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11 FOUNDATIONS OF GENETICS
CHAPTER OUTLINE
LEARNING OBJECTIVES
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Know who Gregor Mendel was and why his work is foundational to modern genetics.
Define “true-breeding,” “P generation,” “F1 generation,” and “F2 generation.”
Understand the basic tenets of genetics, and distinguish between dominant and recessive, between
homozygous and heterozygous, and between genotype and phenotype.
Be able to use a Punnett square to visualize the outcome of a genetic cross.
Be able to state Mendel's two laws of heredity.
Describe various inheritance patterns, such as epistasis, codominance, multiple alleles, continuous
variation, pleiotropy, and incomplete dominance.
Realize that genes are located along chromosomes.
Know how many chromosomes are in normal human cells and what causes aneuploidy.
Understand what is meant by “sex-linked” genes, and give examples.
Describe the role of mutations in human heredity and give examples.
Explain how genetic counselors can help a couple decide whether or not they are at risk for passing
along genetic disorders.
Mendel (p. 184)
11.1
Mendel and the Garden Pea (p. 184; Figs. 11.1, 11.2, 11.3)
A. Heredity is the passing along of traits from one generation to the next, and genetics is the
study of heredity.
B. A scientist and monk by the name of Gregor Mendel contributed to the understanding of
genetics in the 1800s by being the first to actually count numbers of offspring in crosses
involving pea plants.
C. Early Ideas About Heredity
1. Over 200 years before Mendel’s work, British farmers had tried similar crosses and
noted that some types had a stronger tendency to pass along their traits.
D. Mendel’s Experiments
1. Gregor Mendel was born in 1822 and educated in a monastery.
2. He became a monk and later joined a science club for which he undertook a number of
studies.
E. Mendel’s Experimental System: The Garden Pea
1. Mendel used the garden pea because many varieties were available, previous work had
been done with these plants, peas are small and grow quickly, and left alone, the flowers
self-pollinate.
F. Mendel’s Experimental Design
1. Mendel began his crosses with true-breeding varieties that contained only one type of
gene for each trait.
2. The first generation in a succession of crosses is the P or parental generation; their
offspring are the F1 generation or first filial generation.
3. Offspring of two members of the F1 generation comprise the F2 generation.
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11.2
11.3
11.4
What Mendel Observed (p. 186; Figs. 11.4, 11.5; Table 11.1)
A. Mendel repeatedly came up with the same results when examining seven pairs of contrasting
traits.
B. The F1 Generation
1. Mendel called the trait expressed in the F1 plants the dominant trait and the trait not
expressed was recessive.
C. The F2 Generation
1. When the F1 plants were allowed to self-fertilize, Mendel found 3:1 dominant to
recessive phenotype in the F2 generation.
2. Mendel’s studies were unique because he actually counted the number of offspring with
each trait.
D. A Disguised 1:2:1 Ratio
1. When F2 plants were allowed to self-fertilize, Mendel found a 1:2:1 ratio of truebreeding dominant to not true-breeding dominant to true-breeding recessive.
Mendel Proposes a Theory (p. 188; Figs. 11.6, 11.7, 11.8, 11.9; Table 11.2)
A. In his crosses, Mendel found dominant traits expressed in his F1 generations and recessive
traits showing up in the F2 generation.
B. Each individual has two genes (Mendel called these “factors”) for each trait.
C. When both genes are the same, the individual is said to be homozygous for that trait. If the
two genes are different, the individual is heterozygous.
D. Alternate types of genes for each trait are alleles.
E. The genetic makeup of an individual is the genotype.
F. Phenotype refers to the outward expression of the genes.
G. Analyzing Mendel’s Results
1. A modified multiplication table, called a Punnett square, is an easy way to organize all
possible genotypes when conducting a genetic cross.
2. The likelihood of getting one particular genotype in the offspring is expressed as a
probability.
H. Punnett Squares
1. Analyses using Punnett squares demonstrate that Mendel’s results reflect independent
segregation of gametes.
I. The Testcross
1. When Mendel did not know the genotype of an individual expressing a dominant trait,
he did a testcross by crossing the individual with a homozygous recessive for the trait.
2. Testcrosses can also be used to determine the genotype of an individual when two genes
are involved.
Mendel’s Laws (p. 191; Fig. 11.10)
A. Mendel’s First Law: Segregation
1. Mendel's First Law, or the Law of Segregation, says that only one of a pair of alleles is
passed to a gamete, and gametes join randomly when uniting to form offspring.
B. Mendel’s Second Law: Independent Assortment
1. Mendel's Second Law was determined when he worked with two traits at a time in
dihybrid crosses. This law, called the Law of Independent Assortment, states that genes
located on different chromosomes are inherited independently.
From Genotype to Phenotype (p. 192)
11.5
How Genes Influence Traits (p. 192; Fig. 11.11)
A.
From DNA to Protein
1. DNA is transcribed into RNA in the nucleus.
2. RNA is translated into protein on ribosomes in the cytoplasm.
B.
How Proteins Determine the Phenotype
1. The specific sequence of amino acids determines a protein’s functional shape, thus
specifying phenotype.
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C.
11.6
How Mutation Alters Phenotype
1. A change in a single nucleotide within a gene can alter the amino acid encoded, thus
changing the shape of a protein.
D.
Natural Selection for Alternative Phenotypes Leads to Evolution
1. Random mutations occurring in populations, although rare, may result in a better
adaptation in a changing environment.
Why Some Traits Don’t Show Mendelian Inheritance (p. 194; Figs. 11.12–11.19)
A.
The expressions of genotype are not always straightforward.
B.
Continuous Variation
1. Sometimes one trait, such as human height, is determined by the action of several
genes, which results in a continuous variation for the trait within a population.
C.
Pleiotropic Effects
1. When an allele affects more than one trait, it is said to be pleiotropic.
D.
Incomplete Dominance
1. A condition known as incomplete dominance is seen when offspring exhibit a
phenotype intermediate to that of both parents.
E.
Environmental Effects
1. The degree to which an allele is expressed can sometimes depend on the environment.
2. Some alleles are heat-sensitive, resulting in different pigmentation during seasonal
weather changes.
F. Epistasis
1. Epistasis is an interaction between the products of two genes in which one of the genes
modified the phenotypic expression produced by the other.
2. Why Was Emerson’s Ratio Modified?
3. In Emerson’s experiments on corn seed color, his modified ratios were the result of
epistasis.
G. Codominance
1. Sometimes more than two alleles, or multiple alleles, exist for a given trait in a
population of individuals.
2. An example is the human ABO blood designation.
a. In the human ABO blood group, A and B are equally dominant and can occur
together as codominants.
b. The Rh designation also refers to a cell surface marker on red blood cells.
c. About 85% of the population is Rh+, meaning they have the marker.
d. Problems may arise when an Rh-negative mother bears an Rh+ fetus.
Chromosomes and Heredity (p. 200)
11.7
Chromosomes Are the Vehicles of Mendelian Inheritance (p. 200; Figs. 11.20, 11.21, 11.22)
A. The Chromosomal Theory of Inheritance
1. Observations that similar chromosomes paired with each other during meiosis led to the
chromosomal theory of inheritance.
2. Diploid individuals have two copies of each heritable gene, and gametes each have one.
3. Segregation and independent assortment were observed, and were consistent with
Mendel’s model.
B. Problems with the Chromosomal Theory
1. The observation that the number of traits that assorts independently in an organism
exceeds the number of chromosome pairs the organism possesses made scientists
wonder whether the chromosomal theory was correct.
C. Morgan’s White-Eyed Fly
1. Morgan’s 1910 studies of a white-eyed fruit fly showed that certain genes could be sexlinked.
D. Sex Linkage Proves the Chromosomal Theory
1. Male fruit flies have only one X chromosome while female fruit flies have two X
chromosomes.
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2.
11.8
Eye color in this instance was sex-linked, which explained why males were white-eyed
and females had red eyes.
3. Morgan’s experiments were important because they illustrated that genes are carried on
chromosomes, and Mendel’s laws are true.
Human Chromosomes (p. 202; Figs. 11.23, 11.24, 11.25, 11.26)
A. Humans have 23 pairs, or 46, chromosomes that vary by size, shape, and appearance.
B. Photographing the chromosomes produces a karyotype.
C. Nondisjunction
1. Sometimes during meiosis, the homologous chromosomes or the sister chromatids do
not separate properly, a mistake known as nondisjunction.
2. This leads to aneuploidy, which means having an abnormal number of chromosomes.
3. Humans have 23 pairs of chromosomes, of which 22 pairs are called autosomes.
4. Monosomics have only one of a pair of a particular set of chromosomes, and trisomics
have three copies of a chromosome, rather than the normal two.
5. Down syndrome is an example of a trisomic condition in which the individual is born
with an extra copy of chromosome 21.
6. This condition results in mental impairment and a host of other physical defects.
D. Nondisjunction Involving the Sex Chromosomes
1. Humans have 23 pairs of chromosomes, of which 22 pairs are called autosomes; the
remaining pair determines the sex of the individual and are the sex chromosomes.
2. Two kinds of sex chromosomes exist, an X chromosome and a Y chromosome.
3. Human females have two X's, and males have an X and a Y.
4. It is the human male, then, that determines the sex of his offspring.
5. Just as aneuploidy can occur with the autosomes, so it can also occur with the sex
chromosomes, resulting in a number of abnormal conditions.
Human Hereditary Disorders (p. 204)
11.9
11.10
The Role of Mutations in Human Heredity (p. 204; Figs. 11.27–11.33; Table 11.3)
A. Many human hereditary disorders reflect the presence of mutations within human
populations.
B. Family trees, or pedigrees, can indicate the mode of inheritance of the mutation.
C. Hemophilia, an autosomal or sex-linked recessive trait, results from mutations of genes
encoding blood-clotting proteins.
D. Sickle cell anemia is due to a recessive mutation of hemoglobin that is common in Africa
because heterozygotes are resistant to malaria.
E. Tay-Sachs Disease
1. Tay-Sachs disease is an incurable hereditary disorder that progressively destroys the
brain of those affected.
2. It is carried as a recessive and is most common in Jews from Eastern and Central Europe.
F. Huntington’s Disease
1. Huntington’s disease is a dominant condition that does not express itself until later in life,
after the trait has been passed on to the next generation.
Genetic Counseling and Therapy (p. 208; Figs. 11.34, 11.35)
A. Genetic counseling can help couples predict the risk of bearing children with genetic defects.
B. High Risk Pregnancies and Genetic Screening
1. Pregnant women who fear they may be at risk of carrying a fetus with a genetic defect can
undergo a procedure called amniocentesis in which a minute amount of amniotic fluid
surrounding the fetus is removed and checked for genetic defects.
2. Chorionic villi sampling involves removal of a small portion of the chorionic villi for
genetic testing.
3. Ultrasound allows for viewing of the fetus without harming it.
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KEY TERMS
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heredity (p. 184) Heredity is the passing of traits from parents to offspring.
dominant (p. 186) Of a pair, the trait that is expressed.
recessive (p. 186) Of a pair, the trait that remains hidden.
gene (p. 188)
heterozygous (p. 188) Heterozygotes possess alternate forms for an allele; homozygotes have two
copies of the same form of a gene.
allele (p. 188)
phenotype (p. 188) Phenotype refers to the outward expression of the genes.
genotype (p. 188) The actual genetic makeup of an individual is the genotype.
pleiotropy (p. 194) When an allele affects more than one trait, it is said to be pleiotropic.
incomplete dominance (p. 195)
epistasis (p. 196) Epistasis occurs because each gene blocks the expression of the other.
codominant (p. 198) Human blood groups are an excellent example of codominance.
X chromosome (p. 200)
sex-linked (p. 201) Some biologists prefer “X-linked” or “Y-linked” to avoid confusion, although most
sex-linked traits are X-linked.
nondisjunction (p. 202) Nondisjunction can occur either at meiosis I or II and is more common in
female gametes, but can also occur in male gametes.
aneuploidy (p. 202) Euploidy means the true or normal number of chromosomes; aneuploidy means
more or less than the normal number.
pedigree (p. 204)
hemophilia (p. 205)
sickle-cell anemia (p. 206)
Tay-sachs disease (p. 207)
Huntington’s disease (p. 207)
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
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The study of genetics requires students to learn many new terms. When discussing the basic ideas of
genetics, it is helpful to jot down the main terms with their definitions in simple language at the top or
side edge of the chalkboard, and then leave them there during the rest of the discussion. These give
students something to refer to and it helps them to learn concepts with less confusion. Similarly, it is
helpful to do a number of genetics problems using Punnett squares, rather than doing just one or two
examples.
Genetics Problem— A Dihybrid. In Mendel's pea plants, purple flower color is dominant to white, and
green pod color is dominant to yellow pod color. Have students cross a pea plant that is homozygous
dominant for both traits with one that is homozygous recessive (PPGG x ppgg). All F1's will be
heterozygous for both traits, and all will show both dominant phenotypes. Next, have students cross
two F1's using a 4 x 4 Punnett square. They should come up with a phenotypic ratio of 9:3:3:1.
CRITICAL THINKING QUESTIONS
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2.
You are a research scientist working in a genetics lab, and the institute that funds your research wants
you to quickly determine the genetics of a newly discovered tropical plant that seems to have three
flower color variations. The flowers have chemicals that could potentially be the source of life-saving
medications. Describe the nature of the crosses you would make to determine a pattern of inheritance.
Skin and hair color in humans exhibit a wide range in colors. After reading this chapter, what
inheritance pattern do you believe is involved with color?
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3.
Your sister has cystic fibrosis and you are contemplating having children of your own. Your mate
recalls having a cousin with the disease. What is the risk that one or more of your children will have
cystic fibrosis or is there any way to know for certain?
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