The Human Heritage: Genes and the Environment The Development of Children (5th ed.) Cole, Cole & Lightfoot Chapter 2

A World of Possibilities… “If we go to all the trouble it takes to mix our genes with those of somebody else, it is in order to make sure that our child will be different from ourselves and from all our other children.” —François Jacob, The Possible and the Actual

Overview of the Journey Sexual Reproduction and Genetic Transmission Genotype and Phenotype Mutations and Genetic Abnormalities Biology and Culture

Sexual Reproduction and Genetic Transmission Mitosis: A Process of Cell Replication Meiosis: A Source of Genetic Variation Sexual Determination: Another Source of Variation

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)

Cell Replication Mitosis The process of somatic (body) cell duplication and division that generates all the individual’s cells except sperm and ova

Genetic Variation Meiosis The twice-over process of germ cell (i.e., sperm and ova) division that results in a zygote (with 46 chromosomes) at conception Variation enhanced by “crossing over” during first phase...

The Process of Crossing Over

The Case of Twins Monozygotic twins: Come from a single fertilized egg and have exactly the same genes (i.e., identical genotypes) Sometimes called “identical twins”, but may not actually look identical due to the fact that phenotype interacts with the environment Dizygotic twins: Come from two ova that have been fertilized at the same time, and consequently are no more alike than any two siblings

Sexual Determination In 22 of the 23 pairs of chromosomes found human somatic cells, the two chromosomes are of the same size and shape, and carry corresponding genes Chromosomes of pair 23 can differ, however, and this determines a person’s sex Females: Both members of chromosome pair 23 are of the same type and are called X chromosomes (i.e., XX) Males: In chromosome pair 23, one X chromo-some is paired with a different, much smaller chromosome called a Y chromosome (i.e., XY)

Sexual Determination Human X and Y chromo-somes

Sexual Determination Since a female is always XX, each of her eggs contains an X chromosome In contrast, half of a man’s sperm carry an X chromosome and half carry a Y chromosome If a sperm containing an X chromosome fertilizes the egg, the resulting child will be XX, a female If the sperm contains a Y chromosome, the child will be XY, a male

Genotype and Phenotype The Laws of Genetic Inheritance Genes, the Person, and the Environment The Study of Genetic Influences on Human Behavior

Genotype + Environment  Phenotype An Interaction Genotype + Environment  Phenotype 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

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

The Case of Blood Type

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 Examples: Red-green color blindness, hemophilia, muscular dystrophy

Principles of Gene-Environment Interactions Gene-environment interaction is a two-way process. Interactions between organisms and their environments need to be studied in a broad, ecological framework because variations in the environment can have profound effects on the development of the phenotype. Genetic factors often play a role in determining what environments individuals inhabit and how individuals shape and select their own experiences.

Range of Reaction The effect of environment on the expression of a gene for fur color in the Himalayan rabbit. Under normal conditions only the rabbit’s feet, tail, ears, and nose are black. If fur is removed from a patch on the rabbit’s back and an ice pack is placed there, creating a cold local environment, the new fur that grows in is black.

Canalization The process that makes some traits relatively invulnerable to environmental events

Genes and Behavior Only in cases where it has been possible to identify a specific gene that controls a specific pattern of phenotypes across all known environments where humans live has it been possible to make clear causal inferences from genes to behavior. As a substitute in the vast majority of cases where multiple, unknown, genes are likely to be involved, behavioral geneticists rely on the study of various kinship relations to estimate the relative influences of the genotype and the environment on the phenotype.

Types of Kinship Methods Family studies: Relatives who live together in a household are compared Twin studies: Monozygotic and dizygotic twins are compared Adoption studies: Children living apart from their biological parents are studied

Family and Adoption Results for Extroversion Type of Relative Correlation % Shared Genes MZ twins raised together .51 100% DZ twins raised together .18 50% MZ twins raised apart .38 DZ twins raised apart .05 Parents/child. living together .16 Adoptive parents & children .01 0% Siblings raised together .20 Siblings raised apart -.07 Loehlin, 1992

Mutations and Genetic Abnormalities Down Syndrome Phenylketonuria Sickle-Cell Anemia Klinefelter Syndrome

Causes of Genetic Abnormalities

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

Phenylketonuria (PKU): A Treatable Genetic Disease Cause: Defective recessive gene that reduces the body’s ability to convert one amino acid (phenylalanine) into another (tyrosine), resulting in retarded development of prefrontal cortex Traits: Severe retardation Incidence: 1:10,000 (more prevalent among whites than blanks; 1:100 people of European descent is a carrier of the recessive mutant gene) Outlook: Feeding PKU infants a diet low in phenylalanine (e.g., avoiding milk, eggs, bread, fish), reduces the severity of mental retardation, although this does not eradicate its effects entirely

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

Survival Strategies Coevolution Biology and Culture Survival Strategies Coevolution

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

Coevolution of Tool-Making Abilities