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Mendelian Genetics
Mendel’s Law of Segregation
 Definition: The idea that, of the two copies of each
gene everyone carries, only one of the two alleles
(letters) gets put into each gamete
Each parent puts a single set of instructions for building a
particular trait into every sperm or egg it makes
You may get a “A” allele or “a” allele from each parent, for example
The trait observed in an individual depends on the two copies
(alleles) it inherits from both its parents
Punnett Square Practice
 Mom is heterozygous for pigmented skin, whereas
dad displays albinism. What are the genotypic ratios
and phenotypic ratios of the offspring?
Draw a punnett square in your notes
 Use the example above to explain Mendel’s law of
Mendel’s Law of Independent Assortment
 Mendel got lucky. Each of the 7 traits he studied in
pea plants were located on their own chromosomes.
Thus, they did not get inherited together!
In other words, pea flower color was not influenced by plant
height or pea color and vice versa.
 Definition: Neither trait influences the
inheritance pattern for the other trait; all traits are
inherited independently of each other
Law of Independent Assortment (continued)
 Is Mendel’s law of independent assortment always
Example: Most redheads have pale skin and freckles.
 Mendel’s 2nd law is not true for every pair of traits.
Sometimes the alleles for two genes are inherited
together and expressed as a package.
Linked Genes = when genes of different traits are close or
right next to each other on a chromosome and are often
inherited together
Example: Skin pigmentation and hair color
Dihybrid Cross
 Definition = a cross between individuals that
involves two pairs of contrasting traits
More complicated than “monohybrid crosses” because there
are more possible combinations of alleles to work out
Dihybrid Cross Example: A homozygous dominant purple
flowered, wrinkly seeded pea plant is crossed with a
heterozygous purple flowered pea plant that is also
heterozygous for smooth round seeds. Complete the cross.
What are the genotypic and phenotypic ratios?
It isn’t as simple as we thought…
 Unfortunately, the world in which each trait is coded
for by a single gene with two alleles- one completely
dominant and one recessive- and no environmental
effects at all doesn’t quite capture the complexity of
the world beyond Mendel’s pea plants.
Incomplete Dominance
 Definition = The phenotype of a heterozygote is
intermediate between the phenotypes of the two
 Flower color in four o’clocks shows incomplete
dominance, where both the allele for red flowers (R)
and the allele for white flowers (r) influence the
What would a heterozygote four o’clock flower look like?
 Definition = occurs when both alleles for a gene
are expressed in a heterozygous offspring
Neither allele masks the effect of the other
A mom blue fish (BB) is crossed with a dad red fish (RR). Fish
color displays codominance.
What color would the offspring be?
 What would their genotype be?
and which
What would
look like for
Transfusion Reaction
 Clumped blood cells clog arteries
 Hemolysis
 Lack of oxygen
 Organ damage or failure
 Death
Red Blood Cells
Blood Types
 Multiple allelism- when a single gene has more than
two alleles (letters)
An example of this is blood type in which there are 3 alleles
Each individual still carries only two alleles- one from mom
and one from dad
Blood Types
 Three alleles:
 A
 B
 O
 The A and B alleles are completely dominant to O,
but are codominant to each other.
Thus A and B are equally strong but overpower O, which is
Blood Types
 An individual’s blood type alleles carry instructions that
direct construction of antigens that protrude from
every red blood cell. Antigens are proteins that stick
out of a cell surface and play a role in defense.
 Phenotypes:
Type A instructs for A antigens to be made
Type B instructs for B antigens to be made
Type O does not instruct for any type of antigen
If a person has blood type AB, they make both A and B antigens on
their red blood cells
Blood Types
Blood Types
 If a red blood cell with the wrong antigens enter your
bloodstream, your immune system will attack it.
This can cause destruction of red blood cells, low
blood pressure, and even death. Under normal
conditions your immune system will not encounter a
red blood cell with foreign antigens.
When may this occur?
Blood Types
 This is why matching blood types is very important!
 Type A individuals would attack Type B blood cells
and vice versa.
 What about Type AB individuals?
 What about Type O individuals?
Blood Types
Polygenic Traits
 Definition = traits that are influenced by many
different genes
 Polygenic traits may have additive effects when the
effects of alleles from multiple genes all contribute to
the ultimate phenotype
Are there traits that are influenced by
both our genetics and our environment?
 Complex traits are influenced by both genetics
and the environment.
 Nature versus Nurture
 Give a few examples!
Wow, this is getting complex!
Sex-Linked Genes
 Sex-linked traits- traits caused by genes located on the sex
chromosomes (x or y)
 There are more X-linked than Y-linked traits.
What do you notice about the size of the X and Y chromosomes?
 X and Y chromosomes do not have all of the same genes.
 Since a male has an XY chromosome combination, if the
X chromosome carries a recessive allele for a trait, then
the male will show the recessive phenotype.
What??? Explain why!
Sex-Linked Genes
 Red-green colorblindness is a recessive X lined
Brain Games on National Geographic