Inheritance Patterns in
Mendelian Genetics
Lesson 6.4
14.3 – 14.4
A Single Gene
• Some characters controlled by a single gene
don’t necessarily behave in ways as cut and
dry as Mendel found
A Spectrum of Dominance
• Complete Dominance – completely dominant or
completely recessive phenotypes
– Either end of the spectrum
• Codominance- when both alleles affect the
phenotype in separate, distinguishable ways
– Ex: MN Blood group – MM individuals have only an M
group; NN individuals have only an N group, MN
individuals have BOTH M and N groups – not a mesh
of them, but separate groups
• Incomplete Dominance – when the phenotype of
heterozygous individuals is an intermediary of the
dominant and recessive phenotypes
– Ex: Red and white snapdragons breed and yield pink
snapdragons
Understanding Dominance
• Dominance does not mean that, in a
heterozygote, the recessive allele is inactive.
– Ex: Round (R) vs. wrinkled (r) seeds
• Starch/sugar production
– Ex: Tay Sachs disease
• Homozygous show symptoms
• Heterozygous are incomplete dominance, so although
some of the enzyme necessary is absent, there is
enough present that the child can live symptom-free
Multiple Alleles
• Having multiple options for the same gene
(more than 2)
• Blood type
• Pleiotropy – multiple phenotypic effects from
the same gene
– Ex: all the symptoms of Cystic Fibrosis are from a
pleiotropic gene
Multiple Genes
• Sometimes two or more genes are involved in
determining a particular phenotype
• Epistasis – a gene at one locus alters the
phenotypic expression of a gene at a second
locus
– Ex: mice fur color – BB/Bb – black bb – brown
• Second gene determines whether the color gene makes
it to the coat fur. CC/Cc sends the gene color to the fur,
cc means the fur will be albino.
Polygenic Inheritance
• Quantitative Characters - characters that are
not either/or, but are along a continuum
– Usually indicates polygenic inheritance
• An additive effect of 2 or more genes on a single
phenotypic character (opposite of pleiotropy – singe
gene affecting several characters)
Environmental Impact
• In humans, nutrition influences height,
exercise alters build, tanning darkens skin, etc.
– Think about identical twins and some of their
differences
• Most genotypes have a range of phenotypic
possibilities, norm of reaction
Studying Genetics
• Since it’s considered unethical to manipulate
human mating patterns, geneticists collect
data from many families to analyze matings
that have already occurred
• Pedigrees usually follow one trait through a
family
Extra Credit
• Create a pedigree for hair color going back as
far as you can. Include aunts, uncles and
cousins, if possible. If you need to go home
and ask your parents or look at old pictures
(although not too old, otherwise they’ll all be
black or white), that’s fine.
• Due Monday.