chapter 2 the start of life: genetics & prenatal development

Chapter 2

The Start of Life: Genetics & 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 something resembling a person by the human genetic

code!• How is the human genetic code

transferred??– GAMETES

• 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 go form a single new cell, called a zygote

– Some important things to remember about fertilization:• Females are born with all their ova (about

400, 000!; the ova mature only when the female reaches puberty; an ovum ripens every 28 days)

• Males produce several hundred million sperm daily!

• Fertilization typically takes place in the fallopian tubes

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

(More about genes)

• Humans have over 100, 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)

Some more things to know about chromosomes…

• One pair of chromosomes is provided (through the gametes) by the mother

• One pair of chromosomes is provided by the father at fertilization– Gametes (sperm & ova) are formed by a process

called meiosis– ALL other cells replicate the genetic code by a

process called mitosis• THERE ARE 10’S OF TRILLIONS OF

POSSIBLE GENETIC COMBOS!

Some important things to know about multiple births…

• Less than 2% of ALL pregnancies produce twins!! (the odds are even slimmer for triplets, quadruplets, etc.!)

A Comparison of Twins…

• MONOZYGOTIC Twins– Genetically

identical; form when cluster of cells in the ovum splits off within the first 2 weeks following fertilization

• DIZYGOTIC Twins– 2 separate ova are

fertilized by 2 separate sperm; no more genetically similar than 2 siblings

Other kinds of multiple births…• Triplets, quadruplets, etc. can be either monozygotic or

dizygotic• The use of fertility drugs increases the chance of having

multiple births]– 1 in 10 that the birth will be dizygotic– Twin births are up 42 % under these circumstances!

• Racial & Ethnic differences affect the rate of multiple births– African American: 1 out of 70 births are dizygotic– Caucasians: 1 out of 86 dizygotic– Chinese Americans 1 out of 300 dizgotic

Some important facts:

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

The Basics of Genetics…

• Gregor Mendel– 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

(More basics of genetics…)

• 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 genes 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)

[Alleles (genes for traits that make take alternate forms) continued]

• A recessive allele from each parent the child displays the recessive characteristic

• A dominant allele received from each parent a dominant characteristic is displayed

* 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– AB blood type

• 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

PKU is an inherited disorder in which a child is unable to use an essential amino acid, and allows a build up of toxins causing brain damage and mental retardation. We can see the transmission of genetic information in humans by considering the transmission of this disorder.

-neither parent carries gene: child cannot develop PKU-one parent carries: child cannot develop-both parents: child has 1 in 4 chance

The MOST recent approach to the study of the effects of heredity on behavior &

development:

Behavioral Genetics

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!

- # of human genes less than thought (30,000 rather than 100,000+)

- Will also help id disorders

Uniquely Human?

The human genome project has lead to the realization that humans are less complex than originally thought, and not much different that some primitive species!

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…

• Down Syndrome– A disorder produced by the presence of an extra

chromosome on the 21st chromosome pair

• Sickle-Cell Anemia– A blood disorder (named for the shape of the

disordered blood cells)

(More genetic disorders include…)

• Tay-Sachs Disease– An untreatable disorder; produces blindness,

muscle degeneration prior to death

• Klinefelter’s Syndrome-- Results from the presence of an extra X

chromosome that produces underdeveloped genitals, extreme height, and enlarged breasts.

Inheriting Hemophilia…

Hemophilia is a blood-clotting disorder that has been an inherited problem in the royal families of Europe, as illustrated above in the descendants of Queen Victoria of Britain.

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.

Other tests that can take place once a woman is pregnant: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).

• About 15 % of couples suffer from infertility (inability to conceive after 12 to 18 months of trying)

• There are several causes:– Parental age– Previous use of birth control pills, illicit drugs

or cigarettes, STDs – men who have an abnormally low sperm

count – the woman's mother taking certain drugs

during pregnancy

Some Facts About Infertility…

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.

Evidence suggests that the quality of parenting in families who have used artificial means to conception 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

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

Intelligence may be explained by a range of possible causes, spanning the nature-nurture continuum. Which explanation do you find most convincing, based on information in this chapter?

Possible Causes of Intelligence: Nature vs. Nurture

How do scientists investigate the effects of heredity & environment on development

anyway??

A. Nonhumans studies

- Laboratory animals

* bred to share genetic backgrounds and placed in different environments to explore the effects of these environments.

* conversely, they use genetically different animals in similar environments to determine the role

(How scientists investigate the effects of heredity & environment, continued)

• Human studies 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.

The closer the genetic link between two individuals, the more similar their IQ scores are…

No issue looking at the influence of heredity and environment has more research than the topic of intelligence, because it is a core human characteristic!

More about the interaction of heredity & environment in effecting 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)

Genetic & Environmental Influences On Personality

• Increasing evidence supports the conclusion that at least some personality characteristics are affected by genetic factors– Neuroticism (emotional reactivity)– Extroversion (outgoing/sociability)~How do we know which personality traits

reflect genetics? – Examination of genes– Twin studies

Inheriting personality traits…These personality traits have been found to have genetic components. The higher the percentage, the greater the influence of heredity.

Some Psychological Disorders at Least Partially Related to Genetic Factors…

A. schizophrenia

B. major depression

C. alcoholism

D. autism

E. attention-deficit hyperactivity disorder

The psychological disorder of schizophrenia has clear genetic components. The closer the genetic link, the more likely a relative is to develop schizophrenia.

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

Can Genes Influence Environments?• Developmental psychologist Sandra Scarr says yes—

genetics can influence environment!

– 3 ways…

1. Active genotype-environment effects (kids focus on aspects of the environment that are congruent with their genetic abilities)

– Noticing the music tryouts flyer over the sports tryout flyer

(Sandra Scarr, genetics & environment continued…)

2. Passive genotype-environment effects (parents genes are associated with environment where kids are raised)

– Parents are sports oriented= more opportunities for kids in this area

3. Evocative genotype-environment effects (kids genes elicit a type of environment

– Kid with athletic ability draws attention to this so parents will nurture

The Start of Life…Genetics & Prenatal Developmental Development

• Fertilization: The Moment of Conception– The joining of the sperm and 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!

Anatomy of the Sex Organs…

Routes to Pregnancy…~Sexual Activity

• During sexual activity, the sperm released wind their way to the ovum. One sperm fertilizes an ovum, and together contain all the genetic data to produce a new human.

• Artificial insemination (sperm placed in vagina by physician)

~In vitro fertilization (ova removed and fertilized by sperm in a lab)

~Surrogate Mother (another woman carries a couples baby)

Prenatal Growth & Change

• Developmentalists divide the prenatal period into three phases: the germinal, embryonic, and fetal stages.

The Stages of the Prenatal Period: The Onset of Development

The 3 Phases of the Prenatal Period:

1) 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.

- The baby is called a zygote at this stage.

(The 3 Phases of the Prenatal Period, continued)

2) The EMBRYONIC STAGE- the period from two to eight weeks following fertilization

during which significant growth occurs in the major organs and body systems.

- At this point the child is called an embryo. - 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, continued)

3) The FETAL STAGE begins about eight weeks after conception and continues until birth.

~ The developing child from eight weeks after conception until birth is called a FETUS.

~ The fetus dramatically increases in size and weight.

~ 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

During the fetal period, the proportions of the body change dramatically!

Threats to Prenatal Development…Certain aspects of mothers' and fathers'

behavior, both before and after conception, can produce lifelong consequences for the child.

TERATOGENs (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.

Teratogen SensitivityVarious parts of the body are more sensitive to teratogens at different stages of development

Other threats to prenatal development…-- A mother's diet clearly plays an important role

in bolstering the development of the fetus. -- Research shows that mothers over 30 and

adolescent mothers are at greater risk for a variety of pregnancy and birth complications:

• premature birth • low birth weight • Down syndrome • higher infant mortality rates • Illness

Other threats to prenatal development… Illness in a pregnant woman can have devastating

consequences: - 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 miscarriage, respectively. - Syphilis and 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 percent have neurological delays and deficits in motor coordination, speech, and facial expression. In addition, they are susceptible to infection. Survival past infancy is rare.

More threats to prenatal development

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

-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.

Still more threats to prenatal developmentBoth alcohol and cigarettes can disrupt the

development of the fetus:

-just two drinks a day has been associated with lower intelligence;

~~FETAL ALCOHOL SYNDROME (FAS) is a disorder caused by the pregnant mother consuming substantial quantities of alcohol during pregnancy potentially resulting in mental retardation, delayed growth, and facial deformities;

(threats to prenatal development)

--smoking reduces the oxygen content and increase carbon monoxide; babies can miscarry or are born with abnormally low birth weight; babies born to smokers are shorter and may be intellectually delayed.

Fathers can affect the prenatal environment

(second hand smoke and the environment)

•Don’t forget to read chapter 3 for next time!!!!!