2.1 Principles of Genetic Inheritance

Genetic Basics

Genetic inheritance explains how traits pass from parents to children through DNA, genes, and chromosomes. These mechanisms are central to developmental psychology because they set the biological starting point for every aspect of human development, from physical characteristics to predispositions for certain behaviors and abilities.

DNA, Genes, and Chromosomes

DNA (deoxyribonucleic acid) is the molecule that carries genetic instructions for building and running an organism. It's made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). The specific order of these bases encodes all the information your cells need.

Genes are segments of DNA that code for specific proteins, which in turn shape traits like eye color or blood type. Genes sit on chromosomes, which are thread-like structures found inside the nucleus of every cell. Humans have 23 pairs of chromosomes (46 total). You get one chromosome in each pair from your mother and one from your father.

Alleles and Mutations

Alleles are different versions of the same gene. For example, the gene for eye color has alleles that can produce brown, blue, green, and other variations. You inherit two alleles for each gene, one from each parent. When those two alleles differ, one may be expressed over the other (more on dominant vs. recessive below).

Mutations are changes in the DNA sequence that can alter how a gene functions. Some mutations are harmless, some are beneficial, and some can cause disorders. They can result from errors during DNA replication or from environmental factors like UV radiation or chemical exposure.

Inheritance Patterns

Dominant and Recessive Traits

Whether a trait actually shows up depends on which alleles you carry and how they interact:

• A dominant allele is expressed whenever it's present. You only need one copy. For example, brown eye color (B) is dominant.
• A recessive allele is expressed only when you have two copies of it. Blue eye color (b) is recessive, so you need two blue-eye alleles (bb) for blue eyes to appear.

Two key terms to keep straight:

• Genotype = the actual alleles you carry (e.g., BB, Bb, or bb)
• Phenotype = the observable trait that results (e.g., brown eyes or blue eyes)

A person with genotype Bb has brown eyes (because B is dominant), but they carry a recessive blue-eye allele and could pass it to their children. This is why two brown-eyed parents can have a blue-eyed child.

Mendelian and Polygenic Inheritance

Mendelian inheritance refers to the patterns Gregor Mendel first described through his pea plant experiments. The core idea: each parent contributes one allele per trait, and these combine in predictable ways.

A Punnett square is a simple tool for predicting the probability of offspring genotypes. Here's how to use one:

1. Write one parent's two alleles across the top (e.g., B and b).
2. Write the other parent's two alleles down the side.
3. Fill in each box by combining the allele from the column with the allele from the row.
4. Count the resulting genotype ratios to find the probability of each outcome.

Not all traits follow this simple pattern. Polygenic inheritance involves traits influenced by multiple genes acting together. Height and skin color are classic examples. Because many genes contribute, these traits produce a continuous range of phenotypes (a spectrum of heights, for instance) rather than a few distinct categories. Most traits relevant to developmental psychology, including temperament and intelligence, are polygenic.

Cell Division

Mitosis and Meiosis

Two types of cell division matter here, and they serve very different purposes.

Mitosis produces two genetically identical daughter cells. Your body uses it for growth and tissue repair. The stages are prophase, metaphase, anaphase, and telophase. The key point: the daughter cells are clones of the original, with the full 46 chromosomes.

Meiosis produces four genetically unique daughter cells, each with only 23 chromosomes (half the usual number). These cells are gametes, meaning sperm and egg cells. Meiosis happens in two rounds:

1. Meiosis I: Homologous chromosome pairs separate (prophase I, metaphase I, anaphase I, telophase I).
2. Meiosis II: Sister chromatids separate, similar to mitosis (prophase II, metaphase II, anaphase II, telophase II).

During meiosis I, crossing over occurs: homologous chromosomes exchange segments of DNA. This genetic recombination is a major reason why siblings (other than identical twins) look different from each other. Combined with the random assortment of which chromosome from each pair ends up in which gamete, meiosis generates enormous genetic diversity. That diversity is the raw material for individual differences in development.