Development and Inheritance
Genetics and Inheritance
• Every somatic cell carries copies of the original 46
chromosomes in the zygote.
• These chromosomes and their component genes
constitute the individual’s genotype.
• The physical expression of the genotype is the
• Every somatic cell contains 23 pairs of
• 22 pairs are autosomes and one pair is the sex
chromosome (XY in males and XX in females).
Genes and Chromosomes
• Chromosomes contain DNA, and genes are
functional segments of DNA.
• The various forms of a gene are called alleles. If
both homologous chromosomes carry the same
allele of a particular gene, the individual is
homozygous;if they carry different alleles, the
individual is heterozygous.
• Alleles are considered dominant or recessive
depending on how their traits are expressed.
Genes and Chromosomes
• Combining maternal and paternal alleles in a
Punnett Square diagram allows us to predict the
characteristics of offspring.
• Simple inheritance vs. polygenic inheritance.
• Genetic recombination, or crossing over and
translocation, that occurs during meiosis, increases
the variation of male and female gametes.
• The human genome project has identified more
than 6800 of our estimated 100,000 genes,
including some of those responsible for inherited
• Fertilization or conception is the fusion of the male and
female gametes, that normally occurs in the uterine tube.
• Development is the gradual modification of physical and
physiological characteristics from conception to maturity.
• The creation of different cells types is differentiation.
• Inheritance is the transfer of genetically determined
characteristics from generation to generation. Genetics is
the study of the mechanism of inheritance.
• Prenatal development occurs before birth.
• Postnatal development begins at birth and continues to
maturity, when aging begins.
• Fertilization, or conception, normally occurs in the
uterine tube within a day after ovulation.
• Sperm cannot fertilize an oocyte until they have
• The acrosomal caps of the spermatozoa release
hyaluronidase and acrosin, enzymes required to
penetrate the corona radiata and zona pellucida.
• When a single spermatozoan contacts the oocyte
membrane, fertilization begins, and oocyte
• during activation, the oocyte completes meiosis II,
and polyspermy is prevented by membrane
depolarization and the cortical reaction.
• After activation, the female pronucleus and male
pronucleus fuse in a process called amphimixis.
• During prenatal development, differences in
the cytoplasmic composition of the
individual cells trigger changes in genetic
• The chemical interplay between developing
cells is called induction.
• The 9-month gestation period can be
divided into three trimesters.
The first trimester
• Cleavage and blastocyst formation.
• Cleavage subdivides the cytoplasm of the
zygote in a series of mitotic divisions;the
zygote becomes a pre-embryo and then a
• The blastocyst consists of an outer
trophoblast and an inner cell mass.
• During implantation, the blastocyst burrows
into the uterine endometrium.
• Implantation occurs about 7 days after
• As the trophoblast enlarges and spreads,
maternal blood flows through lacunae.
• After gastrulation, the blastodisc becomes
an embryo composed of endoderm,
ectoderm and mesoderm. (germ layers)
• Germ layers help form four extraembryonic
membranes:the yolk sac, amnion, allantois and chorion.
• The yolk sac is an important site for blood cell formation.
• The amnion encloses fluid that surrounds and cushions the
• The base of the allantois gives rise to the urinary bladder.
• Circulation within the vessels of the chorion provides
provides a rapid-transit system that links the embryo with
Placentation and Embryogenesis
• Chorionic villi extend outward into the maternal
tissues, forming an intricate branching network
through which maternal blood flows.
• As development proceeds, the umbilical cord
connects the fetus to the placenta.
• The trophoblast synthesizes hCG, estrogens,
progesterone, hPL, placental prolactin and relaxin.
• The first trimester is critical because events in the
first 12 weeks establish the basis for
Second and Third Trimesters
• In the second trimester, the organ systems increase in
complexity. During the third trimester, many of the organ
systems become fully functional.
• The fetus undergoes its largest weight gain in the last
• The developing fetus is fully dependent on maternal organs
for nourishment, respiration and waste removal.
• Maternal adaptations include increased respiratory rate,
tidal volume, blood volume, nutrient and vitamin intake,
GFR and uterine and mammary gland size.
Labor and delivery
• Progesterone produced by the placenta has an inhibitory
effect on uterine muscles;its calming action is opposed by
estrogens, oxytocin, and prostaglandins.
• At some point, multiple factors interact to produce labor
contractions in the uterine wall.
• The goal of labor is parturition.
• Labor can be divided into three stages:dilation, expulsion
• Premature labor may result in immature delivery or
• Twin births may be dizygotic or monozygotic.
• Postnatal development involves a series of life stages: the
neonatal period, infancy, childhood, adolescence and
• Senescence begins at maturity and ends in the death of an
• The neonatal period extends from birth to 1 month after.
• In the transition from fetus to neonate, the respiratory,
circulatory, digestive and urinary systems begin
• The newborn must also begin thermoregulation.
• Mammary gland cells produce protein rich colostrum-then
convert to milk.
Adolescence and Maturity
• Adolescence begins at puberty:1.the hypothalamus
increases its production of GnRH2. Circulating levels of
FSH and LH rise rapidly3.ovarian or testicular cells
become more sensitive to FSH and LH.
• These changes initiate gametogenesis and a sudden rise in
• These also produce gender-specific differences in the
structure and functions of many systems.
• Further changes occur when sex hormone levels decline at
menopause or the male climateric.
• Senescence then begins, producing gradual changes in the
functional capabilities of all systems.