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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 something resembling 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.

More about genes

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.

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.


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.

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.

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

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 Genetic

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.


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).

Alleles (gene pairs for traits that may take alternate forms)

A recessive allele from each parent à the child displays the recessive characteristic,

A dominant allele received from each parent à 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.

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:

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 genetic.

-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+)

Will also help identify disorders.


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.


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.

Prenatal Testing

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 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.



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).

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

Some Psychological Disorders at Least Partially Related to Genetic Factors…


Major depression



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.

Can Genes Influence Environments?

Developmental psychologist Sandra Scarr says yes—genetics can influence environment in 3 ways…

1. Active genotype-environment effects (children 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 children in this area.

3. Evocative genotype-environment effects (children’s genes elicit a type of environment.

Children with athletic ability draws attention to this so parents will nurture.