Transcript Exceptions to Mendel`s Principles

12-4 Beyond Medelian
Genetics

Alleles are the possible “options” for a trait.
Summary of Mendel’s Principles
1. The inheritance of traits is determined by
genes which are passed from parent to
offspring.
2. When 2 or more alleles for a gene exist, some alleles
may be dominant and other alleles may be
recessive. (_______________________)
3. Genes are segregated from each other when
gametes are formed. (____________________)
4. The alleles for different genes usually segregate
independently of one another.
(__________________________________)
Exceptions to Mendel’s Principles
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Not all genes show simple patterns of
dominant and recessive alleles.
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The majority of genes have more than 2
alleles

Traits are sometimes controlled by more than
one gene.
Exceptions to Mendel’s Principles

Incomplete Dominance – one allele is not
completely dominant over another. There is an
appearance of a 3rd phenotype. Heterozygous
phenotypes show up somewhere in between.
Example snapdragon flowers
Red Flowers
RR
x
White flowers
rr
Pink Flowers
Rr
Exceptions to Mendel’s Principles
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Codominance – both alleles contribute to the
phenotype of the organism. The offspring have a
phenotype that expresses both alleles equally.
Examples
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Chickens:
Black feathers x white feathers
speckled
black and white
Blue roan
Red roan
Exceptions to Mendel’s Principles
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Multiple Alleles – genes that have more than
2 alleles for a specific trait
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Examples:
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Rabbit fur color has 4 different alleles
C = full color
Cch = chinchilla
Ch = himalayan
c = albino – no color
Rabbit Fur Colors
Exceptions to Mendel’s Principles
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Multiple Alleles continue:
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Examples: The alleles IA and IB are always expressed –
they are codominant. Both IA and IB are dominant to i.
Genotypes
Phenotypes
IA IA or IA i
A
IB IB or IB i
B
IA IB
AB
ii
O
Exceptions to Mendel’s Principles

Polygenic Traits – traits controlled by 2 or
more genes. The genes can be on the same
chromosome or on different chromosomes.
In polygenic inheritance, all heterozygotes
are intermediate in phenotype.
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Examples:
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Eye color in fruit flies – 3 genes
Skin color in humans – there are three or four
genes involved in the inheritance of skin color in
humans
Linkage
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Linkage: Some genes that are close on the
same chromosomes are less likely to be
separated during crossing-over.
i.e. they end up getting shuffled together
most of the time.
Example: red hair and
freckles.
Sex Linked
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Traits that can only be passed to males or
females.
The gene can be found on the X or Y
chromosome.
Example: color blindness
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Can be found on the X chromosome because
females have two, males only have one chance
for a “good” color vision gene.