Patterns of Inheritance
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Gregor Mendel
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Austrian monk
In 1860, developed laws of
heredity
Performed crosses
between garden pea plants
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He concluded that plants
transmitted distinct
factors (genes) to
offspring
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Genes are found on
chromosomes
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Chromosomes come in
pairs called homologous
chromosomes
 Same length
 Same centromere
location
 Same gene location
(locus)

May have different forms
of the gene (allele)
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Letters on homologous chromosomes stand for alleles
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Ex. H = widow’s peak h = straight hairline
Capital letters represent dominant traits
Lowercase letters represent recessive traits
Based on his studies, Mendel came up with the law of
segregation:
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2.
3.
4.
Each individual has two factors (genes) for each trait
The factors segregate (separate) during the formation of the
gametes.
Each gamete contains only 1 factor (gene) from each pair of
factors (genes).
Fertilization results in each new individual having 2 factors
(genes) for each trait.
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Phenotype: describes what the person actually looks like
 Ex. Right handed or left handed
 Purple petals or white petals
Genotype: refers to the alleles the chromosomes carry
that are responsible for that trait
 Ex. H = right hand, h = left hand
Dominant alleles will mask the expression of the
recessive allele when they are together.
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Homozygous: two alleles in the pair are the same
 Ex. AA = homozygous dominant
 Ex. aa = homozygous recessive
Heterozygous: two alleles in the pair are different
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Ex. Aa
The gametes only have one allele for each trait.
One-trait Crosses
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To see the probability of
outcomes of crossing two
parents, a Punnett square is
used.
Monohybrid crosses:
crossing of parent genotypes
for 1 trait
Make a key to indicate parent
genotypes
Combine all possible
gametes using a Punnett
square
Determine genotypes &
phenotypes of all offspring
Write results as a ratio,
fraction, or percent
The One-Trait Testcross
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Testcross:
the cross of a known homozygous recessive
with an unknown genotype to see if the
offspring have any dominant traits
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Example: Both you and your brother are left-handed, but
your parents are both right-handed. Righthandedness (H) is dominant over left-handedness
(h). What are the genotypes of your parents?
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Example: My husband and my two kids are left-handed, but I
am right-handed. What is my genotype?
The Inheritance of Two Traits
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Mendel’s law of independent assortment:
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Each pair of factors (genes) separates independently (without
regard to how the others separate).
All possible combinations of factors can occur in the gametes.
Dihybrid crosses: two different traits crossed together
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A 16 square Punnett square is used
Example: Cross a father who is heterozygous for dimpled
cheeks and a cleft chin with a mother who is the same.
D= dimples
d= no dimples
R= cleft chin
r= no cleft chin
Pedigrees
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A pedigree is a chart of a
family’s history with regard to a
particular genetic trait.
 Males = squares
 Females = circles
 A line between the two
represents mating
 A vertical lines branching
off represents offspring
 Affected individuals are
shaded
 Carriers are half shaded
Incomplete Dominance
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Incomplete dominance:
heterozygote is a mixture
of the two alleles
because neither allele
dominates.
Example: Red four o’clock plants mixed with white four
o’clock plants produce pink offspring. Cross
a red four o’clock plant with a white four
o’clock plant.
R1R1= red
R2R2= white R1R2= pink
Codominance
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Both alleles show up in the
heterozygote
 Roan cattle are brown and
white spotted cows
 The allele for brown coat
color is equally dominant to
white coat color
 Example:
R1R1= brown coat
R2R2= white coat
R1R2= brown & white
spots (roan)
 Example:
AB blood type
Multiple Alleles
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There are several alleles for the same gene that control
the inheritance of the trait.
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A person can only have two of the possible alleles for the trait
ABO blood types are an example
IA= A antigen on red blood cells
IB= B antigen on red blood cells
i= neither A or B antigen on red blood cells
What are the possible genotypes for a person with A type,
B type, AB type, and O type blood?
Is it possible for a man who is type A blood and a woman
who is type B blood to have a child who is type O blood?
Sex-Linked Inheritance
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Traits controlled by genes on the
sex chromosomes
 Male sex chromosomes= XY
 Female sex chromosomes=
XX
X-linked genes are alleles found
on the X chromosome
 Most sex-linked genes are on
X chromosome
 The male Y chromosome is
mostly blank for these
diseases
 Examples of sex-linked
diseases: colorblindness,
hemophilia, Duchenne’s
Muscular Dystrophy
Polygenic Inheritance
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Two or more sets of alleles determine the trait
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Each dominant allele codes for a product
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The more dominant alleles the “stronger” the phenotype
Results in a variety of phenotypes
Examples: skin color, height, weight, eye color
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Aa and Bb alleles coding for skin color
AABB= very dark skin
AaBb= medium color skin
aabb= very light skin
Environmental factors, such as temperature and
nutrition, can affect phenotypes.
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Examples: height and nutrition, Himalayan rabbits & temperature
Inheritance of Linked Genes
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All the alleles on one chromosome form a linkage group
because they tend to be inherited together.
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Example: red hair and freckles, blond hair and blue eyes
Genes that are farther apart on the chromosome are less
likely to be linked because they separate more often in
crossing over.