1
Chapter 12
Patterns of InheritanceWho is the Father of Modern Genetics?
Answer: Johann Gregor Mendel
Chapter 12: Inheritance
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
2
P (parental)
Generation
F1 (filial 1)
Generation
F2 (filial 2)
Generation
Progeny (offspring)
Intercross or self
F1 offspring
Controlled genetic crossesInheritance 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
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
3
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
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:
Chapter 12: Inheritance
4
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
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
5
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
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 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
6
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
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
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
Chapter 12: Inheritance
7
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
Chapter 12: InheritancePedigrees:
(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)
Chapter 12: Inheritance
8
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