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Non-Mendelian Genetics
Complex Inheritance
Focus Question:
What can we infer about genes and
traits based on different heredity
patterns?
Simple Inheritance Patterns
• Mendel’s experiments showed the inheritance
patterns of traits that were controlled by a single
gene with alleles that are either dominant OR
recessive with no other possibilities
Simple Dominance
In simple dominance, one form of a gene is completely
dominant and the other is completely recessive. A
heterozygous organism (hybrid) would have the dominant
phenotype.
Non-Mendelian Inheritance Patterns
• But MOST traits are more complicated and don’t
actually follow Mendel’s patterns of inheritance
• Non-Mendelian or Complex Inheritance
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Incomplete Dominance
Codominance
Multiple Alleles
Sex-Linked Traits
Polygenic Traits
Incomplete dominance
In incomplete dominance, the heterozygous
individual (hybrid) would have a blend of the two
traits. Neither allele is dominant.
Codominance
In codominance, the heterozygous organism
(hybrid) would express BOTH phenotypes
because both alleles are dominant
Multiple Alleles
• Often, more than two different forms of a gene
will be possible
• An individual will still only have two alleles
Codominance and Multiple Alleles
• Human blood types are controlled by a gene
that has 3 different alleles: IA, IB, and i
• IA and IB alleles are codominant
Sex-Linked Traits
• Controlled by genes found on the sex
chromosomes, usually X
• Since males and females have different sex
chromosomes, it affects how these traits are
inherited
• Remember- Males are XY and Females are XX
– Red-green color blindness
– Hemophilia
– Male pattern baldness
Sex-Linked Traits - Hemophilia
• Hemophilia is caused by the recessive allele of a
gene carried by the X chromosome only
• Females will have two alleles for the gene, but
males will only inherit one allele (from mom)
– XH = normal allele
– Xh = allele for hemophilia
– Y = no allele present
Problem: A healthy man marries a
woman who is a carrier for hemophilia.
What are the chances that they have a
child with the disease?
Polygenic Inheritance
• Most human traits are controlled by more that one
gene
• Individuals inherit two alleles for each gene
• The effects of these alleles combine to create the
incredible variety of traits that we see
– Height
– Skin color
Question 1• Explain the difference between an
organism’s phenotype and its genotype.
Include an example.
– The phenotype is the physical trait that is
exhibited. It is determined by the genotype
– The genotype is the actual genetic makeup, the
alleles that an organism has inherited
– A corn kernel that is heterozygous for color has
a genotype of Pp and a phenotype of purple.
Question #2
• Explain the difference between simple
dominance, incomplete dominance, and
codominance