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Chapter 10
The Basic Principles of Heredity
Lecture Outline
I. Mendel first demonstrated the principles of inheritance
A. Other plant breeders at the time of Mendel knew
two main facts
1. Hybrid offspring with the same two kinds of
parents are similar
2. When hybrid offspring are self-fertilized, their
offspring show a variety of traits
B. Mendel did his experiments primarily on the garden
pea, Pisum sativum
1. Peas were easy to grow, and many varieties
were readily available
2. Peas may be self or cross fertilized as they
have perfect flowers
a. Pollen from an anther may be dusted onto
a stigma
b. Cross fertilization may be prevented by
bagging the flower
3. Mendel developed true-breeding lines
True-breeding plants, when self-pollinated,
produce offspring all possessing the same
visible trait as the parent (e.g. the same
phenotype)
4. Mendel chose 7 clearly identifiable pairs of
contrasting traits and accompanying
true-breeding plants
5. Mendel first crossed true-breeding plants with
contrasting traits
a. These parental plants (the P generation)
produced the first filial generation
(the
F1 generation)
b. The F1 generation plants all possessed the
trait of only one of two parents
c. When the F1 plants self pollinate, the
resulting F2 generation had characteristics
of both of the P generation plants
d. These experiments showed that the
hereditary factors had not been lost in the
F1 generation, but somehow masked
C. Mendel’s experiments disproved the ideas of
blending inheritance
Mendel proposed that inherited characteristics are
controlled by two “factors”
(genes in
modern terminology)
D. The principle of segregation states that alleles
separate before gametes are formed
1. The alleles from the male and female gametes
do not mix in any way
(the law
of segregation)
2. Segregation is due to the separation of
homologous chromosomes during meiosis
E. Mendel’s results were published in 1866 in the
transactions of Brünn Society for the Study of
Natural Sciences
F. Mendel’s work was independently rediscovered by
DeVries, Correns, and von Tschermak.
*Sutton, in 1903 associated Mendel’s work and
meiosis
G. Alleles occupy corresponding loci on homologous
chromosomes
1. The term locus may refer to the location of a
gene on a chromosome, as well as the
type of
gene controlling a characteristic
2. Alleles are the variations of a gene that govern
the same feature
3. Alleles are denoted by a letter or letters,
dominant genes are typically denoted
by a
capital letter, recessive genes with a lower case letter
II. A monohybrid cross involves individuals with different
alleles of a given locus
A. A monohybrid cross is the mating of two
individuals that have different genes at
a single locus
B. Heterozygotes carry two different alleles of a
particular locus; homozygotes carry
identical alleles
Heterozygotes have a recessive and dominant
allele for a particular characteristic
Both Parents are Homozygous
Both Parents are Heterozygous
Father is Homozygous Dominant
Mother is Homozygous Recessive
C. A Punnett square predicts the ratios of genotypes
and phenotypes of the offspring of a cross
1. Heterozygous organisms produce equal
numbers of gametes possessing
dominant or recessive alleles
2. Punnett squares allow one to determine the
likely outcome of a genetic cross
Go to :
http://www.changbioscience.com/genetics/punnet
t.html
for a Punnett Square calculator
D. The phenotype of an individual does not always
reveal its genotype
1. Heterozygotes express the dominant trait; the
recessive trait is masked
2. The appearance of the individual is the
phenotype
3. The genetic makeup of the individual is the
genotype
4. The phenotype of the homozygous dominant
and the heterozygous is typically identical
5. Traits that are dominant in one species may be
recessive in another
E. A test cross can detect heterozygosity
A test cross is a cross between an individual
expressing the dominant phenotype, and
a homozygous recessive
Note that the homozygous dominant and
heterozygote have indistinguishable
phenotypes, but different genotypes
III. The laws of probability predict the likelihood of genetic
events
A. The product rule predicts the combined
probabilities of independent events
1. Events are independent if the occurrence of
one does not affect the probability
of the
occurrence of the other
2. To determine the outcome of such a cross, the
independent probabilities are multiplied
B. The sum law predicts the combined probabilities of
mutually exclusive events
Outcomes that may be the result of either of two
different events may be predicted
by
summing the probabilities
C. The laws of probability can be applied to a variety
of calculations
Recall that the rules of chance have no “memory”
Go to:
http://www.athro.com/evo/gen/punexam.html
for probability practice Punnet squares
IV. A dihybrid cross involves individuals that have
different alleles of two loci
Practice dihybrid crosses and tutorial info is available at the
following website:
http://www.biology.arizona.edu/mendelian_genetics/proble
m_sets/dihybrid_cross/dihybrid_cross.html
A. If a dihybrid cross involves different loci on nonhomologous chromosomes, independent assortment
occurs
B. The principle of independent assortment states that
the alleles of different loci on non-homologous
chromosomes are randomly distributed into
gametes
Mendel’s dihybrid crosses illustrated this principle
C. The mechanics of meiosis are the basis for
independent assortments
This process occurs during metaphase I of meiosis
D. Linked genes do not assort independently
1. The principle of independent assortment does
not apply to loci on the same homologous pair
of chromosomes
2. Crossing-over may skew the proportions of
expected phenotypic outcomes
V. The linear order of linked genes on a chromosome is
determined by calculating the frequency of crossing-over
Go to the link below to do a practice problem:
http://www.ansci.cornell.edu/courses/as221/pp/pp24.html
A. During crossing-over during prophase I, segments
of chromosomes are exchanged
The percentage of crossing-over is computed by
adding the number of
individuals in the recombinant classes and dividing by
the total number
of offspring
and multiplying by 100
B. Genes that are close together often are exchanged
together
C. The distance between two genes of a chromosome
is measured in map units
1. Map units measure the percentage of crossingover
2. One map unit represents 1% recombination
D. A linkage group is composed of all the genes on a
particular chromosome; they typically are inherited
together
The number of linkage groups is equal to the
number of pairs of chromosomes
E. Linkage maps are being determined for the fruit fly,
the mouse, yeast,
Neurospora (a
mold), and humans to name a few
VI. Sex is commonly determined by special chromosomes
A. The majority of animals have sex chromosomes
which determine the sex of the individual
1. The homogametic sex will have a pair of
identical sex chromosomes
Human females are homogametic; having
two X chromosomes
2. The heterogametic sex has two different sex
chromosomes
Human males are heterogametic; having one
X and one Y chromosome
3. The autosomes are not sex-determining
chromosomes
B. The Y chromosome determines male sex in most
species of mammals
1. A person with the XXY condition (Klinefelter
syndrome) is male
2. A person with Turner syndrome (a single X
chromosome) is phenotypically female
3. The x and Y chromosomes are not truly
homologous, carrying different
genetic components
4. Sperm containing an X chromosome and those
containing a Y chromosome are
produced
in equal numbers; slightly more male embryos are
produced, slightly more
male babies are
born (106 boys for 100 girls)
C. X-linked genes have unusual inheritance patterns
1. The Y chromosome contains few genes, but
important, genes for maleness
2. Genes located on the X chromosome are called
X-linked (older terminology; sex-linked)
3. Males are hemizygous for X-linked traits; they
cannot be carriers for X-linked traits
4. X-linked traits may be denoted as XC for a
dominant allele and Xc for a recessive allele;
the Y
chromosome has no superscripts
5. For most X-linked genes, the dominant form is
a normal form
6. A female expressing an X-linked trait must
have had a father with that trait and
a
heterozygous (“carrier”) mother; therefore, they are rare
7. A male expressing an X-linked trait typically
had a father without that trait and
a
heterozygous (“carrier”) mother
D. Dosage compensation equalizes the expression of
X-linked genes in males and females
1. Typical females have two X chromosomes;
males have one
a. Dosage compensation in mammals
involves the inactivation of one X chromosome
in female cells
b. Fruit fly males accomplish this by making
their single X chromosome more active
2. The Barr body is a dark area of highly
condensed chromatin, and is the single
inactive X chromosome in female cells
Barr bodies (b)
3. The expression of a single X chromosome
results in mottled color of a calico cat
(nearly always they are females)
4. Variegation is the result of X chromosome
inactivation
a. Inactivation happens randomly in cells,
relatively early in embryological
development
b. Resulting clones of cells have the same X
chromosome inactivated
E. Sex-influenced genes are autosomal, but their
expression is affected by the individual’s sex
1. Pattern baldness in humans is an example, as it
is most common in males,
although
not unknown in females
2. Persons with the genotype B1 B1 show
baldness in either sex
3. Persons with the genotype B1 B2 show
baldness if male
VII. The relationship between genotype and phenotype is
often complex
A. A single pair of alleles may control several
characteristics; in contrast,
many alleles may control a single trait
B. Dominance is not always complete
1. Pink four o’-clock flowers result from a cross
between a plant with red flowers
and
a plant with pink flowers
Incomplete dominance results when neither
allele is dominant to the other,
and
the phenotype is intermediate
2. Codominance results when the offspring has
characteristics of both alleles
a. In cows, a reddish coated cow mating
with a white coated cow will produce a
calf
with a roan coloration, with both red and white hairs
b. The human ABO blood group alleles are
an example
C. Multiple alleles for a locus may exist in a
population
1. The human ABO blood group is also an
example of multiple alleles
2. In rabbits, variations of the “C” gene result in
rabbits with quite varied coat colors
D. A single gene may affect multiple aspects of the
phenotype
Multiple effects of a single gene is referred to as
pleiotropy
Many genetic diseases are pleiotropic, e.g.
cystic fibrosis and sickle cell anemia
E. Alleles of different loci may interact to produce a
phenotype
1. These interactions result in variations from
typical expected Mendelian ratios of crosses
2. Epistasis is the interaction between genes, such
as that one gene can influence the
affect of
another gene
The gene causing albinism would hide the gene controlling color of a person's hair
F. Polygenes act additively to produce a genotype
1. Multiple, separate genes have similar and
additive effects on the morphological feature
2. Height and skin color are simple examples in
humans
3. The F1 generation has phenotypes intermediate
between the homozygous parents,
and the
F2 generation shows wide variation in phenotype
4. Bell-shaped curves (a normal distribution) are
indicative of polygenes
VIII. Selection, inbreeding, and out-breeding are used to
develop improved strains
A. Inbreeding leads to organisms homozygous for
many genes, some of which may be harmful
In humans, marriage between close relatives is
forbidden
B. Out-breeding results more heterozygous allelic
pairs, resulting in hybrid vigor
1. The cause of hybrid vigor is not fully
understood
a. The vigor may be due to the lack of
expression of deleterious recessive genes
b. Most of the corn, wheat, and other crops
grown in the United States are
hybrids; produced each year by crossing the original
strains
c. The seeds of the F1 generation are eaten,
not planted the following year
2. Hybrid vigor may also be due to heterozygote
advantage
a. Heterozygote advantage refers to some
positive attribute possessed by
the heterozygote which is not possessed by either the
dominant or
recessive
homozygote
b. Sickle cell anemia is an example; diseased
persons have the genotype ss,
noncarriers SS, and heterozygotes Ss
c. Heterozygotes for the sickle-cell trait are
more resistant to malaria
d. The sickle-cell trait is relatively common
in persons living in areas with
high rates of malaria (or persons descended from those
areas)