The human body contains 100 trillion cells. There is a nucleus inside each human cell (except red blood cells). Each nucleus contains 46 chromosomes, arranged in 23 pairs. One chromosome of every pair is from each parent. The chromosomes are filled with tightly coiled strands of DNA. Genes are segments of DNA that contain instructions to make proteins— the building blocks of life.

Gametes and Zygote Sperm Sperm Ovum Gametes (reproductive cells) Fertilization Zygote

The Process of Meiosis for Sperm Cells Cell with 46 chromosomes (only one pair of homologous chromosomes is shown here). Each member of the pair has begun to replicate similar to mitotic cell division. First meiotic cell division begins, but does not proceed as in mitosis. Instead of the replicated chromosome splitting apart, one member of each homologous pair becomes a part of the first-generation daughter cell. The second meiotic division proceeds after the first is completed; now the replicated chromosome acquired in the first-generation daughter cell splits apart. Each of the four gametes produced by the two-step process now has acquired one member of the pair of homologous chromosomes.

The Process of Mitosis Each daughter cell now has a pair of chromosomes that is identical to the original pair Cell nucleus with a pair of chromosomes Chromosomes split and replicate to produce two identical pairs The pairs separate, and the cell divides

GENOTYPE AND PHENOTYPE GENOTYPE: Set of genetic traits a person inherits; a person’s inborn capacity or potential PHENOTYPE: Set of traits a person actually displays, resulting from a combination of the person’s genotype (potential) and life experiences that modify that potential

Inheritance of Hemophilia, a Sex-Linked Disorder Carrier Mother X X XX XX Normal Carrier X Daughter Daughter (25%) (25%) Normal Father XY XY Normal Hemophilic Y Son Son (25%) (25%)

FREQUENCY OF DOWN SYNDROME (PER 1000) Relationship Between Maternal Age and the Incidence of Down Syndrome 100 90 80 70 60 FREQUENCY OF DOWN SYNDROME (PER 1000) 50 40 30 20 10 15 20 25 30 35 40 45 50 MATERNAL AGE (YEARS)

Inheritance of a Dominant Gene Disorder Affected Parent (Has the Disorder) D r rr normal (25%) Dr Affected (25%) r Normal Father Dr Affected (25%) rr normal (25%) r (50%) (50%)

Inheritance of a Recessive Gene Disorder Carrier Mother D r Dr Affected (25%) rr normal (25%) D Carrier Father Dr Affected (25%) rr normal (25%) r

Risk of Selected Genetic Disorders Chromosomal Down Syndrome Klinefelter syndrome (XXY) Fragile X syndrome Turner syndrome (XO) Dominant Gene Polydactyly Achondroplasia Huntington disease Recessive Gene Cystic fibrosis Sickle-cell disease Tay-Sachs disease X Linked Hemophilia Multifactorial Congenital heart disease Neural tube defect Cleft lip/cleft palate 1/800 1/800 men 1/1,200 male births 1/2,000 female births 1/3,00 women 1/300 - 1/100 1/2,300 1/15,000 - 1/5,000 1/2,500 white persons (risk of being a carrier is 1/25) 1/625 African Americans (risk of being a carrier is 1/10) 1/3,600 Eastern European Jews(risk of being a carrier is 1/30 - 1/300) 1/2,500 male babies 1/125 1 - 2/1,000 1/1,000 - 1/5,000 Sources: ACOG (1990); Blatt (1988); Diamond (1989(; Hagerman (1996); Selekman (1993); Stratford (1994).

Who Should Seek Prenatal Counseling? 1. Couples who already have a child with some serious defect such as Down syndrome, spina bifida, congenital heart disease, limb malformation, or mental retardation 2. Couples with a family history of a genetic disease or mental retardation 3. Couples who are blood relatives (first or second cousins) 4. African Americans, Ashkenzzi Jews, Italians, Greeks, and other high-risk ethnic groups 5. Women who have had a serious infection early in pregnancy (rubella or toxoplasmosis) or who have been infected with HIV 6. Women who have taken potentially harmful medications early in pregnancy or habitually use drugs or alcohol 7. Women who have had X rays taken early in pregnancy 8. Women who have experienced two or more of the following: stillbirth, death of a newborn baby, miscarriage 9. Any woman thirty-five years or older Source: Adapted from Fienbloom & Forman (1987) p. 129

Intellectual Performance (IQ) The Concept of Range of Reaction for Intellectual Performance child A child B child C High Intellectual Performance (IQ) Reaction Range Average Low Restricted Average Enriched Type of Environment

Measuring the effects of Nature and Nurture: Twin and Adoption Studies

Correlations of IQ Scores +1.00 +0.90 +0.80 +0.70 +0.60 Correlation of IQ scores +0.50 +0.40 +0.30 +0.20 +0.10 Identical twins reared together Siblings reared apart Identical twins reared apart Unrelated children reared together Non-identical twins reared together Unrelated children reared apart Siblings reared together