Chapter 2

Natural Selection

Individuals that best adapted to their environment are most likely to survive and reproduce
Their characteristics are passed on to the next generation
Over generations, organisms with characteristics best suited for survival make
up an increased percentage of the population

DNA (deoxyribonucleic acid)

• Complex molecule with a double helix shape
• Contains genetic information that gives instructions to cells to manufacture proteins
• Promotes growth and sustain life

Chromosomes

• Threadlike structures located in the nucleus of each human cell
• Made up of DNA

Genes

• Basic units of hereditary information
• Short segment of the DNA strand
• Direct cells to reproduce and assemble proteins (based on DNA instructions) that direct body processes

Mitosis

Cell’s nucleus, including chromosomes, duplicates itself and divides
• Two new cells are formed with identical DNA makeup

Meiosis

• Specialized process of cell division in which gametes are created
• Gametes: Reproductive cells – sperm and ovum (egg) cells
• Cell’s nucleus duplicates and divides twice, producing four cells
• Each cell contains 23 unpaired chromosomes
• Half of the genetic material of parent cell

Zygote

• Formed when 2 gametes combine to produce new individual cell with 46 total chromosomes in 23 pairs (one from each parent)
• Process is called fertilization

Humans usually possess 46
chromosomes

44 autosomes and 2 sex
chromosomes
Females: XX
Males: XY
Sex of offspring depends on whether the father's Y sperm or X sperm fertilizes the ovum.

Genome

*Involves the full set of genes that are the instructions to make an individual member of a certain species
*Human Genome Project mapped the genome
*Only 22,000 genes today in humans
- Similarity between two people: 99.5%
- Similarity between humans and chimpanzees: 98%
- Similarity between humans and every other mammal: 90%

Genotype

A person’s actual genetic material or potential; unique for each individual

Phenotype

Observable and measurable characteristics of an individual
Ex: Height, hair color, and intelligence

Genetic imprinting

Occurs when genes have differing effects depending on whether inherited
from mother or father
• Chemical process “silences” one member of the gene pair

Polygenetic inheritance

Most human characteristics are determined by interaction of several genes

Allele

Variation of a gene or any of the possible forms in which a gene for a particular trait can occur.
• Effects of variations vary greatly from causing life-threatening conditions to having no
detectable effect at all.

Dominant and recessive genes

Dominant gene overrides the expression of the recessive gene
• Recessive gene exerts its influence only if both genes in the pair are recessive

Sex-linked genes

• X-linked inheritance results when a mutated gene is carried on the X chromosome
• Females are carriers, while males may exhibit an X-linked disease

Sibling Similarities

• Each sibling gets abut 10,000 genes from each parent.
• Millions of variations and billions of combinations are possible.
• Full siblings are not necessarily alike.

Monozygotic (identical)twins

Originate from one zygote that splits
apart very early in development
Incomplete split results in conjoined
twins.
Same genotype but slight variations in
phenotype are possible due to
environmental influences.

Dizygotic (fraternal) twins

Result from fertilization of two separate
ova by two separate sperm.
Dizygotic twins have half their genes in
common and occur twice as often as
monozygotic twins.
Incidence is genetic and varies by
ethnicity and age.

Additive genes

Genes that add something to some aspect of the phenotype
Add up to make the phenotype
Ex:
Height is affected by the contributions of about 100 genes.

Carrier

Person whose genotype includes a gene that is not expressed in the phenotype.
• Unexpressed gene occurs in half of the carrier's gametes and is passed on to
half of the carrier's offspring.
• Offspring can be carrier or express the gene in the phenotype (e.g., when
unexpressed gene is inherited by both parents).

Behavior genetics

Investigates the influence of heredity and environment on individual
differences in human traits and development
• Twin study - Behavioral similarity of identical twins compared with fraternal
twins
• Identical twins share 100% of DNA, while fraternal twins share 50%

Adoption study

Seek to discover whether adopted children’s behavior and psychological
characteristics are more like adoptive or biological parents
• May also compare adoptive and biological siblings

Heredity-environment correlations

Individuals’ genes may influence environments to which they are exposed
• As child ages, experiences extend beyond family
• Some environments can mute or strengthen genetic traits

Three types of heredity and environment correlations

• Passive genotype-environment correlations
• Biological parents, who are genetically related to child, provide rearing environment
• Evocative genotype-environment correlations
• Child’s characteristics elicit certain types of environmental characteristics
• Active (niche-picking) genotype-environment correlations
• When children seek out environments they find compatible and stimulating

Shared environmental experience

Siblings’ common experiences
Ex: parents’ personalities or family’s socioeconomic status

No shared environmental experience

• Child’s own unique experiences, both within and outside of family, not shared by
sibling
• Experiences occurring within the family may be part of nonshared environment
• Examples: different peer groups and different friends

Gene X environment (G X E)

the interaction of a specific measured variation
in DNA and the specific measured aspect of
the environment

Epigenetic view

development is the result of an ongoing, bidirectional interchange between heredity and environment