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Concept 14.2: The laws of probability govern
Mendelian inheritance
• Mendel’s laws of segregation and independent
assortment reflect the rules of probability
• When tossing a coin, the outcome of one toss
has no impact on the outcome of the next toss
• In the same way, the alleles of one gene
segregate into gametes independently of
another gene’s alleles
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Probability Line
Probability Line
Fig. 14-9

Rr
Segregation of
alleles into eggs
Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
R
1/
2
r
R
R
Eggs
4
r
2
r
2
R
1/
1/
1/
1/
4
r
r
R
r
1/
4
1/
4
Parents
Normal
Aa

Normal
Aa
Sperm
A
a
A
AA
Normal
Aa
Normal
(carrier)
a
Aa
Normal
(carrier)
aa
Albino
Eggs
Fig. 14-17
Parents
Dwarf
Dd

Normal
dd
Sperm
D
d
d
Dd
Dwarf
dd
d
Dd
Dwarf
Eggs
Normal
dd
Normal
dominant disorder
Huntington’s Disease –dominant disorder
Huntington’s Disease
Effects of HD
Fig. 14-8
EXPERIMENT
YYRR
P Generation
yyrr
Gametes YR

F1 Generation
YyRr
Hypothesis of
dependent
assortment
Predictions
yr
Hypothesis of
independent
assortment
Sperm
or
Predicted
offspring of
F2 generation
1/
4
Sperm
1/ YR 1/
2
2 yr
1/
4
1/
2
YR
1/
4
1/
4
Yr
yR
1/
4
yr
YR
YYRR YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
YYRR
Eggs
1/
2
YyRr
1/
4
Yr
Eggs
yr
YyRr
3/
4
yyrr
1/
4
yR
1/
4
Phenotypic ratio 3:1
1/
4
yr
9/
16
3/
16
3/
16
1/
16
Phenotypic ratio 9:3:3:1
RESULTS
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
Concept 14.3: Inheritance patterns are often more
complex than predicted by simple Mendelian
genetics
• The relationship between genotype and
phenotype is rarely as simple as in the pea
plant characters Mendel studied
• Many heritable characters are not determined
by only one gene with two alleles
• However, the basic principles of segregation
and independent assortment apply even to
more complex patterns of inheritance
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Extending Mendelian Genetics for a Single Gene
• Inheritance of characters by a single gene may
deviate from simple Mendelian patterns in the
following situations:
– When alleles are not completely dominant or
recessive
– When a gene has more than two alleles
– When a gene produces multiple phenotypes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Degrees of Dominance
• Complete dominance occurs when phenotypes of the
heterozygote and dominant homozygote are identical
• Gene Linkage – Genes found on same chromosome and
stay together during meiosis (unless cross-over occurs)
• In incomplete dominance, the phenotype of F1 hybrids is
somewhere between the phenotypes of the two parental
varieties
• In codominance, two dominant alleles affect the
phenotype in separate, distinguishable ways
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Gene Linkage:
Drosophilia melanogaster –has
only 4 pairs of chromosomes.
Therefore must have large
# of genes on each.
Dr. Thomas Hunt Morgan (1910)
mapped these genes.
Genes stay together during
process of meiosis unless
crossing over occurs.
Chromosome #2 has genes controlling
This does not support Mendel’s
Eye color, wing type, body color, etc.
Crosses involving linked genes do not give
The same phenotypic ratio as unlinked.
Principle of Independent Assortment
Fig. 15-3
Morgan’s Choice of Experimental Organism
Several characteristics make fruit flies a
convenient organism for genetic studies:
They breed at a high rate
A generation can be bred every two weeks
They have only four pairs of chromosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Morgan’s Experimental Evidence: Scientific
Inquiry
The first solid evidence associating a specific
gene with a specific chromosome came from
Thomas Hunt Morgan, an embryologist
Morgan’s experiments with fruit flies provided
convincing evidence that chromosomes are the
location of Mendel’s heritable factors
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Degrees of Dominance
Complete dominance occurs when phenotypes of the
heterozygote and dominant homozygote are identical
Gene Linkage – Genes found on same chromosome and
stay together during meiosis (unless cross-over occurs)
In incomplete dominance, the phenotype of F1 hybrids is
somewhere between the phenotypes of the two parental
varieties
In codominance, two dominant alleles affect the phenotype
in separate, distinguishable ways
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Degrees of Dominance
Complete dominance occurs when phenotypes of the
heterozygote and dominant homozygote are identical
Gene Linkage – Genes found on same chromosome and
stay together during meiosis (unless cross-over occurs)
In incomplete dominance, the phenotype of F1 hybrids is
somewhere between the phenotypes of the two parental
varieties
In codominance, two dominant alleles affect the phenotype
in separate, distinguishable ways
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Degrees of Dominance
Complete dominance occurs when phenotypes of the
heterozygote and dominant homozygote are identical
Gene Linkage – Genes found on same chromosome and
stay together during meiosis (unless cross-over occurs)
In incomplete dominance, the phenotype of F1 hybrids is
somewhere between the phenotypes of the two parental
varieties
In codominance, two dominant alleles affect the phenotype
in separate, distinguishable ways
Multiple alleles – when more than 2 versions of a gene
(alleles) exist in a population. Ex. IA, IB, I (for blood type)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Codominance and Multiple alleles
Multiple Alleles
• Most genes exist in populations in more than
two allelic forms
• For example, the four phenotypes of the ABO
blood group in humans are determined by
three alleles for the enzyme (I) that attaches A
or B carbohydrates to red blood cells: IA, IB,
and i.
• The enzyme encoded by the IA allele adds the
A carbohydrate, whereas the enzyme encoded
by the IB allele adds the B carbohydrate; the
enzyme encoded by the i allele adds neither
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-11
Allele
IA
IB
Carbohydrate
A
B
i
none
(a) The three alleles for the ABO blood groups
and their associated carbohydrates
Genotype
Red blood cell
appearance
Phenotype
(blood group)
IAIA or IA i
A
IBIB or IB i
B
IAIB
AB
ii
O
(b) Blood group genotypes and phenotypes
•Rh factor is another protein (antigen) on the surface of a RBC
•Rh factor, like the blood types A, B, and O, is inherited from one's
parents.
•A simple blood test can determine blood type, including the
presence of the Rh factor.
•Having the factor is Rh positive (is considered dominant)
•Those without the Rh factor are Rh-negative (recessive)
•Negative blood types can not receive from positive blood types b/c
negative would not recognize the Rh antigen and would produce
Rh antibodies.
Polygenic Inheritance
• An additive effect of two or more genes on a
single phenotype
• Skin color in humans is an example of
polygenic inheritance
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings