Difficulty: Intermediate | Prerequisites: Evolution and natural selection study notes, basic cell biology
Darwin needed heritable variation for natural selection to work, but he had no idea how inheritance actually functioned. This set of topics fills that gap. Mendel's experiments with pea plants revealed that inheritance is particulate (discrete units, not blending), and meiosis is the cellular machinery that makes it possible. Once you understand segregation, independent assortment, and the various ways genotype maps to phenotype, you have the tools to see why populations maintain variation and how mutation generates new variation for selection to act on.
Inheritance is particulate, not blending: genes are passed as discrete units through meiosis, which preserves variation across generations. Mendel's laws of segregation and independent assortment explain how alleles sort into gametes, and Punnett squares let you predict offspring ratios. The genotype-phenotype relationship is more complex than simple dominance, involving incomplete dominance, codominance, pleiotropy, epistasis, polygenic inheritance, and environmental effects (norm of reaction).
Gene
A unit of hereditary information located at a specific position on a chromosome. Think of it as one instruction in the organism's blueprint.
Locus
The specific physical location of a gene on a chromosome. In simple terms, it is the gene's "address."
Allele
One of two or more versions of a gene. Different alleles produce variations of the same trait (e.g. purple vs. white flower colour in peas).
Chromosome
A long strand of DNA carrying many genes. In simple terms, it is a package of genetic instructions.
Chromatid
One copy of a duplicated chromosome, joined to its sister chromatid at the centromere. After DNA replication, each chromosome consists of two sister chromatids.
Diploid
Having two complete sets of chromosomes (one from each parent). Human body cells are diploid (2n = 46).
Haploid
Having one complete set of chromosomes. Gametes (sperm and eggs) are haploid (n = 23 in humans).
Homologous chromosomes
A pair of chromosomes, one inherited from each parent, that carry genes for the same traits at the same loci. They pair up during meiosis I.
Tetrad
The structure formed when two homologous chromosomes (each made of two sister chromatids) pair up during meiosis I. Four chromatids in total, hence "tetrad."
Mitosis
Cell division that produces two genetically identical diploid daughter cells. This is how the body grows and repairs itself.
Meiosis
Cell division that produces four genetically unique haploid cells (gametes). It involves two rounds of division: meiosis I separates homologous chromosomes, meiosis II separates sister chromatids.
IPMAT
The stages of cell division: Interphase, Prophase, Metaphase, Anaphase, Telophase. A mnemonic for remembering the sequence.
Blending inheritance
The incorrect theory that offspring traits are a smooth blend of parental traits, like mixing paint. If this were true, variation would be destroyed in a few generations.
Particulate inheritance
Mendel's correct model: inherited information is transmitted as discrete particles (genes) that do not blend. Variation is preserved across generations.
Segregation
Mendel's first law. The two alleles for a gene separate during gamete formation, so each gamete carries only one allele. This is physically caused by homologous chromosomes separating in meiosis I.
Independent assortment
Mendel's second law. Genes on different chromosomes sort into gametes independently of one another. This is why a dihybrid cross produces a 9:3:3:1 ratio.
Recombination (crossing over)
The exchange of DNA segments between homologous chromosomes during meiosis I. It creates new combinations of alleles on the same chromosome, increasing genetic variation.
Genotype
The specific combination of alleles an organism carries for a given gene. Think of it as the genetic recipe.
Phenotype
The observable characteristics of an organism resulting from its genotype and environmental influences. Think of it as the finished dish.
Trait
A specific feature or characteristic of an organism (e.g. flower colour, seed shape).
Homozygous
Having two identical alleles for a gene (e.g. AA or aa). In simple terms, both copies of the gene say the same thing.
Heterozygous
Having two different alleles for a gene (e.g. Aa). The organism carries one of each version.
Dominant
An allele whose phenotype is expressed in a heterozygote. One copy is enough to produce the dominant trait.
Recessive
An allele whose phenotype is expressed only when homozygous (two copies). In a heterozygote, the dominant allele masks it.
Incomplete dominance
A pattern where the heterozygote shows a phenotype intermediate between the two homozygotes. Neither allele fully masks the other (e.g. red x white = pink flowers in snapdragons).
Codominance
A pattern where both alleles are fully expressed in the heterozygote. Neither is dominant or recessive (e.g. AB blood type, where both A and B antigens appear).
Monohybrid cross
A cross between two organisms that are both heterozygous for one gene (Aa x Aa). Used to demonstrate segregation. Expected phenotype ratio: 3:1.
Test cross
A cross between an organism with a dominant phenotype and a homozygous recessive organism. Used to determine whether the dominant individual is homozygous (AA) or heterozygous (Aa).
Dihybrid cross
A cross between organisms heterozygous for two genes on different chromosomes (AaBb x AaBb). Used to demonstrate independent assortment. Expected phenotype ratio: 9:3:3:1.
Punnett square
A grid used to predict the genotypic and phenotypic ratios of offspring from a cross. Gametes go on the edges; possible offspring fill the cells.
Pleiotropy
When one gene affects multiple, seemingly unrelated traits. In simple terms, one gene, many effects.
Epistasis
When one gene masks or modifies the expression of another gene at a different locus. Think of it as a gene that acts as a gatekeeper for another gene's effect.
Polygenic inheritance
When multiple genes contribute to a single trait, producing a continuous range of phenotypes (e.g. height, skin colour). The opposite of Mendel's one-gene-one-trait model.
Norm of reaction
The range of phenotypes a single genotype can produce across different environments. This is a very important concept: it captures the idea that phenotype = genotype + environment.
Point mutation
A change in a single nucleotide of DNA. Subtypes include silent (no amino acid change), missense (different amino acid), and nonsense (premature stop codon).
Insertion/deletion
The addition or removal of nucleotides in a DNA sequence. These often cause a frameshift, scrambling all downstream amino acids.
Duplication
A mutation in which a segment of a chromosome is copied, resulting in extra genetic material.
Inversion
A mutation in which a segment of a chromosome is reversed end to end.
Translocation
A mutation in which a segment of one chromosome breaks off and attaches to a different chromosome.
Non-disjunction
The failure of chromosomes to separate properly during meiosis, resulting in gametes with too many or too few chromosomes (e.g. Down syndrome from trisomy 21).
Darwin's theory required heritable variation, but the prevailing theory of his day was blending inheritance: offspring traits were thought to be a smooth mix of parental traits. If blending were real, variation would be halved every generation and natural selection would quickly run out of material to work with.
Mendel solved this problem (though Darwin never knew). Inheritance is particulate: genes are discrete units that do not blend. They are passed intact from parent to offspring.
The physical basis for particulate inheritance is meiosis:
Genes sit on chromosomes. After DNA replication, each chromosome consists of two sister chromatids joined at the centromere.
In mitosis (ordinary cell division), sister chromatids separate, producing two diploid daughter cells identical to the parent.
In meiosis (gamete formation), two rounds of division occur:
Meiosis I: homologous chromosomes pair up (forming tetrads), crossing over occurs, and homologues separate. Each daughter cell is now haploid.
Meiosis II: sister chromatids separate (as in mitosis). The result is four haploid gametes.
Three features of meiosis create and preserve genetic variation:
Segregation. The two alleles for each gene separate into different gametes during meiosis I.
Independent assortment. Genes on different chromosomes sort independently, so maternal and paternal chromosomes are shuffled randomly into gametes.
Recombination (crossing over). During meiosis I, homologous chromosomes exchange segments of DNA, producing new allele combinations on the same chromosome.
Mendel worked out segregation and independent assortment by breeding peas, without knowing about meiosis. His key insight: traits are controlled by pairs of "factors" (alleles), one inherited from each parent.
Dominance relationships determine how alleles map to phenotype:
Complete dominance. The heterozygote looks the same as the homozygous dominant (Aa looks like AA).
Incomplete dominance. The heterozygote is intermediate (red x white = pink).
Codominance. Both alleles are fully expressed in the heterozygote (AB blood type).
Key crosses and what they demonstrate:
Monohybrid cross (Aa x Aa). Demonstrates segregation. Expected phenotype ratio 3:1 (with complete dominance). Genotype ratio 1:2:1 (AA : Aa : aa).
Test cross (dominant phenotype x aa). Reveals whether the dominant individual is AA or Aa. If all offspring show the dominant phenotype, the parent was likely AA. If roughly half show the recessive phenotype, the parent was Aa.
Dihybrid cross (AaBb x AaBb). Demonstrates independent assortment. Expected phenotype ratio 9:3:3:1.
Punnett squares are the tool for predicting these ratios. Gamete types go along the edges; each cell inside represents one possible offspring genotype, all equally likely.
A crucial takeaway: variation is not lost even when it disappears from the phenotype. A recessive allele can be hidden in heterozygotes for generations and then reappear when two carriers mate.
Mendel's system was elegant but simplified. The real relationship between genotype and phenotype is more involved:
Pleiotropy. One gene affects multiple traits. A single mutation in the gene responsible for sickle-cell haemoglobin affects red blood cell shape, oxygen transport, and malaria resistance.
Epistasis. One gene modifies or masks the expression of another gene at a different locus. The coat colour in Labrador retrievers is a textbook example: one gene controls pigment colour (brown vs. black), but a second gene controls whether pigment is deposited at all.
Polygenic inheritance. Multiple genes contribute to a single trait, producing a continuous distribution of phenotypes (e.g. human height or skin colour).
Norm of reaction. The same genotype can produce different phenotypes in different environments. This is important: phenotype = genotype + environment. A plant with genes for tall growth will still be short if starved of nutrients.
All genetic variation ultimately arises through mutation. The main types:
Point mutations: changes in a single nucleotide. Silent (no effect on protein), missense (different amino acid), or nonsense (premature stop codon).
Insertions/deletions: adding or removing nucleotides, often causing frameshifts.
Chromosomal mutations: duplication, inversion, translocation.
Non-disjunction: failure of chromosomes to separate during meiosis, leading to aneuploidy (wrong chromosome number).
Genetic testing for inherited diseases relies on Mendelian principles. Carrier screening for conditions like cystic fibrosis or sickle-cell disease uses the logic of test crosses: if both parents are heterozygous carriers, each child has a 1-in-4 chance of being affected. Agricultural breeding programmes use the same principles to select for desirable traits in crops and livestock.
Students often think dominant means "more common" or "better." Dominant simply describes which allele is expressed in the heterozygote. A dominant allele can be rare, and a recessive allele can be very common in a population.
Students often confuse mitosis and meiosis. Mitosis produces two identical diploid cells (growth and repair). Meiosis produces four unique haploid cells (gametes). The key difference: meiosis has two rounds of division and involves homologous chromosome pairing, crossing over, and independent assortment.
Students often assume one gene controls one trait. Pleiotropy (one gene, many traits), epistasis (gene-gene interactions), and polygenic inheritance (many genes, one trait) all complicate this picture.
Students often forget the environment's role in determining phenotype. The norm of reaction is the range of phenotypes a single genotype can produce across environments. Genes set the potential; the environment shapes the outcome.
Expect fill-in-the-blank questions on key terms. "Segregation," "independent assortment," "recombination," "norm of reaction" are all likely blanks.
Know how to set up and solve a Punnett square for both monohybrid and dihybrid crosses. Be able to state the expected ratios (3:1 for monohybrid, 9:3:3:1 for dihybrid).
Be prepared to distinguish complete dominance, incomplete dominance, and codominance with examples.
Norm of reaction is flagged as a "very important concept" in the review guide. Expect at least one question testing whether you understand that phenotype depends on both genotype and environment.
Know the difference between meiosis I and meiosis II, and which events happen in each (homologue pairing and crossing over in meiosis I; sister chromatid separation in meiosis II).
Mutation types (point mutations, insertions/deletions, chromosomal mutations, non-disjunction) are likely to appear. Be able to distinguish silent, missense, and nonsense point mutations.
True or false: In meiosis I, sister chromatids separate.
False. In meiosis I, homologous chromosomes separate. Sister chromatids separate in meiosis II.
Fill in the blank: The expected phenotype ratio of a monohybrid cross (Aa x Aa) with complete dominance is ________.
3:1.
True or false: Codominance means the heterozygote is intermediate between the two homozygotes.
False. That describes incomplete dominance. In codominance, both alleles are fully and separately expressed.
Fill in the blank: The range of phenotypes a single genotype can produce across different environments is called the ________.
Norm of reaction.
True or false: A recessive allele is lost from a population when it is masked by a dominant allele in heterozygotes.
False. Recessive alleles persist in heterozygous carriers and can reappear in future generations.
Q: Why was blending inheritance a problem for Darwin's theory?
A: If traits blended, variation would be halved each generation, and natural selection would quickly run out of heritable differences to act on. Particulate inheritance solved this by showing that alleles remain intact across generations.
Q: What is the physical basis for Mendel's law of segregation?
A: The separation of homologous chromosomes during meiosis I. Each gamete receives one member of each homologous pair, so each gamete carries only one allele for each gene.
Q: In a test cross, the offspring include roughly 50% dominant phenotype and 50% recessive phenotype. What was the genotype of the dominant parent?
A: Heterozygous (Aa). If the dominant parent were homozygous (AA), all offspring would show the dominant phenotype.
Q: How does incomplete dominance differ from codominance?
A: In incomplete dominance, the heterozygote shows a blended or intermediate phenotype (e.g. pink from red x white). In codominance, both alleles are fully and separately expressed (e.g. AB blood type shows both A and B antigens).
Q: A nonsense point mutation occurs in the middle of a gene. What is the likely effect on the protein?
A: A premature stop codon is introduced, producing a truncated (shortened) protein that is likely non-functional.
Q: Two parents of normal height have a child who is significantly shorter. Assuming height is polygenic, how is this possible?
A: In polygenic inheritance, each parent may carry a mix of "tall" and "short" alleles across multiple genes. By chance, a child can inherit a disproportionate number of "short" alleles, resulting in a phenotype outside the parental range. The environment (norm of reaction) also plays a role.
This material connects backward to evolution and natural selection: Mendel's particulate inheritance explains how heritable variation is maintained, which is essential for natural selection to work. It connects forward to population genetics, where you will use allele frequencies, Hardy-Weinberg equilibrium, and the forces of evolution (selection, drift, mutation, gene flow) to model how populations change over time. Mutation, covered here, is the ultimate source of all new genetic variation.
Mendel, meiosis, mitosis, segregation, independent assortment, recombination, crossing over, genotype, phenotype, allele, gene, locus, chromosome, chromatid, diploid, haploid, homologous chromosomes, tetrad, dominant, recessive, incomplete dominance, codominance, monohybrid cross, dihybrid cross, test cross, Punnett square, pleiotropy, epistasis, polygenic inheritance, norm of reaction, point mutation, missense, nonsense, silent mutation, insertion, deletion, duplication, inversion, translocation, non-disjunction, particulate inheritance, blending inheritance, BBIO 180, intro biology, University of Washington