Chapter 27 - Development & Heredity

What are the most important changes that occur during the 4th month of the fetal period?

- Body growth is rapid. - The lower limbs lengthen and joints are forming as the skeleton continues to ossify. - The fetus has nipples and some hair. = The kidneys are well formed, the digestive glands are forming, and the heartbeat can be heard with a stethoscope.

What are the most important changes that occur during the 5th month of the fetal period?

- Growth slows and the lower limbs achieve their correct proportions relative to the body. - Hair grows on the head, and the skin is covered by a downy hair called lanugo and a white to gray secretion composed of shed epithelial cells and sebum known as vernix caseosa.

What are the most important changes that occur during the 7th month of the fetal period?

- The eyelids open completely, and fat is deposited in subcutaneous tissue so the skin is slightly smoother, but still wrinkled and red. - The fetus usually turns upside down, assuming the vertex position.

What are the most important changes that occur during the 3rd month of the fetal period?

- The fetal body lengthens as its head growth slows; the upper limbs grow to their birth length. - Ossification begins in most bones. - The genitals are distinguishable as male or female.

What are the most important changes that occur during the 6th month of the fetal period?

- The fetus gains significant weight. - Eyebrows and eyelashes appear, and the eyelids, which have been fused since the third month, are partially open. - The fetus' skin is wrinkled and translucent and the lungs begin to produce surfactant, which is key to survival if the infant is born prematurely.

What changes occur in the urinary system in mothers during pregnancy?

- The fetus produces metabolic wastes that the mother's kidneys must excrete. - The glomerular filtration rate can rise by up to 50% during pregnancy. - The uterus tends to compress the urinary bladder as it expands, sometimes leading to stress incontinence, or unintentional loss of urine.

What changes occur in the female cardiovascular system during pregnancy?

- The growing placenta requires increased blood volume, and as the offspring grows, it needs more oxygen and produces more waste that must be removed. - The mother's blood volume and cardiac output increase while blood pressure may rise to some extent. - As the uterus grows and presses on pelvic blood vessels, venous return from the lower limbs may be impaired and sometimes results in varicose veins and leg swelling, or edema.

How does the female reproductive system change during pregnancy?

- The uterus enlarges greatly over the 9-month pregnancy and eventually extends upward from its normal location in the pelvic cavity to reach the level of the xiphoid process of the sternum. - By about 28 weeks, the uterus reaches above the umbilicus, and as the uterus continues to expand, the abdominal organs are displaced upward, compressing the diaphragm, and pressing on the urinary bladder. - The mammary glands also undergo significant changes in preparation for lactation after the newborn is born.

What are the criteria of the Apgar score?

1. Skin color 2. Pulse rate 3. Respiratory level 4. Muscle activity 5. Grimace (reflexes)

What pair of chromosomes are the sex chromosomes?

23rd pair

How long is the expulsion stage of labor?

30 mins to 1 hour

How long is the placental stage of labor?

30 minutes

What does a punnett square predict?

A Punnett square can predict the possible offspring genotypes that might occur if a man and woman have a child.

What happens during week 3 in gastrulation?

A bilaminar embryonic disc contains two cell layers: the epiblast and hypoblast

What does each pair of diploid homologous chromosomes contain?

A copy from each parent, so in total our cells have 23 total maternal chromosomes and 23 total paternal chromosomes. Each member of a pair of autosomes, chromosomes 1 through 22, carries the same genes.

What is a karyotype?

A full set of the 23 chromosome pairs can be arranged and displayed in what is called a karyotype

What is the corona radiata?

A layer of granulosa cells that surrounds an ovulated secondary oocyte

What is cleavage?

A series of rapid mitotic divisions that produce small, genetically identical cells called blastomeres

What forms between the pronuclei?

A spindle forms between the pronuclei, and the chromosomes intermix. Spindle fibers form between the male and female pronuclei, and the pronuclear membranes break down, allowing the chromosomes to mingle

What is meconium?

A tarry, green fecal material made up of bile and other wastes that is first excreted by the newborn.

What is the primitive streak?

A thin groove on the dorsal surface of the epiblast, which elongates along the cephalic-caudal (head-tail) line of the embryo, establishing the head and tail regions, the right and left sides, and the dorsal and ventral surfaces of the embryo

When does gastrulation begin?

After the primitive streak has completed its development and starts as cells detach from the epiblast layer and move into the primitive streak, a process called ingression.

What are the kidneys like after birth?

Although the kidneys are almost fully developed at birth, they are less capable than adult kidneys of concentrating urine. Babies are born with a near-adult level of IgG antibodies acquired passively from the mother through the placenta.

What is a genotype?

An individual's genetic makeup

What is a genome?

An individuals entire DNA

What are the inner cell mass?

An inner cluster of rounded cells, the inner cell mass, or embryoblasts form the embryo proper—the developing body.

What are trophoblast cells?

An outer layer of large, flattened cells, called trophoblast cells surround the fluid-filled cavity, participate in forming the placenta, the temporary organ that provides nutrients and oxygen to the conceptus, and also suppress the mother's immune system, preventing it from attacking the conceptus

When is a fetus considered full-term and ready to be born?

Approximately 266 days after fertilization

When does cleavage happen?

Approximately 30 hours after fertilization occurs, the zygote begins cleavage

What happens during the skeletal system during the first year of life?

As for the skeleton, various areas of it are cartilaginous rather than ossified at birth; these will be converted to bone over time.

When has the embryonic period begun?

At the beginning of the third week of development

When does human development begin?

Begins at fertilization, with the union of gametes, and continues throughout life

How does breast milk impact meconium?

Breast milk also acts as a mild laxative and helps cleanse meconium

What is the nutrition side of breast milk?

Breast milk has a high fat content and contains substances such as essential fatty acids, growth factors, enzymes to help digest the milk, IgA, and immune chemicals such as complement proteins and interferon, which makes it easily digestible.

When have all three primary germ layers finished forming?

By the end of week three

What is the Apgar score?

Calculated to evaluate whether a newborn is adjusting appropriately, its physical status is evaluated at 1 minute and 5 minutes after birth, based on five criteria.

What happens to sperm as they migrate to the oocyte?

Capacitation

Where do the most conspicuous anatomical changes occur between the prenatal and postnatal state of the fetus?

Cardiovascular system

What happens to fetal circulation and the fetal cardiovascular system after birth?

Change rapidly soon after birth in order to adjust to life without a placenta

What happens when the first sperm cell penetrates the zona pellucida and binds to the oocyte;s plasma membrane?

Changes in the oocyte and zona pellucida occur so that no more sperm can enter

What changes occur in the integumentary system in mothers during pregnancy?

Changes in the skin during pregnancy are more apparent in some women than in others. - Some experience increased pigmentation around the eyes and cheekbones in a masklike pattern called chloasma. - Stretch marks or striae, over the abdomen can occur as the uterus enlarges rapidly and the elastic fibers in the dermis break.

What are the sex chromosomes?

Consist of one X chromosome and one Y chromosome. The X and Y chromosomes determine the sex of the female.

What does sperm entry stimulate?

Cortical reaction

What are common fetal measurements of length?

Crown-rump length (CRL) or crown-heel length (CHL)

What is the cortical reaction?

Destroys sperm-binding receptors. Prevents additional sperm cells from binding

What do prostaglandins do in labor?

Dilate the cervix and, along with oxytocin, increase the strength of uterine contractions

What is the more common type of twining that results from the ovulation of two secondary oocytes at the same time?

Dizygotic or fraternal twins

When does the placenta form?

During the embryonic period; begins to provide nutrition and oxygen to the embryo and remove wastes

What happens to the nervous system during the first year of life?

During the first year of life, the nervous system grows and many neurons develop a myelin sheath

When is the conceptus referred to as an embryo?

During the five-week stage of the embryonic period

When do the flattened trilaminar embryonic disc change shape?

During the fourth week of development, when two types of folding cause the flat embryonic disc to become cylindrical: 1. Cephalocaudal folding 2. Transverse, or lateral folding

When do the extra embryonic membranes first appear?

During the second week of development, but continue to develop during the embryonic and fetal periods

What happens during the pre-embryonic period?

During this stage, the zygote divides mitotically to produce a multicellular structure, the blastocyst, which implants in the endometrium of the uterus

When does labor take place?

During week 38 of pregnancy, but is variable

What are the remaining cells of the epiblast after the endoderm and mesoderm have formed?

Ectoderm

What are the first cells to migrate in ingression?

Endoderm, which replaces the hypoblast

How long is the gestation period of pregnancy?

Extends from the mother's last menstrual period until birth, which is approximately 280 days or 40 weeks

How long does the embryonic period last?

Extends from week 3 through week 8 of gestation

What is colostrum?

For the first few days after birth, mammary glands secrete a thick yellowish fluid called colostrum.

What process does the embryonic period start with?

Gastrulation

What changes occur in the digestive system of mothers during pregnancy?

Good nutrition is necessary during all trimesters of pregnancy for the health of the conceptus. - A pregnant woman typically needs about 300 extra calories each day to supply both herself and the fetus. Specifically, protein, calcium salt, iron salt, and folic acid requirements increase from the recommended daily levels required for nonpregnant women. - During the first trimester, many pregnant women suffer from morning sickness, nausea and sometimes vomiting, thought to be related to elevated hCG, estrogen, and progesterone levels, but no definitive cause has been identified.

What are the most important changes during months 8 and 9 of the fetal period?

In the final months, fetal neurons form networks, and organs grow and develop. Blood cells form in bone marrow, and the digestive and respiratory systems complete development during the ninth month.

How is pregnancy subdivided?

Into three, 3-month long trimesters

What hormones are released by the placenta?

Involve the release of hCG, relaxin, and hormones released by the placenta, including human placental lactogen (hPL), or human chorionic somatomammotropin (hCS), melanocyte-stimulating hormone (MSH), and corticotropin-releasing hormone (CRH)

What are the series of events that expel the fetus called?

Labor

What long does the fetal period last?

Lasts from week 9 through week 8 of gestation, or until birth

What is the pre-embryonic period?

Lasts the fist 2 weeks after fertilization

What do the rising levels of progesterone during pregnancy lead to?

Leads to the formation of a mucus plug at the cervix. When the cervix begins to dilate, the mucus is discharged into the vagina. Labor may begin within hours after the mucus plug is discharged, or it may not occur for up to 2 weeks

What is the zona pellucida?

Located underneath the oocyte's corona radiata; enzymes released by the sperm catalyze reactions that digest holes in the zona pellucida

What are the anatomical and physiological changes to mothers post partum?

Many adjustments occur after birth to reverse changes made during pregnancy and to restore homeostasis to the mother's system.

Why do anatomical and physiological changes happen in women?

Many anatomical and physiological changes take place to support pregnancy and prepare a woman's body for childbirth and lactation, or breastfeeding.

What are multiple-allel traits?

Many human traits involve the interaction of multiple alleles.;ABO blood types exhibit the phenomenon of codominance, meaning that some alleles are equally dominant and are therefore equally expressed. IA and IB are codominant alleles, while i is recessive.

What are the second cells to migrate to a position between the epiblast and endoderm?

Mesoderm

What structures does the mesoderm differentiate into?

Mesodermal cells form a structure called the notochord, which serves to support and organize the embryo around a central axis.

What is the first trimester of pregnancy?

Months 1-3; In the first 12 weeks of development, all pre-embryonic and embryonic development is completed and fetal development begins.

What is the second trimester of pregnancy?

Months 4-6; During the second trimester, the fetus continues to grow and develop, and the pregnancy usually becomes obvious as the uterus and abdomen expand.

What is the third trimester of pregnancy?

Months 7-9; The fetus grows rapidly and gains a significant amount of weight.

How does the respiratory system change during pregnancy for the mother?

More oxygen is needed during pregnancy, and more carbon dioxide is produced that must be exhaled.

What happens when the head of the fetus pushes on and stretches the cervix?

More oxytocin is released! As more oxytocin is released, the myometrium contracts more forcefully and the placenta secretes more prostaglandins. Both effects cause the cervix to stretch more, which stimulates the release of more oxytocin in a positive feedback loop.

What are polygenic inheritance patterns?

Most inherited traits are controlled by the combined effects of two or more genes, a condition called polygenic inheritance.

What are patterns of inheritance?

Most inherited traits are determined by multiple alleles or by the interaction of several genes. The following features the different patterns of inheritance, and the genotypes and phenotypes in which they result.

How do sperm move?

Move by a whip-like movement of their flagella, or "tails."

What is multifactorial inheritance?

Multifactorial inheritance occurs when the combined effects of many genes are further influenced by environmental factors.

What form the 46 chromosomes?

Nucleotides, which are found in all somatic cells with a nucleus. Chromosomes are paired so that there are 23 pairs of homologous chromosomes in each nucleus.

What is implantation?

Occurs approximately 4-7 days after fertilization when the blastocyst begins to attach to the endometrium of the uterus

How does blood flow between the mother and fetus through the placenta?

Occurs without allowing blood cells to move through the placental barrier

What happens when placentation begins?

Once placentation has begun, the stratum functionalis of the uterus is known as the decidua. The region of the endometrium that lies beneath the fetus becomes the decidua basalis. The region that surrounds the uterine cavity forms the decidua capsularis.

What do other endocrine glands secrete during pregnancy?

Other endocrine glands increase secretion during pregnancy, including the release of aldosterone, parathyroid hormone, prolactin, and oxytocin.

What do high levels of estrogen from the placenta do?

Override the quieting influence of progesterone on the myometrium, which becomes "irritable," leading to irregular contractions called Braxton-Hicks contractions

What is placentation?

Placentation is the process of forming the disc-shaped placenta, which attaches to the uterine wall and to the embryo or fetus through the umbilical cord.

What are the three stages of prenatal development that occur during the 38-week time period?

Pre-embryonic period, the embryonic period, and the fetal period

When does pregnancy end?

Pregnancy ends with childbirth, or parturition, when the fetus is expelled from the uterus through the vagina.

What does the fetal adrenal cortex produce and then stimulate?

Produces cortisol, which stimulates the placenta to secrete a high level of estrogen.

What do the extra embryonic membranes do?

Protect the embryo and assist with vital functions, including nutrition, gas exchange, and storage and removal of waste

What happens during the postnatal period?

Rapid developmental changes continue to occur in the postnatal period, which is broadly divided into the following five stages: neonatal period, infancy, childhood, adolescence, and adulthood.

What are Braxton-Hicks contractions?

Referred to as false labor because they do not result in the three stages of labor

What is lactation?

Refers to the production and release of breast milk from the mammary glands

What does the acrosomal reaction do?

Releases enzymes from the head of the sperm

What is oxytocin responsible for in the mammary glands?

Responsible for the actual ejection of milk from the alveoli of the mammary glands, a process called the let-down reflex

What happens when an oocyte completes meiosis II?

Results in ovum swelling to form a female pronucleus

What is the syncytiotrophoblast responsible for?

Secreting the hormone human chorionic gonadotropin (hCG)

What are genes?

Segments of the DNA that code for a specific protein

What does the inner cell mass do in implantation?

Simultaneously, the inner cell mass, or developing offspring, separates from the trophoblast and differentiates into 2 layers: (1) the superior epiblast and (2) the inferior hypoblast together form a flat bilaminar embryonic disc.

How does fertilization begin?

Sperm cells are ejaculated in semen, which is a mixture of sperm and fluid secretions from the male reproductive glands. Several events must occur before fertilization can occur between the sperm cell and the oocyte.

What is the important of the corona radiata?

Sperm must penetrate this cell layer by a combination of sperm movements and hyaluronidase, an enzyme released from the membrane-bound sac on the head of the sperm called the acrosome.

What arises from the allantois?

The allantois is a small outpocketing of the yolk sac. It forms the base for the umbilical cord and ultimately will become part of the urinary bladder.

What arises from the amnion?

The amnion is a transparent membrane that develops from the epiblast. It eventually encloses the embryo in the fluid-filled amniotic cavity, which is penetrated only by the umbilical cord, the vessels that will connect the growing embryo or fetus to the mother's blood supply. The amnion secretes amniotic fluid into the cavity, which protects the embryo from trauma and drying out, and helps maintain a constant temperature.

What has all happened before the embryonic period?

The blastocyst is fully implanted, the inner cell mass has differentiated into the bilaminar embryonic disc, and the extraembryonic membranes have developed. We then enter the embryonic period, which begins at the third week of development and continues through the eighth week.

What does the first breath of a newborn cause?

The blood pressure in the lungs to drop and decrease pressure in the right side of the heart

What happens to the cells within the first 8 days of fertilization?

The cells separate into two groups and develop two individuals, called monozygotic, or identical twins, or dizygotic, or fraternal twins.

What arises from the chorion?

The chorion is the outermost extraembryonic membrane which encloses all the other membranes and the embryo. As the chorion develops, it forms outgrowths called the chorionic villi. By late week four of development, the chorionic villi are blended with the stratum functionalis layer and begin to form the principal embryonic part of the placenta.

What happens to the conceptus during the embryonic period?

The conceptus grows, folds, and forms the rudimentary organ systems that is called an embryo

What does human chorionic gonadotropin (hCG) stimulate?

The corpus luteum in the ovary to secrete estrogen and progesterone and helps promote placental development

What happens to the cells during cleavage?

The divisions occur too quickly for cells to grow; for this reason, the cell number increases, but the cell size becomes progressively smaller. Each subsequent division takes less time than the previous one, so that by the second day there are 4 cells and by the end of the third day there are 16 cells. At this point, the conceptus is known as a morula and remains covered by the zona pellucida.

What structures will the ectoderm differentiate into?

The ectoderm forms the epidermis of the skin and is responsible for forming the majority of the nervous system and sense organ

What structures does the endoderm differentiate into?

The endoderm becomes the internal epithelial layers of the digestive, respiratory, urinary, and reproductive systems. Endoderm forms the cavity of the middle ear and the auditory tube. Several glands and accessory digestive organs—including the thyroid gland; parathyroid glands; thymus; parts of the palatine tonsils; and the majority of the liver, gallbladder, and pancreas—also form from the endoderm.

What do the bilaminar embryonic disc and trophoblast produce together?

The extra embryonic membranes

What are changes in a newborn?

The first 4 weeks after birth constitute the neonatal period for the newborn, or neonate, and is a time of significant physiological adjustments to adapt to life outside the uterus

What are changes to the mother after birth?

The first 6 weeks following birth makeup the postpartum period, when the mother's body undergoes several changes to return the body to pre-pregnancy homeostasis and to begin and then maintain the production of breast milk, the process of lactation

What is the most dramatic even after birth?

The first breath; inflating the alveoli of the lungs for the first time

What is the first major event of organogenesis?

The first major event of organogenesis is neurulation, which produces the brain and spinal cord.

What is the cenptus?

The growing offspring is referred to as the conceptus and develops during the gestation period of pregnancy

What layers to trophoblast cells divide into in implantation?

The inner cytrophoblast and the outer syncytiotrophoblast

What is fetal development characterized by?

The maturation of tissues and organs. Fetal growth is rapid and fetal size increases dramatically

What is the major change for the newborn after birth?

The neonate must now perform all of the physiological functions that the mother had been doing, including breathing, obtaining nutrients, excreting waste, and maintaining body temperature.

What are traits with incomplete dominance?

The pattern known as incomplete dominance, the heterozygous genotype results in a phenotype that is "between" the phenotypes of individuals who are homozygous dominant and homozygous recessive.

What are dominant and recessive traits?

The pattern of inheritance known as dominant-recessive inheritance refers to the interaction of alleles that are strictly dominant or recessive.

What is a phenotype?

The physical expression of an individual's genotype in the form of a trait

How does the placenta function in addition to its nutritive functions?

The placenta also functions as an endocrine organ. The placenta assumes the production of hCG as it grows and results in the subsequent reduction of the syncytiotrophoblast

What is the function of the placenta?

The placenta is a temporary organ, shed after the infant is born, that is the site of exchange of oxygen, nutrients, and waste between the mother and fetus. In addition, the placenta produces hormones to support the pregnancy.

Why is the placenta a unique organ?

The placenta is a unique organ because it develops from both fetal and maternal structures: the fetal portion is formed by the chorionic villi and the maternal portion by the decidua basalis.

Where does sperm bind to?

The plasma membrane of hte oocyte

When does the postnatal period begin?

The postnatal period begins at birth and continues through all of life's stages.

What is fertilization?

The pre-embryonic period begins with fertilization, the fusion of a sperm cell and secondary oocyte to form a zygote, and continues until the blastocyst moves into the uterus and implants in the endometrium of the uterine wall.

What time period does the prenatal period include?

The prenatal period is the developmental stage of life that occurs during approximately 38 weeks duration within the mother's womb. The prenatal period is what people usually refer to as pregnancy.

What is organogenesis?

The process where the three primary germ layers differentiate into organs and organ systems

What is gastrulation?

The rearrangement and migration of the cells of the bilaminar embryonic disc to form the trilaminar embryonic discs

What characteristics does a fetus have when it is considered full-term?

The scalp usually has hair, and the fingers and toes have well-developed nails. If, however, a fetus is born before that time, its tissues and organs may not have developed completely.

What is developmental biology?

The science that studies the changes in form and function from the fertilized egg through old age

What is an example of incomplete dominance in humans?

The sickling gene that causes sickle-cell anemia.

What is the male pronucleus?

The sperm nucleus swells to form a male pronucleus.

What is genetics?

The study of heredity and how it is transmitted

What is embryology?

The study of the events that occur during the approximately 38 weeks of development inside the mothers womb

What does the bilaminar embryonic disc eventually become?

The three primary germ layers that differentiate to produce all tissues in the body: ectoderm, mesoderm, and endoderm

How does implantation begin?

The trophoblast invades the stratum functionalis, the layer of the endometrium by secreting digestive enzymes that catalyze reactions to degrade the endometrial lining.

What arises from the yolk sac?

The yolk sac arises from hypoblast cells and is the first extraembryonic membrane to develop. In humans, cells from the yolk sac form part of the digestive tract and are the source of the first blood cells and blood vessels. The yolk sac also produces the first germ cells, the precursors of the gametes, which will eventually migrate into the developing ovaries and testes.

What is the placental stage of labor summarized?

This is the time after delivery of the newborn when the placenta and the attached fetal extraembryonic membranes, collectively called the afterbirth, are delivered. Complete removal of the placenta is important to stop uterine bleeding. A retained placenta can lead to significant blood loss for the mother and an increased risk for infection.

What are maternal changes during pregnancy?

This period in the mother's life is one of dramatic anatomical and physiological changes that occur to her body during the nine month process of pregnancy.

What is the expulsion stage of labor summarized?

This stage is the time from full dilation to delivery of the newborn. When the first part of the fetus' head distends the vagina, crowning has occurred. The fetus' head is followed by the rest of the body, which is delivered more easily. The umbilical cord is then clamped and cut.

What is the dilation stage of pregnancy summarized?

This stage is the time from the onset of labor until the cervix is fully dilated, or expanded, to 10 cm (3.9 in.) in diameter. When labor begins, contractions originate in the upper part of the uterus and move toward the vagina. Dilation is the longest and most variable stage of labor. As the fetus' head is forced against the cervix, the cervix thins and dilates. At some point during this stage, the amnion ruptures and releases the amniotic fluid, which is the event known as the "water breaking" or "rupture of membranes." Toward the end of the dilation stage, the fetus' head enters the true pelvis and the fetus rotates so that its head can pass through the pelvic outlet.

What develops during gastrulation?

Three germ layers develop during this period that will become all of the major organ systems in the process of organogenesis by the end of week 8

What is heredity?

Transmission of genetic characteristics from parent to offspring through genes.

What are the cell populations of blastocyst?

Trophoblast cells and inner cell mass

What are the prenatal cardiovascular structures present during development?

Unique cardiovascular structures present during prenatal development include the umbilical arteries and umbilical vein and three circulatory "shortcuts" called vascular shunts: the ductus venosus, foramen ovale, and ductus arteriosus.

When does the embryo have recognizable organ systems?

When the embryonic period ends at the end of week 8

What sex chromosomes does a human female normally have?

XX

What sex chromosomes does a human male normally have?

XY

What are the extra embryonic membranes?

Yolk sac, amnion, allantois, and the chorion

What are sex linked traits?

are those that are specifically expressed on the X chromosome or Y chromosome. The Y chromosome contains genes that determine male phenotypic traits, which explains why females have the genotype XX and males have the genotype XY. There can be alleles for genes on the X chromosome that encode harmful traits, producing what are called sex-linked, or X-linked disorders.

What is the umbilical cord?

connects the center of the placenta to the fetus' umbilicus (bellybutton). The cord normally contains two umbilical arteries that carry deoxygenated fetal blood away from the fetal heart to the placenta, and one umbilical vein that carries oxygen and nutrients toward the fetal heart.

What does pregnancy include?

embraces all of the events from conception, when an egg is fertilized and begins to develop, until birth, and includes the anatomical and physiological changes to the mother during this time.

What is the fetal period?

extends from the beginning of week 9 until birth, which usually occurs near week 38

What happens during the fetal period?

he conceptus is now called a fetus; it grows larger and continues to develop until its organ systems can function without assistance from the mother. After birth, the fetus becomes a newborn, and the postnatal period of life begins

What are genes and alleles?

homologous chromosomes have pairs of genes that code for the same trait and these are found at the same location, or locus, of that chromosome. These genes are not necessarily identical—they may have slightly different sequences of nucleotides. These variants of a gene are called alleles. If two alleles code for the same trait, they are said to be homozygous for that trait. If the two alleles are Copyright © 2019 Pearson Education, Inc. 13different they are heterozygous. An allele that has the ability to mask another is called dominant, and the allele that is masked is known as recessive.

What happens to the blastocyst when the embryonic period begins?

the blastocyst has implanted in the uterus and chorionic villi have penetrated uterine blood vessels to form lacunae filled with maternal blood, which merge into a single blood-filled cavity called the placental sinus

What happens by the 12th week of the fetal period?

the placenta is fully functional, providing nutrition to the fetus for the rest of the gestation period.

What happens to the fetus cardiovascular structures after birth?

these shunts and the umbilical vessels close, and a normal circulatory pattern begins within a year in most infants, forming the fossa ovalis from the foramen ovale, ligamentum arteriosum from the ductus arteriosus , and the ligamentum venosum from the ductus venosus.