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Lecture Outline
I.
Historical Overview: Hypotheses to Explain How Traits Are Transmitted from Parent to
Offspring
A.
B.
II.
Blending Hypothesis—favored by the botanist Carl Nägeli, 1800s, mentor of Gregor
Mendel
1. Hereditary determinants from two parents blend together in their offspring.
2. Example—a white sheep mates with a black sheep, producing gray offspring.
Inheritance of Acquired Characteristics—Proposed by Lamarck (1809; see Chapter
21) and favored by Charles Darwin.
1. Traits present in the parents are modified through use and passed on to
offspring.
2. Example—Adult giraffes strain their necks to reach food in trees, necks become
longer, and their offspring inherit the longer neck trait.
Rules of Inheritance: The Experiments of Gregor Mendel (1865)
A.
Mendel selected an appropriate model organism, the garden pea, which has many
useful qualities. (Fig. 13.1a)
1. It is cheap and readily available due to use in agricultural practices.
2. Peas are easy to propagate with a short reproductive cycle.
3. Mating can be controlled by manual pollination. (Fig. 13.1b)
4. Peas had multiple traits that exhibited one of two phenotypes. (Fig. 13.2)
5. True-breeding lines could be obtained.
B.
Mendel’s monohybrid crosses provide evidence for the particulate inheritance
hypothesis.
1. When two true-breeding pea plants (round vs. wrinkled seeds) are crossed, all
of the offspring are round seeded. (Fig. 13.3)
2. Reciprocal crosses demonstrated that the inheritance of seed shape is not
affected by whether the round trait is donated by the male or female parent.
(Fig. 13.3)
3. Self-fertilization of the hybrids (F1 generation) illustrated that they did “carry”
the wrinkled trait, inherited though their seeds appeared round.
a. The trait that was visible in the hybrids is called “dominant.”
b. The trait that reappeared in the F2 generation is called “recessive.”
4. Mendel repeated these experiments on pea plants following six other traits and
found the same results.
5. These data do not support the blending hypothesis.
C.
From this data, Mendel developed the particulate inheritance hypothesis:
(Fig. 13.5)
1. Genetic determinants (genes) act like discrete particles.
2. Genetic determinants maintain their integrity from generation to generation:
a. Do not become blended
b. Do not acquire new or modified characteristics through use
3. Each individual has two versions, or alleles, of each gene.
4.
5.
6.
All the alleles present in one individual = genotype.
Genotype influences the expression traits (phenotype) in the individual.
Allele hypothesis explains the framework for dominance/recessiveness:
a. RR = round seeded, Rr = round seeded, rr = wrinkled seeded
b. Phenotype depends on which two alleles of the gene are paired in the
individual.
D.
Mendel proposes the principle of segregation to explain particulate inheritance.
1. The alleles of a gene segregate to different gamete cells during egg and sperm
formation. (Fig. 13.4a)
2. Each gamete receives one allele of each gene (= haploid state).
3. Fertilization restores the diploid state—Fusion of egg and sperm brings
together two alleles of each gene. (Fig. 13.4b)
a. Homozygous—individual with two copies of the same allele for a
particular gene
b. Heterozygous—individual with two different alleles of a particular gene
E.
R. C. Punnett developed a technique for predicting genotypes and phenotypes from
a given cross.
1. A Punnett square is a diagram that displays gamete genotypes from which all
possible offspring genotypes can be predicted.
2. Rules of probability explain why Punnett squares work (Box 13.1)
a. “Both-and” rule (multiplication rule) used to determine the probability
that two or more independent events will occur together. (Probability that
one event will occur  probability the other event will occur.)
b. “Either-or” rule (addition rule) used to determine the probability of an
event occurring, when it can occur in several different ways.
(Sum of the probabilities of each way an event can occur.)
F.
Testing Mendel’s Particulate Inheritance Model
1. If traits are inherited as units, the phenotypes of the offspring should indicate
the genotype of the parents.
2. Experiment—Allow the F2 generation to self-fertilize.
3. Results and conclusions:
a. Only wrinkled seeds were produced by self-fertilization of wrinkledseeded F2 plants.
b. Genotypes of the individuals with the dominant trait can be inferred from
phenotypes of the offspring.
(1) Parents that produced all round seeds were RR.
(2) Parents that produced a 3:1 ratio of round and wrinkled seeds were
Rr.
G.
Does the principle of segregation hold true if parents differ in two or more traits (a
dihybrid cross)?
1. Mendel generated a dihybrid (RrYy) by crossing a pure line of round, yellowseeded parents with a pure line of wrinkled, green-seeded parents (RRYY 
rryy) (Fig. 13.6a)
a. Selfing the F1 generation produced phenotypes in the ratio of 9 round
yellow to 3 round green to 3 wrinkled yellow to 1 wrinkled green.
b.
2.
Considering the two traits separately, selfing of the F1 generation produced
a 3:1 ratio for round:wrinkled and for yellow:green.
c. The “Both-and” rule explains the 9 round, yellow individuals—Overall
probability of R and Y together = 3/4  3/4 = 9/16.
Conclusion: Probability theory accurately predicts the phenotypic ratios that
were seen in crossing pea plants that differed in two traits, thus the data
support the principle of segregation for two traits.
H. Mendel developed the principle of independent assortment to explain phenotypic
ratios in his dihybrid cross.
1. In his dihybrid experiment, Mendel noticed that the alleles for the two different
traits seemed to segregate independent of one another. (Fig. 13.6b)
2. Therefore, he predicted that the R gamete will be obtained from an Rr parent
1/2 of the time, and the Y gamete will be obtained from a Yy parent 1/2 of the
time. Neither is dependent on the other. (Fig. 13.6c)
3. To test the hypothesis, Mendel performed a testcross
a. A testcross determines the type of gametes a parent produces by crossing a
parent with the dominant phenotype to a parent that is recessive for the
same trait(s). (Fig. 13.8)
b. If the R and Y alleles assort independently . . . (Fig. 13.6a)
(1) then ♀ RrYy  ♂ rryy predicts four types of gametes (RY, Ry, rY, and ry)
from the female parent, and one type of gamete (ry) from the male parent.
(2) and thus predicts four types of offspring RrYy, Rryy, rrYy, and rryy (in
a 1:1:1:1 ratio).
c. If the R and Y genes assort dependently . . . (Fig. 13.6b)
(1) then ♀ RrYy  ♂ rryy predicts two types of gametes (RY, ry) from the
female and one type of gamete from the male (ry).
(2) and thus predicts two types of offspring RrYy and rryy.
d. Results: Round yellow (31), round green (26), wrinkled yellow (27), and
wrinkled green (26). Ratio ≈ 1:1:1:1.
e. Conclusion: The data support the principle that alleles for different genes
assorted independently of one another.
I.
Mendel’s Contributions to the Process for Conducting Heredity Studies
1. Importance of selecting an appropriate model organism
2. Use of true-breeding lines with discrete trait differences
3. Testing large sample sizes reduces chance interactions, makes data easier to
interpret. (Box. 13.2, Fig. 13.7)
4. Application of the rules of probability to predict the number and types of
offspring
5. Development of a simple system for notation of genes and phenotypes
J.
Mendel identified the two fundamental patterns of transmission genetics.
1. Segregation of alleles into separate gametes
2. Independent assortment of alleles that control different traits
III. The Chromosome Theory of Inheritance
A.
Sutton and Boveri (1903) independently developed the chromosome theory of
inheritance, based on Mendel’s work. (Fig. 13.9a)
1. Pattern component—Mendel’s patterns of inheritance
2. Process component—Mendel’s rules can be explained by the independent
assortment of homologous chromosomes at meiosis I. (Fig. 13.9b)
B.
Testing and Extending Chromosome Theory—Thomas Hunt Morgan
1. Used Drosophila as a model organism (Fig. 13.10a)
2. Discovered white-eyed Drosophila male in the red-eyed wild-type population.
(Fig. 13.10b)
a. Inferred the white-eyed phenotype was due to a mutation.
b. Mated white-eyed male to red-eyed female:
(1) All F1 flies had red eyes.
(2) Conclusion: The white-eyed phenotype is recessive.
c. Mated F1 males to F1 females:
(1) F2 population 3:1 red-eyed:white-eyed flies.
(2) Unusual result: All the white-eyed flies were male.
3. Morgan et al. tested the association of eye color and sex:
a. Mated F1 red-eyed females to white-eyed males—obtained some female
offspring with white eyes.
b. Reciprocal cross—mated white-eyed females to red-eyed males:
(1) All the F1 female flies were red eyed.
(2) All the F1 males were white eyed.
c. This data indicated that there was some linkage between gender and eye
color.
4. N. Stevens discovered sex chromosomes by studying karyotypes of the beetle
Tenebrio.
a. 2n females have 20 large chromosomes; 2n males have 19 large and one
small chromosome (denoted the “Y” chromosome). (Fig. 13.11a)
b. All eggs have 10 large chromosomes, but 50% of sperm have 10 large
chromosomes and the other 50% have 9 large and 1 small Y chromosome.
(Fig. 13.11b)
5. Morgan confirmed that the transmission pattern of X and Y chromosomes
could account for the white-eyed phenotype.
a. Proposed the gene for eye color is on the X chromosome in Drosophila. (Fig.
13.12)
b. Sex-linked inheritance
(1) Some traits are transmitted by X-linked inheritance patterns because
the gene is located on the X chromosome.
(2) The shortened Y chromosome does not carry alleles for these genes, so
the phenotype encoded by the X chromosome is observed.
c. Inheritance of genes located on non-sex chromosomes is called autosomal
inheritance.
d. Morgan’s data confirmed the chromosomal theory of inheritance, because
it indicated that chromosomes carried genes that governed certain traits.
IV. Linkage: The Physical Association of Genes on the Same Chromosome
A.
Later experiments confirmed that genes are physical components of chromosomes.
1. Genes located on the same chromosome will not assort independently of one
another.
2. If this is true, then . . .
a. linked genes should always be inherited together during gamete
formation.
b. the F1 heterozygotes should produce only two classes of gametes.
3. In Drosophila eye color and body color do not assort independently, because
they are both X-linked. (Fig. 13.13)
a. Wild-type female—red eyes, gray body (w+ y+/ w+y+).
b. Predicted F1 genotypes and phenotypes for a cross between white-eyed,
gray-bodied (wy+/wy+) ♀  red-eyed, yellow-bodied (w+y/ Y) ♂
(1) All females, wy+/w+y, red eyes, gray bodies.
(2) All males, wy+/Y, white eyes, gray bodies.
c. Predicted F2 genotypes and phenotypes for a cross between red-eyed,
gray-bodied (wy+/w+y) ♀  white eyed, gray bodied (wy+/Y) ♂
(1) Number of white-eyed, gray-bodied (wy+/wy+) ♀ = number of redeyed, gray-bodied (w+y/wy+) ♀.
(2) Number of white-eyed, gray-bodied ♂ (wy+/ Y) = number of red-eyed,
yellow-bodied ♂ (w+y/ Y).
d. Actual results of the F2 generation:
(1) Four different phenotypes were seen in the male when only two were
expected.
(2) Recombinant phenotypes resulted from crossover in the female,
between the wy+ alleles and the w+y alleles, to form recombinant w+y+
and wy alleles. (Fig. 13.14)
B.
Morgan’s Hypothesis
1. Genetic loci are arranged in a linear array on each chromosome.
2. The physical distance between two loci dictates the frequency of crossover
between the loci. The smaller the distance, the less chance of crossover.
(Fig. 13.15)
C.
Linkage Mapping—A. H. Sturtevant (1911)
1. Proposed that recombination frequency can be used to map distances between
genetic loci
a. Recombination frequency indicates how often crossing over occurs
between two linked genes.
b. Recombination frequency = # of recombinant offspring/total # of
offspring.
c. 1 unit of distance = frequency of recombination between two loci.
2. Example—frequency of recombinants of three genes on X chromosome:
(Fig. 13.16a)
a. 1.4% between eye-color and body-color genes
b. 20% between body-color and cut-wing genes
c. 18.6% between eye-color and cut-wing genes
3. Conclusions (Fig. 13.16b)
a. Body-color and cut-wing loci are 20 map units apart.
b.
4.
V.
Eye-color locus is between body-color and cut-wing loci, 1.4 map units
from body-color locus, 18.6 map units from cut-wing locus.
Exceptions: Can linked genes assort independently?
a. When two genes are far apart on a chromosome, crossing over will always
separate them.
b. These genes will appear to assort independently because their
recombination frequency will be 50%.
Extending Mendel’s Rules: Traits That Do Not Produce Expected Phenotype Frequency
Patterns (Table 13.2)
A.
Incomplete Dominance and Codominance
1. Heterozygotes exhibit a phenotype intermediate between those of the parental
generation.
2. Incomplete dominance—four o’clock flowers (Fig. 13.17)
a. Cross between pure lines of red-flowered plants and white-flowered
plants; RR  rr.
b. F1 all produce pink flowers, genotype Rr.
c. Self-fertilize the F1 plants—Obtain 1/4 red RR, 1/2 pink Rr, and 1/4 white
rr.
3. Codominance—M/N blood gene in humans
a. When alleles are codominant, both alleles are expressed in heterozygotes.
b. Three genotypes: MM, MN, NN
c. Phenotypes
(1) MM individuals have only M-type proteins on red blood cells.
(2) MN individuals have both M-type and N-type proteins on red blood
cells.
(3) NN individuals have only N-type proteins on red blood cells.
4. The molecular biology of the various dominance relationships
a. Genes encode for protein, and different alleles encode for different forms
of the same protein.
b. Codominance results when two different forms of a membrane protein are
expressed in a single cell (MN proteins on RBCs).
c. Complete and incomplete dominance usually involve alleles that encode
for enzymes.
(1) Enzyme function is dependent on amino acid sequence.
(2) Usually one allele for an enzyme makes a functional protein, and the
other allele makes a dysfunctional protein.
(3) Complete dominance will occur in heterozygotes for traits in which
half the amount of functional enzyme is enough to generate the full
dominant phenotype (growth hormone production and plant height in
peas).
(4) Incomplete dominance will occur in heterozygotes when half the
amount of functional enzyme on produces half the dominant
phenotype (red pigment production in four o’clock flowers).
B.
Multiple Allelism—More Than Two Alleles of the Same Gene
1.
2.
Polymorphism—more than 2 phenotypes, due to multiple combinations of
multiple alleles.
Example—ABO blood group is an example of multiple allelism.
a. Blood-type differences result from variation in carbohydrates attached to
lipids in the red blood cell membrane.
b. Three alleles—i (recessive); IA and IB (codominant).
c. Each individual can only have two of the three alleles.
d. Six possible diploid combinations, leading to four different phenotypes:
(Table 13.1)
(1) Type O blood (ii)
(2) Type A blood (IAIA) or (iIA)
(3) Type B blood (IBIB) or (iIB)
(4) Type AB blood (IAIB)
C.
Pleiotropy
1. When a single gene affects more than one trait.
2. Example: Marfan syndrome in humans
a. Caused by a mutation (recessive allele) in a single gene
b. That single mutated gene has many phenotypic effects (increased height,
long limbs and fingers, backbone problems, etc.).
D.
Phenotype is affected by the genetic and/or physical environment in which it is
expressed.
1. Phenotype can be affected by the environment in which a gene is expressed.
a. Example—Phenylketonuria (PKU), a genetic mutation, causes lack of
enzyme that breaks down phenylalanine.
(1) Phenylalanine and a related molecule accumulate in the body.
(2) Symptoms—mental retardation, impaired nervous system
development.
(3) Prevention—Place individual on a low-phenylalanine diet; no
symptoms occur.
b. Conclusion: The PKU phenotype can be blocked by altering the physical
environment.
2. Epistasis: When the expression of one gene is affected by the expression of
other genes.
a. How is fruit color inherited in bell peppers? (Fig. 13.18a)
(1) Experiment—Cross pure-line, brown-fruited parent with a pure-line,
yellow-fruited parent, then allow F1 progeny to interbreed.
(2) F2 progeny expressed four phenotypes with ratios that were in
sixteenths, suggesting that two genes interact to determine fruit color.
(3) Inferences based on F2 results: (Fig. 13.18b)
(a) Red color in F2 may be a combination of dominant alleles from each
gene, frequency = 9/16.
(b) Green may be a combination of recessive alleles from each gene,
because frequency = 1/16.
(4) Young fruits start out green, may develop another color as they
mature.
b.
(a) One locus may affect whether chlorophyll synthesis stops as the
fruit matures; if so, a different color dominates.
(b) Second locus may code for another color, such as red or yellow.
(c) Model—Several phenotypes can result from the interactions of
these two hypothetical loci. (table in Fig. 13.18c)
Conclusion: Pepper color is likely due to interactions between multiple
genes.
E.
Organelle Genomes
1. Mitochondria and chloroplasts have their own circular genomes.
2. Because sperm have little cytoplasm, mitochondria and chloroplasts are usually
inherited from the mother.
3. Since these organelle genomes are haploid and inherited from only one parent,
there is no segregation, independent assortment, or recombination.
F.
Quantitative Inheritance
1. Phenotypes of the trait show a continuous, bell-shaped distribution of
frequencies.
2. Examples—human height and wheat kernel color (Fig. 13.19, 13.20)
3. Experiment—Nilsson-Ehle crossed pure lines of plants with dark red kernels
versus white kernels.
a. F2 progeny—Frequencies of color shades distribute on a bell curve.
(Fig. 13.20)
b. Proposed a model—Three genes control kernel color in wheat; each assorts
independently.
(1) AABBCC produces dark red kernels.
(2) aabbcc produces white kernels.
(3) Alleles A, B, and C contribute to pigment production in equal and
additive ways.
(4) The more alleles associated with the dominant trait (A, B, C) that is
present, the darker the red pigmentation.
c. Conclusion: Quantitative traits result from the independent action of many
genes.
VI. Applying Mendel’s Rules to Humans
A.
Controlled genetic crosses cannot ethically be performed to study inheritance
patterns in humans.
B.
Scientists use genetic family trees (pedigrees) to study inheritance patterns in
humans. (Fig. 13.21)
C.
Are alleles dominant or recessive?
1. Autosomal recessive traits (Fig. 13.22a)
a. Individuals with the recessive trait must be homozygous for that allele.
b. Parents of those individuals may not exhibit the trait, but are carriers
(heterozygotes).
c. Affected individuals, if mating with a unaffected individual can have
unaffected children (who would be carriers).
d. Example: Cystic fibrosis
2.
D.
Autosomal dominant traits (Fig. 13.22b)
a. Both homozygous dominant and heterozygous individuals will express
the phenotype.
b. All children of homozygous dominant individuals will express the
phenotype.
c. Affected individuals have at least one affected parent.
d. Example: Huntington’s Disease and polydactyly
Is the trait autosomal or sex-linked?
1. If a trait appears equally in males and females, it is likely autosomal.
2. If males are much more likely to express the trait, then it is likely an X-linked
recessive trait (hemophilia). (Fig. 13.23)
a. If males have an X-linked recessive allele, they will express the trait.
b. Females can be carriers of an X-linked recessive trait without expressing
the trait.
3. If an affected male has all affected daughters and no affected sons, then the trait
is likely X-linked dominant (rickets).