12.3 Laws of Inheritance

Mendelian Genetics

Mendelian genetics lays the foundation for understanding inheritance patterns. It explains how traits pass from parents to offspring through discrete units (genes), introducing key concepts like segregation and independent assortment. These principles let you predict offspring characteristics and understand where genetic diversity comes from.

Laws of Segregation and Assortment

Law of Segregation: Each organism carries two alleles for a given gene, but alleles separate during gamete formation so that each gamete receives only one allele. This directly reflects what happens in meiosis I, when homologous chromosomes are pulled apart. The result is that each egg or sperm carries just one copy of each gene.

Law of Independent Assortment: Alleles for different genes sort into gametes independently of one another, meaning inheriting one trait doesn't influence the inheritance of a different trait. This happens because homologous chromosome pairs line up randomly at the metaphase plate during meiosis I. Each pair's orientation is independent of every other pair.

One important caveat: independent assortment only holds for genes on different chromosomes (or genes far apart on the same chromosome). Genes that are close together on the same chromosome tend to be inherited as a unit, which is covered in the Gene Linkage section below.

Probability in Genetic Crosses

Two main tools help you predict outcomes of genetic crosses: diagrams and probability rules.

Forked-line method and Punnett squares are visual approaches. A Punnett square lists the possible gametes from each parent along the rows and columns, then fills in every possible offspring genotype in the grid. The forked-line method works similarly but uses branching lines, which is especially handy for dihybrid or trihybrid crosses where a Punnett square gets unwieldy.

Probability rules let you calculate outcomes mathematically:

• Multiplication rule (AND): Use this when you need two or more independent events to both happen.

$P(A \text{ and } B) = P(A) \times P(B)$

For example, if the probability of getting a dominant allele from the mother is $\frac{1}{2}$ and the probability of getting a recessive allele from the father is $\frac{1}{2}$, the probability of an offspring getting both is $\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$.

• Addition rule (OR): Use this when either of two mutually exclusive outcomes would satisfy your question.

$P(A \text{ or } B) = P(A) + P(B)$

For example, in a monohybrid cross of two heterozygotes ($Aa \times Aa$), the probability of offspring showing the dominant phenotype is the probability of being homozygous dominant ($\frac{1}{4}$) plus the probability of being heterozygous ($\frac{2}{4}$), giving $\frac{3}{4}$.

Genetic Terminology and Concepts

• Allele: An alternative form of a gene. Different alleles arise from mutations and can produce different versions of a trait.
• Genotype: The combination of alleles an organism carries (e.g., $Aa$, $BB$). This is the genetic blueprint.
• Phenotype: The observable characteristics that result from the genotype interacting with the environment. Two organisms with the same genotype can sometimes show different phenotypes if environmental conditions differ.
• Homozygous: Carrying two identical alleles for a gene ($AA$ or $aa$).
• Heterozygous: Carrying two different alleles for a gene ($Aa$).
• Dominance: The relationship where one allele (dominant) masks the expression of the other allele (recessive) in a heterozygote. A heterozygous individual ($Aa$) will display the dominant phenotype.

Genetic Interactions

Genetic interactions add complexity beyond simple Mendelian patterns. Gene linkage, recombination, and epistasis all influence how traits are expressed and inherited. These phenomena explain why you sometimes see ratios that deviate from the classic 3:1 or 9:3:3:1 predictions.

Gene Linkage and Recombination

Gene linkage occurs when two or more genes sit close together on the same chromosome. Because they're physically connected, they tend to be inherited as a unit rather than assorting independently. This violates the law of independent assortment. A classic example: genes for red hair and freckles in humans are linked, which is why these traits often appear together.

Recombination (crossing over) is the exchange of DNA segments between homologous chromosomes during prophase I of meiosis. This can break up linked genes, producing gametes with new allele combinations that neither parent had.

A key relationship: recombination frequency depends on the physical distance between genes on a chromosome.

• Genes far apart on the same chromosome have more opportunities for a crossover event between them, so they recombine more often.
• Genes very close together rarely get separated by crossing over, so they behave as though they're linked.

Impact on gamete formation:

1. Linked genes that don't undergo recombination are inherited together, maintaining the parental combination of alleles in offspring.
2. When crossing over does occur between linked genes, it produces recombinant gametes with new allele combinations. This is a major source of genetic diversity.

Epistasis in Phenotypic Expression

Epistasis is a gene interaction where one gene (the epistatic gene) controls whether another gene (the hypostatic gene) can be expressed. The epistatic gene effectively masks or modifies the hypostatic gene's effect on phenotype.

A well-known example is coat color in Labrador retrievers. One gene determines pigment color (black vs. brown), but a second epistatic gene controls whether pigment is deposited in the fur at all. Dogs homozygous recessive at the epistatic locus ($ee$) are yellow regardless of their genotype at the pigment color gene.

Types of epistasis:

• Dominant epistasis: A single dominant allele at the epistatic locus is enough to mask the hypostatic gene. Example: fruit color in summer squash, where a dominant allele at one locus produces white fruit regardless of the genotype at a second color locus. This gives a modified 12:3:1 ratio instead of the expected 9:3:3:1.
• Recessive epistasis: The hypostatic gene is masked only when the epistatic locus is homozygous recessive. Example: certain forms of albinism in humans, where homozygous recessive alleles at a pigment-pathway gene block all pigment production regardless of other color genes. This produces a modified 9:3:4 ratio.

Why this matters for problem-solving: Whenever you see a dihybrid cross that doesn't produce the standard 9:3:3:1 ratio, consider epistasis. The modified ratios (12:3:1, 9:3:4, 9:7, etc.) still add up to 16 total parts because they're rearrangements of the same 16 possible genotypic combinations. Recognizing which ratio you're seeing tells you what type of epistasis is at work.