Difficulty: Intermediate | Prerequisites: Meiosis, chromosome structure
This final set of notes ties together how traits are inherited (genetics) and how cells talk to each other and hold tissues together (cell communication and junctions). You need a solid understanding of meiosis and chromosome behaviour to follow inheritance patterns. Cell communication is the bridge between molecular biology and whole-organism physiology.
Mendelian genetics explains how alleles segregate and assort independently during meiosis, producing predictable inheritance ratios. Non-Mendelian patterns (incomplete dominance, codominance, epistasis, polygenic traits, sex-linkage) add complexity. Cells communicate through signal transduction pathways and are physically connected by junctions (gap junctions in animals, plasmodesmata in plants) and adhesion molecules.
Genotype
The genetic information (alleles) an individual carries and can pass to offspring. Think of it as the instruction set.
Phenotype
The observable physical trait that results from the genotype plus environmental influences. Think of it as what you can see.
Homozygous
Having two identical (or functionally identical) alleles of a gene, one on each homologous chromosome.
Heterozygous
Having two different alleles of a gene, one on each homologue.
Dominant
An allele that controls the phenotype even when only one copy is present (heterozygous condition).
Recessive
An allele that controls the phenotype only when two copies are present (homozygous condition).
Independent assortment
The principle that when two or more genes are on different chromosomes, the way one pair of alleles segregates during meiosis does not affect how another pair segregates.
Pedigree
A diagram of family relationships used to trace the inheritance of a trait across generations.
Sex-linked
Describing genes located on a sex chromosome (usually the X chromosome).
Barr body
An inactivated X chromosome, visible as a condensed structure in the nucleus of female mammalian cells.
Incomplete dominance
An inheritance pattern where the heterozygote has a phenotype intermediate between the two homozygotes. In simple terms, the traits blend.
Codominance
An inheritance pattern where both alleles are fully expressed in the heterozygote, producing two distinct phenotypic effects simultaneously.
Polygenic trait
A trait influenced by multiple genes, often resulting in continuous quantitative variation (e.g. height, skin colour).
Epistasis
An interaction where one gene modifies or masks the phenotypic expression of another gene.
Nondisjunction
The failure of chromosomes (or chromatids) to separate properly during anaphase of meiosis, resulting in daughter cells with abnormal chromosome numbers.
Translocation
The transfer of a segment from one chromosome to a non-homologous chromosome.
Cell adhesion molecule
A cell-surface protein that attaches cells to one another or to the extracellular matrix. Example: cadherin binds to cadherin on neighbouring cells.
Integrin
A transmembrane protein found on virtually all animal cells that connects the cytoskeleton to the extracellular matrix.
Plasmodesmata
Channels between adjacent plant cells that allow molecules to pass directly from cytoplasm to cytoplasm.
Gap junctions
Protein channels between adjacent animal cells that allow ions and small molecules to pass directly between cytoplasms.
Ligand
A signalling molecule that binds to a receptor, typically a protein on the cell surface.
Signal transduction
The process by which an extracellular signal (ligand) activates a receptor, which then relays information through a cascade of intracellular molecules to produce a cellular response.
Extracellular matrix (ECM)
A meshwork of proteins (collagen, elastin, fibronectin) and polysaccharides outside the cell that provides structural support and signalling cues.
Pili
Hollow, threadlike structures on the surface of bacteria used to transfer plasmids between cells during conjugation.
Plasmid
A small, circular DNA molecule in bacteria that replicates independently of the main chromosome and often carries genes for antibiotic resistance or other traits.
Each individual carries two alleles for every autosomal gene, one from each parent.
Dominant alleles mask recessive alleles in the heterozygous state.
Law of Segregation: The two alleles of a gene separate during meiosis so each gamete carries only one.
Law of Independent Assortment: Genes on different chromosomes sort independently during meiosis. The segregation of one pair does not influence the segregation of another.
A pedigree tracks inheritance across generations and can reveal whether a trait is dominant, recessive, autosomal or sex-linked.
Incomplete dominance: The heterozygote shows a blended phenotype. Classic example: red × white snapdragons → pink.
Codominance: Both alleles are fully and separately expressed. Classic example: AB blood type, where both A and B antigens appear on red blood cells.
Polygenic traits: Multiple genes contribute to one trait, producing continuous variation (e.g. height, skin colour). These do not follow neat Mendelian ratios.
Epistasis: One gene's product affects the expression of another gene. Example: a gene controlling pigment deposition can mask the effect of a gene controlling pigment colour.
Sex-linked inheritance: Genes on the X chromosome follow a distinctive pattern. Males (XY) express every X-linked allele, whether dominant or recessive, because they have only one X.
Nondisjunction during meiosis produces gametes with too many or too few chromosomes. This can result in conditions such as trisomy (e.g. Down syndrome, trisomy 21).
Translocation moves a segment from one chromosome to a non-homologous chromosome, potentially disrupting gene function or regulation.
Barr bodies are inactivated X chromosomes in female cells, ensuring dosage compensation between XX and XY individuals.
Gap junctions (animal cells): channels of connexin proteins that allow ions and small molecules to flow directly between adjacent cells. This is how cardiac muscle cells coordinate contraction.
Plasmodesmata (plant cells): membrane-lined channels through the cell wall that connect the cytoplasm of adjacent plant cells.
Cell adhesion molecules (e.g. cadherins): surface proteins that bind cells to each other. Cadherin-cadherin interactions are calcium-dependent.
Integrins: transmembrane proteins that link the intracellular cytoskeleton to the extracellular matrix, anchoring cells in place and transmitting mechanical signals.
The general pattern:
A ligand (signalling molecule) binds to a receptor on the cell surface.
The receptor changes shape and activates intracellular signalling cascades, often involving second messengers (Ca²⁺, cAMP, cGMP, lipid-derived signals).
The cascade amplifies the signal and produces a cellular response (gene expression change, enzyme activation, cytoskeletal rearrangement, etc.).
Bacteria carry their main genome on a circular chromosome and may also carry plasmids, small circular DNA molecules with additional genes.
Pili are surface structures that allow bacteria to transfer plasmids to one another during conjugation.
Colonies are collections of individual prokaryotic or protist cells.
Students often assume dominant means "more common" or "better." Dominant simply means the allele is expressed in the heterozygote. A dominant allele can be rare and harmful.
Incomplete dominance and codominance are frequently confused. In incomplete dominance the phenotype blends (pink flowers); in codominance both phenotypes appear simultaneously (AB blood type).
Sex-linked does not mean the trait only affects one sex. Males are more often affected by X-linked recessive conditions because they have only one X chromosome, but females can be carriers or, if homozygous recessive, can show the phenotype.
Students sometimes think gap junctions and plasmodesmata are the same thing in different kingdoms. The structures are analogous (both allow direct cytoplasmic exchange) but structurally distinct.
⚠️ Be able to work Punnett squares for monohybrid and dihybrid crosses, and predict offspring ratios.
⚠️ Know the difference between incomplete dominance and codominance, with examples of each.
⚠️ Understand how nondisjunction leads to aneuploidy and be able to explain at which stage of meiosis it occurred.
⚠️ Know the signal transduction pattern: ligand → receptor → intracellular cascade → response.
⚠️ Be able to distinguish gap junctions (animal) from plasmodesmata (plant) and explain what each allows.
True or False: A heterozygous individual always shows the dominant phenotype in simple Mendelian inheritance. (True.)
Fill in the blank: The failure of chromosomes to separate during meiosis is called __________. (Nondisjunction.)
True or False: In codominance, the heterozygote shows a blended intermediate phenotype. (False, that is incomplete dominance. In codominance both alleles are fully expressed.)
Fill in the blank: Direct cytoplasmic connections between plant cells are called __________. (Plasmodesmata.)
True or False: A ligand typically enters the cell before activating a signalling cascade. (False, most ligands bind to a surface receptor and the signal is relayed internally by second messengers.)
Q: In a cross between two heterozygous parents (Aa × Aa), what are the expected genotypic and phenotypic ratios of the offspring?
A: Genotypic ratio: 1 AA : 2 Aa : 1 aa. Phenotypic ratio (assuming complete dominance): 3 dominant : 1 recessive.
Q: A woman who is a carrier for an X-linked recessive disorder has children with an unaffected man. What fraction of their sons will be affected?
A: Half. The mother is X^A X^a. She passes X^a to half her sons, who will be X^a Y (affected), and X^A to the other half, who will be X^A Y (unaffected).
Q: Explain why polygenic traits produce continuous variation rather than discrete categories.
A: Each contributing gene has a small additive effect on the phenotype. With multiple genes and environmental influence, the number of possible phenotypic classes becomes so large that the distribution appears continuous rather than falling into distinct Mendelian ratios.
Q: Describe the basic steps of a signal transduction pathway.
A: A ligand binds to a cell-surface receptor. The receptor undergoes a conformational change that activates intracellular signalling molecules, often generating second messengers (e.g. cAMP, Ca²⁺). These relay and amplify the signal, ultimately triggering a cellular response such as a change in gene expression or enzyme activity.
Q: How do gap junctions and plasmodesmata differ structurally, and what is their shared function?
A: Gap junctions are protein channels (connexins) that span the membranes of two adjacent animal cells. Plasmodesmata are membrane-lined channels that pass through the cell walls of adjacent plant cells. Both allow small molecules and ions to move directly between the cytoplasms of connected cells, enabling cell-to-cell communication.
Genetics builds directly on meiosis (covered in the Cell Division notes): independent assortment and crossing over during meiosis are the physical basis for Mendel's laws. Signal transduction connects to gene expression, because many signalling cascades ultimately switch transcription on or off. Cell junctions and adhesion molecules connect to the extracellular matrix and cytoskeleton topics.
Genotype, phenotype, allele, homozygous, heterozygous, dominant, recessive, Punnett square, monohybrid cross, dihybrid cross, independent assortment, law of segregation, pedigree, sex-linked, X-linked, Barr body, incomplete dominance, codominance, polygenic, epistasis, nondisjunction, aneuploidy, trisomy, translocation, cell adhesion molecule, cadherin, integrin, extracellular matrix, gap junction, connexin, plasmodesmata, ligand, receptor, signal transduction, second messenger, cAMP, plasmid, pili, conjugation, colony, BIO 151 Exam 2