Heredity and Prenatal Development
Chapter 3
Heredity and Prenatal Development
Learning Objectives
When students have studied the material in this chapter, they will be able to answer the following:
• Introduction
1. Explain how differentiation and reorganization produce qualitative change during prenatal development.
2. How do factors at each level in Bronfenbrenner’s model affect prenatal development?
3. What practical implication does information about genetics and prenatal development have for parents-to-be?
• Genetic processes
4. Describe the basic structure of human genes and chromosomes.
5. Explain the processes of mitosis and meiosis.
6. Discuss how genes and the environment interact during prenatal development.
• Conception
7. Describe the process of conception and the events leading up to it.
• Prenatal development
8. List the three stages of prenatal development and summarize the events in each one.
• The mother's experience of pregnancy
9. Explain what is meant by trimesters of pregnancy and what occurs during each one.
• Problems in prenatal development
10. Explain the major sources of birth defects and give examples of each type.
11. Discuss the use of prenatal diagnostic techniques and genetic testing and counseling.
• Birth
12. Describe what happens during each of the three stages of labor.
13. Discuss the most common birth complications and their developmental outcomes.
• Prenatal development in context
14. Explain how considering context enhances understanding of prenatal development.
Chapter Summary Outline
Note: Terms in bold print are chapter vocabulary words.
Introduction
Four themes are emphasized regarding the prenatal period of development:
1. Genes are only one part of a complex developmental system.
2. Development involves differentiation, the process by which parts of an organism progressively take on specialized forms and functions.
3. Development involves repeated reorganization and qualitative change.
4. New structures and capacities emerge in an orderly way from those that existed before.
Genetic Processes
• Genes are segments of DNA that contain the code for producing a particular protein.
• Chromosomes, threadlike structures in the nucleus of cells, are composed of long molecules of DNA. A single chromosome may have as many as 20,000 genes.
• Genome, the complete DNA sequence for an organism, has 30,000-40,000 genes.
• Mechanisms of cell division
5. Body cells, called somatic cells, contain 23 pairs of chromosomes.
6. These cells were formed by means of cell division called mitosis:
¨ DNA in the original cell duplicates itself, producing double-stranded chromosomes
¨ Double-stranded chromosomes line up along the cell’s center; each one splits into two single-stranded chromosomes.
¨ Single-stranded chromosomes separate and move to opposite sides of the cell.
¨ The original cell divides down the middle, producing two new cells that are exact copies of the original, with 46 chromosomes.
7. Two chromosomes in a pair are homologues, similar in size and shape, containing the same type of genes. One member of this pair is from the sperm; the other is from the egg. Germ cells (from which egg and sperm are produced) contain 23 chromosomes.
8. Germ cells divide by a process called meiosis:
¨ The DNA in the germ cells duplicates itself.
¨ The homologous chromosomes arrange themselves in pairs, one from each parent.
¨ These chromosomes then separate and move to opposite ends of the cell.
¨ The cell divides; each of the two resulting cells has 23 double-stranded chromosomes (first meiotic division).
¨ The double-stranded chromosomes in these two cells split
¨ These cells divide, resulting in four cells, each with a single set of 23 chromosomes (second meiotic division). For males, all four cells become mature sperm (gamete). For females, only one becomes a mature egg (gamete).
9. During the second meiotic division, crossing over occurs, producing new genetic combinations (homologues exchange genes), and random assortment of mother’s or father’s chromosomes may occur when they move to opposite ends of the cell. Thus, the chance that a couple will produce two identical children from separate sperm-egg combinations is 1 in 64 trillion.
• How Genes Influence Development
- The environment can influence how a gene is expressed. Genes also are influenced by one’s developmental history.
- The Interaction of Genes and Environment: The case for Physical Gender Development
¨ Gender is shaped by the sex chromosomes (the 23rd pair). Two X chromosomes result in a female, one X and one Y chromosome result in a male. Y chromosomes contain less genetic material.
¨ Differentiation into testes or ovaries occurs during week seven after conception, its critical period.
¨ After the critical period, gender development is shaped by hormones.
¨ Testes secrete male sex hormones (androgens), causing differentiation of the male reproductive tract. The absence of androgens allows development of the female reproductive tract.
¨ Prenatal hormones during the critical period can alter genital development (e.g., genetic females can develop male genitals with excessive exposure to androgens).
¨ Genes and environment interact to guide development; how genetic information is actually used depends on the environment.
• How Genes Affect One Another
1. Genes come in several forms. Alternate forms of genes for the same trait (e.g., blood type) are alleles. Some alleles are homozygous (identical alleles) while some are heterozygous (different alleles).
2. Sometimes one allele is dominant and the other recessive. The dominant will be expressed. Sometimes alleles are codominant where both are expressed.
3. Phenotypes (observable traits) do not always match genotypes (genetic makeup) as dominant alleles mask the presence of recessive alleles.
4. Some recessive traits are sex-linked traits; they are carried on the X chromosome and are commonly expressed in males (e.g., red-green color blindness, hemophilia). A male has no dominant allele to cancel out the recessive allele’s effects. A female needs homozygous alleles to express the trait. Otherwise, she is a carrier of the trait.
5. Many human characteristics are polygenic, influenced by a number of gene pairs (e.g., intelligence, weight).
• Summing Up the Influence of Genes
- Phenylketonuria (PKU) is a classic example of the interaction of genes and environment on development. If a low-phenylalanine diet is started in the first 6 weeks of life, few harmful effects will be suffered. If after this time, IQ scores are generally below 70.
Conception
• Conception depends on appropriate timing of a chain of events. An ovum ripens in one of the woman’s ovaries over a period of about 28 days. When it is ready for fertilization, ovulation occurs; the ovum is released into the fallopian tube and travels to the uterus over the course of several days. Without fertilization within 24 hours, the ovum disintegrates. If conception occurs, a zygote is produced.
• More than one ovum may be released and fertilized by different sperm, resulting in dizygotic (di = two, zygotic = fertilized egg) twins if two ova were released.
• If a fertilized egg splits into two, identical twins are formed (monozygotic twins; mono = one, zygotic = fertilized egg)
Prenatal Development
• Prenatal development, conception to birth at about 38 weeks gestational age, occurs along a predetermined schedule and involves incredible differentiation and rapid growth.
• Stages of Prenatal Development: Germinal period , Embryonic period, Fetal period
• The Germinal Period: Conception Through Week 2
1. The single celled zygote divides into two cells about 30 hours after conception – the beginning of mitosis.
2. After 60 hours, there are four cells and at the end of a week, there are hundreds of cells clustered together into a blastocyst; cell differentiation has begun.
3. The embryoblast, inner cell mass, develops into the embryo.
4. The trophoblast, outer cell mass, forms the embryo’s life support system.
5. Implantation occurs about the sixth day after fertilization; uterine lining is rich with blood. Attachment has occurred by the end of the second week.
• The Embryonic Period: Weeks 3 Through 8
1. Once the zygote is firmly implanted, it is referred to as an embryo.
2. Period of organogenesis when all of the vital organs and other major body structures are formed.
3. The Embryo’s Life Support System
¨ The placenta forms partly from cells of the uterine lining and partly from cells of the trophoblast. Oxygen and nutrients are transferred and waste removed through cell membranes. The placenta offers protection from many bacteria, for instance.
¨ The umbilical cord links the placenta to the embryo and to the mother’s uterine wall via separate sets of blood vessels
¨ Amniotic sac is a fluid-filled protective environment that cushions and regulates temperature.
4. Embryonic Cell Differentiation
¨ In week 3, cells on the surface migrate toward an indentation (beginning of mouth and digestive tract) and form a central layer between an inner and outer layer: endoderm (internal organs), mesoderm (bones, muscles, and blood) and ectoderm (central nervous system, sensory organs, and skin).
¨ Embryonic inductions – caused by chemical substances that create critical tissue interactions triggering developmental changes. The mesoderm induces the changes in the developing embryo. Another example of embryonic induction pertains to the formation of the lens of the eye.
¨ There are critical periods in embryonic cell differentiation.
5. The Timetable for Embryonic Development
¨ There are critical periods in development during the embryonic and fetal periods, for which there is a predictable timetable.
¨ Cephalocaudal development – head to tail.
¨ Proximodistal development – near to far from the center of the body.
• The Fetal Period: Week 9 to Birth
1. Major body parts grow rapidly and become refined in structure, which is distinct from the type of development during the embryonic period.
2. Growth in body length reaches its maximum rate early in fetal development, while weight gains are greatest as delivery approaches.
3. The fetus is more responsive than the embryo. The fetus, by week 10, will flex if touched. The responses become more specialized with development, reflecting qualitative changes in the central nervous system.
4. Behaviors become increasingly regular and integrated.
Mother’s Experience of Pregnancy
Trimesters – three, three-month periods of the pregnancy.
1. First trimester – fatigue and drowsiness are common as are breast swelling, frequent urination, and morning sickness.
2. Second trimester – fatigue and nausea usually disappear. Quickening, first fetal movements, is usually experienced by the fourth month. Bulging abdomen.
3. Third trimester – the increase in the size of the fetus and the uterus puts pressure on other organs. Fetal kicking can cause discomfort.
Problems in Prenatal Development
• Congenital defect (birth defect) – abnormality present at birth. 25% are from genetic defects, 3% are from the prenatal environment, 25% are caused by a combination of genes and environment and in over 40% of the cases, the causes are unknown.
• Single-gene disorders (Mendelian disorders) – most are the result of inheriting a recessive allele from each parent (such as with PKU, sickle-cell anemia, Tay-Sachs disease).
• A few disorders are due to inheritance of a dominant allele (e.g., Huntington’s disease).
• Other genetic defects involve chromosomal abnormalities, due to errors in meiosis (extra, missing, or damaged chromsomes). 1 in 160 newborns have some type of chromosomal abnormality.
1. Best known example is Down syndrome (trisomy 21): physical features are abnormal as are many of their organs, and they present with mental retardation. 1 in 800 births, with the number increasing with maternal age.
2. Sex chromosome abnormalities – errors in meiosis,1 in 500 births. Examples: Klinefelter syndrome where males have an extra X, XYY syndrome, Turner syndrome (females with only one X), trisomy X. Some result in developmental problems, many do not.
· Environmental influences
1. Teratogens – substances in the environment that can cause abnormalities during prenatal development. Teratology – study of the effects of these substances.
2. Usually cause abnormalities by preventing or modifying normal cell division and differentiation, generally posing the greatest danger during the embryonic period.
3. Alcohol, Tobacco, and Illicit Drugs
¨ Can impact development at least three ways:
a. By passing through the placenta and interfering directly with the developing embryo or fetus.
b. By affecting the mother physically and psychologically in ways that can have an indirect impact on the embryo or fetus.
c. By causing the baby to be born with a physical addiction, which is greatest in likelihood for narcotics.
¨ Alcohol
a. 20 % of newborns in the U.S. have prenatal exposure and nearly 1% of newborns show some degree of neurological problems as a result.
b. Alcohol severely disrupts brain development in a variety of ways, up to the time of birth.
c. Neurological problems are quite likely with binge drinking. Those who drink heavily late in pregnancy have babies with the most severe retardation in physical growth.
d. 1 in 3 babies born to heavy drinkers have fetal alcohol syndrome (FAS) – 2 in every 1000 births; have a distinctive pattern of unusual facial characteristics, poor growth, and central nervous system problems.
e. Those exposed to lower levels of alcohol prenatally can have neurological and behavioral problems.
¨ Tobacco
a. About 25% of unborn children are exposed to maternal tobacco use in the U.S. Babies are more likely to be delivered prematurely and to have low birth weight. Later cognitive functioning, language development, and reading skills may be impacted with heavy smoking.
b. Smoking directly exposes the fetus to nicotine and indirectly impacts the mother’s body, affecting prenatal brain development by activating nicotine receptors, raising carbon monoxide levels in the mother’s blood, and interfering with the functioning of the placenta.
¨ Cocaine
a. Cocaine-exposed infants are estimated to account for over half of the newborns affected by illicit drugs in the U.S. Cocaine use increases miscarriage and stillbirths as well as premature delivery.
b. Exposed newborns show a variety of behavioral effects, depending on degree of exposure: impaired arousal, irritability, social interaction problems, and impaired motor development and cognitive functioning.
¨ Heroin
a. Babies of addicted mothers often are addicted and suffer classic withdrawal symptoms.