CHAPTER
9: Patterns of Inheritance
I. Introduction
A. Dogs are one of man’s longest genetic experiments.
1. Over thousands of years, humans have chosen and mated dogs with specific traits.
2. The result has been an incredibly diverse array of dogs with distinct
a. body types and behavioral traits.
II. Mendel’s Laws
9.1 The science of genetics has ancient roots
1. Pangenesis, proposed around 400 B.C. by Hippocrates, was an early explanation for
inheritance that suggested that
a. particles called pangenes came from all parts of the organism to be incorporated into eggs or
sperm, and characteristics acquired during the parents’ lifetime could be transferred to the
offspring.
2. Aristotle rejected pangenesis and argued that instead of particles, the potential to
produce the traits was inherited.
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.
9.2 Experimental 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, deduced the principles of genetics by breeding garden
peas, and relied upon 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. Heritable features that vary among individuals, such as flower color, are called a
character.
6. Each variant for a character, such as purple or white flowers, is a trait.
7. True-breeding varieties result when self-fertilization produces offspring all identical to the parent.
8. The offspring of two different varieties are hybrids.
9. The cross-fertilization is a hybridization, or genetic cross.
10. True-breeding parental plants are the P generation. Hybrid offspring are the F1 generation.
11. A cross of F1 plants produces an F2 generation.
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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 & ¼ white flowers.
3. The all purple F1 generation did not produce light purple flowers, as predicted by the blending hypothesis.
4. Mendel needed to explain why the
a. white color seemed to disappear in the F1 generation and white color reappeared in one quarter of
the F2 offspring.
5. Mendel observed the same patterns of inheritance for six other pea plant characters.
6. 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.
7. 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.
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, alleles of a gene reside at the same locus.
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a. Homozygous individuals have the same allele on both homologues.
b. Heterozygous individuals have a different allele on each homologue.
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. 9/16 had round yellow seeds, 3/16 had wrinkled yellow seeds, 3/16 had round green seeds &
1/16 had wrinkled green seeds
3. Mendel needed to explain why the F2 offspring
a. had new nonparental combinations of traits and a 9:3:3:1 phenotypic ratio.
4. Mendel
a. suggested that the inheritance of one character has no effect on the inheritance of
another, suggested that the dihybrid cross is the equivalent to two monohybrid crosses, and
b. called this the law of independent assortment.
5. Figure 9.5C demonstrates the law of independent assortment as it applies to two
characters in Labrador retrievers:
a. black versus chocolate color, normal vision versus progressive retinal atrophy.
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 genotypes.
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Black coat (B) & Normal vision are dominant traits in Labs. Brown coat (b) & blindness (n) are recessive traits.
B
____
_______
_______
N
____
_______
_______
.
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 the reforming of pairs at fertilization.
2. The probability scale ranges from 0 to 1. An event that is
a. certain has a probability of 1 and 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. (this AND this AND this….)
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. (this OR this OR this…)
Problem: Two plants with genotype AABbCC and AaBbCc are crossed. What is the probability of
AABBCC genotype?
AaBbCc genotype?
aabbccc genotype?
9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees
1. In a simple dominant-recessive inheritance of dominant allele A and recessive allele a,
a. a recessive phenotype always results from a homozygous recessive genotype (aa), but
b. a dominant phenotype can result from either
i. the homozygous dominant genotype (AA) or a heterozygous genotype (Aa).
2. Wild-type traits, those prevailing in nature, are not necessarily specified by dominant alleles.
3. The inheritance of human traits follows Mendel’s laws.
4. 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.
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9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene
1. Inherited human disorders show either
a. recessive inheritance in which
i. two recessive alleles are needed to show disease,
ii. heterozygous parents are carriers of the disease-causing allele, and
iii. the probability of inheritance increases with inbreeding, mating between close relatives.
b. dominant inheritance in which
i. one dominant allele is needed to show disease and
ii. dominant lethal alleles are usually eliminated from the population.
2. Recessive human disorders include
a. cystic fibrosis (c),resulting in excessive thick mucus secretions in the lungs
b. sickle cell (s) resulting in abnormal red blood cell shape and inability to carry oxygen
i. at molecular level sickle cell shows different genetics
c. albinism (a) causes loss of pigmentation
3. Dominant human disorders include
a. achondroplasia, (A) resulting in dwarfism
b. Huntington’s disease (H), a degenerative disorder of the nervous system.
9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy
1. New technologies offer ways to get genetic information before conception, pregnancy,& after birth.
2. Genetic testing of parents can identify potential parents who are 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 and biochemical tests on cultured fetal cells to detect other conditions
b. Chorionic villus sampling removes a sample of chorionic villus tissue from the placenta and
permits similar karyotyping and biochemical tests.
4. Blood tests on the mother at 14–20 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 determining should anything be done as a result of the testing.
Variations on Mendel’s Laws
9.11 Incomplete dominance results in intermediate phenotypes
1. Mendel’s pea crosses always looked like one of the parental varieties, called complete dominance.
2. For some characters, the appearance of F1 hybrids falls between the phenotypes of the two parental
varieties. This is called incomplete dominance, in which neither allele is dominant over the other
& expression of both alleles occurs.
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3. One example of incomplete dominance in humans is hypercholesterolemia (h), in which
a. dangerously high levels of cholesterol occur in the blood and heterozygotes have intermediately
high cholesterol levels. (HH = normal cholesterol Hh = 2x chlolesterol hh = 5x cholesterol)
H
h
h
h
_______
_______
_______
_______
9.12 Many genes have more than two alleles in the population
1. Although an individual can at most carry two different alleles for a particular gene, more than two
alleles often exist in the wider 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 & B alleles are both expressed in heterozygous individuals, known as codominance.
5. In codominance,
a. neither allele is dominant over the other &expression of both alleles is observed as a distinct
phenotype in the heterozygous individual.
b. AB blood type is an example of codominance.
9.13 A single gene may affect many phenotypic characters
1. Pleiotropy occurs when one gene influences many characteristics.
2. Sickle-cell disease is a human example of pleiotropy. This disease
a. affects the type of hemoglobin produced, the shape of red blood cells, causes anemia &organ
damage.
3. Sickle-cell alleles are recessive at the gene level; sickle-cell and nonsickle alleles are codominant at
the molecular level.
4. Carriers of sickle-cell disease are resistant to malaria.
9.14 A single character may be influenced by many genes
1. Many characteristics result from polygenic inheritance, in which a single phenotypic character results from the additive effects of two or more genes.
2. Human skin color and height are examples of polygenic inheritance.
Problem: If an individual has a genotype of AaBbcc for skin color – which of the following individuals would
also have similar shades of skin tone?
AAbbcc,
aaBBcc,
AabbCc,
Aabbcc,
aaBbCc?
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9.15 The environment affects many characters
1. Many characters result from a combination of heredity and the environment.
a. skin color is affected by exposure to sunlight, susceptibility to diseases, such as cancer
b. identical twins show some differences.
2. Nature vs. Nurture- a person's development is predisposed in his/her DNA, or a majority of it is
influenced by this life experiences and his/her environment
The Chromosomal Basis of Inheritance
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 depends on separation of homologous chromosomes in
anaphase I.
b. The law of independent assortment depends on alternative orientations of chromosomes in metaphase I.
9.17 SCIENTIFIC DISCOVERY: 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 F2 generation.
2. What they found was an example of linked genes, which are located close together on the same
chromosome and tend to be inherited together.
Sex Chromosomes and Sex-Linked Genes
9.20 Chromosomes determine sex in many species
1. Many animals have a pair of sex chromosomes,
a. designated X and Y, that determine an individual’s sex.
2. In mammals,
a. males have XY sex chromosomes, females have XX sex chromosomes,
b. the Y chromosome has genes for the development of testes, and an absence of the Y allows
ovaries to develop.
3. Many other organisms have different genetic systems that determine sex
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.
Key
XR = red
Xr = white
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9.22 CONNECTION: Human sex-linked disorders affect mostly males
1. Most sex-linked human disorders are
a. due to recessive alleles and 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 color blindness, 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.
Chapter 9: Patterns of Inheritance
Word Parts
-centesis = a puncture
hetero- = different
co- = together
homo- = same
cyto- = cell
mono- = one
di- = two
pedi- = a child
geno- = offspring
pheno- = appear
hemo- = blood
pleio- = more
poly- = many;
gene- = produce
re- = again;
com- = together;
bin- = two at a time
Vocabulary
1. hybrid- An offspring of parents of two different species or of two different varieties of one species;
an offspring of two parents that differ in one or more inherited traits; an individual that is heterozyous
for one or more pairs of genes.
2. F1 generation- The offspring of two parental (P generation) individuals; F1 stands for first filial.
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3. carrier- An individual who is heterozygous for a recessively inherited disorder and who therefore
does not show symptoms of that disorder but who may pass on the recessive allele to offspring.
4. pedigree- A family genetic tree representing the occurrence of heritable traits in parents and offspring across a number of generations. A pedigree can be used to determine genotypes of matings that
have already occurred.
5. true-breeding- Referring to organisms for which sexual reproduction produces offspring with inherited traits identical to those of the parents. The organisms are homozygous for the characteristics
under consideration.
6. law of segregation- A general rule in inheritance (originally formulated by Gregor Mendel) that
individuals have two alleles for each gene and that when gametes form by meiosis, the two alleles
separate, each resulting gamete ending up with only one allele of each gene; also known as Mendel's
first law of inheritance.
7. cystic fibrosis- A genetic disease that occurs in people with two copies of a certain recessive allele;
characterized by an excessive secretion of mucus and vulnerability to infection; fatal if untreated.
8. Huntington's disease- A human genetic disease caused by a single dominant allele; characterized
by uncontrollable body movements and degeneration of the nervous system; usually fatal 10 to 20
years after the onset of symptoms.
9. rule of addition- A rule stating that the probability that an event can occur in two or more alternative ways is the sum of the separate probabilities of the different ways. (this OR this OR this….)
10. character- A heritable feature that varies among individuals within a population, such as flower
color in pea plants or eye color in humans.
11. homozygous- Having two identical alleles for a given gene.
12. hemophilia- A human genetic disease caused by a sex-linked recessive allele; characterized by
excessive bleeding following injury.
13. test cross- The mating between an individual of unknown genotype for a particular characteristic
and an individual that is homozygous recessive for that same characteristic. The testcross can be used
to determine the unknown genotype (homozygous dominant versus heterozygous).
14. achondroplasia- A form of human dwarfism caused by a single dominant allele; the homozygous
(AA) condition is lethal.
15. ABO blood groups- Genetically determined classes of human blood that are based on the presence or absence of carbohydrates A and B on the surface of red blood cells. The ABO blood group
phenotypes, also called blood types, are A, B, AB, and O.
16. codominant- Inheritance pattern in which a heterozygote expresses the distinct trait of both alleles.
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17. P generation- The parent individuals from which offspring are derived in studies of inheritance; P
stands for parental.
18. incomplete dominance- A type of inheritance in which the phenotype of a heterozygote (Aa) is
intermediate between the phenotypes of the two types of homozygotes (AA and aa).
19. complete dominance- A type of inheritance in which the phenotypes of the heterozygote and
dominant homozygote are indistinguishable.
20. phenotype- The expressed traits of an organism.
21. polygenic inheritance- The additive effects of two or more gene loci on a single phenotypic characteristic.
22. recessive allele- An allele that has no noticeable effect on the phenotype of a gene when the individual is heterozygous for that gene.
23. ultrasound imaging- A technique for examining a fetus in the uterus. High-frequency sound
waves echoing off the fetus are used to produce an image of the fetus.
24. red-green colorblindness- A category of common, sex-linked recessive human disorders
involving several genes on the X chromosome; characterized by a malfunction of light-sensitive
cells in the eyes; affects mostly males but also homozygous females.
25. linked genes- Genes located near each other on the same chromosome that tend to be inherited
together.
26. Punnett square- A diagram used in the study of inheritance to show recessive allele the results of
random fertilization.
27. sex chromosome- A chromosome that determines whether an individual is male or female; 23rd
pair.
28. genotype- The genetic makeup of an organism.
29. trait- A variant of a character found within a population, such as purple or white flowers in pea
plants.
30. dominant allele- The allele that determines the phenotype of a gene when the individual is heterozygous for that gene.
32. cross-fertilization- The fusion of sperm and egg derived from two different individuals.
33. Duchenne muscular dystrophy- A human genetic disease caused by a sex-linked recessive
allele; characterized by progressive weakening and a loss of muscle tissue.
34. heterozygous- Having two different alleles for a given gene.
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35. cross- A mating of two sexually reproducing individuals; often used to describe a genetics experiment involving a controlled mating (a "genetic cross").
36. dihybrid cross- An experimental mating of individuals differing in two characters.
37. law of independent assortment- A general rule in inheritance (originally formulated by Gregor
Mendel) that when gametes form during meiosis, each pair of alleles for a particular characteristic
segregate independently of other pairs; also known as Mendel's second law of inheritance.
38. rule of multiplication- A rule stating that the probability of a compound event is the product of
the separate probabilities of the independent events. (this AND this AND this…)
39. inbreeding- Mating between close blood relatives.
40. F2 generation- The offspring of the F1 generation; F2 stands for second filial.
41. monohybrid cross- An experimental mating of individuals differing in a single character.
42. allele- An alternative version of a gene.
43. sex-linked gene- A gene located on a sex chromosome. In humans, the vast majority of sex-linked
genes are located on the X chromosome.
44. pleiotropy- The control of more than one phenotypic characteristic by a single gene.
45. amniocentesis- A technique for diagnosing genetic defects while a fetus is in the uterus. A sample
of amniotic fluid, obtained by a needle inserted into the uterus, is analyzed for telltale chemicals and
defective fetal cells.
46. self-fertilize- A form of reproduction that involves fusion of sperm and egg produced by the same
individual organism.
47. chorionic villus sampling (CVS)- A technique for diagnosing genetic defects while the fetus is in
an early development stage within the uterus. A small sample of the fetal portion of the placenta is
removed and analyzed.
48. chromosome theory of inheritance- A basic principle in biology stating that genes are located on
chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns.
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