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Developmental Psychology Unit 2: Genetics, Prenatal Development and Birth

This unit covers how genes are passed on and expressed, how a single cell becomes a newborn, what can disrupt prenatal development, and what happens at birth. Tests often combine simple inheritance problems with questions on teratogens and prenatal timing.

Covers: Chromosomes, genes and inheritance, Genetic and chromosomal disorders, Behavior genetics and heritability, Gene-environment interplay and epigenetics, Prenatal periods, Teratogens, Birth and the newborn

Study guide

Human cells normally carry 46 chromosomes in 23 pairs: 22 pairs of autosomes and one pair of sex chromosomes (XX in most females, XY in most males). Each chromosome is a long strand of DNA divided into genes, the units that code for proteins. Gametes (sperm and ova) form through meiosis and carry 23 single chromosomes, so conception restores the full set of 46. Different versions of a gene are called alleles. A person's genotype is the genetic makeup they carry; their phenotype is the trait actually expressed. In dominant-recessive inheritance, a dominant allele is expressed whenever it is present, while a recessive trait appears only when a person inherits two recessive alleles. Many traits, such as height and intelligence, are polygenic and shaped by many genes plus environment.

Genetic and chromosomal disorders illustrate these rules. Recessive disorders such as phenylketonuria (PKU), sickle-cell disease and cystic fibrosis appear when both parents pass on a recessive allele; two carrier parents have a 1 in 4 chance with each pregnancy of an affected child. Huntington disease is dominant, so one affected heterozygous parent gives each child a 1 in 2 chance. Fragile X syndrome is X-linked and is the most common inherited cause of intellectual disability; X-linked recessive conditions affect males more often because they have only one X. Chromosomal disorders involve missing or extra chromosomes: Down syndrome (trisomy 21), Turner syndrome (XO) and Klinefelter syndrome (XXY). The risk of Down syndrome rises with maternal age.

Behavior genetics asks how much of the variation in a trait across a population comes from genetic differences. Heritability is a proportion from 0 to 1 that describes a population, not an individual, and it can change when environments change. Twin studies compare identical (monozygotic) twins, who share nearly all their genes, with fraternal (dizygotic) twins, who share about half on average; a rough estimate is h² = 2(r for identical twins - r for fraternal twins). Adoption studies compare children with biological and adoptive parents. Genes and environments are linked: in passive gene-environment correlation, parents provide both genes and a home; in evocative correlation, a child's traits draw out reactions; in active correlation (niche-picking), people seek settings that fit their genes. Epigenetics describes experiences that switch genes on or off without changing the DNA sequence.

Prenatal development has three periods. The germinal period (about weeks 1-2) runs from conception through implantation of the blastocyst in the uterine wall. The embryonic period (weeks 3-8) is organogenesis, when the major organs and body structures form; the neural tube closes early, which is why folic acid matters before and just after conception. The fetal period (week 9 to birth) is mostly growth and refinement, and the age of viability is about 22-24 weeks. Teratogens are agents that cause birth defects, including alcohol, nicotine, certain drugs, rubella and Zika. Their effects depend on dose, genetic susceptibility and timing: structural damage is most likely during the embryonic period, when each organ has its own sensitive window.

Labor has three stages: dilation of the cervix (the longest stage), delivery of the baby, and delivery of the placenta (afterbirth). Newborns are checked with the Apgar scale at 1 and 5 minutes, which rates appearance (color), pulse, grimace (reflex irritability), activity (muscle tone) and respiration from 0 to 2 each, for a total of 0 to 10; 7 or above is normal. The Brazelton Neonatal Behavioral Assessment Scale looks more broadly at reflexes, responsiveness and state changes. Preterm infants are born before 37 weeks, and low birth weight means under 2,500 grams (about 5.5 pounds). Kangaroo care (skin-to-skin contact) and responsive caregiving improve outcomes for at-risk newborns.

Key ideas

• Genotype is the genes a person carries; phenotype is the observable trait that results from genes plus environment.
• A recessive trait appears only with two recessive alleles, so two carrier parents have a 25% chance per child.
• Heritability describes the share of variation in a population due to genetic differences and says nothing about one individual.
• Gene-environment correlations can be passive, evocative or active, and active niche-picking grows stronger with age.
• The embryonic period (weeks 3-8) is the most sensitive window for major structural birth defects.
• Teratogen effects depend on dose, timing and genetic susceptibility.
• The Apgar scale scores five signs from 0 to 2 at 1 and 5 minutes after birth.

Formulas and rules

Common mistakes

• Saying a heritability of 0.60 means 60% of one person's trait comes from genes.
• Assuming the fetal period is the most dangerous time for teratogens; major structural defects usually arise in the embryonic period.
• Thinking a 1/4 risk means that if the first child is affected, the next three will not be; each pregnancy is independent.
• Confusing trisomy 21 (Down syndrome) with sex-chromosome disorders such as Turner (XO) or Klinefelter (XXY).
• Treating the Apgar score as a measure of long-term intelligence rather than immediate newborn condition.

Before the test, make sure you can

• Explain the difference between genotype and phenotype and between dominant and recessive alleles.
• Calculate the probability that a child inherits a dominant or recessive condition.
• Identify common chromosomal and single-gene disorders and their causes.
• Interpret twin and adoption data and estimate heritability.
• Distinguish passive, evocative and active gene-environment correlations.
• Name the three prenatal periods with their timing and main events.
• Explain how dose, timing and susceptibility shape teratogen effects.
• Calculate an Apgar score and define preterm and low birth weight.

Words to know

Genotype

The genetic makeup a person carries, including alleles that may not be expressed.

Phenotype

The observable traits that result from the genotype interacting with the environment.

Allele

One of the alternative versions of a gene.

Polygenic trait

A trait influenced by many genes, such as height or skin color.

Carrier

A person with one recessive allele for a condition who does not show the condition but can pass it on.

Down syndrome

A chromosomal condition caused by an extra copy of chromosome 21 (trisomy 21).

Fragile X syndrome

An X-linked condition and the most common inherited cause of intellectual disability.

Heritability

The proportion of variation in a trait within a population that is due to genetic differences.

Monozygotic twins

Identical twins from one fertilized egg that split, sharing nearly all genes.

Niche-picking

Active gene-environment correlation, in which people seek settings that suit their genetic tendencies.

Epigenetics

Changes in whether genes are expressed, caused by experience, without changes to the DNA sequence.

Germinal period

The first two weeks after conception, ending with implantation in the uterine wall.

Embryonic period

Weeks 3-8 of pregnancy, when the major organs and structures form.

Teratogen

An environmental agent, such as alcohol or rubella, that can cause prenatal damage.

Fetal alcohol spectrum disorder

A range of physical and cognitive problems caused by prenatal alcohol exposure.

Age of viability

The point, about 22-24 weeks, at which a fetus may survive outside the womb with intensive care.

Apgar scale

A 0-10 rating of a newborn's color, pulse, reflexes, muscle tone and breathing at 1 and 5 minutes.

Low birth weight

A birth weight under 2,500 grams, or about 5.5 pounds.

Sample practice questions

30 more questions like these, from level 1 to 5, are in the practice set.