Patterns of
Inheritance
Inheritance Hypotheses
• Blending Hypothesis
– parental
contributions
combined
• Particulate
Hypothesis – parents
pass along discrete
heritable units
Some Important Vocabulary
• Allele= an alternative form of a gene (one
member of a pair) that is located at a
specific position on a specific chromosome.
• Homozygous = two identical alleles (Ex. tt,
TT)
• Heterozygous = two different alleles (Ex. Tt)
Genotype
– The two genes that an organism inherits
for a certain trait (Example Tt, TT, or tt)
• Phenotype
– Physical characteristics (example: tall
and short)
Analyzing Patterns
• Genetic Cross –
controlled
experiment to
determine the
inheritance of a
trait
– P – parental
generation
– F1 – first generation
– F2 – second
generation
P
F1
F2
Review Meiosis
• Of your father’s
46 chromosomes
how many do you
receive?
• Of your mother’s
46 chromosomes
how many do you
receive?
• So for each allele
you share __(#)
copy from mom
and ___(#) copy
from dad.
Y
y
Homologous
chromosomes
• If this is one of
your
chromosomes
what two
possible alleles
would you
have at this
locus to pass
on to your
offspring?
General Patterns of Inheritance
• Alternative versions of genes cause
variation
• Offspring inherit one copy from
each parent
• Dominant alleles are expressed in
phenotype
• Recessive alleles are expressed in
phenotype in the absences of
dominate alleles
• Alleles for genes separate during
meiosis
• Gametes fuse randomly
Mendel’s Laws
• Father of modern genetics
• Researched pea plants
• Developed ideas of
dominance and trait
segregation
–
–
–
–
–
Allelic Interactions
Pleiotropy
Epistasis
Environment
Polygenics
Mendel's Law of Segregation
• Each allele separates from
the other so that the
offspring get only one allele
from each parent for a given
trait.
• Let’s cross a heterozygous
tall plant (Tt) with a short
plant (tt).
• Each plant will give only one
of its’ two genes to the
offspring or F1 generation.
Plant 2 (short)
Plant 1 (tall)
T
t
t
T
X
Possible zygotes after meiosis
T
T
t
t
t
t
T
T
Law of Segregation and Punnett Squares
Plant 2 (short)
Plant 1 (tall)
T
t
t
T
X
Possible zygotes after meiosis
T
T
t
t
t
t
T
T
Tt
TT
tt
Tt
Developed ideas of dominance and
trait segregation
• Allelic Interactions
– Complete dominance
– Incomplete dominance
– Co-dominance
•
•
•
•
Pleiotropy
Epistasis
Environment
Polygenics
Allelic Interactions
• Complete dominance:
dominant allele fully
expressed
• Incomplete dominance:
neither allele fully
expressed
• Co-dominance: both
alleles fully expressed
Allelic Interactions: Complete Dominance
– The dominant allele is
the only one seen in the
phenotype
– Dominant vs. Recessive
Gene
– Dominant Gene: A gene
that always expresses
itself. It is symbolized by
a CAPITAL letter
– Recessive Gene: a gene
that expresses itself only
when a dominant form
of the gene is NOT
present. It is symbolized
by a lower case letter
– Monohybrid cross is
looking at 1 gene
– Dihybrid cross is looking
at 2 genes
Monohybrid Cross
X
Allelic Interactions: Complete Dominance
• Practice Problem:
– Let's say that in seals, the gene for the length of the whiskers has two
alleles. The dominant allele (W) codes long whiskers & the recessive
allele (w) codes for short whiskers.
a) What percentage of offspring would be expected to have short
whiskers from the cross of two long-whiskered seals, one that is
homozygous dominant and one that is heterozygous?
b) If one parent seal is homozygous long-whiskered and the other is
short-whiskered, what percent of offspring would have short whiskers?
c) Is this a mono- or dihybrid cross?
Complete Dominance Dihybrid Cross
What are the possible gametes from each of
these pea plants?
YyRr
YyRr
X
Y=yellow
y=green
R=round
r=wrinkly
Allelic Interactions: Incomplete Dominance
• Neither allele fully
expressed, they are mixed
• The phenotype of the
heterozygote will be
intermediate between
the phenotypes of the
two homozygotes
• Example:
RR = Red snapdragon flower
Rr = pink snapdragon flower
rr = white snapdragon flower
Allelic Interactions: Incomplete Dominance
• Practice Problem: Cross a red
snapdragon with a pink snapdragon
flower. What will the genotype and
phenotype % be for the offspring.
Co-Dominance
• If two alleles have a co dominant
relationship, in the heterozygote both
alleles will be completely expressed.
• Example: blood type
Example: Blood Types
Co-Dominance
What would happen if you crossed a man with AB blood type with a
women with O blood type?
(a) Give the genotypes and phenotypes for the offspring.
(b) How many of the offspring would have AO? _____%
(c) How many of the offspring would have BO? _____%
(d) How many of the offspring would have AB? _____%
(e) How many of the offspring would have O? _____%
Sex Linked (X-Linked) Traits
• The genes that are located
on the chromosomes are
called sex-linked traits
• Many traits that are carried
on the X chromosome do
not have a corresponding
spot on the Y chromosome.
This causes for some unique
possibilities for the
offspring.
• Ex. Color blindness in
humans, fruit fly eye color,
hemophilia.
Analyzing Patterns
• Pedigree – illustration of relationships among
family members over multiple generations
Inheritance and Genetic Disorders
• Hereditary disorders
– Autosomal or sex-linked
– Recessive or dominant
• Genetic disorders are
often polygenic and
influenced by
environment
Autosomal Recessive Disorders
• Carriers –
heterozygotes that
have one allele but not
the disorder
– Examples – Cystic
Fibrosis, Tay Sachs
Autosomal Dominant Disorders
• Disorders that are expressed
with only one copy of the
allele
– Example – polydactyly
– Example – Huntington’s
Sex-linked Recessive Disorders
• Disorders that are on the sex chromosome
– Example – Hemophilia
Sex-linked Dominant Disorders
Pleiotropy
• Condition where a single gene influences
multiple traits
– Example: Marfan’s Syndrome
Polygenics
• Traits that are
determined by many
genes
– Examples – eye color,
skin color, height, etc.
Environment
• Phenotype = Genotype
+ Environment
• Epigenetics – study of
changes in phenotype
due to mechanisms
other than changes in
DNA sequence
Epigenetics