The Human Heritage:Genes and the

Environment

The Development of Children (5th ed.)

Cole, Cole & Lightfoot

Chapter 2

“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

“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

A World of Possibilities…

Overview of the Journey

Sexual Reproduction and Genetic Transmission

Genotype and Phenotype

Mutations and Genetic Abnormalities

Biology and Culture

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

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

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

Genetic Variation

MeiosisThe 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...

MeiosisThe 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

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

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

1. Dominant gene: The allele that is expressed when an individual possesses two different alleles for the same trait

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

3. 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

1. Gene-environment interaction is a two-way process.

2. 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.

3. 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 The process that makes that makes some traits some traits relatively relatively invulnerable to invulnerable to environmental environmental eventsevents

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 .38 100%

DZ twins raised apart .05 50%

Parents/child. living together .16 50%

Adoptive parents & children .01 0%

Siblings raised together .20 50%

Siblings raised apart -.07 50%

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

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