Chapter 12
Patterns of Inheritance
Chapter 12: Inheritance
Who is the Father of Modern Genetics?
Answer:
Johann Gregor Mendel
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Definitions:
Character - a heritable feature
Trait - each variant for a character
Definitions:
• True/pure breeding
consistently gives rise to
offspring with same trait
when selfed (or bred with
same type).
x
• Hybrid cross
cross between parents truebreeding for different trait(s)
Black lab with a golden lab.
Chapter 12: Inheritance
1) Self-fertilization:
• Same flower provides
pollen and eggs
• True-breeding (homozygous)
2) Cross-fertilization:
• Different flowers provide
pollen and eggs (by hand)
Carpel
(eggs)
Stamen
(pollen)
2
Controlled genetic crosses
P (parental)
Generation
F1 (filial 1)
Generation
Progeny (offspring)
Intercross or self
F1 offspring
F2 (filial 2)
Generation
Chapter 12: Inheritance
Inheritance of Single Traits:
• Mendel raised true-breeders
for different forms of a trait
and then cross-fertilized the
forms:
• Mendel then allowed F1
to self-fertilize:
Chapter 12: Inheritance
Modern Genetic Concepts:
Locus: Specific location of a gene on
a chromosome
Locus
Locus
Alleles: Alternate forms of a nucleotide
sequence for a single gene
Heterozygous (“different pair”)
• Homologous chromosomes have
different alleles for a gene
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Chapter 12: Inheritance
Modern Genetic Concepts:
Locus: Specific location of a gene on
a chromosome
Locus
Alleles: Alternate forms of a nucleotide
sequence for a single gene
Locus
Heterozygous (“different pair”)
• Homologous chromosomes have
different alleles for a gene
Homozygous (“same pair”)
• Homologous chromosomes have
same allele for a gene
Chapter 12: Inheritance
Mendel’s Hypothesis:
1) Traits are determined by pairs of discrete physical units
• Physical units = genes
• One allele of each gene / homologous chromosome
2) Alleles of a gene segregate from one another during meiosis
• Law of Segregation
• Which allele enters which gamete occurs by chance
3) Differing alleles can mask one another if occupying same cell
• Dominant Allele: Allele expressed (observed)
• Recessive Allele: Allele mask (not observed)
4) True-breeding individuals have two of the same alleles
• Hybrids have two different alleles for the gene
Mendel’s model:
1. Hereditary units (genes) come in different forms
(alleles)
Gene = hair color
Alleles = blonde hair, red hair, black hair, brown hair.
2. Two hereditary units for each trait (per individual)
One from each parent
One may be
dominant to
the other.
purple
purple
white
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Chapter 12: Inheritance
Inheritance of Single Traits:
Phenotype:
• Outward expression of
a trait (appearance, behavior)
Genotype:
• Actual combination of alleles
carried by an individual
B. The law of segregation
• Dominant trait - The trait exhibited by the F1 generation
• Recessive trait - the trait NOT seen in the F1 generation
P
x
F1
Chapter 12: Inheritance
Inheritance of Single Traits:
Individual 1 = Pp
Individual 2 = Pp
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Chapter 12: Inheritance
Punnett Square Method: (“Genetic Bookkeeping”)
Phenotype:
3 purple : 1 white
Genotype:
1 PP : 2 Pp : 1 pp
Keep in Mind:
Predicted proportions
will fluctuate in real life
Chapter 12: Inheritance
Mendel’s hypothesis can be used to predict cross outcomes:
Test Cross:
Crossing a dominate phenotype individual with a homozygous
recessive individual to determine what the genotype of the
dominate individual
P
P
p
Pp
Pp
p
Pp
Pp
P
p
p
Pp
pp
p
Pp
pp
PP or Pp
100% Purple
pp
50% Purple / 50% White
Chapter 12: Inheritance
Inheritance of Multiple Traits:
• Mendel raised true-breeders
for different forms of two traits
and then cross-fertilized the
forms:
YYSS x yyss
YySs
• Mendel then allowed F1
to self-fertilize:
YySs x YySs
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Chapter 12: Inheritance
Inheritance of Multiple Traits:
• Resulting ratio of self-fertilization:
Y
Y
y
YY
Yy
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3
3
1
Yellow, smooth seeds
Yellow, wrinkled seeds
Green, smooth seeds
Green, wrinkled seeds
9:3:3:1
3/4 yellow
1/4 green
y
Yy
yy
Yy = Seed Color
Ss = Seed Texture
S
S
s
SS
Ss
3/4 smooth
1/4 wrinkled
s
Ss
ss
9/16 Yellow, smooth seeds
3/16 Yellow, wrinkled seeds
3/16 Green, smooth seeds
1/16 Green, wrinkled seeds
9:3:3:1
Chapter 12: Inheritance
Law of Independent Assortment:
• The alleles for one trait may be distributed to the gametes
independently of the alleles for other traits
• Occurs via random assortment of chromosomes
during Meiosis I
• Traits located on separate chromosomes
Law of independent assortment
Each pair of alleles segregates independently
of other allele pairs.
as long as they are
unlinked
i.e., not on the same
chromosome
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Chapter 12: Inheritance
Punnett Square (Multiple Traits):
YySs x YySs
Sperm
YS
Ys
yS
ys
YS
Egg
Ys
yS
ys
Independent
Assortment
of Alleles:
(Figure 12.7)
Chapter 12: Inheritance
Punnett Square (Multiple Traits):
YySs x YySs
Sperm
YS
Ys
yS
ys
YS YYSS
Egg
Ys
yS
ys
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Punnett Square
for Two Traits:
9
3
3
1
Yellow, smooth seeds
Yellow, wrinkled seeds
Green, smooth seeds
Green, wrinkled seeds
9:3:3:1
(Figure 12.6)
Chapter 12: Inheritance
Genes on the Same Chromosome Tend to be Inherited Together
• Linked Genes = Genes on same chromosome
Chapter 12: Inheritance
Linked genes may be separated
by crossing over events:
• Crossing Over = Exchange
of segments of homologous
chromosomes
• Occurs during metaphase
of meiosis I
Genetic
Recombination
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Chapter 12: Inheritance
Sex Chromosomes:
• Determine sex of individual:
• Humans = X and Y chromosomes
• Female = XX : Male = XY
• Birds/Reptiles = W and Z chromosomes
• Female = WZ : Male = ZZ
• Only one pair per species:
• Humans = 23 chromosome pairs (1 pair sex)
• Dogs = 39 chromosome pairs (1 pair sex)
• Autosomes = All other chromosome pairs other than
sex chromosomes (usually same size)
Chapter 12: Inheritance
Sex-linked Genes:
• Genes present on one sex chromosome but not the other
• Human Y chromosome ∼ 80 genes
• Human X chromosome ∼ 1500 genes
• Color vision genes
• Blood clotting factors
• Females = Normal dominant/recessive relationships
exist for alleles on X chromosomes
• Males = Fully express alleles on X chromosome
Cross:
female Bb x male B
XB
X
XB
A difference in phenotypic ratios
between male and female
progeny always indicates sex
linkage.
Y
Xb
XB XB
XB Xb
XB Y
Xb Y
i.e., more males have the disorder than
females.
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Example of Sex-linked
Trait:
(Figure 12.10)
Chapter 12: Inheritance
Variations on the Mendelian Theme:
• Assumptions so far:
1) One allele is completely
dominant over another allele
Fact: Alleles can display
Incomplete Dominance:
• Heterozygous phenotype is
intermediate between
homozygous phenotypes
Flower Color in Snapdragons
(Figure 12.11)
Chapter 12: Inheritance
Variations on the Mendelian Theme:
• Assumptions so far:
2) There are only two possible
alleles for each gene
Fact: Genes can have multiple
alleles:
AA or AO
BB or BO
AB
O
• Most genes have > 10 alleles
• Alleles arise from mutation
Codominance:
Remember: An individual will
have only a maximum of two
unique alleles for a gene(diploid)
• Both phenotypes expressed
equally in heterozygotes
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Chapter 12: Inheritance
Variations on the Mendelian Theme:
• Assumptions so far:
3) Each trait is completely controlled by a single gene
Fact: Many traits are influenced by several genes
• Polygenic Inheritance:
• Interaction of 2 or more genes contribute to a
single phenotype
• Skin Color = 3 or 4 genes
• Eye Color = 2 genes
Example of Polygenic
Inheritance:
(similar idea to Figure 12.12)
Chapter 12: Inheritance
Additional Variations on the Mendelian Theme:
• Pleiotrophy: Single genes may have multiple phenotypic
effects
• SRY gene: Codes for protein that activates other
proteins in male cascade
Fertilization
Hormones
Sex Determining Region of the Y
Gonad Formation
Sexual Differentiation of phenotype
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Pleiotropy
• Marfan’s syndrome
Pleiotropic trait that causes
Tall, lanky bodies
Detached retinas
Collapsed lungs
Aortic dissections
Nearsightedness (myopia)
Chapter 12: Inheritance
Additional Variations on the Mendelian Theme:
• The Environment influences Gene Expression:
• Temperature in Himalayan Rabbits
• Enzyme deactivated at high temperature
Humans:
• Skin color (exposure)
• Height (nutrition)
• Intelligence (?)
Chapter 12: Inheritance
Human Genetic Disorders:
How are Human Genetic Disorders Investigated?
Answer: Pedigrees
• Diagrams which show the genetic relationship among
related individuals
• Can reveal whether trait is dominant, recessive, or
sex-linked
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Pedigrees:
Recessive Trait
Dominant Trait
(Figure 12.14)
Chapter 12: Inheritance
Inheritance of Human Disorders (Single Gene):
1) Recessive Alleles:
• Disease if individual inherits two recessive alleles
• Carrier = Heterozygous individual (not affected)
Albinism (no pigmentation; pink eyes)
Sickle-cell Anemia (defective hemoglobin)
Chapter 12: Inheritance
Inheritance of Human Disorders (Single Gene):
2) Dominant Alleles:
• Potential Effects:
• Produce abnormal protein
• Carry out toxic reactions
• Produce “overactive” proteins
• Huntington Disease (deterioration of brain)
• Abnormal protein production
3) Sex-linked:
• Genes associated with sex chromosomes
• Frequently in males… often skip generations
• Color blindness (defective color receptors)
• Hemophilia (defective clotting proteins)
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Royal Families of Europe:
Chapter 12: Inheritance
Errors in Chromosome Number can Affect Humans:
• Nondisjunction: Homologous pairs don’t separate during
meiosis (affects gamete chromosome counts)
1) Sex Chromosomes:
• XO (Female) = Turner Syndrome (Infertile; lack 2°sexual char.)
• XXX (Female) = Trisomy X (Fertile; usually tall; potential ↓ IQ)
• XXY (Male) = Klinefelter Syndrome (Infertile; often unnoticed)
• XYY (Male) = Fertile; Excess testosterone (violent?); ↓ IQ)
Chapter 12: Inheritance
Errors in Chromosome Number can Affect Humans:
• Nondisjunction: Homologous pairs don’t separate during
meiosis (affects gamete chromosome counts)
2) Autosomal Chromosomes:
• Down Syndrome (Trisomy 21)
• Physical characteristics
• Mental retardation
• Heart malformations
• Frequency increase with age
at pregnancy
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