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Chapter 12
Patterns of Inheritance
Who is the Father of Modern Genetics?
Answer: Johann Gregor Mendel
Chapter 12: Inheritance
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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).
• Hybrid cross
�cross between parents true-
breeding for different trait(s)
�Black lab with a golden lab.
x
Stamen
(pollen)
Carpel(eggs)
1) Self-fertilization:
• Same flower providespollen and eggs
2) Cross-fertilization:
• Different flowers providepollen and eggs (by hand)
• True-breeding (homozygous)
Chapter 12: Inheritance
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P (parental)
Generation
F1 (filial 1)
Generation
F2 (filial 2)
Generation
Progeny (offspring)
Intercross or self
F1 offspring
Controlled genetic crosses
Inheritance of Single Traits:
• Mendel raised true-breeders
for different forms of a traitand then cross-fertilized the
forms:
• Mendel then allowed F1
to self-fertilize:
Chapter 12: Inheritance
Chapter 12: Inheritance
Modern Genetic Concepts:
Locus
Locus: Specific location of a gene ona chromosome
Alleles: Alternate forms of a nucleotide sequence for a single gene
Locus
Heterozygous (“different pair”)
• Homologous chromosomes havedifferent alleles for a gene
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Modern Genetic Concepts:
Locus
Locus: Specific location of a gene ona chromosome
Alleles: Alternate forms of a nucleotide sequence for a single gene
Heterozygous (“different pair”)
• Homologous chromosomes havedifferent alleles for a gene
Homozygous (“same pair”)
• Homologous chromosomes havesame allele for a gene
Locus
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
Chapter 12: Inheritance
purple
purple
One may be
dominant to
the other.
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
white
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Inheritance of Single Traits:
Phenotype:
• Outward expression of
a trait (appearance, behavior)
Genotype:
• Actual combination of alleles
carried by an individual
Chapter 12: Inheritance
B. The law of segregation
• Dominant trait - The trait exhibited by the F1 generation
• Recessive trait - the trait NOT seen in the F1 generation
xP
F1
Individual 1 = Pp
Individual 2 = Pp
Inheritance of Single Traits:
Chapter 12: Inheritance
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Punnett Square Method: (“Genetic Bookkeeping”)
Keep in Mind:Predicted proportionswill fluctuate in real life
Phenotype:3 purple : 1 white
Genotype:1 PP : 2 Pp : 1 pp
Chapter 12: Inheritance
Mendel’s hypothesis can be used to predict cross outcomes:
Crossing a dominate phenotype individual with a homozygous recessive individual to determine what the genotype of the dominate individual
Test Cross:
PP or Pp
pp
P P
p
p
Pp Pp
Pp Pp
P p
p
p
Pp
Pp
pp
pp
100% Purple 50% Purple / 50% White
Chapter 12: Inheritance
Inheritance of Multiple Traits:
• Mendel then allowed F1
to self-fertilize:
YySs
YYSS x yyss
YySs x YySs
• Mendel raised true-breeders for different forms of two traits
and then cross-fertilized the
forms:
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Inheritance of Multiple Traits:
• Resulting ratio of self-fertilization: 9 Yellow, smooth seeds3 Yellow, wrinkled seeds3 Green, smooth seeds1 Green, wrinkled seeds
9 : 3 : 3 : 1
Yy = Seed Color
Y y
S s
y
Y
Ss = Seed Texture
s
S
YY Yy
Yy yy
SS Ss
Ss ss
3/4 yellow
1/4 green
3/4 smooth
1/4 wrinkled
9/16 Yellow, smooth seeds3/16 Yellow, wrinkled seeds3/16 Green, smooth seeds1/16 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
Chapter 12: Inheritance
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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Punnett Square (Multiple Traits):
YySs x YySsSperm
Egg
YS
Ys
yS
ys
YS Ys yS ys
Chapter 12: Inheritance
Independent Assortmentof Alleles:
(Figure 12.7)
Punnett Square (Multiple Traits):
YySs x YySsSperm
Egg
YS
Ys
yS
ys
YS Ys yS ys
Chapter 12: Inheritance
YYSS
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Punnett Squarefor Two Traits:
(Figure 12.6)
9 Yellow, smooth seeds3 Yellow, wrinkled seeds3 Green, smooth seeds1 Green, wrinkled seeds
9 : 3 : 3 : 1
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
GeneticRecombination
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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 thansex 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 relationshipsexist for alleles on X chromosomes
• Males = Fully express alleles on X chromosome
Chapter 12: Inheritance
Cross:
female Bb x male B
A difference in phenotypic ratios
between male and female
progeny always indicates sex
linkage.
� i.e., more males have the disorder than
females.
XB Xb
XB
Y
XB XB
XB Y
XB Xb
XbY
X
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Example of Sex-linkedTrait:
(Figure 12.10)
Variations on the Mendelian Theme:
• Assumptions so far:
• Heterozygous phenotype is intermediate betweenhomozygous phenotypes
1) One allele is completely dominant over another allele
Fact: Alleles can display Incomplete Dominance:
Flower Color in Snapdragons(Figure 12.11)
Chapter 12: Inheritance
Variations on the Mendelian Theme:
• Assumptions so far:
• Most genes have > 10 alleles
• Alleles arise from mutation
2) There are only two possiblealleles for each gene
Fact: Genes can have multiple alleles:
Remember: An individual willhave only a maximum of twounique alleles for a gene(diploid)
AA or AO BB or BO
AB O
Codominance:
• Both phenotypes expressedequally in heterozygotes
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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
Chapter 12: Inheritance
Example of PolygenicInheritance:
(similar idea to Figure 12.12)
Additional Variations on the Mendelian Theme:
• Pleiotrophy: Single genes may have multiple phenotypic effects
• SRY gene: Codes for protein that activates otherproteins in male cascade
Fertilization Sex Determining Region of the Y
Gonad FormationHormones
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)
Additional Variations on the Mendelian Theme:
• The Environment influences Gene Expression:
• Temperature in Himalayan Rabbits
• Enzyme deactivated at high temperature
• Skin color (exposure)
• Height (nutrition)
• Intelligence (?)
Humans:
Chapter 12: Inheritance
Human Genetic Disorders:
How are Human Genetic Disorders Investigated?
Answer: Pedigrees
• Diagrams which show the genetic relationship amongrelated individuals
• Can reveal whether trait is dominant, recessive, orsex-linked
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Pedigrees:
(Figure 12.14)
Dominant Trait
Recessive Trait
Inheritance of Human Disorders (Single Gene):
1) Recessive Alleles:
• Disease if individual inherits two recessive alleles
• Carrier = Heterozygous individual (not affected)
Sickle-cell Anemia (defective hemoglobin)
Albinism (no pigmentation; pink eyes)
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:
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 ageat pregnancy
Chapter 12: Inheritance