Since Mendel
Incomplete dominance
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When the offspring of two homozygous parents
show an intermediate phenotype.
Individuals DO NOT have to exhibit the trait in
order for it to appear in their offspring.
Multiple Alleles
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Many traits are controlled by more than 2 alleles.
A trait that is controlled by more than two
alleles is said to be controlled by multiple
alleles.
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Produce more than 3 phenotypes of that trait.
Ex. Blood Type in humans- controlled by 4
alleles.
A, B, AB, O
O is recessive to both A and B
Multiple Alleles
Phenotype A blood = genotype AA or AO
 Phenotype B blood = genotype BB or BO
 Phenotype O blood = genotype OO
Ex. What possible genotypes will the offspring
have if the parents’ blood types are O and AB?
Answer: AO or BO.
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Polygenic Inheritance
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* Eye color is an example of a trait that is
produced by a combination of many genes.
Polygenic Inheritance
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Occurs when a group of gene pairs acts together
to produce a trait.
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Ex. Height, color of eyes, and skin
Impact of the Environment
Genes determine many of your traits
 Some people may have genes for developing cancer
 Whether they get the caner or not might depend on
external environmental factors.
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Human Genes and Mutations
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Mutation
Error that occur in DNA
 Ex. X-rays, radioactive substances
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Recessive Genetic Disorders
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Cystic Fibrosis is a homozygous recessive disorder
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Thick fluid instead of thin fluid lubricates the lungs and
intestinal tract. Makes it hard to breathe.
Sex Determination
XX- Females
 XY- Males
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Sex-Linked Disorders
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Some inherited conditions are linked with
the X and Y chromosomes.
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Sex-linked gene
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An allele inherited on a sex chromosome.
Ex. Color blindness
Color blindness- recessive allele on the X chromosome.
Males only have one X chromosome so if the allele is on
this X chromosome then he will be color-blind.
Females have two X chromosomes so the allele would need
to be on both X chromosomes for the female to be colorblind.
It is more common for boys to be color-blind than
girls.