Meiosis and Mendelian Genetics

This is Unit Five of Biology, Part Three: the laws, their physical basis, and the exceptions. Mendel generalised three laws from breeding data, and two of them turned out to have a physical cause visible in meiosis — discovered decades after his death, from work he never saw. Segregation happens at anaphase I, where the two homologs carrying an organism's two alleles separate. Independent assortment happens at metaphase I, where each tetrad orients independently of every other. The two laws govern different scopes, and confusing them is the standing error. Segregation concerns one locus — the two alleles at it separate. Independent assortment concerns two loci relative to each other. Then the exceptions, which are exceptions to independent assortment rather than to segregation. Linkage is about physical proximity on a chromosome. Epistasis is about one gene masking another gene's product. They produce departures from the expected ratio for entirely different reasons. The part closes on sex chromosomes, X-inactivation, and reading a family pedigree backward to a mode of inheritance. By the end you will be able to do five things. One: say what each law governs. Two: name the meiotic event behind each. Three: say where nine-three-three-one comes from. Four: tell linkage from epistasis. Five: say why X-inactivation gives a mosaic.

Five entries. The laws come first, each followed where applicable by the meiotic event that turned out to cause it. Keep the historical order in mind — Mendel had the laws without the mechanism, and the mechanism arrived from other people long afterward.

One conceals the other.

Bio 1049 The Law of Dominance

The law of dominance holds that in a heterozygote one allele's trait conceals the other's, so the two do not blend. It describes phenotype in a heterozygote only — incomplete dominance and codominance are exceptions to this law, not violations of segregation itself.

Two alleles, one each.

Bio 1051 The Law of Segregation

The law of segregation holds that an organism's two alleles at a locus separate during gamete formation, so each gamete carries exactly one — never both. It separates the two alleles at one locus, and says nothing about how two different loci sort relative to each other.

Where it happens.

Bio 1053 The Physical Basis of Segregation in Meiosis I

Segregation's physical basis is anaphase I, where the homologous chromosomes carrying an organism's two alleles separate to opposite poles. Mendel deduced segregation from breeding ratios alone — the meiotic mechanism behind it was described only after his death.

Two loci.

Bio 1055 The Law of Independent Assortment

The law of independent assortment holds that alleles at one locus sort into gametes independently of alleles at another — a dihybrid plant produces all four gamete combinations. The law holds for loci on different chromosomes, or far apart on one; loci close together are a separate case.

Where that happens.

Bio 1057 The Physical Basis of Independent Assortment in Meiosis I

Independent assortment's physical basis is the random orientation of different tetrads at metaphase I, each pair facing the poles independently of every other. It applies chromosome by chromosome — loci sharing one chromosome do not assort independently.

Two entries. A cross designed to test whether two traits sort independently, and the ratio that appears when they do. The ratio is a result, not a definition — it holds under a condition that can fail.

Two traits.

Bio 1059 The Dihybrid Cross

A dihybrid cross tracks two characteristics at once between parents differing at both, revealing whether the traits assort independently. It requires both parental lines to be true-breeding at both loci — an already-mixed starting genotype breaks the design.

Four classes.

Bio 1061 The 9:3:3:1 Ratio

Self-crossing a dihybrid heterozygote gives an F2 generation with four phenotype classes in a nine-three-three-one ratio — roughly nine yellow-round, three yellow-wrinkled, three green-round and one green-wrinkled per sixteen. That exact ratio holds only under independent assortment; linked loci give a skewed proportion instead.

Two entries covering what to do when the grid gets unmanageable. Three traits would need a sixty-four-square Punnett square. Both methods here avoid drawing it, and both rest on the same assumption.

Branching.

Bio 1063 The Forked-Line Method

The forked-line method diagrams a multi-trait cross by branching each trait's probability separately and multiplying across branches, avoiding an unwieldy full Punnett square. It assumes independent assortment at every branch — linked loci break its multiplication step.

Three traits, no diagram.

Bio 1065 Solving a Trihybrid Cross with the Probability Method

The procedure solves a trihybrid cross straight from the probability rules. Crossing two individuals heterozygous at three independently assorting loci, each locus separately gives a three-quarters chance of the dominant trait, and multiplication across the three gives twenty-seven sixty-fourths for all three dominant traits together — matching the twenty-seven squares a sixty-four-square Punnett grid would show, reached without drawing one. The method extends to any combination: multiply across loci, add across mutually exclusive alternatives. It depends entirely on the loci assorting independently.

Three entries. Two departures from independent assortment and one interaction that is a different kind of thing entirely. Keep the causes separate: linkage is about where genes sit, epistasis is about what their products do.

Close together.

Bio 1067 Genetic Linkage

Genetic linkage occurs when two loci sit close on the same chromosome, so their alleles tend to be inherited together rather than assorting independently — as with flower-colour and pollen-shape genes in sweet peas. Linkage is a documented exception to independent assortment tied to chromosome proximity, not a failure of segregation.

How often they separate.

Bio 1069 Recombination Frequency

Recombination frequency measures how often crossing over separates two linked alleles onto different chromatids, rising with the distance between the loci. Two linked loci recombining near fifty percent achieve genuine independent assortment — not merely its appearance — without ever moving to separate chromosomes.

One gene over another.

Bio 1071 Epistasis

Epistasis occurs when one gene masks or alters a second gene's phenotype, producing an F2 ratio departing from nine-three-three-one — a pigment gene's recessive homozygote blocks all pigment in a mouse, hiding its coat-pattern genotype entirely. This is an interaction between two genes' products, not one gene's dominant-recessive pair.

One capstone, and it does something the ratio alone does not: it shows the sixteen gamete combinations sorting into nine genotypes and then collapsing into four phenotypes. That collapse is where genotype and phenotype come apart.

The capstone. Parents to ratio.

Bio 1073 Full Dihybrid Cross: Genotype to Phenotype

The capstone crosses two true-breeding pea lines, yellow-round with green-wrinkled. Each parent makes one gamete type, so every F1 plant is identical in genotype and yellow and round in phenotype. Self-crossing an F1 plant gives four gamete types, and a sixteen-square grid sorts into nine yellow-round, three yellow-wrinkled, three green-round and one green-wrinkled. The ratio is a phenotype ratio built from sixteen combinations sorting into nine genotypes, not four — genotype and phenotype diverge exactly where dominance masks a heterozygous locus. Linked genes would skew it.

Three entries closing the unit. How sex is assigned chromosomally, what a cell does with a second X, and a procedure reading a family diagram backward to infer how a trait is inherited.

Three systems.

Bio 1075 Sex-Determination Systems

Sex-determination systems assign sex chromosomally: humans and most mammals use X and Y, some insects use X and O, and birds and butterflies use Z and W — where females rather than males carry the two different chromosomes. Which system an organism has is what X-linked crosses assume, rather than derive.

One X switched off.

Bio 1077 X-Inactivation and the Barr Body

In each cell of an X-and-Y-system female, one of the two X chromosomes is randomly inactivated early in development, condensing into a visible Barr body — a calico cat's patchwork coat shows each allele active where that X stayed on. Inactivation is random per cell rather than uniform, so a heterozygous female is a mosaic of both alleles, not a blend.

Reading a family backward.

Bio 1079 Reading a Pedigree to Infer a Mode of Inheritance

The procedure reads a pedigree — a diagram of matings and offspring across generations — to infer whether a trait is autosomal or X-linked, dominant or recessive. Take an affected mother, unaffected father, affected sons and unaffected daughters in every generation. No skipped generation rules out simple autosomal recessive inheritance. The sharp sex split rules out an autosomal locus, since autosomal traits affect both sexes at similar rates. A mother transmitting to all sons and no daughters, with the father carrying nothing, points to X-linked dominant. An even sex split alone would not rule out X-linkage — only tracing which parent transmits to which sex settles it.

Five things to carry out of Part Three, and out of Unit Five. The two laws govern different scopes. Segregation concerns one locus: the two alleles there separate, so each gamete carries exactly one. Independent assortment concerns two loci relative to each other, and it holds when they sit on different chromosomes or far apart on the same one. Each law has a meiotic event behind it, found long after Mendel deduced the law from breeding data. Segregation happens at anaphase I, where homologs carrying the two alleles separate. Independent assortment happens at metaphase I, where each tetrad's orientation is set independently of every other tetrad's. The nine-three-three-one ratio comes from sixteen equally likely gamete combinations sorting into nine genotypes, which dominance then collapses into four phenotype classes. It holds only under independent assortment — linkage skews it toward the parental combinations, and epistasis distorts it a different way. Linkage and epistasis depart from the expected ratio for unrelated reasons. Linkage is physical proximity on one chromosome, and crossing over can still separate linked alleles — at fifty percent recombination they assort genuinely independently. Epistasis is one gene's product masking a second gene's phenotype, with the two genes anywhere at all. And X-inactivation makes a heterozygous female a mosaic rather than a blend, because the choice of which X to switch off is made independently in each cell early in development — which is why a calico cat's coat comes out in patches. Unit Six turns from how alleles are inherited to how D N A carries and expresses the information those alleles encode.