Chapter 2

The Human Heritage: Genes and Environment

Genetic Code• Chromosome: A threadlike

structure made up of genes – 46 in all human cells, except

sperm and ova which have 23• Deoxyribonucleic acid (DNA):

A long double-stranded molecule that makes up chromosomes

• Genes: The segments on a DNA molecule that act as hereditary blueprints for the organism’s development

Model of DNA

DNA Replication• A strand of DNA replicates

by splitting down the middle of the rungs of its ladderlike structure

• Each free base picks up a new complementary partner:– Cytosine (C) pairs with

guanine (G)– Adenine (A) pairs with

thymine (T)

Mitosis: A Process of Cell Replication

Meiosis: A Source of Variability

Sexual Determinism: A Case of Variability

Sexual Reproduction and Genetic Transmission

Mitosis

• Mitosis is the process of cell division that generates all the cells of the body except the germ cells.

Meiosis

• Meiosis is the process of cell division that generates the germ cells.

Sexual Determinism: A Case of Variability

The Five Sexes Fausto-Sterling (1993)

1- Male = testes 2- Female = ovaries3- Hermaphrodites = 1 testes & 1 ovary4- Merms = Male pseudohermaphrodites i.e.,

testes & some aspect of female genitalia5- Ferms = Female pseudohermaphrodites

i.e., ovaries & some aspect of male genitalia

An Interaction

• Genotype: An individual’s genetic endowment

• Environment: The totality of conditions and circumstances that surround an individual

• Phenotype: The observable characteristics of an individual

– Physical and psychological traits, health, behavior

Genotype + Environment Phenotype

Laws of Genetic Inheritance

Gregor Mendel (1822-1884): Garden peas• Allele: The specific form of a gene coded for a

particular trait

• Homozygous: Having inherited two genes of the same allelic form for a trait

• Heterozygous: Having inherited two genes of different allelic forms for a trait

Heterozygous Possibilities1. Dominant gene: The allele that

is expressed when an individual possesses two different alleles for the same trait

2. Recessive genes: The allele that is not expressed when an individual possesses two different alleles for the same trait

3. Codominance: An outcome in which a trait that is determined by two alleles is different from the trait produced by either of the contributing alleles alone

Sex-Linked Traits• Given that the X chromosome is much larger than the Y

chromosome, most inherited sex-linked characteristics are carried on the X chromosome

• Males are more susceptible to genetic defects than are females– If a daughter has a harmful recessive gene on one X chromosome,

she will usually have a normal dominant gene on the other X chromosome to override it

– A son who inherits a harmful recessive gene on his X chromosome has no such complementary allele to override the recessive gene’s harmful effects

Genotype and Phenotype

The Laws of Genetic Inheritance

•Sex-Linked Genetic Effects•Examples: Red-green color blindness, hemophilia, muscular dystrophy, night blindness,Hypogammaglobulin (lack of production of anitbodies to fight bacteria), Vitamin D Resistance (Rickets)

Genes, The Organism, and the Environment

• Range of Reaction- all possible gene/env relationships compatible w/ organism’s continued life

• Canalization- certain characteristics typical of a species may be restricted to a narrow range despite wide variations in environmental conditions

• The Study of Genetic Influences on Human Behavior- heritability I.e., genetically influenced v. genetically caused

• Estimating Genetic Influence Through Kinship Studies- twin & adoption studies

Mutations and Genetic Abnormalities

Sickle-Cell Anemia: An Example of Gene-

Environmental Interaction

Down Syndrome: A Chromosomal Error

Sex-Linked Chromosomal

Abnormalities

Phenylketonuria: A Treatable Genetic

Disease

Down Syndrome: A Chromosomal Error

• Cause: More than 95% of children born with Down syndrome have 3 (vs. 2) copies of chromosome 21

• Traits: Mentally/physically retarded; distinctive physical characteristics (e.g., slanting eyes, rather flat facial profile, ears lower than normal, short neck, protruding tongue, dental irregularities, small curved fingers, wide-spaced toes); more likely to die young

• Incidence: 1:1000 births (increases with mother’s age: 1:100 by age 40, 1:32 by age 45, 1:12 by age 49)

• Outlook: Supportive intervention that includes special education by concerned adults can markedly improve the intellectual functioning of some of these children

Down Syndrome: A Chromosomal Error

• Cause: More than 95% of children born with Down syndrome have 3 (vs. 2) copies of chromosome 21

• Traits: Mentally/physically retarded; distinctive physical characteristics (e.g., slanting eyes, rather flat facial profile, ears lower than normal, short neck, protruding tongue, dental irregularities, small curved fingers, wide-spaced toes); more likely to die young

• Incidence: 1:1000 births (increases with mother’s age: 1:100 by age 40, 1:32 by age 45, 1:12 by age 49)

• Outlook: Supportive intervention that includes special education by concerned adults can markedly improve the intellectual functioning of some of these children

Sickle-Cell Anemia: Gene–Environment Interaction

• Cause: Recessive gene (victims are homozygous, but heterozygous subjects are also mildly affected)

• Traits: Abnormal blood cells cause circulatory problems (e.g., heart enlargement) and severe anemia

• Incidence: 8-9% of U.S. blacks• Outlook: Crippling, but

treatable with medication

Klinefelter Syndrome: A Sex- Linked Abnormality

• Cause: Extra X chromosome (i.e., XXY)

• Traits: Males fail to develop secondary sex characteristics (e.g., facial hair, voice change), are sterile, and most have speech and language problems

• Incidence: 1:900 males

• Outlook: May be treatable with testosterone replacement therapy

Prenatal Testing

• Ultrasound

• Amniocentesis

• Chorionic Villus Sampling

Ultrasound

• Ultrasound creates images of the fetus by using high-frequency sound waves to "see" inside your body. This ultrasound image reveals the normal development of healthy twins.

Amniocentesis

• During an amniocentesis, samples of amniotic fluid are removed using a needle and then tested for genetic abnormalities in the laboratory.

Chorionic Villus Sampling

• During CVS, samples of the cells that line the placenta, known as chorionic villi cells, are removed and tested for genetic abnormalities in the laboratory.

Survival Strategies: Culture• Cultural artifacts, such as tools and

clothing, greatly extend the range of environments in which human beings can reproduce and thrive

• Cultural knowledge, such as the creation of “hazard prevention strategies,” serves to protect and support children to enable them to mature and reproduce