Difficulty: Introductory | Prerequisites: Basic cell biology, familiarity with meiosis
This is the foundation of classical genetics, the bit everything else in the course builds on. Mendel figured out that traits are inherited as discrete particles (genes), not blended together, and that those particles follow predictable rules when passed from parent to offspring. If you understand segregation and independent assortment, the rest of genetics (linkage, population genetics, molecular biology) slots into place. You should already be comfortable with the basics of meiosis before tackling this.
Genes come in different versions called alleles, and every individual carries two copies of each gene (one from each parent). During gamete formation the two copies separate (segregation), and genes on different chromosomes sort independently (independent assortment). These two principles, combined with dominant/recessive relationships, predict the classic 3:1 and 1:1:1:1 ratios Mendel observed.
Gene
The hereditary determinant of a trait. In simple terms, a gene is a stretch of DNA that codes for a particular characteristic.
Allele
One of two or more versions of the same gene. Think of alleles as different flavours of the same gene, such as "round" vs "wrinkled" for pea seed shape.
Genotype
The combination of alleles an individual carries for a given gene. This is what you have at the DNA level.
Phenotype
The observable trait produced by a genotype. This is what you can see or measure.
Homozygous
Carrying two identical alleles for a gene (e.g. AA or aa).
Heterozygous
Carrying two different alleles for a gene (e.g. Aa). In a simple dominant/recessive system, the heterozygote shows the dominant phenotype.
Dominant allele
The allele whose phenotype is expressed in the heterozygote. In simple terms, the version that "wins" when paired with a recessive allele.
Recessive allele
The allele whose phenotype is masked in the heterozygote. It only shows up when present in two copies (homozygous recessive).
Locus
The specific physical position of a gene on a chromosome. Think of it as the gene's address.
Zygote
The first cell of a new organism, formed when egg and sperm fuse.
Wild type
The most common phenotype in a natural population. Used as the baseline for comparison.
Mutant
An individual with an unusual phenotype caused by a heritable change (mutation) in a gene.
Particulate inheritance
Mendel's model: hereditary determinants remain discrete, unchanging particles across generations rather than blending together.
Principle of segregation
The two alleles of each gene separate into different gametes during meiosis, so each gamete carries only one allele per gene.
Principle of independent assortment
Alleles of genes on different chromosomes are transmitted independently of one another. Over many meiotic divisions this produces all possible allele combinations in equal proportions.
Test cross
A cross between an individual with a dominant phenotype (unknown genotype) and a homozygous recessive individual. The offspring phenotypes reveal the unknown parent's genotype.
Reciprocal cross
A pair of matings in which the sex associated with each phenotype is swapped. If results are identical, the gene is autosomal; if they differ, the gene may be sex-linked.
Before Mendel, the prevailing view was blending inheritance: traits from two parents mix like paint. Mendel's experiments with pea plants showed this was wrong.
Hereditary determinants (genes) stay intact across generations. They do not blend or dilute.
This explains why a trait can vanish in the F1 generation and reappear in the F2: the recessive allele was there the whole time, just masked by the dominant one.
Each individual carries two alleles per gene (one on each homologous chromosome).
During meiosis, homologous chromosomes separate, so each gamete receives only one allele.
At fertilisation, two gametes fuse, restoring the pair. Both parents contribute equally to the offspring's genotype.
This is the mechanism behind the 3:1 phenotypic ratio in a monohybrid cross of two heterozygotes.
Three predictions flow from this principle: (1) alleles segregate during meiosis, (2) one allele is dominant over the other, (3) fertilisation is random.
Key point: genes do not change when transmitted between generations.
Alleles of genes located on different chromosomes sort independently during meiosis.
There are two equally likely ways for homologous pairs to line up at metaphase I. Over many divisions this produces four gamete types in equal proportions.
Independent assortment predicts a 1:1:1:1 ratio of phenotypes in a dihybrid test cross.
This principle holds only for genes on separate chromosomes. Genes on the same chromosome can be linked (covered in the next set of notes).
Purpose: determine whether an individual showing the dominant phenotype is homozygous dominant (AA) or heterozygous (Aa).
Method: cross the individual with a homozygous recessive (aa) partner.
If all offspring show the dominant phenotype, the unknown parent is likely AA.
If roughly half the offspring show the recessive phenotype, the unknown parent is Aa.
Swap which parent carries which phenotype: if the mother had trait A and the father trait B, repeat with the mother having trait B and the father trait A.
When results are identical in both directions, the gene is autosomal (the parent's sex does not matter).
When results differ, the gene may be sex-linked (covered in the chromosomal inheritance notes).
A mutation is a heritable change in a gene's DNA sequence.
The common, "normal" phenotype in a population is called the wild type.
Genetic variants (mutants) are essential for informative crosses: without variation there is nothing to track.
"Dominant" does not mean "better" or "more common." Huntington disease is caused by a dominant allele, yet it is rare. Dominant and recessive describe which phenotype appears in the heterozygote, nothing more.
Students often assume the dominant allele will eventually replace the recessive one in a population. It will not, on its own. Allele frequency changes require selection, drift, migration or mutation.
A 3:1 ratio is expected in large samples. With small sample sizes the observed ratio can deviate quite a bit from 3:1 by chance alone.
Independent assortment applies only to genes on different chromosomes (or far apart on the same chromosome). Genes close together on the same chromosome are linked and do not assort independently.
⚠️ Be able to predict offspring ratios from a monohybrid cross (3:1 phenotypic, 1:2:1 genotypic) and a dihybrid cross (9:3:3:1).
⚠️ Know how to set up and interpret a test cross.
⚠️ Understand why segregation happens (homologous chromosomes separate during meiosis I).
⚠️ Be clear on the difference between genotype and phenotype: the genotype is the allele combination, the phenotype is the observable outcome.
⚠️ Remember that "dominant" and "recessive" say nothing about allele frequency or fitness.
Test crosses are the basis of genetic screening in agriculture: breeders cross an organism of unknown genotype with a known homozygous recessive to figure out what alleles it carries. The same logic underpins genetic counselling in humans, where carrier status for recessive disorders (e.g. cystic fibrosis) is inferred from family pedigrees.
True or false: a dominant allele is always more common in a population than a recessive allele.
False. Dominance describes phenotype expression, not frequency.
Fill in the blank: the principle of ______ states that the two alleles of each gene separate during gamete formation.
Segregation.
True or false: in a test cross, both parents must be heterozygous.
False. One parent shows the dominant phenotype (unknown genotype) and the other is homozygous recessive.
Fill in the blank: the specific position of a gene on a chromosome is called its ______.
Locus.
True or false: Mendel's principle of independent assortment applies to all genes, regardless of their chromosomal location.
False. It applies only to genes on different chromosomes (or very far apart on the same one).
Q: A pea plant heterozygous for seed shape (Rr) is crossed with a homozygous recessive plant (rr). What phenotypic ratio do you expect in the offspring?
A: 1:1 (half round, half wrinkled). This is a test cross.
Q: In a cross between two heterozygotes (Aa x Aa), what fraction of offspring are expected to be homozygous recessive?
A: 1/4 (25%).
Q: Why does the principle of segregation occur?
A: Homologous chromosomes (which carry the two alleles of each gene) separate during meiosis I, so each gamete ends up with only one allele.
Q: You cross two pea plants that are both heterozygous for two independently assorting genes (AaBb x AaBb). What phenotypic ratio do you expect?
A: 9:3:3:1 (9 dominant for both, 3 dominant A recessive B, 3 recessive A dominant B, 1 recessive for both).
Q: A reciprocal cross produces different results depending on which parent is the mother. What does this suggest about the gene?
A: The gene is likely sex-linked (located on a sex chromosome rather than an autosome).
This material connects directly to chromosomal inheritance and linkage: once you understand that genes sit on chromosomes, you can see why linked genes violate independent assortment and why crossing over restores some recombination. It also underpins population genetics (Hardy-Weinberg equilibrium uses allele frequencies derived from Mendelian ratios) and molecular genetics (what a "gene" physically is at the DNA level).
Mendelian genetics, Mendel's laws, law of segregation, law of independent assortment, monohybrid cross, dihybrid cross, Punnett square, test cross, reciprocal cross, dominant, recessive, genotype, phenotype, homozygous, heterozygous, allele, locus, wild type, mutant, particulate inheritance, gamete, meiosis, F1 generation, F2 generation, 3:1 ratio, 9:3:3:1 ratio