Beyond Dominant and
Recessive Alleles
Exceptions to Mendel’s Principles
Mendel’s Principles
1. The inheritance of traits is determined by
individual units known as genes. In
organisms that reproduce sexually, genes
are passed from parents to their offspring
(children).
2. In cases in which two or more forms of the
gene for a single trait exist, some forms of
the gene may be dominant and others may
be recessive.
Mendel’s Principles
3. In most sexually
reproducing organisms,
each adult has two alleles
for each gene – one from
each parent.
These alleles are
segregated from each
other when gametes are
formed. One allele goes
into each gamete cell.
But…
Genetics is more complicated than this!!
There are exceptions to Mendel’s principles…
Some alleles are neither
dominant nor recessive, and
many traits are controlled by
multiple alleles or multiple genes.
Incomplete Dominance
In the case of incomplete dominance one allele
is not completely dominant over the other.
Both show in the offspring but they are
blended.
Incomplete Dominance
Curly Hair (CC)
Wavy Hair (Cc)
Straight Hair (cc)
Codominance
Codominance occurs when both alleles
contribute to the phenotype of the organism.
Both alleles can be seen in the offspring.
White Cow
Red Bull
Roan Cow
Polygenic Traits
Many traits are produced by the interaction of
several genes. These are called polygenic
traits, which means having many genes.
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Height
Skin Color
Weight
This allows for a lot of variation in a population.
Gene-Chromosome Theory
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States that genes are located in specific
spots on specific chromosomes.
If genes are on the same chromosome they
are said to be “linked” and passed onto the
next generation together.
Crossing Over
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Crossing over can create even more genetic
variation by creating new combinations of
alleles.
Environmental Influence on Heredity
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Article on Fingerprints of Twins
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