Brian is a college student. His genetics assignment is to do a family
pedigree for dimples in the cheeks. Absence of dimples is recessive;
presence of dimples reﬂects a dominant allele. Brian has dimples,
as do his three brothers. His mother and maternal grandmother are
dimple free, but his father and all other grandparents have dimples.
Construct a pedigree spanning three generations for Brian’s family.
Show the phenotype and genotype for each person.
Mr. and Mrs. Lehman have sought genetic counseling. Mrs.
Lehman is concerned because she is unexpectedly pregnant and
her husband’s brother died of Tay-Sachs disease. She can recall
no incidence of Tay-Sachs disease in her own family. Do you
think biochemical testing should be recommended to detect the
deleterious gene in Mrs. Lehman? Explain your answer.
±e Browns are both carriers of the recessive allele that causes
the metabolic disorder called phenylketonuria. What is the
probability of each of the following occurring? (a) All three
children will have the disorder. (b) None of their three children
will have the disorder. (c) One or more of their children will
have the disorder. (d) At least one of their children will be
A woman with blood type A has two children. One has type O
blood and the other has type B blood. What is the genotype of
the mother? What are the genotype and phenotype of the father?
What is the genotype of each child?
In skin color inheritance, what will be the relative range of
pigmentation in oﬀspring arising from the following parental
Compare and contrast amniocentesis and chorionic villus sampling
as to the time at which they can be performed and the techniques
used to obtain information on the fetus’s genetic status.
and Clinical Application
A color-blind man marries a woman with normal vision. ±e
woman’s father was also color-blind. (a) What is the chance that
their ﬁrst child will be a color-blind son? A color-blind daughter?
(b) If they have four children, what is the chance that two will be
color-blind sons? (Be careful on this one.)
Related Clinical Terms
Chromosomal aberration in which part of a chromosome
Formerly called mongolism, this condition
usually reﬂects the presence of an extra autosome (trisomy of
chromosome 21). ±e child has slightly slanted eyes, ﬂattened
facial features, a large tongue, and a tendency toward short
stature and stubby ﬁngers. Some, but not all, aﬀected people
are mentally retarded. Down syndrome individuals have
mitochondrial defects, which have been associated with
neurodegeneration in other disorders. A distinctive feature
of Down syndrome is early onset of Alzheimer’s disease. ±e
most important risk factor appears to be advanced maternal (or
change) A permanent structural change in a
gene. ±e mutation may or may not aﬀect function, depending
on the precise site and nature of the alteration.
Abnormal segregation of chromosomes during
meiosis, resulting in gametes receiving two or no copies of
a particular parental chromosome. It is more common in
female meiosis than in male meiosis. If the abnormal gamete
participates in fertilization, the resulting zygote will have an
abnormal chromosomal complement (monosomy or trisomy)
for that particular chromosome (as in Down syndrome).
AT T H E C L I N I C
We will now return to the case of
Maria Rodriguez, whom we met in
Chapter 28. Recall that Mrs. Rodriguez
had found out that she was pregnant
and was concerned about the health of her baby. Mrs. Rodriguez
went on to have a normal pregnancy without complications. During
Mrs. Rodriguez’s treatment in the hospital, she discovered that her
blood is type O. During one of her regular visits to her doctor, Mrs.
Rodriguez asked what type of blood her baby will have, given that
her husband’s blood type is type A. Her doctor explained to Mrs.
Rodriguez that she would need to know more about the blood type
of Mr. Rodriguez’s parents.
Describe the difference between genotype and phenotype.
What are the phenotypes and genotypes of Mr. and Mrs.
Describe the type of inheritance that is represented by blood
Mr. Rodriguez’s mother has O blood type and his father has A
blood type. Explain how this information helps you understand
Mr. Rodriguez’s genotype.
What is the probability that Mrs. Rodriguez’s baby will have O
blood type? A? B? AB?
(Answers in Appendix H)