Genes Genes consists of instructions via pairs of four chemicals called bases (adenine, thiamine, cytosine, and guanine; abbreviated A, T, C, and G) Instructions to build proteins

Genes  Most genes are identical for every human being.  Some genes vary slightly in their codes from one person to another. 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.

Genes Genetic diversity  Distinguishes each person  Allows the human species to adapt to pressures of the environment Genome  Involves the full set of genes that are the instructions to make an individual member of a certain species

The Beginnings of Life Zygote  Two gametes (sperm and ovum) combine and produce a new individual with 23 chromosomes from each parent. Gametes  Reproductive cells (sperm and ova)  Each gamete consists of 23 chromosomes.

The Beginnings of Life  Genotype  Organism’s entire genetic inheritance, or genetic potential.  Homozygous  Two genes of one pair that are exactly the same in every letter of their code  Heterozygous  Two genes of one pair that differ in some way The Moment of Conception This ovum is about to become a zygote. The Moment of Conception This ovum is about to become a zygote.

Male or Female?  Females: XX  Males: XY  Sex of offspring depends on whether the father's Y sperm or X sperm fertilizes the ovum.

First Stages of the Germinal Period (a) (b) (c) The original zygote as it divides into (a) two cells, (b) four cells, and (c) eight cells. Occasionally at this early stage, the cells separate completely, forming the beginning of monozygotic twins, quadruplets, or octuplets.

Siblings and Twins Monozygotic (identical) twins  one zygote that splits apart  Incomplete split results in conjoined twins.  Same genotype but slight variations in phenotype are possible due to environmental influences. Dizygotic (fraternal) twins  fertilization of two separate ova by two separate sperm.  Dizygotic twins have half their genes in common  occurs twice as often as monozygotic twins.  Incidence is genetic and varies by ethnicity and age.

From One Cell to Many Epigenetics – environmental factors that surround the genes, affecting genetic expression All diseases known to be genetic (schizophrenia, cancer etc.) are actually epigenetic No trait is determined by genes alone Genes affect everything but determine nothing

Phenotype  Observable characteristics of an organism, including appearance, personality, intelligence, and all other traits

Current consensus  Genes affect every aspect of behavior.  Most environment influences on children raised in the same home are not shared.  Genes produce responses that shape development.  Lifelong, people choose friends and environments that encourage their genetic predispositions. Shyness is inherited, but this mother seems not to have the gene. What about her daughter?

Genotype and Phenotype Alcoholism  Genes can make addiction more or less likely through biochemical processes and psychological traits  Differential sensitivity  Society and culture are important

Chromosomal Problems Down syndrome  Called trisomy-21 because the person has three copies of chromosome 21  Involves around 300 distinct characteristics from third chromosome; unique individuals

Down Syndrome  Thick tongue, round face, slanted eyes  Distinctive hands, feet, and fingerprints.  Hearing problems  Heart abnormalities  Muscle weakness  Short stature  Slower to develop intellectually, especially in language  Often reach their maximum intellectual potential at about age 15

Genetic Testing for Disorders Genetic counseling  Involves consultation and testing by trained experts  Enable individuals to learn about their genetic heritage  Helps couples clarify values and determine genetic risks Ethical rules  Tests are confidential  Decisions are made by the clients, not the counselors

End of Chapter 3