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Mendelian Genetics
Chapter 14
Gregor Mendel
1850s, Mendel studied
inheritance in pea plants
by breeding them
Peas are good because
they have distinctive
traits, male and female
parts, and are easy to
manipulate
He crossed true breeding
with other true breeding
as well as with hybrids
Gregor Mendel
P = parental generation
F1 = First filial generation (P X P)
F2 = Second filial (F1 X F1)
True breeding (homozygous) = plants
that only breed to produce one
phenotype
Hybrid (heterozygous) = results of
crosses between two plants that breed
true for different phenotypes for the
same trait
Gregor Mendel
Genotype = genetic makeup of an
organism
– Sequence of the gene
Phenotype = expressed genotype
– Visible or measurable traits – based
upon genotype
Gregor Mendel
Gregor Mendel
He crossed a true breeding purpleflowered plant with a true breeding
white-flowered plant
All of the F1 generation was purple
Gregor Mendel
He then crossed two F1’s with each
other
The results (F2’s) were roughly 3
purples to 1 whites
White trait was
absent in F1’s but
reappeared in the
F2’s
Gregor Mendel
The reappearance of the white trait
meant that it was not altered in the F1,
just “hidden”
Mendel did the same crosses, and got
similar results, with other traits of the
pea plant
Gregor Mendel
 Four ideas spawn from Mendel’s
work:
1. Alleles account for different
traits
2. Organisms inherit two alleles,
one from each parent (may
be same or different)
3. If alleles are different,
then one is dominant over
the other
4. The two alleles segregate (law of
segregation) during gamete formation
Punnett Square
 Predicts the
statistical results
of a cross
Punnett Square
 A test cross, breeding a
homozygous recessive with a
dominant
phenotype, but
unknown
genotype, can
determine the
identity of the
unknown allele
Punnett Square
 One Factor Crosses = crosses with
only one trait
– Monohybrid crosses = crossing
one heterozygote with another
 Two Factor Crosses = crosses with
two traits
– Dihybrid crosses = crossing one
organism that is heterozygous for
both traits with another that is
heterozygous for both traits
Punnett Square
 In monohybrid crosses, he noticed
that genotypic and phenotypic ratios
could be predicted
– Genotypic ratio = 1:2:1
– Phenotypic ratio = 3:1
Punnett Square
 Example of a dihybrid cross:
– Crossing one plant that is
heterozygous for seed shape and
yellow seed coat (RrYy) with
another plant that is also
heterozygous for both traits (RrYy)
– RrYy X RrYy
 Dihybrid results point to the law of
independent assortment
Independent Assortment
 In our example (RrYy X RrYy), both
parents can produce four possible
gametes
– RY
– Ry
– rY
– Ry
 Since both parents have four
possible gametes, there are 16
possible combinations for fertilization
Independent Assortment
Independent Assortment
 These possible combinations
produce a genotypic ratio of
1:2:1:2:4:2:1:2:1
 It also produces a phenotypic ratio of
9:3:3:1
 The law of independent assortment
simply means that during gamete
formation, allelic pairs that code for
different traits assort independent of
each other
Dominance
 Mendel was very lucky to have
studied peas
– They follow simple dominance
laws
– Not all cells follow the simple laws
of dominance vs. recessiveness
Variations
 Incomplete dominance = neither allele
is shown in its dominant form; an
intermediate
phenotype is
shown
– Genotypic and
phenotypic
ratios are the
same (1:2:1)
Variations
 Codominance = heterozygote
expresses a phenotype that is distinct
from and not intermediate between
those of the two homozygotes
Ex. Human AB blood type
Variations
 Codominance
Variations
 Multiple alleles – gene exists as more
than two alleles in the population
– Rabbit coat color gene has 4 alleles:
C, c, cch & ch
– 5 phenotypes
– 10 genotypes
Variations
 Pleiotropy – one gene affects more
than one phenotypic characteristic
– Ex: sickle-cell disease affects
much more than just the overall
conformation of the hemoglobin
protein
Variations

Epistasis – one gene at one locus affects the
phenotype of another gene at another locus
– Ex: mice coat color depends on two genes
– One, the epistatic gene, determines
whether or not pigment will be deposited
in hair
• Presence (C) is dominant to absence (c)
– The second determines whether the
pigment to be deposited is black (B) or
brown (b)
– An individual that is cc has a white (albino)
coat regardless of the genotype of the
second gene
Variations – epistasis
 A cross between two black mice that
are heterozygous (BbCc) will follow the
law of independent assortment
 However, unlike the
9:3:3:1 offspring
ratio of an normal
Mendelian
experiment, the
ratio is 9 black, 3
brown, and 4 white
Variations
Polygenic – one phenotype determined
by the combined
effect of more than
one gene
– Ex: human skin
color, eye color,
height
Variations
Variations
Multifactorial Traits – determined by the
combined effect of one or more genes
plus the environment
– Ex: heart disease, body weight,
intelligence
Pedigrees
 Autosomal Recessive Traits:
– located on non-sex chromosomes
– affects males and females
– parents must be carriers or
affected
– affected individuals are
homozygous recessive
Ex: Albinism, Cystic fibrosis,
Phenylketonuria, Sickle cell disease
Autosomal Recessive
Pedigrees
 Autosomal Dominant Traits:
– located on non-sex chromosomes
– affects males and females
– at least one parent is affected
– does not skip generations
– affected individuals are
homozygous dominant or
heterozygous
Ex. Achondroplasia, Huntington disease,
Lactose intolerance, Polydactyly
Autosomal Dominant