Chapter 2: Genetics

Genetic FoundationsHeredity & Environment

Genetic Foundations

Chromosomes: limits

“Each of us carries a “genetic code” that we inherited from our parents. Because a fertilized egg carries this human code, a fertilized human egg cannot grow into an egret, eagle, or elephant.”

Genetics and Behavior

Nucleus Chromosome Gene

Cell DNA

Genes: Our Biological Blueprint

Human Genome Project Initial phase completed about the year 2000 Humans have 20,000 – 25, 000 genes (21,667) There are far more proteins than genes – 10-20 million Genes (DNA) are dependent- collaborate with other

sources of information Gene expression/activity is affected by context or

environment Context is affected by hormones, light, nutrition, etc.

Sex cells are formed by meiosis rather than mitosis.

Gametes (sperm and ova) have only 23 chromosomes total.

At conception, these two unite resulting in a full complement of 46 chromosomes (23 pairs).

A fertilized egg is called a zygote.

The Sex Cells

• Alleles are normal variations of a gene, found at the same location.

• A child who inherits the same allele (type of gene) from both parents is homozygous for that trait.

• A child who inherits different alleles from each parent is heterozygous for that trait.

Sources of Genetic Variation

Genetic ExpressionInfluenced by the environment

hormoneslightnutritionbehaviorstress (cortisol may cause a fivefold increase in DNA damage)

Sources of Variation

• Patterns of Genetic Inheritance•

Dominant-recessive: the dominant gene (allele) will determine the characteristic

Sources of Genetic Variation

• Examples of dominant genes Dark hair, curly hair, dimples,

types A & B blood (vs. type O), traits for normality in vision, hearing, pigmentation, etc.

Huntington’s Disease

Patterns of Genetic InheritanceDominant-recessive inheritance

• Examples of recessive genes: Cystic fibrosis, PKU, Tay-sachs

disease. Sickle-cell anemia

Patterns of Genetic InheritanceDominant-recessive inheritance

• Co-dominance: both alleles contribute to the phenotype, although not to the same degree.

• Additive: They contribute about equally (50%-50%).

• Example of Co-dominance; Sickle-cell anemia

Patterns of Genetic InheritanceCo-dominance and Additive

• Female children receive an X chromosome from the father which matches locations on the mother’s X.

• Male children receive a Y from the father, which does not have all the gene locations of an X.

• The defective gene on the mother’s X is offset by the gene on the normal X in females, but not in males.

• So, males will show evidence of the defective gene (e.g., hemophilia, RG colorblindness).

• Females will be normal, but carriers of the defective gene.

X-linked (sex-linked) inheritance

Polygenic Inheritance

Many genes interact to influence the characteristic

Most psychological characteristics are polygenic

(Where environmental factors are included, traits are said to be multi-factorial.)

Usually happen during meiosis

Involve breakage and failure to separate

Usually result in miscarriage

Those most commonly survived are:Down syndrome (trisomy 21)

Sex-linked abnormalities

Chromosomal Abnormalities

XXY (Klinefelter) may have verbal difficulties. Tall, underdeveloped testes, possible breasts. 1/800 live male births.

XO (Turner) have trouble with math and spatial skills. Short and have webbed neck; may be infertile. 1/2500 live female births

XYY (Are they more aggressive, antisocial?)

Sex Chromosome Abnormalities

Gene-linked Abnormalities

Over 7000 known (most rare), including: Cystic fibrosis Diabetes Hemophilia Huntington PKU (phenylketonuria) Sickle-cell anemia Spina bifida Tay-sachs disease

Genetic Counseling – for whom?

Family history of disease, mental retardation, physical defects

History of miscarriages

Mother over age 35 (rate of abnormality begins to rise sharply)

May cause miscarriage (except ultrasound, maternal blood samples)

Is the problem correctible?

Genetic engineering is still in the future.

Often the only decision is whether or not to abort the fetus.

Prenatal Diagnostic Methods

Chorionic villi sampling (6-8 weeks);detects genetic defects; risk of miscarriage, limb deformity

Amniocentesis – (11 weeks, best after 15 weeks); detects genetic defects; smaller risk of miscarriage

Prenatal Diagnostic Methods

Infertility

1 in 6 couples in U.S. Waiting too late Sexually transmitted diseases

Fertility technology (IVF, donors) Adoption

Babies culturally unavailable

Environmental Influence

Environmental Influence

Rats reared in an environment enriched with playthings show increased development of the cerebral cortex

Impoverished environment

Enriched environment

Twins Identical Twins

develop from a single fertilized egg that splits in two, creating two genetically identical organisms

Fraternal Twins develop from separate eggs genetically no closer than

brothers and sisters, but they share a fetal environment

Identicaltwins

Fraternaltwins

Samesex only

Same oropposite sex

Dizygotic (two zygotes)

Share approximately 50% of their genetic heritage like any two siblings.

Major causes are maternal age and fertility drugs.

Twinning dramatically on the increase since the 1970s.

Multiple Births – fraternal twins

Multiple Births – identical twins

Monozygotic – one zygote (same fertilized egg)

Share 100% of genetic heritage

Occurs about 3 per 1000 live births worldwide

Factors may include temperature and oxygen levels and late fertilization

Genetics Research

Behavior Genetics

study of the power and limits of genetic and environmental influences on behavior

Molecular Genetics subfield of biology that studies the

molecular structure and function of genes

Nature-nurture Research

• Molecular genetics Human Genome Project

• Behavioral genetics Twin studies

Equal environment assumptions Adoption studies Concordance rates

Epigenesis – ongoing nature/nurture exchanges (bi-directional)

Reaction range

Canalization

Genetic-environmental correlation• Passive• Evocative• Active (niche-picking)