Transcript Chapter 14

Chapter 14
Mendel and the Gene Idea
A. Gregor Mendel’s Discoveries
Mendel brought an scientific and mathematical
approach to studying heredity  this is the field of Genetics.
He studied peas. Why?
-Peas have a variety of characters that were easily
studied. Characters are heritable features (eg. flower
color). Each variant of a character is called a trait (eg. purple
or white flower).
-Some selected traits used by Mendel were (See Table 14.1
for complete list): flower color, seed color, seed shape, and
stem length.
Mendel was extremely lucky in choosing the pea plant with
which to work. This is because, the pea plant traits that he
studied are all discontinuous traits.
This means that they are either one way or the other, there is
no in between. For example, pea plants have either purple or
white flowers; smooth or wrinkled seeds. These traits have
no gradations.
This is important, because it allowed Mendel to discern how
traits are passed from one generation to the next. There are
many traits that have gradations and we will see some of
these later in the Chapter.
One example is the carnation flower’s colors (See next)
There were other reasons that Mendel used pea plants:
- Stamens (male reproductive organs) could be removed
to control mating. (There would be no self-fertilization.)
Thus, he could mate male and female gametes as he
chose and could control his experiments.
- This was be done by taking pollen (sperm) from one
plant, and adding it to the carpel (female organ) of
another plant that had its stamen removed (Figure 14.1)
- By fertilizing plants by hand, the parents of each pea
seed would be known.
- In addition, Mendel used only true-breeding plants.
With these plants, the traits remain constant after selffertilization. (This means that the plants contain two identical
genes  both genes encode the same trait.) For example,
because a pea plant has only genes for white flowers, if it
self-fertilizes, all the offspring will only have genes for white
flowers. Thus, the trait is constant in each generation.
For his breeding experiments, Mendel did the following in
which he tracked heritable characteristics for three
generations (Figure 14.3):
- Produced offspring by hybridization. Hybridization is the
mating of two (2) true-breeding individuals.
- True-breeding parents are called the P generation.
- Hybrid offspring are called the F1 generation.
-He then allowed the F1 generation to self-pollinate, the
offspring of this group are called the F2 generation.
 Note the ratio of three purple to one white flower!!
By observing those three generations, Mendel laid the
foundation for two important principles:
1. Law of Segregation
2. Law of Independent Assortment
Let’s look at the tenets of the Law of Segregation in detail:
a. There are multiple versions of the same gene (each
version is a different allele; See Figure 14.4).
b. Each organism inherits two (2) alleles for each character;
one allele from each parent.
c. If the two alleles are different, then the dominant allele is
fully expressed; the recessive allele has no noticeable effect
on the organism’s appearance.
d. The two alleles for each character separate during gamete
production (Occurs during meiosis)  Segregation
A Punnett Square is a device for predicting the results of a
genetic cross between two individuals of known
genotypes. It is used to illustrate the 3:1 ratio that Mendel
observed in the F2 generation.
The Punnett Square and how to use it is described in Figure
14.5 (p. 255) – Mendel’s law of segregation.
Remember the following vocabulary words and apply them to
Fig. 14.5 and 14.6:
- Homozygous: contains identical alleles for a character
- Heterozygous: contains two different alleles for a character
- Phenotype: an organism’s traits
- Genotype: an organism’s genetic makeup
One of the things we can do with a Punnett Square is to
devise a way to reveal the genotype of an unknown organism.
This is done by doing a Testcross 
- By breeding an organism of unknown genotype with an
organism with a homozygous recessive individual, we can
determine the genotype of the unknown individual. The ratio
of phenotypes in the offspring is used to determine unknown
genotype. For example:
Figure 14.7 (p. 256) – A testcross.
2. Law of Independent Assortment
Mendel did his experiments by following only a single
character at a time. Instead, one can follow two characters at
a time, to demonstrate the Law of Independent Assortment 
Each allele pair segregates independently from other allele
pairs during gamete formation.
These experiments use what’s called a dihybrid cross.
Figure 14.8 (p. 257) – Testing two hypotheses for segregation
in a dihybrid cross. Note that the combination of two traits
gives a 9:3:3:1 ratio!
B. Extending Mendelian Genetics  There are many factors
that make genetics not as straight forward as Mendel saw.
These include:
1. Incomplete dominance
- F1 hybrids have a phenotype somewhere in between the
phenotypes of the two parents.
Figure 14.10 (p. 261) – Incomplete dominance in snapdragon
color.
 Can you determine how one can see if it’s incomplete
dominance?
2. Dominance vs. co-dominance
- Both alleles are equally expressed. Example: Tay-Sachs
disease
Homozygous recessive do not produce an enzyme to
metabolize lipids that accumulate in brain cells, which causes
the cells to die.
Heterozygotes produce the enzyme but only at half the
amount produced in homozygous dominants. You don’t see
the symptom of the disease, because half the normal amount
of enzyme is sufficient.
3. Multiple alleles
- Most genes have more than two (2) alleles. Example: Blood
type = A; B; AB; O
4. Pleiotropy
- Most genes affect an organism in many ways; they don’t
affect just one phenotypic character. Example  The many
effects of sickle cell anemia
5. Epistasis
- One gene affects the expression of another gene.
Figure 14.11 (p. 263) – An example of epistasis. In this
case, the gene for color is B where BB = black, and bb =
brown. But a second gene, C, determines whether
pigment can be produced. C allows for pigment to be
produced, c does not allow pigment to be produced
(albino).
6. Polygenic inheritance
- Two or more genes affect one phenotypic character; the
opposite of pleiotropy. These are called quantitative
characters. Example, skin color, where three genes impart
color.
Figure 14.12 (p. 263) – A simplified model for polygenic
inheritance of skin color.
7. Environmental impact on gene expression
- Environmental factors/conditions may alter gene
expression.
Figure 14.13 (p. 264) – The effect of environment on
phenotype.
C. Mendelian Inheritance in Humans
- We are unable to manipulate mating patterns of humans for
experimentation. For this reason,
- Traits are studied by gathering information and placing it
into a family tree.
- The interrelationships between parents and children are
called the family pedigree.
Figure 14.14 (p. 265) – Pedigree analysis.
Many Human Disorders Follow Mendelian Patterns of
Inheritance
1. Recessively Inherited Disorders
- Heterozygous individuals exhibit normal phenotype
because one copy of the normal allele is typically sufficient.
- Heterozygotes, who are phenotypically normal, are called
carriers. They may transmit the recessive allele to their
offspring.
- Cystic fibrosis
- Tay-Sachs disease
- Sickle-cell disease
2. Dominantly Inherited Disorders
- Lethal dominant alleles are uncommon because, if they
cause death before maturity, then the allele will not be passed
to future generations.
- Huntington’s disease (nervous disorder) is caused by a lateacting allele and is sometimes passed to future generations.
3. Multifactorial Disorders
- Most diseases are influenced not only by genetics, but also
by environmental factors
- Heart disease, diabetes, cancer, alcoholism, mental
illnesses
Technology for Genetic Testing and Counseling
1. Carrier Recognition
- Determine whether prospective parents are heterozygous
carriers of a recessive trait
- Identify carriers of diseases such as Tay-Sachs, sickle cell,
or cystic fibrosis
- Ethical issues?
2. Fetal Testing
a. By inserting a needle into the uterus, physicians can
extract amniotic fluid. In some cases, the fluid can be used
to detect genetic disorders. The technique is known as
amniocentesis. ** In rare cases, amniocentesis can result in
complications or fetal death. Therefore, it is reserved for
cases in which the risk for defect is greatest.
b. An alternative technique is called chorionic villus
sampling (CVS). A tube is inserted through the cervix and
fetal tissue from the placenta is extracted.
c. Other techniques, such as ultrasound, can be used to
examine the fetus directly for physical abnormalities.
Figure 14.17 (p. 270) – Testing a fetus for genetic disorders.
3. Newborn Screening
- Some genetic disorders can be detected by simple tests
perfomed soon after birth.
- Phenylketonuria (PKU): inability to break down
phenylalanine
Chapter 15
You should review and understand Figure 15.1