Mendel and the Foundations of Inheritance

Core Concepts

Key genetic terms

• Gene: DNA segment coding for a product, located at a specific locus. Allele: Alternative version of a gene. Genotype: Alleles carried. Phenotype: Observable traits (genotype + environment). Homozygous: Identical alleles (AA or aa). Heterozygous: Different alleles (Aa). Dominant: Expressed in heterozygote; masks recessive. Recessive: Expressed only in homozygote (aa).

Important: Dominant does NOT mean "stronger," "healthier," "better," or "more common." It simply means the allele's effect is visible in a heterozygote. Many recessive alleles are the "normal" or "wild-type" versions of genes (e.g., the allele for typical skin pigmentation is recessive to certain albinism alleles in some contexts — wait, no: actually albinism is typically recessive. The point is: dominance describes expression, not value.)

Mendel's experiments: He used true-breeding (homozygous) pea plants with contrasting traits. Monohybrid cross: P (purple × white) → F1 (all purple) → F2 (~3:1 purple:white).

Explaining the 3:1 ratio: PP (purple) × pp (white) → F1 all Pp (purple, dominant). F1 gametes: ½ P, ½ p. Random fertilization → F2: 1 PP : 2 Pp : 1 pp genotype, 3 purple : 1 white phenotype.

The principle of segregation

Mendel's principle (law) of segregation states: The two alleles for each trait separate (segregate) during gamete formation, so that each gamete carries only one allele for each gene. This segregation corresponds to the separation of homologous chromosomes during anaphase I of meiosis.

Punnett squares

A Punnett square is a diagram that predicts the genotypes and phenotypes of offspring from a genetic cross. The possible gametes of one parent are written across the top; the possible gametes of the other parent are written down the side. The squares show the possible combinations.

Example: Monohybrid cross (Pp × Pp)

Genotypic ratio: 1 PP : 2 Pp : 1 pp

Phenotypic ratio: 3 purple : 1 white

Test cross

A test cross determines the genotype of an individual showing the dominant phenotype (but whose genotype is unknown — it could be PP or Pp). The individual is crossed with a homozygous recessive individual (pp):

• If the unknown is PP, all offspring will show the dominant phenotype (all Pp).

• If the unknown is Pp, half the offspring will show the recessive phenotype (½ Pp, ½ pp).

Probability: Product rule — multiply probabilities of independent events (e.g., probability of pp from Pp × Pp = ½ × ½ = ¼). Sum rule — add probabilities of mutually exclusive events (e.g., probability of Pp = ¼ + ¼ = ½).

Expected ratios and sample size

The 3:1 ratio Mendel observed is a statistical expectation based on large sample sizes. In small samples, chance deviations from the expected ratio are common — just as flipping a coin 10 times will not always produce exactly 5 heads and 5 tails. This does not mean the underlying probabilities are wrong; it means that probabilities describe long-term frequencies, not guaranteed outcomes in small samples.

Genotype → phenotype

How does a genotype produce a phenotype? Genes code for proteins (or functional RNAs). A dominant allele typically codes for a functional protein; a recessive allele often codes for a nonfunctional protein or no protein at all. In a heterozygote (Aa), the dominant allele produces enough functional protein to produce the dominant phenotype. This is why many recessive disorders result from loss-of-function mutations — one functional copy (from the dominant allele) is sufficient for normal function.

ELI Example

Think of alleles as recipes. You inherit two recipe books — one from each parent — for making a particular dish. The dominant recipe is printed in bold, clear text. The recessive recipe is printed in faint, small text. If you have at least one bold recipe, that is the one the kitchen (your cells) follows. Only if you have two faint recipes do you end up making the faint version. The recipe itself is not "better" in any absolute sense — it is just the one that gets read when present. Segregation is the process of randomly putting one recipe book (not both) into each delivery box (gamete) you send out.

Do Not Confuse

High-Yield Memory Anchors

• Segregation: alleles separate during gamete formation; each gamete gets one.

• Monohybrid cross (heterozygote × heterozygote): genotype 1:2:1, phenotype 3:1.

• Test cross: cross with homozygous recessive to reveal unknown genotype.

• Genotype = genetic makeup. Phenotype = observable trait.

• Dominant ≠ common, better, or stronger. It only describes expression in heterozygotes.

Quick Check

Q1 (Foundational): Define the following terms: gene, allele, homozygous, heterozygous, genotype, phenotype.

Q2 (Application): In pea plants, tall (T) is dominant to dwarf (t). A tall plant of unknown genotype is crossed with a dwarf plant. Half the offspring are tall and half are dwarf. What is the genotype of the tall parent? Explain using a Punnett square.

Q3 (Comparison/Reasoning): Two parents, both with the dominant phenotype for a trait, have a child who expresses the recessive phenotype. Explain how this is possible. What must the genotypes of the parents be? What is the probability that their next child will also express the recessive phenotype?

Quick Check Answers

A1: Gene: a unit of heredity — a segment of DNA that codes for a product. Allele: an alternative version of a gene. Homozygous: carrying two identical alleles for a gene (AA or aa). Heterozygous: carrying two different alleles (Aa). Genotype: the genetic makeup — the alleles an organism carries. Phenotype: the observable trait.

A2: The tall parent must be heterozygous (Tt). The dwarf parent is homozygous recessive (tt). Cross: Tt × tt. Gametes from tall parent: T and t (equal probability). Gametes from dwarf parent: all t. Offspring: ½ Tt (tall) and ½ tt (dwarf). If the tall parent were TT, all offspring would be Tt (tall). The 1:1 ratio of tall to dwarf confirms the tall parent is Tt.

A3: The parents must both be heterozygous (Aa) for the trait. Each carries one dominant allele (A, producing the dominant phenotype) and one recessive allele (a, not expressed). When both are Aa, each gamete has a ½ chance of carrying a. The probability of a child being homozygous recessive (aa) is ½ × ½ = ¼ (25%). Each pregnancy is an independent event, so the probability for the next child is also ¼, regardless of the genotype of any previous children.

Chapter Summary

Segregation: alleles separate during gamete formation. Monohybrid cross (Aa × Aa): 1:2:1 genotype, 3:1 phenotype. Test cross reveals unknown genotype. Dominant describes expression in heterozygotes — not commonness or superiority. Genotype + environment = phenotype.

Common Mistakes

Mistake: "A 3:1 ratio in offspring means each individual family of four will have three with the dominant trait and one with the recessive."

Reality: The 3:1 ratio is the expected probability for each offspring independently. In small samples, chance deviations are common. Just as four coin flips will not always produce two heads and two tails, four offspring from a monohybrid cross will not always show a 3:1 ratio.

Mistake: "Dominant alleles are always more common, better, or the 'normal' version."

Reality: Dominance describes expression in a heterozygote — nothing more. Many disease-causing alleles are dominant (e.g., Huntington's disease). Many recessive alleles are the wild-type (normal) version. Frequency in a population is independent of dominance.

Mistake: "A recessive trait can skip generations only because it hides."

Reality: A recessive trait can appear to skip generations because heterozygous individuals (carriers) do not express it, but they can pass the recessive allele to their offspring. When two carriers mate, 1/4 of their offspring (on average) will express the recessive trait.