Check Your Understanding 1.
Chromosomes are not visible during
interphase. Te DNA-containing material is in the form of dispersed
strands of chromatin. As mitosis begins, the chromatin coils and con-
denses, becoming visible as chromosomes. Chromosomes continue to
condense throughout prophase and are most visible during metaphase.
Chromosomes other than sex chromosomes are called autosomes.
An allele represented by a capital letter is presumed to be a dominant
Descriptions of his phenotype are blond, blue-eyed, and hairy
Te alleles on diﬀerent pairs of homologous chromosomes are
segregated independently of each other to diﬀerent gametes.
separations of linked genes on the same chromosome, producing gam-
etes with variable genomes.
Dominant genes are expressed. Tus if the
dominant gene is lethal, the carrier will probably not live very long or will
die during development.
In incomplete dominance, the heterozygote
has a phenotype intermediate between that of the dominant and reces-
sive alleles [for example, for the sickling gene, the dominant homozygote
(SS) has no evidence of sickling; the heterozygote (Ss) has sickle-cell trait;
and the homozygote for the recessive gene (ss) has sickle-cell anemia].
In codominance, both dominant alleles are expressed (as in ABO blood
A male always expresses an X-linked recessive allele because,
unlike a female, he does not have a second X containing homologous al-
leles to blunt or counteract the eﬀect. Te Y chromosome lacks most of
the genes carried on the X chromosome.
Height is an example of a
trait conferred by polygene inheritance in which several genes on diﬀer-
ent chromosomes contribute to the trait. Such traits show a distribution
of phenotypes that yields a bell-shaped curve.
Other genes, measles
in a pregnant woman, and lack of key dietary nutrients all may alter gene
Genomic imprinting labels genes as maternal or paternal.
Maternal mitochondrial DNA confers extranuclear inheritance.
Amniocentesis analyzes chemicals and cells in amniotic ﬂuid.
Ultrasound imaging is used to determine some aspects of fetal
development (fetal age for example) and is noninvasive.
Review Questions 1.
(1)d, (2)g, (3)b, (4)a, (5)f, (6)c, (7)e, (8)h;
(2)a, (3)b, (4)d, (5)c, (6)d, (7)f
Case Study 1.
A person’s genotype is a description of the alleles that they
carry. Phenotype refers to the way that those genes are expressed and
show up as traits in the body. Mrs. Rodriguez’s genotype is
phenotype is O. Mr. Rodriguez’s genotype is either
. Either one
of these genotypes would result in his phenotype, which is represented by
his A blood type.
Blood type is determined by multiple-allele inheri-
tance. Tree alleles (
represent the three diﬀerent blood types (A,
B, O). Te alleles for A and B blood type are codominant, which means
that they are both expressed when combined together. Te result of this
combination would be the AB blood type. Te allele for O blood type is
recessive to both the A and B blood type alleles.
Mr. Rodriguez’s geno-
type can be either
. We now know that Mr. Rodriguez’s mother
had O blood type, and that means that she carries two copies of the O
blood type allele (
). She must have passed on one of these alleles to Mr.
Rodriguez. Tis means that Mr. Rodriguez’s father must have passed on
the allele for the A blood type. Mr. Rodriguez’s genotype
We know that Mrs. Rodriguez’s genotype is
and Mr. Rodriguez’s
. Te results of a Punnett square reveal that there is a 50%
chance that Mrs. Rodriguez’s baby will have the O blood type and a 50%
chance that the baby will have the A blood type. Te B and AB blood type
require that one of the parents is carrying the B allele (
). Neither Mr.
Rodriguez nor Mrs. Rodriguez carries this allele, so there should be a 0%
chance of the baby having the B or AB blood type.
growth between divisions, so the resulting cells get smaller and smaller.
Te result is a large number of cells with a high surface-to-volume ratio.
Te trophoblast cells contribute to the formation of the placenta.
Te syncytiotrophoblast actually accomplishes implantation.
blastocyst secretes the hormone human chorionic gonadotropin, which
is detectable in the urine.
Te chorion develops from the trophoblast
and a layer of extraembryonic mesoderm.
Te decidua basalis coop-
erates with the chorionic villi to form the placenta.
Te placenta is
usually fully functional by the end of the third month of pregnancy.
Te amnion helps to maintain a constant temperature for the de-
veloping fetus, protects it from physical trauma, and prevents adhesion
of fetal parts.
Te allantois provides the basis of the umbilical cord,
which provides a pathway for the embryonic blood vessels to reach the
Before organogenesis can occur in earnest, the embryonic
body must fold and undercut to form a tubular embryo.
derm gives rise to essentially all body tissues except neural and epidermal
tissue and mucosae.
Te fetal period begins at the end of 8 weeks.
Diﬃcult breathing during late pregnancy is due to the fact that the
uterus is pressing against and crowding the diaphragm (and hence the
lungs). Te waddling gait is due to the relaxing eﬀect of relaxin on the
pelvic ligaments and the pubic symphysis.
While the exact cause of
morning sickness is unknown, it is thought to be due to the rising levels
of female sex hormones and hCG in the mother’s blood, which some-
times takes a while to get used to.
Te hormone hPL works with es-
trogen and progesterone to stimulate breast maturation in the pregnant
woman, promotes fetal growth, and exerts a glucose-sparing eﬀect on
Breech presentation is a buttock-ﬁrst presentation of the
baby during labor.
Prostaglandins are most responsible for triggering
Te descent of the baby’s head (the largest part of its body)
follows the widest dimensions of the bony pelvis.
Te foramen ovale
and ductus arteriosus allow most of the blood to bypass the heart.
the most part, the special fetal circulatory modiﬁcations are occluded at
birth or shortly therea±er.
Oxytocin causes the let-down reﬂex.
Review Questions 1.
(1)a, (2)b, (3)b, (4)a;
(1)e, (2)f, (3)a, (4)d, (5)h, (6)b, (7)
Case Study 1.
A hospital pregnancy test works by detecting human cho-
rionic gonadotropin (hCG) in the blood. Home pregnancy test kits detect
this hormone in urine.
Human chorionic gonadotropin is a hormone
produced by the trophoblast cells that helps to maintain the viability of
the corpus luteum. Te corpus luteum is essential during early pregnancy
because it is responsible for the production of estrogen and progesterone.
Tese two hormones are required to maintain the uterus during the early
stages of a pregnancy.
While any amount of trauma can be cause for
concern with a pregnancy, Mrs. Rodriguez’s accident occurred prior to
the formation of the placenta. If her accident had occurred a few weeks
later, her blood loss may have had an impact on the oxygenation of her
First, the egg that Mrs. Rodriguez released during her
most recent ovulation is fertilized to form the zygote. Over the next week,
embryonic development features many rapid rounds of cell division as the
embryo moves through the uterine tube. Cleavage is the very early period
of rapid cell division. By 72 hours a±er fertilization, the embryo is a ball
of 16 or more cells called the morula. Around day 4, a ﬂuid-ﬁlled cavity
appears, which signals the formation of the blastocyst. Approximately
one week a±er fertilization, the embryo has made its way into the uterus
and begins the process of implantation.
At this time, Mrs. Rodriguez’s
embryo transforms into a three-layered structure as the result of a process
called gastrulation. Tis process involves the rearrangement and migration
of cells as the three primary germ layers (ectoderm, mesoderm, and en-
doderm) are formed. Tese three layers serve as the primitive tissues from
which all of the body organs will be formed as development continues.