BIOL10008 · Foundations Of Biology: Life's Machinery
Genetics
Heredity is the passing of information across generations, and its unit is the gene — a sequence of DNA expressed as an RNA or polypeptide product. This chapter defines the gene and its address (the locus) on a chromosome, then bridges to function: a gene cannot make a trait directly; it must be read out as gene → mRNA → protein → trait, which is just the central dogma with a phenotype on the end. It separates genotype (the alleles an organism carries) from phenotype (the observable trait), and sets the inheritance vocabulary — allele, homozygous/heterozygous, dominant/recessive — with the molecular meaning of dominance (a working protein from one copy masks a non-working recessive one). It covers other dominance patterns (incomplete dominance, e.g. the lilac primula giving a 1:2:1 ratio; co-dominance) and ends with mutation as the primary source of variation: a base change can ripple up to the protein and the trait, though code redundancy makes some mutations silent. The thread throughout is structure → function, all the way down from sequence to trait.
What this chapter covers
- 01What a gene is — sequence, locus, genome
- 02Gene → mRNA → protein → trait: the central dogma as the bridge
- 03Genotype vs phenotype
- 04Alleles & basic inheritance: dominant, recessive, homozygous, heterozygous
- 05Other dominance patterns: incomplete dominance and co-dominance
- 06Mutation as the source of variation — and why some are silent
Worked example: how a base change reaches the trait (and why some don't)
- +1(a) The chain: the gene's DNA sequence is transcribed to mRNA, translated into the pigment enzyme, which builds the purple pigment — the working protein's function is the trait (gene → mRNA → protein → trait).
- +1(b) A trait-changing mutation: a base change alters the mRNA, which may insert a different amino acid (or an early stop), so the enzyme misfolds or loses function; no pigment is made → a white/pale phenotype (a new allele).
- +1(b) Why it propagates: the trait is downstream of protein function, so a sequence change matters only insofar as it changes the protein — here it disables the enzyme.
- +1(c) Why another mutation is silent: the genetic code is redundant — several codons specify the same amino acid — so some base changes leave the amino-acid sequence (and the protein, and the trait) unchanged.
- +1(c) How it is inherited: a mutation in a cell that becomes a gamete is passed to offspring as a new allele, whether it is silent or trait-changing.
Key terms
- Gene
- A sequence of DNA that is expressed as an RNA or polypeptide product. It sits at a fixed position (its locus) on a chromosome; the order of its bases is the information, and changing the order can change the product.
- Genotype vs phenotype
- Genotype is the alleles an organism carries (e.g. Aa); phenotype is the observable trait (e.g. purple). Two organisms can share a phenotype but differ in genotype — AA and Aa both look purple, but only Aa can pass on the recessive allele.
- Allele; homozygous / heterozygous
- An allele is a variant version of a gene. A diploid organism carries two copies: homozygous = two of the same allele (AA or aa); heterozygous = two different alleles (Aa).
- Dominant vs recessive
- A dominant allele is expressed in the heterozygote (Aa shows the A trait); a recessive allele is masked in the heterozygote and shows only when homozygous (aa). Dominance is about expression in the heterozygote, not about being commoner or 'stronger.'
- Incomplete dominance vs co-dominance
- In incomplete dominance the heterozygote is a blend (purple × white → all lilac; lilac × lilac → 1:2:1 purple:lilac:white). In co-dominance both alleles are fully and separately expressed at once, with no blend.
Genetics FAQ
What exactly is a gene?
A gene is a sequence of DNA that is expressed as an RNA or polypeptide product — a stretch of the DNA molecule carrying instructions to build one specific thing. It sits at a fixed address (its locus) on a particular chromosome, and because chromosomes are copied and divided faithfully by replication and mitosis, genes are transmitted between generations. The DNA itself does nothing on the shelf; it is a stored instruction that must be read out to have any effect.
How does a gene actually produce a trait?
Through the central dogma with a phenotype on the end: gene → mRNA → protein → trait. The gene's DNA is transcribed into mRNA, the mRNA is translated into a protein, and the protein's function (an enzyme, a pigment, a receptor) produces the observable trait. The trait is not 'in' the DNA as a finished feature — it is the downstream consequence of whether the encoded protein can do its job.
Does 'dominant' mean an allele is more common or stronger?
No — dominance is only about which allele is expressed in the heterozygote. It says nothing about how frequent the allele is in a population, nor that it is 'better.' A rare allele can be dominant and a common one recessive. At the molecular level, a dominant allele typically makes a working protein, and one working copy often supplies enough product to give the phenotype, hiding a non-functional recessive allele.
Why do some mutations change a trait while others do nothing?
Because a mutation is a change in information at the bottom of the structure → function chain, and whether it matters depends on how it propagates. The genetic code is redundant (several codons specify the same amino acid), so some base changes are silent — the protein and trait are unchanged. Others swap a single amino acid (which may or may not affect folding), and a few are catastrophic (e.g. an early stop codon that destroys the protein). The DNA change is the cause; the protein and trait changes are the consequences.
Exam move
Treat the whole chapter as one structure → function spine: base sequence sets the amino-acid sequence, which sets the fold, which sets the protein's function, which is the trait. Be able to define a gene precisely (a DNA sequence expressed as an RNA/polypeptide product), separate genotype from phenotype, and trace gene → mRNA → protein → trait both forwards and (for mutations) when broken at the start. Lock the inheritance vocabulary and the molecular meaning of dominance, and have the dominance patterns ready (incomplete dominance → 1:2:1; co-dominance → both shown separately). Keep the trap on a sticky note: dominant ≠ common or stronger, only expressed in the heterozygote.
