Chapter 8
Mendel and Heredity
Section 1: The Origins of Genetics
Section 2: Mendel’s Theory
Section 3: Studying Heredity
Section 4: Complex Patterns of Heredity
Section 1
The Origins of Genetics
Objectives:
•Identify the investigator whose studies formed the
basis of modern genetics.
•List characteristics that make the garden pea a good
subject for genetic study.
•Summarize the three major steps of Gregor
Mendel's garden-pea experiments.
•Relate the ratios that Mendel observed in his crosses
to his data.
Section 1
The Origins of Genetics
Mendel’s Studies of Traits
•Mendel’s Breeding Experiments Gregor Mendel
bred varieties of the garden pea in an attempt to
understand heredity.
•Useful Features in Peas The garden pea is a
good subject for studying heredity for the
following reasons: several traits show discrete
forms, self-fertilization and cross-fertilization are
possible, the garden pea is easy to cultivate, and
the plants produces many offspring.
Section 1
The Origins of Genetics
Traits Expressed as Simple Ratios
•Monohybrid Crosses A monohybrid cross is a
cross that involves one pair of contrasting traits.
•Mendel’s Results Mendel observed that
contrasting traits appear in offspring according to
simple ratios. In Mendel’s experiments, only one
of the two contrasting forms of a trait was
expressed in the F1 generation. The other form
reappeared in the F2 generation in a 3:1 ratio.
Section 2
Mendel’s Theory
Objectives:
•Describe the four major hypotheses Mendel
developed.
•Define the terms homozygous, heterozygous,
genotype, and phenotype.
•Compare Mendel's two laws of heredity.
Section 2
Mendel’s Theory
A Theory of Heredity
Mendel’s Hypotheses Different versions of a gene are
called alleles. An individual usually has two alleles for
a gene, each inherited from a different parent.
Mendel’s Findings in Modern Times Individuals
with the same two alleles for a gene are homozygous;
those with two different alleles for a gene are
heterozygous.
Genotype and Phenotype The set of alleles that an
individual has is called its genotype. The physical
appearance of a trait or outward expression of the
genotype is called the phenotype.
Section 2
Mendel’s Theory
The Laws of Heredity
•The Law of Segregation The law of segregation
states that the two alleles for a trait separate when
gametes are formed.
•The Law of Independent Assortment The law of
independent assortment states that two or more
pairs of alleles separate independently of one
another during gamete formation.
Section 3
Studying Heredity
Objectives:
•Predict the results of monohybrid genetic crosses by
using Punnett squares.
•Apply a test cross to determine the genotype of an
organism with a dominant phenotype.
•Predict the results of monohybrid genetic crosses by
using probabilities.
•Analyze a simple pedigree.
Section 3
Studying Heredity
Punnett Squares
•Function of Punnett Squares The results of
genetic crosses can be predicted with the use of
Punnett squares.
•One Pair of Contrasting Traits Punnett squares
can be used to predict the outcome of a monohybrid
cross with one pair of contrasting traits.
•Determining Unknown Genotypes A test cross
can be used to determine whether an individual
expressing a dominant trait is heterozygous or
homozygous.
Section 3
Studying Heredity
Outcomes of Crosses
•Probability of a Specific Allele in a Gamete The
probability of a specific allele in a gamete can be
predicted with the use of probabilities. For a gene with
two alleles, the chance of contributing one allele or the
other to the gamete is 1/2.
•Probability of the Outcome of a Cross The results
of a genetic cross can be predicted with the use of
probabilities. To find the probability that a
combination of two independent events will occur,
multiply the separate probabilities of the two events.
Section 3
Studying Heredity
Inheritance of Traits
•Pedigrees A trait’s pattern of inheritance within a
family can be determined by analyzing a pedigree.
• Patterns of Inheritance Scientists use pedigrees to
determine whether a trait is autosomal or sex-linked,
dominant or recessive, and heterozygous or
homozygous.
Section 4
Complex Patterns of Heredity
Objectives:
•Identify five factors that influence patterns of
heredity.
•Describe how mutations can cause genetic
disorders.
•List two genetic disorders, and describe their causes
and symptoms.
•Evaluate the benefits of genetic counseling.
Section 4
Complex Patterns of Heredity
Complex Control of Traits
•Traits Influenced by Several Genes Many traits—
weight, hair color, and skin color—are polygenic traits
that involve several genes influencing the trait.
•Intermediate Traits A trait that is intermediate
between the two parental types is a condition known
as incomplete dominance.
•Traits Controlled by Genes with Three or More
Alleles Some traits, such as the ABO blood type
alleles, are controlled by three or more alleles.
Section 4
Complex Patterns of Heredity
Complex Control of Traits continued
•Traits with Two Forms Displayed at the Same
Time Codominance occurs if two alleles are dominant
and thus both forms of the trait are expressed at the
same time.
•Traits Influenced by the Environment A phenotype
often depends on conditions in the environment.
Section 4
Complex Patterns of Heredity
Genetic Disorders
•Sickle Cell Anemia Sickle cell anemia is a recessive
genetic disorder that causes an abnormal form of
hemoglobin protein.
•Cystic Fibrosis Cystic fibrosis is a fatal recessive trait
that causes a defective chloride-ion transport protein.
•Hemophilia Hemophilia is a recessive genetic disorder
that leads to a defective blood-clotting factor.
•Huntington’s Disease Huntington’s disease is a
dominant genetic disorder that leads to the production
of an inhibitor of brain-cell metabolism.
Section 4
Complex Patterns of Heredity
Treating Genetic Disorders
•Genetic Counseling Genetic counseling can help
patients concerned about a genetic disorder.
•Gene Therapy A promising new gene technology
called gene therapy will allow scientists to replace
defective genes with copies of healthy genes.