Notes on Hereditary Influences on Development

Genotype, Phenotype, and Basic Genetic Concepts

• Genotype: the genetic endowment that an individual inherits.

• Phenotype: the observable or measurable expression of the genotype.

• Conception: the moment of fertilization, when a sperm penetrates an ovum, forming a zygote.

• Zygote: a single cell formed at conception from the union of a sperm and an ovum.

• Genetic material: chromosome — threadlike structure made up of genes; in humans there are $46$ chromosomes in the nucleus of each body cell.

• Genes: basic units of heredity, hereditary blueprints for development transmitted unchanged from generation to generation.

• Deoxyribonucleic acid (DNA): long, double‑stranded molecules that make up chromosomes.

Cell Division and Growth

• Growth of the zygote and production of body cells rely on mitosis:

  • Mitosis: a cell duplicates its chromosomes and then divides into two genetically identical daughter cells.

• Stepwise illustration (simplified):
  1) Original cell with four chromosomes (illustrative).
  2) Each chromosome splits lengthwise, producing a duplicate.
  3) Duplicates move to opposite ends; the cell begins to divide.
  4) Division completes; two new cells have identical sets of chromosomes.

Germ Cells, Gametes, and Meiosis

• Germ (sex) cells: also present; produce gametes (sperm in males, ova in females).

• Gametes: produced by meiosis, a germ cell division producing haploid cells that contain half of the parent cell's original chromosome complement.

• In humans, each gamete contains $23$ chromosomes (half of the 46 in somatic cells).

Meiosis and Genetic Variation

• Meiosis process includes:

  • Duplication of the germ cell's 46 chromosomes.

  • Crossing over: adjacent duplicated chromosomes cross and exchange segments, creating new hereditary combinations.

  • The original cell divides to form two new cells, each with 23 duplicated chromosomes (some altered by crossing‑over).

  • In the final gametes, each chromosome and its duplicate segregate into separate gametes, so each gamete has half the chromosomes of the original cell.

• Crossing‑over: during meiosis, genetic material is exchanged between chromosome pairs, producing new combinations.

Sex Chromosomes, Autosomes, and Genetic Inheritance

• Germ cell steps (summary): starting from 46 chromosomes, meiosis yields gametes with 23 chromosomes each.

• Sex chromosomes: X and Y; autosomes: the 22 pairs of chromosomes identical in males and females.

• X chromosome: longer of the two sex chromosomes; females typically have two Xs, males have one X.

• Y chromosome: shorter; males have one Y, females have none.

• Autosomes: 22 pairs, identical in males and females.

• After conception, zygotes carry a full set of chromosomes; inheritance patterns determine phenotypic outcomes.

What Do Genes Do?

• Produce enzymes and proteins necessary for cell creation and function.

• Guide cell differentiation.

• Regulate pace/timing of development.

• Environmental factors (internal and external) influence how genes function.

Genetic Variation: Simple Mendelian Example (Illustrative)

• Parental genotypes: Mother Nn, Father Nn (example with a single gene affecting vision).

• Gametes: N sperm or n sperm; N ovum or novum.

• Possible zygotes (genotypes):

  • NN (homozygous normal vision)

  • Nn (heterozygous normal vision)

  • nN (heterozygous normal vision)

  • nn (homozygous nearsighted)

• Phenotypic outcomes depend on dominance relationships; this example illustrates how genotype combinations produce different vision outcomes.

Heredity Disorders and Genetic Abnormalities

• Heredity disorders can arise from chromosomal abnormalities or genetic abnormalities.

• Detecting, predicting, and treating hereditary disorders is a major area of study.

Chromosomal Abnormalities

• Chromosomal abnormalities involve too many or too few chromosomes (aneuploidy) due to uneven distribution during meiosis.

• Consequence: gametes with incompatible chromosome numbers lead to zygotes with abnormal development.

• Autosomes vs. Sex Chromosomes: abnormalities can involve autosomes (22 autosome pairs) or sex chromosome (23rd pair).

• Down syndrome (trisomy‑21): an autosomal abnormality caused by an extra 21st chromosome; individuals typically have distinctive physical features and moderate to severe intellectual disability.

• Most frequent autosomal abnormality: Down syndrome (trisomy 21).

• 

Genetic Abnormalities

• Sex chromosome abnormalities involve the 23rd pair (the sex chromosomes).

• Common sex chromosome abnormalities are listed in Table 3.2 (not reproduced here).

• Other genetic abnormalities can involve autosomes or sex chromosomes.

Recessive and Dominant Genetic Diseases

• Many disorders are passed when both parents carry recessive alleles.

• Some disorders are caused by dominant alleles inherited from either parent; the contributing parent may display the disorder.

• Table 3.3 summarizes major recessive hereditary diseases (not reproduced here).

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Detecting Genetic Abnormalities

• Amniocentesis: extraction of amniotic fluid to test fetal cells for chromosomal abnormalities and other defects.

  • Conducted around weeks $11$–$14$ of pregnancy.

  • Risks: miscarriage risk higher than risk of birth defect for women under age 35.

  • Results: typically available in $2 ext{--}3$ weeks.

  • Figure 3.10 illustrates the needle insertion and sample collection.

• Chorionic Villus Sampling (CVS): sampling fetal cells from the chorion.

  • Conducted around weeks $8$–$9$ of pregnancy.

  • Results: available within $24$ hours.

  • Risk of miscarriage roughly 1 in 50(2%)

  • Figure 3.11 shows sampling methods guided by ultrasound.

• Ultrasound: uses sound waves to produce an image outline of the fetus; useful after week $14$; noninvasive. To determine any structural abnormalities.

Treating Genetic Abnormalities

• Special diets for metabolic disorders (e.g., phenylketonuria, PKU).

• PKU: a genetic disease in which the child cannot metabolize phenylalanine; if untreated, leads to hyperactivity and mental retardation.

• Germline gene therapy: a theoretical/experimental approach to repair or replace harmful genes; not yet perfected or approved for humans.

Methods of Studying Hereditary Influences

• Heritability: the amount of variability in a trait attributable to hereditary factors.

Selective Breeding

• Selective breeding experiments study genetic influences by testing whether traits can be bred via mating.

• Example: maze‑learning differences in inbred (maze‑bright vs maze‑dull) lines over generations.

• Figure 3.12 (and related concordance data) illustrates maze performance and concordance across identical and fraternal twins.

Family Studies and Kinship

• Kinship: the extent to which two individuals share genes.

• Twin design: compare twins differing in zygosity (identical vs fraternal) to estimate heritability of traits.

• Adoption design: compare adoptees with biological and adoptive relatives to estimate heritability.

Estimating Gene–Environment Contributions

• Concordance rate: the percentage of cases in which a trait is present for one twin given it is present for the other.

• Heritability coefficient: a numeric estimate from $0.00$ to $1.00$ of the portion of variation due to genetic factors.

• Figure 3.14 shows concordance rates for identical vs fraternal twins across behavioral dimensions.

• Table 3.4 (average correlations for intelligence test scores from family studies) summarizes findings across kinship levels.

  

Environment and Heredity

• Non-shared environmental influence (NSE): environmental factors that siblings do not share, contributing to differences among them.

• Shared environmental influence (SE): environmental factors that siblings share, contributing to similarities among them.

Hereditary Influences on Behavior and Development

Intellectual Performance

• As children age, the role of genes in intellectual performance tends to increase.

• Nonshared environment tends to increase with age; shared environment tends to decrease.

• Figure 3.15 portrays changes in IQ correlations between identical and fraternal twins over childhood.

Personality

• Genetic influences on personality traits such as introversion/extraversion and empathic concern.

• Introversion/extraversion: opposite ends of a personality spectrum; introverts tend to be shy and withdraw; extraverts are sociable.

• Empathic concern: degree to which a person recognizes others' needs and cares about their welfare.

• Estimated heritability for empathic concern around +0.40; nonshared environmental influences are most important.

• Question: do siblings have different experiences because they have different genes?

Parent, Child, and Transactional Influences

• Parent effects model: parenting influences child outcomes.

• Child effects model: children influence parenting through temperament and personality.

• Transactional model: reciprocal influence between children and parenting; gene–environment interplay fosters development.

Behavioral Disorders and Mental Illness

• Many disorders (schizophrenia, alcoholism, criminality, depression, hyperactivity, bipolar disorder, neurotic disorders) have genetic components; individuals inherit a predisposition, not the disorder itself.

• Schizophrenia: serious mental illness with disturbances in thinking, emotion, and social behavior.

• Bipolar disorder: extreme mood fluctuations.

• Neurotic disorders: irrational patterns of thinking/behavior to cope with stress or anxiety.

Theories of Gene–Environment Interactions

Canalization

• Canalization: genetic restriction of phenotype to a narrow set of developmental outcomes.

• Highly canalized traits are channeled along predetermined pathways, with environment having little effect on the resulting phenotype.

Range of Reaction Principle

• Genotype sets limits on the range of possible phenotypes in response to different environments.

• Example: hypothetical reaction ranges for intellectual performance across restricted, average, and intellectually enriching environments (Figure 3.16).

Genotype–Environment Correlations

• Many behavioral geneticists believe genes influence the environments we experience.

• Passive genotype–environment correlation: rearing environments provided by biological parents are influenced by the parents' genes and correlated with the child's genotype.

• Evocative genotype–environment correlation: heritable attributes evoke responses from others that shape the environment.

• Active genotype–environment correlation: individuals seek out environments compatible with their genotype.

Contributions and Criticisms of the Behavioral Approach

• Contributions:

  • Many attributes thought to be environmentally determined are also influenced by genes.

  • Genetics and environment are intertwined; understanding behavior requires considering both factors.

• Criticisms:

  • Describes correlations or associations but does not fully explain development.

  • Environmental forces remain underspecified or not fully explained by the model.