Chapter 2 The Start of Life: Genetics and Prenatal Development
What is Heredity?
Inheritance: The transmission of qualities genetically derived from one’s ancestors
As humans, we begin life as a single cell--1/20,000,000th of an ounce.
This single cell is transformed into a person by the human genetic code.
How is the human genetic code transferred?
The sex cells from the mother and father that form a new cell at conception (also known as sperm and ovum).
Fertilization is the process by which a sperm and an ovum (the gametes) join to form a single new cell, called a ZYGOTE.
Genes & Chromosomes: The Code of Life
GENES are the basic unit of genetic information.
Composed of DNA (deoxyribonucleic acid) sequences.
DNA determines the nature of every cell in the body and how it will function.
Humans have about 25,000 genes.
They are arranged in specific locations and in a specific order along 46 chromosomes.
CHROMOSOMES are rod-shaped sections of DNA organized into 23 pairs.
One pair of chromosomes is provided by the mother, and the other by the father.
The 23rd chromosome determines the sex of the child.
Females are XX, & males are XY.
The father’s sperm determines the sex of the child.
THERE ARE 10’S OF TRILLIONS OF POSSIBLE GENETIC COMBOS.
A Comparison of Twins…
Genetically identical; form when cluster of cells in the ovum splits off within the first 2 weeks following fertilization.
2 separate ova are fertilized by 2 separate sperm; no more genetically similar than 2 siblings.
Triplets, quadruplets, etc. can be either monozygotic or multizygotic.
The use of fertility drugs increases the chance of having multiple births.
1 in 10 chance that the birth will be dizygotic.
Older mothers are more likely to have multiple births.
Racial & ethnic differences affect the rate of multiple births.
African American: 1 out of 70 births are dizygotic
Caucasians: 1 out of 86 dizygotic
The Basics of Genetics
Worked with peas; discovered that when 2 competing traits were present, only one could be expressed.
The DOMINANT TRAIT is the trait that is expressed.
The RECESSIVE TRAIT is present in the organism but not expressed.
A GENOTYPE is the underlying combo of genetic material present (but not outwardly visible) in an organism.
A PHENOTYPE is an observable trait that is actually seen; physical appearance.
-Alleles are gene pairs for traits that may take alternate forms (example: hair texture/color, skin tone).
Homozygous (inheriting similar genes for a given trait).
Heterozygous (inheriting different forms of a gene for a given trait).
A recessive allele from each parent the child displays the recessive characteristic.
Ex.: bb = blue eyes
A dominant allele received from one or both parents a dominant characteristic is displayed.
Ex.: Bb or BB = brown eyes
* MOST traits are the result of POLYGENIC INHERITANCE (a combo of multiple gene pairs leads to a particular trait).
Some genes are neither dominant nor recessive, but instead are a combination.
Some are X-linked genes, Located on the X chromosome
Males have higher risk for X-linked disorders.
Why? Because the lack a 2nd X chromosome to counteract the genetic info that produces the disorder.
Examples of X-linked disorders: red-green color blindness, hemophilia.
The MOST recent approach to the study of the effects of heredity on behavior & development:
The Human Genome Project
In early 2001 molecular biologists succeeded in mapping the sequence of genes on chromosomes.
One of the most important moments in the history of genetics.
Already leading to important advances in our understanding of genetics.
99.9% of gene sequence is shared by all humans: similarities of people realized.
The number of human genes less than thought (25,000 rather than 100,000+)
Behavioral geneticists investigate several areas…
Behavioral geneticists explore how behavioral difficulties may have a genetic basis (example: schizophrenia).
How genetic defects may be remedied.
The inheritance patterns of genetic disorders.
The focus of behavioral geneticists, continued
How physically damaged genes contribute to genetic disorders.
The role of spontaneous mutation in genetic disorders (how genes sometimes change form on their own).
How environmental factors affect genetic mutation (x-ray exposure, teratogens).
Some genetic disorders include…
A disorder produced by the presence of an extra chromosome on the 21st chromosome pair.
A blood disorder (named for the shape of the disordered blood cells).
(More genetic disorders include…)
An untreatable disorder; produces blindness, muscle degeneration prior to death.
-- Results from the presence of an extra X chromosome that produces underdeveloped genitals, extreme height, and enlarged breasts.
What is Genetic Counseling?
The discipline that focuses on helping people deal with issues related to inherited disorders.
- Blood, skin, urine often used to isolate/examine specific chromosomes.
- Possible genetic defects can be identified by assembling a karyotype, a chart containing enlarged photos of each of the chromosomes.
AMNIOCENTESIS (the process of identifying genetic defects by examining a small sample of fetal cells drawn by a needle inserted into the amniotic fluid surrounding the unborn fetus).
CHORIONIC VILLUS SAMPLING (CVS) (a test used to find genetic defects that involves taking samples of hairlike material that surrounds the embryo).
ULTRASOUND SONOGRAPHY (a process in which high-frequency sound waves scan the mother's womb to produce an image of the unborn baby whose size and shape can then be assessed).
Nature vs. Nurture… The interaction of heredity vs environment
The correct question is not whether behavior is caused by nature or nurture but how much by nature and how much by nurture.
How scientists investigate the effects of heredity & environment
Human twins used to study the effects of genes and the environment.
Differences between monozygotic twins separated at birth (usually most likely but not always due to different environments).
If monozygotic twins are more similar than dizygotic twins on a particular trait than we can assume that genetics plays a role.
Human studies in heredity/environment research, continued
People who are unrelated but share the same environment also tell us about environmental influences.
Researchers also study biological parents and their children versus adoptive parents and their children to see the effects of heredity versus environment.
Bottom line: Virtually all traits, characteristics, and behaviors are the joint result of the combination and interaction of nature and nurture.
More about the interaction of heredity & environment in development…
--The more genetically similar two people are, the more likely they are to share physical characteristics (e.g., height, weight).
--Genetics plays a significant role in intelligence; however, the environment is also a significant factor.
--Increasing evidence supports the conclusion that at least some personality traits have at least some genetic components. Some personality characteristics have been found to be linked to genetic factors (neuroticism, extroversion).
Some Psychological Disorders at Least Partially Related to Genetic Factors…
Attention-deficit hyperactivity disorder
Psychological Disorders: Predisposition or Automatic Inheritance?
Genetics alone does not automatically lead to the development of schizophrenia or other psychological disorders.
If genetics were the sole cause, identical twins would automatically develop schizophrenia, and this is not the case.
Other factors contribute, including structural abnormalities, biochemical imbalances, and stressors.
Interaction of Genes and Environment
Developmental psychologist Sandra Scarr has identified 3 types of gene-environment interaction:
Passive genotype-environment interaction (parent’s genotype influences type of environment where kids are raised)
Ex. of positive passive interaction: parents & child naturally sociable, parents provide social activity; athletic parents provide sports opportunities to athletic child
Ex. of negative passive interaction: sociable parents and introverted child, child uncomfortable in environment; athletic parents pressure
Examples of the Three Types of Interaction
2. Evocative genotype-environment interaction (child's genotype affects how others react to him/her, elicit a type of environment).
Ex. of positive: smiling, sociable baby gets more social stimulation and reinforcement
Ex. of negative: strong-willed, difficult child gets negative attention
3. Active genotype-environment interaction (children focus on or seek out aspects of the environment that are congruent with their genetic abilities).
Also known as “niche-picking,” for example, seeking out involvement in sports or music
Prenatal Growth & Development
Fertilization: joining of sperm & ovum to create the single-celled zygote that develops into a baby
Females are born with 400,000+ ova, which mature at puberty.
Males produce sperm with a much shorter lifespan--several hundred million sperm per day in adult males.
The 3 Phases of the Prenatal Period
The GERMINAL STAGE
The first and shortest stage of prenatal development
Takes place during the first two weeks following conception.
Characterized by methodical cell division and the attachment of the organism (blastocyst) to the wall of the uterus.
At 3 days, the blastocyst consists of about 32 cells, by the end of the first week it is 100-150 cells.
- The blastocyst can divide into multiple individuals during this period.
The 3 Phases of the Prenatal Period, cont.
The EMBRYONIC STAGE
the period from two to eight weeks following fertilization during which significant growth occurs in the major organs and body systems.
The developing child is now composed of three layers:
the ectoderm (the outer layer forming the skin, hair, teeth, sense organs, the brain and spinal cord).
the endoderm (the inner layer producing the digestive system, liver, pancreas, and respiratory system).
the mesoderm (sandwiched between the inner and outer layers and forms the muscles, bones, blood, and circulatory system).
The 3 Phases of the Prenatal Period, cont.
The FETAL STAGE
- begins about eight weeks after conception and continues until birth.
- The fetus dramatically increases in size and weight.
- At 4 months it weighs 4 ounces, at 7 months it weighs 3 pounds.
- Organs become more differentiated and operational.
- By three months the fetus swallows and urinates.
- By four months the mother will be able to feel her fetus move.
About 15 % of couples suffer from infertility (inability to conceive after 12 to 18 months)
There are several causes:
Previous use of birth control pills, illicit drugs or cigarettes, STDs
Most common cause in men is an abnormally low sperm count
Most common cause in women is failure to release an egg through ovulation (hormonal imbalance, structural damage, stress).
Several Alternate Routes to Pregnancy
-- ARTIFICIAL INSEMINATION (fertilization that occurs after a man's sperm is placed directly into a woman's vagina by a physician).
-- IN VITRO FERTILIZATION (IVF) (a procedure in which a woman's ova are removed from her ovaries, and a man's sperm are used to fertilize the ova in a laboratory).
-- SURROGATE MOTHER, a woman who agrees to carry the child to term, may be used in cases where the mother is unable to conceive.
Ethical Issues Related to Infertility Treatments
Some surrogate mothers have refused to give up the child.
Is it ethical for parents to engage in sex selection techniques now being use? What about choosing eye or hair color or trying to “fertilize” for intelligence?
Do children conceived with reproductive technologies fare as well as naturally conceived children?
Evidence suggests that the quality of parenting may even be superior to naturally conceived children.
The psychological adjustment of children conceived artificially is no different than that of children conceived using natural techniques.
Miscarriage and Abortion
Miscarriage (spontaneous abortion) - embryo detaches from wall of uterus and is expelled.
Fifteen to 20 percent of all pregnancies end in miscarriage.
Many occur so early that mother may not have even been aware of pregnancy or miscarriage.
Typically caused by genetic abnormalities.
Abortion - voluntary termination of pregnancy
Involves complex physical, psychological, legal and ethical issues.
Threats to Prenatal Development
Certain aspects of mothers' and fathers' behavior, both before and after conception, can produce lifelong consequences for the child.
TERATOGENS are environmental agents such as a virus, chemical, or other factor that produces a birth defect.
At some phases of prenatal development, a teratogen may have minimal impact; at other periods, consequences can be severe.
Threats to Prenatal Development - Drugs
Mother's use of legal and illegal drugs pose serious risks to the unborn child:
- Aspirin can lead to bleeding.
-Thalidomide caused missing limbs.
-Marijuana restricts oxygen to the fetus, causing irritability, later learning problems
-Cocaine restricts blood flow and oxygen, babies are born addicted and go through withdrawal; they are shorter and weigh less; they have serious respiratory problems and birth defects or seizures; it is often impossible to soothe them
Threats to Prenatal Development – Alcohol and Cigarettes
Alcohol: Just two drinks a day has been associated with lower intelligence.
FETAL ALCOHOL SYNDROME (FAS): a disorder caused by the pregnant mother consuming substantial quantities of alcohol during pregnancy, potentially resulting in mental retardation, delayed growth, & facial deformities.
Smoking reduces the oxygen content and increases carbon monoxide; babies can miscarry or are born with abnormally low birth weight; babies are shorter and may be intellectually delayed
Fathers can affect the prenatal environment.
Second-hand smoke may affect mother and child’s health.
Father’s alcohol and drug use may impair sperm and lead to chromosomal damage that may affect fetus.
Father’s exposure to environmental toxins like mercury or lead may cause damage to sperm and cause birth defects.
Physically and emotionally abusive fathers may cause damage to unborn children by raising maternal stress or causing actual physical damage to child.
As many as 8% of pregnant women are beaten by their partners.
Other Threats to Prenatal Development
Mother's diet clearly plays an important role in bolstering the development of the fetus.
Mother’s age - Research shows that mothers over 30 and adolescent mothers are at greater risk for a variety of pregnancy and birth complications:
Low birth weight
Higher infant mortality rates
Other Threats to Prenatal Development
Illness in a pregnant woman can affect the baby:
- Rubella (German measles) prior to the 11th week can cause blindness, deafness, heart defects, or brain damage.
- Chicken pox and mumps may cause birth defects and
- Syphilis & gonorrhea can be transmitted to the child.
Babies born with AIDS can have birth abnormalities including small, misshapen faces, protruding lips, and brain deterioration; 90% have neurological delays and deficits in motor coordination, speech, and facial expression. Survival past infancy is rare.
Optimizing the Prenatal Environment
Avoid birth control pills for 3 months before trying to conceive
Get vaccinated against German measles
Don’t use alcohol or drugs
Monitor caffeine intake