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

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The Human Heritage:Genes and the Environment

The Development of Children (5th ed.)

Cole, Cole & Lightfoot

Chapter 2

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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

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Overview of the Journey

  • Sexual Reproduction and Genetic Transmission

  • Genotype and Phenotype

  • Mutations and Genetic Abnormalities

  • Biology and Culture

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Sexual Reproduction and Genetic Transmission

Mitosis: A Process of Cell Replication

Meiosis: A Source of Genetic Variation

Sexual Determination: Another Source of Variation

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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

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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)

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Cell Replication


The process of somatic (body) cell duplicationanddivision that generates all the individual’s cells except sperm and ova

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Genetic Variation


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...

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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

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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)

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Sexual Determination

Human X and Y chromo-somes

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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

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Genotype and Phenotype

The Laws of Genetic Inheritance

Genes, the Person, and the Environment

The Study of Genetic Influences on Human Behavior

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An Interaction

  • 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

Genotype + Environment  Phenotype

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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

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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

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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

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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.

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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.

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The process that makes some traits relatively invulnerable to environmental events

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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.

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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

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Family and Adoption Results for Extroversion

Loehlin, 1992

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Mutations and Genetic Abnormalities

Down Syndrome


Sickle-Cell Anemia

Klinefelter Syndrome

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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

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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

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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

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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

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Biology and Culture

Survival Strategies


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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

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Coevolution of Tool-Making Abilities