Topic 11— Genotypes & Phenotypes (Bio 1/2)

What is the difference between phenotype and genotype?

Genotype — the genetic make-up of an organism at a particular gene locus (e.g. Aa, TT, tt). Phenotype — an observable or measurable characteristic that is the product of both genetic and environmental factors (e.g. black hair, tall stem). Key point: same genotype can produce different phenotypes depending on the environment (e.g. PKU).

How does gene expression differ between autosomal and sex-linked genes?

Autosomal genes: both males and females carry 2 copies of every autosomal gene → can be homozygous or heterozygous. Sex-linked (X-linked) genes: males carry only one copy (hemizygous) because they have one X. Females carry two copies (XX). Therefore sex-linked traits appear more often in males (they have no second X to mask a recessive allele).

Define and distinguish homozygous, heterozygous, and hemizygous.

Homozygous — two identical alleles at a locus (e.g. AA or aa). Heterozygous — two different alleles at a locus (e.g. Aa). Hemizygous — only a single allele for a gene on the X or Y chromosome; applies to males for X-linked genes (e.g. X^A Y or X^a Y). Note: hemizygous males cannot be carriers of recessive X-linked conditions — they either express or don't express them.

What is a carrier?

A carrier is a heterozygote (Aa) that carries the allele for a recessive trait but does not express it — because the dominant allele masks the recessive one. Example: a carrier of albinism has genotype Aa → normal pigment phenotype, but can pass the recessive allele 'a' to offspring.

Explain and distinguish complete dominance, codominance, and incomplete dominance. Give an example of each.

Complete dominance: heterozygote looks identical to dominant homozygote. One allele fully masks the other. (e.g. Tt pea plants are tall, same as TT). Codominance: both alleles fully expressed simultaneously in the heterozygote. (e.g. I^A I^B → blood type AB; C^R C^W rhododendron → red AND white patches). Incomplete dominance: heterozygote shows an intermediate phenotype. (e.g. H^S H^C → wavy hair, between straight and curly).

How can the environment influence gene expression? Give three specific examples.

1. PKU — genotype pp cannot make phe hydroxylase. HIGH phe diet → brain damage; LOW phe diet → normal phenotype. Same genotype, different phenotype. 2. Siamese cats / Himalayan rabbits — tyrosinase gene is temperature-sensitive; active only in cooler extremities → darker fur on ears, paws, tail. 3. Hydrangeas — same pigment gene; acidic soil → blue flowers; alkaline soil → pink flowers.

List five environmental factors that can modify phenotype.

1. Temperature (e.g. Siamese cats, turtle sex determination) 2. Soil pH / acidity (e.g. hydrangea colour) 3. Diet / nutrition (e.g. PKU, plant growth) 4. Light (e.g. plant height and development) 5. Water availability, wind exposure, predation, soil type — all can alter phenotype without changing the genotype.

What is the difference between monogenic and polygenic traits?

Monogenic — controlled by a single gene; produces discontinuous variation (distinct, non-overlapping categories); qualitative; shown as a bar chart. (e.g. tongue rolling, ABO blood group). Polygenic — controlled by two or more genes (polygenes) whose effects are small and additive; produces continuous variation (range of phenotypes); quantitative; shown as a bell curve. (e.g. height, skin colour, eye colour).

Why do monogenic traits show discontinuous variation and polygenic traits show continuous variation?

Monogenic → discontinuous: a single gene with only 2 alleles gives very few possible phenotypes (2–4 classes) → discrete categories with no overlap. Polygenic → continuous: many genes each contributing a small additive amount → large number of possible combinations → wide, graded range of phenotypes that form a bell curve. Environmental factors further blur the boundaries.

Define epigenetics.

Epigenetics refers to all changes to genes, apart from changes to their base sequences, which bring about phenotypic changes. Epigenetic factors act on DNA to turn genes permanently on or off — gene expression changes but the DNA sequence itself does not change. Analogy: if genes are the hardware, epigenetics is the software.

Explain two mechanisms by which epigenetic factors turn genes on or off.

1. DNA Methylation — methyl groups (–CH₃) are added to cytosine (C) bases alongside guanine (G). More methyl groups → gene switched OFF. Fewer → gene switched ON. Tags are inherited by daughter cells. 2. Chromatin remodelling (DNA packaging) — tightly packaged chromatin → genes silenced. Loosely packaged (open) chromatin → genes active. Histones themselves can be tagged to control access.

Explain cell differentiation as an example of epigenetics.

All stem cells in an organism have the same genotype. Epigenetic factors (methylation and chromatin remodelling) start stem cells down different developmental paths by switching different sets of genes on or off in each cell type. Result: a liver cell and a neuron have identical DNA but look and function completely differently — because different genes are epigenetically active in each cell type.

Explain X-inactivation as an example of epigenetics.

Female mammals have two X chromosomes. In early embryonic development, one X chromosome is randomly inactivated in each somatic cell — all genes on it are switched off by epigenetic tags. The inactivated X forms a dense structure called a Barr body. The same X remains inactivated in all daughter cells derived from that cell. Result: females are genetic mosaics (visible in tortoiseshell cats — patches of different fur colour reflect which X is active in each skin cell).