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CHAPTER
29
Heredity
Objectives
The Vocabulary of Genetics
1. Define allele.
2. Differentiate between genotype and phenotype.
Types of Inheritance
4. Compare and contrast dominant-recessive inheritance with incomplete dominance and
codominance.
Environmental Factors in Gene Expression
7. Provide examples illustrating how gene expression may be modified by environmental
factors.
Genetic Screening, Counseling, and Therapy
Suggested Lecture Outline
I. The Vocabulary of Genetics (pp. 1096–1097; Fig. 29.1)
A. Introduction (p. 1096; Fig. 29.1)
1. The nuclei of all human cells except gametes contain the diploid number of
a. Homologous chromosomes are pairs of chromosomes, one paternal and one
maternal, which carry the same genes, but do not necessarily express the trait in the
same way.
B. Chromosomes are paired, with one coming from each parent, and the genes on those
chromosomes are also paired (pp. 1096–1097).
1. Alleles are any two matched genes at the same locus (location) on homologous
chromosomes.
a. If the two alleles controlling a trait are the same, the genotype is homozygous.
b. If the two alleles controlling a trait are different, the genotype is heterozygous.
C. Genotype and Phenotype (p. 1097)
1. A person’s genetic makeup is called his or her genotype.
2. The way the genotype is expressed in the body is that individual’s phenotype.
II. Sexual Sources of Genetic Variation (pp. 1097–1099; Figs. 29.2–29.3)
A. During metaphase of meiosis I, the alignment of the tetrads along the center of the cell is
completely random, allowing for the random distribution of maternal and paternal
chromosomes into the daughter nuclei (pp. 1097–1098; Fig. 29.2).
1. In meiosis I, the two alleles determining each trait are segregated, or distributed to
B. During meiosis I homologous chromosomes may exchange gene segments, a process
called crossing over, which gives rise to recombinant chromosomes that have
contributions from each parent (p. 1098; Fig. 29.3).
III. Types of Inheritance (pp. 1099–1101; Figs. 29.4–29.6; Tables 29.1–29.2)
A. Dominant-Recessive Inheritance (pp. 1099–1100; Fig. 29.4; Table 29.1)
1. A Punnett square is used to determine the possible gene combinations resulting from
the mating of parents of known genotypes and the probability of each combination.
3. Genetic disorders caused by dominant genes are uncommon because lethal dominant
genes are always expressed and result in the death of the embryo, fetus, or child.
B. Some traits exhibit incomplete dominance, wherein the heterozygote has a phenotype
intermediate between those of the homozygous dominant and the homozygous recessive
(p. 1100).
D. Inherited traits determined by genes on the sex chromosomes are said to be sex-linked
(pp. 1100–1101; Fig. 29.5).
1. The Y chromosome is much smaller than the X chromosome and lacks many of the
genes present on the X that code for nonsexual characteristics, such as red-green color
blindness.
IV. Environmental Factors in Gene Expression (p. 1102)
A. In many situations, environmental factors override or at least influence gene expression.
V. Nontraditional Inheritance (pp. 1102–1103)
A. Nontraditional inheritance is the result of control mechanisms outside of the coding
portion of DNA and of the chromosome entirely (p. 1102).
B. RNA-only genes are found throughout the non-protein coding DNA and code for RNAs
that regulate gene expression (p. 1102).
D. Genomic imprinting somehow tags genes during gametogenesis as either paternal or
maternal and confers important functional differences in the resulting embryo (pp. 1102–
1103).
VI. Genetic Screening, Counseling, and Therapy (pp. 1103–1105; Fig. 29.7)
A. When one of the parents of a developing embryo displays a recessive disorder, it is
important to determine if the other partner is a heterozygote and thus a carrier for that
trait (p. 1103).
B. Fetal testing is used when there is a known risk of a genetic disorder (pp. 1103–1104;
Fig. 29.7).
1. The most common type of fetal testing is amniocentesis, in which a needle is inserted
into the amniotic sac to withdraw amniotic fluid for testing.
Cross References
Additional information on topics covered in Chapter 29 can be found in the chapters listed below.
1. Chapter 3: Mitosis; chromatin
Lecture Hints
1. Stress that an individual receives a member of an allele from each parent.
2. Students often confuse the terms genotype and phenotype. Mention that the genotype is
the genetic component of a trait, but the phenotype is how those genes are expressed in
the individual.
Activities/Demonstrations
1. Audiovisual materials are listed in the Multimedia in the Classroom and Lab section of
this Instructor Guide (p. 387).
2. Use pipe cleaners and craft balls (with holes in them) to form chromosomes. Then use
those “chromosomes” to demonstrate various genotypes and other genetic patterns.
Critical Thinking/Discussion Topics
1. Describe the tests available to detect various genetic and/or development problems prior
to birth.
Library Research Topics
1. Research several types of birth defects by system category, such as skeletal system,
circulatory system, and so on.
2. Investigate the chromosomal aberrations that result in congenital disorders.
3. Study the multiple-allele inheritance disorders.
List of Figures and Tables
All of the figures in the main text are available in JPEG format, PPT, and labeled & unlabeled
format on the Instructor Resource DVD. All of the figures and tables will also be available in
Transparency Acetate format. For more information, go to www.pearsonhighered.com/educator.
Figure 29.1 Preparing a karyotype.
Figure 29.2 Gamete variability resulting from independent assortment.
Figure 29.3 Crossover and genetic recombination.
Figure 29.4 Genotype and phenotype probabilities resulting from a mating of two
heterozygous parents.
Answers to End-of-Chapter Questions
Multiple-Choice and Matching Question answers appear in Appendix H of the main text.
Short Answer Essay Questions
3. The mechanisms that lead to genetic variations in gametes are segregation and
independent assortment of chromosomes, crossover of homologues and gene recombina-
tion, and random fertilization. Segregation implies that the members of the allele pair
determining each trait are distributed to different gametes during meiosis. Independent
4.
T
TTT Tt
Tt tt
t
t
a. Looking at the Punnett square above, we can see that 75% or 3/4 tasters are possible—
T
tTt tt
Tt tt
t
t
b. As seen in the Punnett square above, tasters make up two out of four of the possible
genetic combinations, with the other two being nontasters. From this, we can see that
the percentage of tasters is 50%, and nontasters 50%. Looking at the possible
combinations of alleles, we can see that 50% are homozygous recessive, 50%
are heterozygous and 0% are homozygous dominant. (pp. 1099–1100)
type B blood, has the IBi genotype. (pp. 1099–1100)
7. a. AABBCC × aabbcc
(very dark × very light)
offspring genotype: AaBbCc
offspring phenotype: medium range of color
b. AABBCC × AaBbCc
(very dark × medium color)
This is an example of polygene inheritance. (p. 1101)
8. Amniocentesis is done after the 14th week. A needle is inserted through the mother’s
abdominal wall to remove fluid (or fetal cells) to be tested. Chorionic villus sampling can
be done at 8 weeks. A tube is inserted through the vagina and cervical os. It is guided by
ultrasound to an area where a piece of placenta can be removed. (p. 1103)
Critical Thinking and Clinical Application Questions
1. Given that the maternal grandfather is color-blind, he has the genotype XcY. This
means the mother, while she has normal vision, carries the color-blind gene and has the
genotype XCXc. The color-blind man she marries has the same genotype as her
father, XcY. This produces the following Punnett square for their offspring:
a. There is a 50% chance that the first child is a son, and a 50% chance he is color-blind,
so the probability of a color-blind son is 0.50 × 0.50 = 0.25 or 25%. Since the
probability of a daughter is also 50%, and the chance she is color-blind is 50%, the
combined probability will be the same for the first child being a color-blind daughter,
25%.
2.
paternal
grandmother
dimples
(D?)
father
dimples
(DD)
mother
no dimples
(dd)
dimples
(Dd)
(Brian)
dimples
(Dd)
dimples
(Dd)
dimples
(Dd)
paternal
grandfather
dimples
(D?)
maternal
grandfather
dimples
(Dd)
maternal
grandmothe
r
no dimples
(dd)
3. Mrs. Lehman should be tested because Tay-Sachs is a recessive disorder. The baby
would have to get both recessive genes for the disease. If there is no incidence of the
disease in her family, the recessive gene could be there but would always be masked by
the dominant gene. If her husband carries one recessive gene and she carries a recessive
gene, the baby would have a chance of getting two recessive alleles and having the
disease. (p. 1100)
Suggested Readings
Barry, Patrick. “Finding the Golden Genes.” Science News 174 (3) (Aug. 2008): 16–21.
Berletch, J. B., et al. “Genes that Escape from X Inactivation.” Human Genetics 130 (2)
(Aug. 2011): 237–245.
Cibelli, Jose B., et al. “The First Human Cloned.” Scientific American 286 (1) (Jan. 2002):
44–51.
Gardner, Thomas A. “Gene Therapy.” Science and Medicine 8 (3) (May/June 2002):
124–125.
Hamzelou, Jessica. “Ma’s Gene Does Different Things to Pa’s Copy.” New Scientist 209
(2797) (Jan. 2011): 8.
Heard, Edith, and James Turner. “Function of the Sex Chromosomes in Mammalian
Fertility.” Cold Spring Harbor Perspectives in Biology 3 (10) (Oct. 2011).
