Download Chapter 9 Genetics

CHAPTER
9
Patterns of Inheritance
Key Terms
ABO blood groups
alleles
amniocentesis
carriers
character
chorionic villus sampling
(CVS)
chromosome theory of
inheritance
codominant
complete dominance
cross
dihybrid cross
dominant allele
F1 generation
F2 generation
genetics genotype
heredity
heterozygous
homozygous
Huntington’s disease
hybrids
incomplete dominance
law of independent assortment
law of segregation
linkage map
linked genes
locus (plural, loci)
monohybrid cross
pedigree
P generation
phenotype
pleiotropy
polygenic inheritance
Punnett square
recessive allele
recombination frequency
rule of addition
rule of multiplication
sex chromosomes
sex-linked gene
sickle-cell disease
testcross
trait
true-breeding
ultrasound imaging
Lecture Outline
I. Introduction
1. The people of Tibet live and work at altitudes above 13,000 feet, where the amount of
oxygen that reaches the blood is 40% less than at sea level.
2. What makes the Tibetan people so able to tolerate their harsh surroundings?
3. Over the last several thousand years, the Tibetan population has accumulated several
dozen genetic mutations that affect their circulatory and respiratory systems.
II. Mendel’s Laws
A. 9.1 The study of genetics has ancient roots
1. The Greek physician Hippocrates explained inheritance by proposing that
a. particles called “pangenes” travel from each part of an organism’s body to eggs or
sperm and
b. characteristics acquired during the parents’ lifetime could be transferred to the
offspring.
2. Hippocrates’s idea is incorrect in several respects.
a. The reproductive cells are not composed of particles from somatic (body) cells.
b. Changes in somatic cells do not influence eggs and sperm.
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3. The idea that hereditary materials mix in forming offspring, called the blending
hypothesis, was
a. suggested in the 19th century by scientists studying plants but
b. later rejected because it did not explain how traits that disappear in one generation
can reappear in later generations.
B. 9.2 The science of genetics began in an abbey garden
1. Heredity is the transmission of traits from one generation to the next.
2. Genetics is the scientific study of heredity.
3. Gregor Mendel
a. began the field of genetics in the 1860s,
b. deduced the principles of genetics by breeding garden peas, and
c. relied on a background of mathematics, physics, and chemistry.
4. In 1866, Mendel
a. correctly argued that parents pass on to their offspring discrete “heritable factors”
and
b. stressed that the heritable factors (today called genes), retain their individuality
generation after generation.
5. A heritable feature that varies among individuals, such as flower color, is called a
character.
6. Each variant for a character, such as purple or white flowers, is a trait.
7. Perhaps the most important advantage of pea plants as an experimental model was that
Mendel could strictly control matings.
a. The petals of the pea flower almost completely enclose the reproductive organs: the
stamens and carpel.
b. Consequently, pea plants usually are able to self-fertilize in nature.
8. When Mendel wanted to cross-fertilize plants, he
a. prevented self-fertilization by cutting off the immature stamens of a plant before
they produced pollen and
b. dusted its carpel with pollen from another plant to cross-fertilize the stamenless
flower.
9. After pollination, the carpel developed into a pod, containing seeds (peas) that he later
planted.
10. True-breeding varieties result when self-fertilization produces offspring all identical
to the parent.
11. The offspring of two different varieties are hybrids.
12. The cross-fertilization is a hybridization, or genetic cross.
13. True-breeding parental plants are the P generation.
14. Hybrid offspring are the F1 generation.
15. A cross of F1 plants produces an F2 generation.
C. 9.3 Mendel’s law of segregation describes the inheritance of a single character
1. A cross between two individuals differing in a single character is a monohybrid
cross.
2. Mendel performed a monohybrid cross between a plant with purple flowers and a
plant with white flowers.
a. The F1 generation produced all plants with purple flowers.
b. A cross of F1 plants with each other produced an F2 generation with ¾ purple and ¼
white flowers.
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3. The all-purple F1 generation did not produce light purple flowers, as predicted by the
blending hypothesis.
4. Mendel needed to explain
a. why white color seemed to disappear in the F1 generation and
b. why white color reappeared in one-quarter of the F2 offspring.
5. Mendel developed four hypotheses, described below using modern terminology.
a. Alleles are alternative versions of genes that account for variations in inherited
characters.
b. For each characteristic, an organism inherits two alleles, one from each parent.
The alleles can be the same or different.
i. A homozygous genotype has identical alleles.
ii. A heterozygous genotype has two different alleles.
c. If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the
organism’s appearance and is called the recessive allele.
i. The phenotype is the appearance or expression of a trait.
ii. The genotype is the genetic makeup of a trait.
iii. The same phenotype may be determined by more than one genotype.
d. A sperm or egg carries only one allele for each inherited character because allele
pairs separate (segregate) from each other during the production of gametes. This
statement is called the law of segregation.
i. The fusion of gametes at fertilization creates allele pairs once again.
6. Mendel’s hypotheses also explain the 3:1 ratio in the F2 generation.
a. The F1 hybrids all have a Pp genotype.
b. A Punnett square shows the four possible combinations of alleles that could occur
when these gametes combine.
D. 9.4 Homologous chromosomes bear the alleles for each character
1. A locus (plural, loci) is the specific location of a gene along a chromosome.
2. For a pair of homologous chromosomes (homologs), alleles of a gene reside at the
same locus.
a. Homozygous individuals have the same allele on both homologs.
b. Heterozygous individuals have a different allele on each homolog.
E. 9.5 The law of independent assortment is revealed by tracking two characters at once
1. A dihybrid cross is a mating of parental varieties that differ in two characters.
2. Mendel performed the following dihybrid cross with the following results:
a. P generation: round yellow seeds  wrinkled green seeds
b. F1 generation: all plants with round yellow seeds
c. F2 generation:
i. 916 had round yellow seeds
ii. 316 had wrinkled yellow seeds
iii. 316 had round green seeds
iv. 116 had wrinkled green seeds
3. Mendel needed to explain why the F2 offspring
a. had new nonparental combinations of traits and
b. had a 9:3:3:1 phenotypic ratio.
4. Mendel
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a. suggested that the inheritance of one character has no effect on the inheritance of
another,
b. suggested that the dihybrid cross is the equivalent to two monohybrid crosses, and
c. called this the law of independent assortment.
5. The following figure demonstrates the law of independent assortment as it applies to
two characters in Labrador retrievers:
a. black versus chocolate color, and
b. normal vision versus progressive retinal atrophy.
F. 9.6 Geneticists can use the testcross to determine unknown genotypes
1. A testcross is the mating between an individual of unknown genotype and a
homozygous recessive individual.
2. A testcross can show whether the unknown genotype includes a recessive allele.
3. Mendel used testcrosses to verify that he had true-breeding varieties of plants.
4. The following figure demonstrates how a testcross can be performed to determine the
genotype of a Lab with normal eyes.
G. 9.7 Mendel’s laws reflect the rules of probability
1. Using his strong background in mathematics, Mendel knew that the rules of
mathematical probability affected
a. the segregation of allele pairs during gamete formation and
b. the reforming of pairs at fertilization.
2. The probability scale ranges from 0 to 1. An event that is
a. certain to occur has a probability of 1 and
b. certain not to occur has a probability of 0.
3. The probability of a specific event is the number of ways that event can occur out of
the total possible outcomes.
4. Determining the probability of two independent events uses the rule of multiplication,
in which the probability is the product of the probabilities for each event.
5. The probability that an event can occur in two or more alternative ways is the sum of
the separate probabilities, called the rule of addition.
H. 9.8 VISUALIZING THE CONCEPT: Genetic traits in humans can be tracked through
family pedigrees
1. The inheritance of human traits follows Mendel’s laws.
2. A pedigree
a. shows the inheritance of a trait in a family through multiple generations,
b. demonstrates dominant or recessive inheritance, and
c. can also be used to deduce genotypes of family members.
I. 9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene
1. The genetic disorders listed in Table 9.9 are known to be inherited as dominant or
recessive traits controlled by a single gene.
2. These human disorders therefore show simple inheritance patterns like the traits
Mendel studied in pea plants.
3. The genes discussed in this module are all located on autosomes.
4. Thousands of human genetic disorders—ranging in severity from mild, such as
albinism, to invariable fatal, such as cystic fibrosis—are inherited as recessive traits.
5. Most people who have recessive disorders are born to normal parents who
a. are both heterozygotes, carriers of the recessive allele for the disorder, but
b. are phenotypically normal.
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6. The most common lethal genetic disease in the United States is cystic fibrosis (CF).
The CF allele is
a. recessive and
b. carried by about 1 in 31 Americans.
7. Cystic fibrosis is
a. characterized by an excessive secretion of very thick mucus from the lungs and
other organs and
b. most common in Caucasians.
8. Dominant human disorders include
a. Huntington’s disease, a degenerative disorder of the nervous system and
b. achondroplasia, a form of dwarfism in which
i. the head and torso of the body develop normally but
ii. the arms and legs are short.
9. Until relatively recently, the onset of symptoms was the only way to know if a person
had inherited the Huntington’s allele.
10. A genetic test is now available that can detect presence of the Huntington’s allele in
an individual’s genome.
11. This is one of the several genetic tests currently available.
J. 9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy
1. Modern technologies offer ways to obtain genetic information
a. before conception,
b. during pregnancy, and
c. after birth.
2. Genetic testing can identify prospective parents who are heterozygous carriers for
certain diseases.
3. Several technologies can be used for detecting genetic conditions in a fetus.
a. Amniocentesis extracts samples of amniotic fluid containing fetal cells and permits
i. karyotyping to detect chromosomal abnormalities such as Down syndrome and
ii. biochemical tests on cultured fetal cells to detect other conditions, such as
Tay-Sachs disease.
b. Chorionic villus sampling (CVS) removes a sample of chorionic villus tissue from
the placenta and permits similar karyotyping and biochemical tests.
4. Blood tests on the mother at 15–20 weeks of pregnancy can help identify fetuses at
risk for certain birth defects.
5. Fetal imaging enables a physician to examine a fetus directly for anatomical
deformities. The most common procedure is ultrasound imaging, using sound waves
to produce a picture of the fetus.
6. Newborn screening can detect diseases that can be prevented by special care and
precautions.
7. New technologies raise ethical considerations that include
a. the confidentiality and potential use of results of genetic testing,
b. time and financial costs, and
c. the determination of what, if anything, should be done as a result of the testing.
III. Variations on Mendel’s Laws
A. 9.11 Incomplete dominance results in intermediate phenotypes
1. Mendel’s pea crosses always looked like one of the parental varieties, a situation
called complete dominance.
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2. For some characters, the appearance of F 1 hybrids falls between the phenotypes of the
two parental varieties. This is called incomplete dominance.
3. One example of incomplete dominance in humans is hypercholesterolemia, in which
a. dangerously high levels of cholesterol occur in the blood and
b. heterozygotes have intermediately high cholesterol levels.
B. 9.12 Many genes have more than two alleles in the population
1. Although each individual carries, at most, two different alleles for a particular gene, in
cases of multiple alleles, more than two possible alleles exist in a population.
2. Human ABO blood group phenotypes involve three alleles for a single gene.
3. The four human blood groups, A, B, AB, and O, result from combinations of these
three alleles.
4. The A and B alleles are both expressed in heterozygous individuals, making both
alleles codominant.
C. 9.13 A single gene may affect many phenotypic characters
1. Pleiotropy occurs when one gene influences many characters.
2. Sickle-cell disease is a human example of pleiotropy.
a. This disease affects the type of hemoglobin produced and the shape of red blood
cells and causes anemia and organ damage.
b. Sickle-cell and nonsickle alleles are codominant.
c. Carriers of sickle-cell disease have increased resistance to malaria.
D. 9.14 A single character may be influenced by many genes
1. Many characters result from polygenic inheritance, in which a single phenotypic
character results from the additive effects of two or more genes.
2. Human skin color is an example of polygenic inheritance.
E. 9.15 The environment affects many characters
1. Many characters result from a combination of heredity and the environment. For
example,
a. skin color is affected by exposure to sunlight and
b. heart disease and cancer are influenced by genes and the environment.
2. Identical twins show that a person’s traits are the results of
a. genetics and
b. the environment.
IV. The Chromosomal Basis of Inheritance
A. 9.16 Chromosome behavior accounts for Mendel’s laws
1. The chromosome theory of inheritance states that
a. genes occupy specific loci (positions) on chromosomes and
b. chromosomes undergo segregation and independent assortment during meiosis.
2. Mendel’s laws correlate with chromosome separation in meiosis.
a. The law of segregation
i. states that pairs of alleles separate from each other during gamete formation via
meiosis and
ii. depends on separation of homologous chromosomes in anaphase I.
b. The law of independent assortment
i. states that each pair of alleles sorts independently of other pairs of alleles during
gamete formation and
ii. depends on alternative orientations of chromosomes in metaphase I.
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B. 9.17 SCIENTIFIC THINKING: Genes on the same chromosome tend to be inherited
together
1. Bateson and Punnett studied plants that did not show a 9:3:3:1 ratio in the F 2
generation. What they found was an example of linked genes, which
a. are located close together on the same chromosome and
b. tend to be inherited together.
C. 9.18 Crossing over produces new combinations of alleles
1. Crossing over between homologous chromosomes produces new combinations of
alleles in gametes.
2. Linked genes can be separated by crossing over, forming recombinant gametes.
3. The percentage of recombinant offspring is the recombination frequency.
D. 9.19 Geneticists use crossover data to map genes
1. When examining recombinant frequency, Alfred H. Sturtevant, one of Morgan’s
students, found that the greater the distance between two genes on a chromosome, the
more points there are between them where crossing over can occur.
2. Recombination frequencies can thus be used to map the relative position of genes on
chromosomes.
3. Such a diagram of relative gene locations is called a linkage map.
V. Sex Chromosomes and Sex-Linked Genes
A. 9.20 Chromosomes determine sex in many species
1. Many animals have a pair of sex chromosomes, designated X and Y, that determine
an individual’s sex.
2. Among humans and other mammals,
a. individuals with one X chromosome and one Y chromosome are males and
b. XX individuals are females.
3. In addition, human males and females both have 44 autosomes (nonsex chromosomes).
4. In mammals (including humans),
a. the Y chromosome has a crucial gene, SRY, for the development of testes, and
b. an absence of the SRY gene directs ovaries to develop.
5. Grasshoppers, roaches, and some other insects have an X-O system, in which
a. O stands for the absence of a sex chromosome,
b. females are XX, and
c. males are XO.
6. In certain fishes, butterflies, and birds,
a. the sex chromosomes are Z and W,
b. males are ZZ, and
c. females are ZW.
7. Some organisms lack sex chromosomes altogether.
8. In most ants and bees, sex is determined by chromosome number.
a. Females develop from fertilized eggs and thus are diploid.
b. Males develop from unfertilized eggs. Males are thus
i. fatherless and
ii. haploid.
9. In some animals, environmental temperature determines the sex.
a. For some species of reptiles, the temperature at which the eggs are incubated during
a specific period of development determines whether the embryo will develop into
a male or a female.
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b. Global climate change may therefore impact the sex ratio of such species.
B. 9.21 Sex-linked genes exhibit a unique pattern of inheritance
1. Sex-linked genes are located on either of the sex chromosomes.
2. The X chromosome carries many genes unrelated to sex.
3. The inheritance of white eye color in the fruit fly illustrates an X-linked recessive trait.
C. 9.22 CONNECTION: Human sex-linked disorders affect mostly males
1. Most sex-linked human disorders are
a. due to recessive alleles and
b. seen mostly in males.
2. A male receiving a single X-linked recessive allele from his mother will have the
disorder.
3. A female must receive the allele from both parents to be affected.
4. Recessive and sex-linked human disorders include
a. hemophilia, characterized by excessive bleeding because hemophiliacs lack one or
more of the proteins required for blood clotting,
b. red-green colorblindness, a malfunction of light-sensitive cells in the eyes, and
c. Duchenne muscular dystrophy, a condition characterized by a progressive
weakening of the muscles and loss of coordination.
D. 9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human
male evolution
1. The Y chromosome provides clues about human male evolution because
a. Y chromosomes are passed intact from father to son and
b. mutations in Y chromosomes can reveal data about recent shared ancestry.
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