Chapter 11:Introduction to GeneticsSection 11-3: Other Patterns of Inheritance

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Beyond Dominant and Recessive Alleles

• There are exceptions to every rule

• In most organisms, genetics is more complicated than Mendelian rules because most genes have more than two alleles, and many important traits are controlled by more than one gene

• Mendel’s principles alone cannot predict these traits

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Incomplete Dominance• A cross between two

four o’clock plants shows a common exception• The F1 generation

produced by a cross between red-flowered (RR) and white-flowered (WW) plants consists of pink-colored flowers (RW) 3

Incomplete Dominance• In this case, neither

allele is dominant• Called incomplete

dominance - the heterozygous phenotype lies somewhere between the two homozygous phenotypes

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Codominance• Codominance occurs when

the phenotypes o both alleles are clearly expressed

• In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers

• Heterozygous chickens have a color described as “erminette,” speckled with black and white feathers 5

Multiple Alleles • A gene with more

than two alleles is said to have multiple alleles

• Many genes have multiple alleles, including the human genes for blood type.

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Multiple Alleles• Type A, Type B, Type AB, Type O• Three alleles: IA, IB, i

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Blood Type Possible GenotypesType A IA IA or IA i

Type B IB IB or IB i

Type AB IA IB (also shows codominance)

Type O ii

Polygenic Traits• Traits controlled by two

or more genes are said to be polygenic traits

• Often show a wide range of phenotypes

• Human traits like skin color, facial appearance, and height are polygenic 8

Genes and Environment

• The characteristics of any organism are not determined solely by the genes that organism inherits – the expression of genes also depends on the environment

• Let’s look at the Western white butterfly

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Genes and Environment• Environment in which the butterflies develop influences the

expression of their genes for wing coloration• Studies revealed that those hatching in springtime had greater

levels of pigment in their wings than those hatching in the summer, causing them to have different phenotypes

• It is related to flight – the body temperature of the butterfly needs to be in a certain range to fly effectively

• More pigment means that the butterflies can reach the optimal body temperature faster (spring)

• Less pigment means the butterflies will not overheat (summer)

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