Concept 10.1: Genetics developed from curiosity about
inheritance
I. The Blending Hypothesis of Inheritance
A. A trait is a variation of a particular characteristic such as
for red flowers or yellow flowers
B. In the early 1800’s many biologists believed in the
blending hypothesis, which stated that offspring would
be a blend of their parents traits. A red and yellow
flower would produce and orange flower
II. Mendel’s Plant Breeding Experiments
A. Mendel’s work gave rise to the branch of Biology called
genetics, the study of heredity
B. Mendel developed the particulate hypothesis which
states that parents pass on separate and distinct factors,
or genes
C. To test this hypothesis, Mendel crossed true-breeding
plants that had distinct and contrasting traits such as
purple and white flowers
D. Mendel cross-fertilized all his pea plants by hand to
control which traits he wanted to control
Concept 10.2: Mendel discovered that inheritance follows rules
of chance
I. Mendel’s Principle of Segregation
A. The offspring of two different true-breeding varieties are
called hybrids
B. When Mendel crossed plants that differed in only onetrait it was called a mono-hybrid cross
C. From these results Mendel developed the following
hypothesis:
1. There are alternative forms of a gene called alleles.
2. For each inherited trait, an organism has two alleles
for the gene controlling that character, one from
each parent. If both alleles are the same the
individual is homozygous, and if the alleles are
different the individual is heterozygous.
3. When only one of the alleles in a heterozygous
individual appears to affect the trait, that allele is
called the dominant trait. The allele that does not
appear to have an affect on the individual is called
the recessive trait
4. The two alleles for a character separate during the
formation of gametes, so each gamete carries only
one allele for each character. This is known as
Mendel’s Principle of Segregation
II. Probability and Punnett Squares
A. The inheritance of alleles follows the laws of probability
B. If you were to flip two pennies the probability of flipping
a head or a tail on one does not affect the probability of
the other one
C. A diagram that shows all the possible outcomes of a
genetic cross is the Punnett Square
III. Genotypes and Phenotypes
A. The way an organism looks is not the same as its genetic
make-up
B. An observable trait is called a phenotype while the
genetic make-up of alleles is called the genotype
IV. The Testcross
A. A testcross breeds individuals of unknown genotypes,
but the dominant phenotype with a homozygous
recessive individual
B. Depending on the ratios of the offspring, the genotype of
the unknown can be determined
V. Mendel’s Principle of Independent Assortment
A. Mendel also did crosses between plants that differed in
two traits called a dihybrid cross
B. From this he developed his Law of Independent
Assortment which states that during gamete formation
the way in which one allele is inherited does not affect
the way another is inherited if they are on separate
chromosomes
Concept 10.3: There are many variations of inheritance patterns
I. Intermediate Inheritance
A. When an organism has two alleles and neither is
dominant the phenotype is intermediate between the two
alleles
B. This pattern of inheritance is called intermediate
inheritance
II. Multiple Alleles
A. Many genes have several alleles for each trait which
expands the number of genotypes and phenotypes
B. Codominance is when a heterozygous individual
expresses both traits equally.
III. Polygenic Inheritance
A. When multiple genes affect a character the variation in
phenotypes can become even greater.
B. When two or more genes affect a single character, it is
called polygenic inheritance
C. Examples of polygenic inheritance would include skin
color, and height
IV. The Importance of Environment
A. An individuals phenotype depends on environment as
well as on genes
B. In humans, nutrition influences height, exercise affects
build, and exposure to sunlight darkens the skin
Concept 10.4: Meiosis Explains Mendel’s Principles
I. Chromosome Theory of Inheritance
A. Biologists worked out the processes of mitosis and
meiosis in the late 1800s and observed the parallels
between the behavior of chromosomes and the behavior
of Mendel’s heritable factors
B. The chromosome theory of inheritance states that
genes are located on chromosomes, and the behavior of
chromosomes during meiosis and fertilization accounts
for inheritance patterns
C. The alleles for a gene reside at the same location or gene
locus
II. Genetic Linkage and Crossing Over
A. Mendel’s principles only work when for genes that are
located on separate chromosomes
B. The tendency for the alleles on one chromosome to be
inherited together is called genetic linkage
C. The closer the two genes are on a chromosome, the
greater the genetic linkage
Concept 10.5: Sex-linked traits have unique inheritance patterns
I. Sex-linked Genes
A. Many species have sex chromosomes, designated X and
Y that are associated with determining an individual’s
sex XX= Female, while XY= Male
B. Any gene located on a sex chromosome is called a sexlinked gene
C. Sex-linked genes were discovered by Thomas Hunt
Morgan while studying fruit flies
II. Sex-Linked Disorders
A. A number of human conditions, including red-green
color blindness and hemophilia are inherited as sex
linked recessive traits
B. It takes two copies of the allele to be present in females
for them to show signs of the disorder while it only
takes one in males
Concept 12.2: Accidents Affecting Chromosomes Can Cause
Disorders
I. Down Syndrome
A. Trisomy 21 results in having three number 21
chromosomes
B. In most cases, a human embryo with an abnormal
number of chromosomes results in a miscarriage
C. People with trisomy 21 have Down syndrome, named
after John Langdon Down
II. Nonseparation of Chromosomes
A. Nondisjunction is when homologous fail to separate
B. A woman’s age will have an effect on the possibility of
nondisjunction occurring
C. This is due to the time line of egg cell development
III. Damaged Chromosomes
A. Duplication is when part of a chromosome is repeated
B. Deletion is when a fragment of a chromosome is lost
C. Inversion involves reversing a fragment of the original
chromosome
D. Translocation occurs when a fragment of one
chromosome attaches to a nonhomologous chromosome
IV. Jumping Genes
A. Single genes may move from one location to another in
a chromosome or to a different chromosome
B. This was discovered by Barbara McClintock in the
1940’s
C. These genes can land in the middle of other genes and
disrupt them
D. These genes are called transposons
Concept 12.3 Mendel’s Principles Apply to Humans
I. Working With Human Pedigrees
A. Human geneticists cannot control matings, but must
analyze the patterns of existing families
B. A pedigree is a family tree that records and traces traits
II. Disorders Inherited as Recessive Traits
A. There are over one thousand genetic disorders that are
inherited as a dominant or recessive trait controlled by a
single gene
B. Most human genetic disorders are recessive
C. A carrier is an individual who has a copy of the
recessive disorder but does not show symptoms
III. Disorders inherited as Dominant Traits
A. Dominant alleles that are lethal are more common than
lethal recessives
B. Usually, the person with the dominant disorder dies
before the allele can be passed on
IV. Sex-Linked Disorders
A. Sex-Linked alleles are usually located on the X
Chromosome in humans
B. A male only needs to inherit one copy of the allele to
exhibit the trait
C. A female must inherit two copies of the allele to exhibit
the trait
V. Predicting and Treating Genetic Disorders
A. A genetic counselor is trained to collect and analyze
data about inheritance patterns
B. This information is used by couples to determine the
risks of passing on genetic disorders to their children
C. Genetic tests are done before and after the baby is born