Difficulty: Introductory | Prerequisites: Basic understanding of meiosis and chromosomes (Ch. 13 or equivalent).
Chapter 14 is the foundation of classical genetics. It covers Gregor Mendel's experiments with pea plants, which established the rules governing how traits pass from parents to offspring. These principles, the law of segregation and the law of independent assortment, underpin every genetics topic that follows in the course, from chromosomal inheritance to population genetics. If you missed the material on meiosis (Ch. 13), read that first: segregation and independent assortment map directly onto what happens to chromosomes during meiosis.
Mendel crossed true-breeding pea plants and tracked how traits like flower colour and seed shape passed through generations. He found that organisms carry two copies of each gene (one from each parent), which separate during gamete formation (law of segregation), and that genes on different chromosomes sort into gametes independently of one another (law of independent assortment). These two laws, combined with simple probability rules, let you predict offspring ratios for single-gene and multi-gene crosses.
Character
A heritable feature that varies among individuals, such as flower colour or seed shape.
Trait
A specific variant of a character. Think of it as one option within a character: purple flowers and white flowers are two traits of the flower-colour character.
True-breeding
An organism that produces offspring identical to itself for a given trait, generation after generation. In simple terms, this means it is homozygous for that gene.
Hybridization
The crossing of two true-breeding varieties. Mendel's core experimental method.
P generation (parental generation)
The true-breeding parents in a genetic cross.
F1 generation (first filial generation)
The hybrid offspring of the P generation cross.
F2 generation (second filial generation)
Offspring produced by crossing or self-pollinating F1 individuals.
Allele
An alternative version of a gene. In simple terms, different "spellings" of the same gene at the same chromosomal location.
Locus
The specific physical location of a gene on a chromosome.
Dominant allele
The allele that determines the phenotype when present in a heterozygote. Represented by a capital letter (e.g. P).
Recessive allele
The allele whose phenotypic effect is masked in a heterozygote. Represented by a lowercase letter (e.g. p). It is only expressed in the homozygous state.
Homozygous
Having two identical alleles for a gene (e.g. PP or pp). Think of it as a "matching pair."
Heterozygous
Having two different alleles for a gene (e.g. Pp). In simple terms, one allele from each parent, and they differ.
Genotype
An organism's genetic makeup for a given character (e.g. PP, Pp, or pp). Can also refer to the entire genome.
Phenotype
An organism's observable traits, including appearance, physiology, and behaviour. Two organisms can share a phenotype but have different genotypes (e.g. PP and Pp both produce purple flowers).
Punnett square
A grid used to predict the genotypes and phenotypes of offspring from a cross. Gametes from each parent line the edges; the boxes show all possible offspring combinations.
Testcross
A cross between an organism of dominant phenotype but unknown genotype and a homozygous recessive individual. Used to reveal whether the dominant organism is homozygous or heterozygous.
Monohybrid cross
A cross that tracks the inheritance of a single character.
Dihybrid cross
A cross that tracks two characters simultaneously.
Law of segregation
The two alleles for a heritable character separate during gamete formation, so each gamete carries only one allele.
Law of independent assortment
Genes on different chromosomes (or far apart on the same chromosome) sort into gametes independently of one another during meiosis.
Many distinct varieties with contrasting traits (purple vs. white flowers, round vs. wrinkled seeds, etc.)
Short generation time and large numbers of offspring per cross, giving statistically meaningful results
Mendel could control mating: each pea flower has both pollen-producing and egg-bearing organs, allowing self-pollination or deliberate cross-pollination
Mendel's work predated the discovery of chromosomes; his conclusions came purely from quantitative analysis of offspring ratios
Mendel crossed two contrasting true-breeding lines (the P generation), e.g. purple-flowered x white-flowered
All F1 hybrids showed only one parental trait (purple flowers), which Mendel called the dominant trait; the hidden trait (white) he called recessive
When F1 hybrids self-pollinated, the F2 generation showed a consistent 3:1 ratio of dominant to recessive phenotypes (e.g. 705 purple : 224 white)
The reappearance of the recessive trait in F2 was key evidence: the heritable factor for white flowers had not been destroyed, only masked in the F1
Alternative versions of genes (alleles) account for variation in characters
The flower-colour gene exists in a purple allele and a white allele; these are different DNA sequences at the same locus
The purple allele codes for an enzyme that produces purple pigment; the white allele does not
An organism inherits two alleles for each character, one from each parent
Diploid cells carry two sets of chromosomes, so each locus appears twice
The two alleles may be the same (homozygous) or different (heterozygous)
If alleles differ, the dominant allele determines the phenotype
Mendel's F1 plants were Pp: they looked purple because P is dominant over p
The law of segregation: the two alleles separate during gamete formation, so each gamete carries only one allele
This corresponds to the separation of homologous chromosomes in meiosis I
A homozygous parent (PP) produces gametes all carrying P; a heterozygous parent (Pp) produces 50% P gametes and 50% p gametes
A Punnett square lines up the gametes from each parent on two axes and fills in all possible offspring combinations
Capital letters = dominant allele; lowercase = recessive allele
For a monohybrid cross Pp x Pp:
1/4 PP (purple), 2/4 Pp (purple), 1/4 pp (white)
Phenotypic ratio: 3 purple : 1 white
Genotypic ratio: 1 PP : 2 Pp : 1 pp
PP and Pp share the same phenotype (purple flowers) but have different genotypes
Phenotype includes physiological and behavioural traits, not just outward appearance (e.g. the ability to self-pollinate is a phenotypic trait in peas)
Problem: a purple-flowered plant could be PP or Pp, because both look the same
Solution: cross it with a homozygous recessive (pp), which contributes only p gametes
If all offspring are purple, the mystery parent is PP (all offspring Pp)
If roughly half the offspring are purple and half white, the mystery parent is Pp (offspring are 1/2 Pp, 1/2 pp)
The expected phenotypic ratio for a Pp x pp testcross is 1:1
A monohybrid cross tracks one character (e.g. Pp x Pp). The F1 offspring heterozygous for that one character are called monohybrids.
A dihybrid cross tracks two characters simultaneously. The F1 offspring heterozygous for both characters are called dihybrids (e.g. YyRr, where Y/y = seed colour and R/r = seed shape).
Mendel crossed a plant with yellow, round seeds (YYRR) with one carrying green, wrinkled seeds (yyrr)
All F1 plants were YyRr and showed both dominant phenotypes (yellow, round)
The key question: do the Y and R alleles stay linked together, or do they sort independently?
Dependent assortment: F1 plants produce only two gamete classes (YR and yr), preserving the parental combinations. This would yield a 3:1 phenotypic ratio in the F2, just as in a monohybrid cross.
Independent assortment: F1 plants produce four gamete classes in equal proportions (YR, Yr, yR, yr). This would yield a 9:3:3:1 phenotypic ratio in the F2.
The F2 data matched the 9:3:3:1 prediction, including two new phenotype combinations not seen in the P generation: yellow-wrinkled and green-round
This confirmed that alleles for seed colour and seed shape segregate independently of each other
With four gamete classes from each parent, there are 16 (4 x 4) equally probable combinations in the F2
Phenotypic breakdown: 9/16 dominant for both, 3/16 dominant for first only, 3/16 dominant for second only, 1/16 recessive for both
You can verify independent assortment by collapsing the dihybrid data to a single character: ignoring seed shape, the yellow-to-green ratio should still be approximately 3:1
Independent assortment applies only to genes on different chromosomes, or to genes very far apart on the same chromosome
Genes close together on the same chromosome tend to be inherited together (linked genes, covered in later chapters)
Mendel happened to choose characters controlled by genes on separate chromosomes, which simplified his results
Students often think "dominant" means "more common in the population." It does not. Dominance is about which allele shows in a heterozygote, not how frequently an allele appears. Polydactyly (extra fingers/toes) is caused by a dominant allele, yet it occurs in only about 1 in 400 births.
Students often assume the dominant allele physically suppresses or silences the recessive allele. It does not. The two alleles in a heterozygote do not interact with each other at all; the dominant allele simply produces enough functional protein on its own to determine the phenotype.
Students sometimes confuse genotypic ratio with phenotypic ratio. In a standard monohybrid cross (Pp x Pp), the genotypic ratio is 1:2:1 (PP:Pp:pp) while the phenotypic ratio is 3:1, because PP and Pp look the same.
Students often forget that independent assortment has a condition: it works for genes on different chromosomes or very far apart on the same chromosome. Genes close together on the same chromosome are linked and do not assort independently.
⚠️ Be able to set up and solve a Punnett square for both monohybrid and dihybrid crosses, including predicting both genotypic and phenotypic ratios.
⚠️ Know the difference between genotype and phenotype, and be prepared to explain why two organisms with different genotypes can share the same phenotype.
⚠️ Understand what a testcross is, when you would use one, and how to interpret its results (all dominant offspring = homozygous parent; 1:1 ratio = heterozygous parent).
⚠️ Be able to explain how the law of segregation maps onto meiosis (alleles separate when homologous chromosomes separate in meiosis I).
⚠️ Know that the 9:3:3:1 ratio from a dihybrid cross is the signature of independent assortment, and be ready to verify it by collapsing the data down to a monohybrid 3:1 ratio for each character separately.
True or false: A heterozygous organism (Pp) is true-breeding. (False: it produces gametes with different alleles, so offspring vary.)
Fill in the blank: The phenotypic ratio of a monohybrid cross between two heterozygotes is ___. (3:1)
True or false: In a testcross, a 1:1 offspring ratio tells you the dominant parent was homozygous. (False: a 1:1 ratio means the dominant parent was heterozygous.)
Fill in the blank: A dihybrid cross between two F1 heterozygotes produces a phenotypic ratio of ___. (9:3:3:1)
True or false: Independent assortment applies to all pairs of genes, regardless of their chromosomal location. (False: it applies only to genes on different chromosomes or very far apart on the same chromosome.)
Q: In a cross between two purple-flowered pea plants (Pp x Pp), what fraction of the offspring are expected to be homozygous dominant (PP)?
A: 1/4 (25%).
Q: You are given a purple-flowered pea plant and asked to determine its genotype. Describe the cross you would perform and how to interpret the results.
A: Perform a testcross by crossing the purple plant with a white-flowered (pp) plant. If all offspring are purple, the plant is PP. If approximately half are purple and half are white, the plant is Pp.
Q: In a dihybrid cross (YyRr x YyRr), what is the probability of an offspring that is homozygous recessive for both traits (yyrr)?
A: 1/16. Each character independently gives a 1/4 chance of homozygous recessive, and by the multiplication rule 1/4 x 1/4 = 1/16.
Q: Mendel observed 705 purple-flowered and 224 white-flowered plants in the F2 generation. Calculate the ratio and explain what it tells you.
A: 705/224 ≈ 3.15:1, which is very close to the predicted 3:1 ratio. This supports the model that the F1 plants were heterozygous (Pp) and that the purple allele is dominant over white.
Q: Why was it important that Mendel used true-breeding parent plants for his initial crosses?
A: True-breeding plants are homozygous (e.g. PP or pp), which means all of their gametes carry the same allele. Starting with homozygous parents ensures that the F1 generation is uniformly heterozygous, giving a clean baseline from which to observe segregation patterns in the F2.
This material connects directly to meiosis (Ch. 13): the law of segregation is what happens when homologous chromosomes separate in meiosis I, and independent assortment is what happens when different chromosome pairs orient randomly at the metaphase plate. Later chapters on chromosomal inheritance (Ch. 15) will explain what happens when genes do not assort independently because they sit close together on the same chromosome (linkage). The probability rules introduced here (multiplication and addition rules) recur in every genetics problem set for the rest of the course.
Mendelian genetics, Gregor Mendel, pea plant genetics, law of segregation, law of independent assortment, dominant trait, recessive trait, homozygous, heterozygous, genotype, phenotype, Punnett square, testcross, monohybrid cross, dihybrid cross, 3:1 ratio, 9:3:3:1 ratio, allele, locus, true-breeding, hybridization, P generation, F1 generation, F2 generation, particulate inheritance, blending hypothesis