28.7 Patterns of Inheritance

Genetic Inheritance

Genetic inheritance determines traits ranging from eye color to disease risk. Genes, the functional units of DNA, carry instructions that shape how our bodies develop and function. Understanding how these genes are passed from parent to offspring explains the diversity we see in human populations.

Two core terms to lock in early: your genotype is your actual genetic code, while your phenotype is what that code produces in the real world. Alleles (different versions of the same gene) can be dominant or recessive, and the way they combine drives the traits you express.

Genotype vs. Phenotype Distinction

Your genotype is the set of alleles you inherit from your parents for a given gene. It's written using letter notation: AA, Aa, or aa. You can't observe a genotype just by looking at someone.

Your phenotype is the observable trait that results from your genotype, things like eye color, hair color, or blood type. Phenotype is also influenced by environmental factors, which is why genotype doesn't always predict phenotype perfectly. Two people with the same genotype can sometimes show slightly different phenotypes depending on nutrition, climate, or other conditions.

Alleles and Observable Traits

Alleles are alternative forms of a gene, located at a specific position (called a locus) on a chromosome. They're typically labeled as dominant (A) or recessive (a).

• A dominant allele is expressed whenever it's present. Both homozygous dominant (AA) and heterozygous (Aa) individuals show the dominant trait.
• A recessive allele is only expressed in the homozygous state (aa), meaning you need two copies for the recessive trait to appear.

Here's how the three possible genotypes map to phenotype:

This is why carriers exist. A person with genotype Aa looks the same as someone with AA, but they carry a hidden recessive allele they can pass to offspring.

Mendelian Inheritance in Humans

Gregor Mendel established the foundational rules of heredity through experiments with pea plants. Two laws form the basis of his work:

• Law of Segregation: During gamete (egg or sperm) formation, the two alleles for each gene separate so that each gamete carries only one allele.
• Law of Independent Assortment: Alleles for different genes sort independently during gamete formation. This allows for many different trait combinations in offspring.

Punnett squares are the standard tool for predicting offspring genotypes and phenotypes. They map out every possible allele combination from a cross.

A monohybrid cross tracks one gene with two alleles. For example, crossing AA × aa produces 100% Aa offspring (all heterozygous, all showing the dominant trait).

A dihybrid cross tracks two genes simultaneously. Crossing AaBb × AaBb produces a classic 9:3:3:1 phenotype ratio in the offspring. The starting cross of AABB × aabb, by contrast, yields 100% AaBb in the first generation.

Autosomal vs. Sex-Linked Inheritance

Autosomal inheritance involves genes on the 22 pairs of non-sex chromosomes (autosomes). These traits affect males and females equally.

• Autosomal dominant disorders require only one copy of the dominant allele to cause disease. An affected individual has at least one affected parent. Examples: Huntington's disease, Marfan syndrome, neurofibromatosis.
• Autosomal recessive disorders require two copies of the recessive allele (aa). Both parents can be unaffected carriers (Aa) and still produce an affected child. Examples: cystic fibrosis, sickle cell anemia, phenylketonuria (PKU).

Sex-linked inheritance involves genes on the X or Y chromosome. X-linked disorders are far more common than Y-linked ones.

Why males are more vulnerable to X-linked disorders: Males have only one X chromosome (XY), so they're hemizygous for X-linked genes. A single defective allele on their X has no second copy to compensate. Females (XX) can be carriers if they're heterozygous, carrying one normal and one defective allele. They're usually unaffected because the normal allele on the other X chromosome compensates, partly through a process called X-inactivation (lyonization).

Examples of X-linked disorders: hemophilia, Duchenne muscular dystrophy, and red-green color blindness.

Genetic Variations and Mutations

Not all inheritance follows simple dominant/recessive rules. Several patterns add complexity beyond what Mendel originally described.

Codominance occurs when both alleles are fully expressed at the same time. The AB blood type is the classic example: a person with one A allele and one B allele expresses both A and B antigens on their red blood cells. Neither allele masks the other.

Incomplete dominance produces a blended phenotype. Neither allele is fully dominant, so the heterozygote shows an intermediate trait. The textbook example uses flower color: crossing a red-flowered plant with a white-flowered plant produces pink offspring.

Mutations are changes in the DNA sequence that can alter how a gene functions. Some mutations are harmless, some are beneficial, and some cause disease. Mutations are the ultimate source of new alleles in a population.