Definition
Chromosomes of eukaryotes are made of DNA wound around special proteins called histones, together forming a complex called chromatin.
In prokaryotes, the chromosome is typically a single, circular strand of DNA, not wound around histones and not enclosed in a nucleus, unlike the more complex, histone-packaged linear chromosomes of eukaryotes.
The point where two sister chromatids of a duplicated chromosome are attached is called the centromere.
A gene is a specific segment of DNA that codes for a particular characteristic/protein. An allele is one of the different possible versions of that same gene — so a gene and its alleles relate as a general category to its specific variants (e.g. the 'flower colour' gene might have a 'purple' allele and a 'white' allele).
The locus of a gene is the specific, fixed physical location of that gene on a chromosome.
Definition
Comparing DNA and RNA: DNA is typically double-stranded, RNA is typically single-stranded; DNA's sugar is deoxyribose, RNA's sugar is ribose; DNA uses the bases adenine, thymine, guanine, and cytosine, while RNA uses adenine, uracil (replacing thymine), guanine, and cytosine.
Definition
The central dogma of molecular biology is best represented as: DNA RNA Protein — genetic information flows from DNA, is copied into RNA, and RNA then directs protein construction.
DNA's role in protein synthesis is to serve as the master template/instructions, held safely in the nucleus, specifying the exact sequence of amino acids needed to build a particular protein.
During transcription, a molecule of messenger RNA (mRNA) is formed, copied directly from a gene's DNA sequence — this mRNA then carries that genetic information out of the nucleus to where proteins are actually built.
Genes control the traits and characteristics of an organism by coding for specific proteins (via this DNA RNA protein pathway), and those proteins (as enzymes, structural components, or regulatory molecules) go on to determine how the organism's cells — and so the whole organism — actually develop and function.
Remember
Different alleles of the same gene can lead to different physical traits because each allele has a slightly different DNA sequence, which (through transcription and translation) can produce a slightly different version of the resulting protein — a small change in protein structure or amount can noticeably change the trait it controls, e.g. an enzyme that works differently or not at all.
Definition
When two different alleles are inherited together, the allele that is actually expressed (visible in the organism) is called the dominant allele; the one whose effect is masked is the recessive allele — a recessive allele's trait only shows up if both inherited copies are recessive.
Genotype is an organism's actual genetic makeup (its combination of alleles, e.g. ). Phenotype is the observable physical trait that results (e.g. purple flowers). Homozygous means having two identical alleles for a gene (e.g. or ); heterozygous means having two different alleles (e.g. ).
Definition
Mendel chose pea plants for his genetic experiments because they're easy to grow in large numbers, have a short generation time, show several clear, easily distinguishable contrasting traits (like tall/short, purple/white flowers), and — crucially — their reproductive structure lets the researcher fully control pollination (self-pollination or cross-pollination) rather than leaving it to chance.
Mendel used both self-pollination (to first establish true-breeding parent lines, where a trait reliably passes unchanged generation after generation) and cross-pollination (to deliberately combine two different true-breeding parent lines and study how their traits combined in the offspring).
A true-breeding plant always produces offspring identical to itself for a given trait when self-pollinated, generation after generation — e.g. a true-breeding tall pea plant, when self-pollinated, always produces only tall offspring.
Rule
Mendel's Law of Segregation: the two alleles for a gene separate (segregate) from each other during gamete formation, so each gamete carries only one allele for that gene — this is why a heterozygous parent () produces two different types of gametes ( and ) in equal proportion.
Mendel's Law of Independent Assortment: alleles of different genes are inherited independently of one another during gamete formation (for genes on different chromosomes) — the allele a gamete receives for one trait doesn't influence which allele it receives for a separate, unrelated trait.
Definition
A monohybrid cross studies the inheritance of just one trait/gene (e.g. flower colour alone). A dihybrid cross studies the inheritance of two traits/genes simultaneously (e.g. both seed shape and seed colour together).
Example 1 — a monohybrid test cross
In peas, purple flower () is dominant over white (). Cross a white flower () with a heterozygous purple flower (). Find the offspring ratio.
- Parent genotypes: .
- gametes: all . gametes: half , half .
- Offspring combinations: and , each in equal proportion.
Answer: (purple) : (white) — a ratio
Example 2 — gametes from a dihybrid genotype
Write the combination of alleles in the gametes of a plant with genotype .
- Since is homozygous, every gamete gets one .
- Since is homozygous, every gamete gets one .
- Because the plant is homozygous at both genes, there's only one possible gamete combination.
Answer: All gametes are
Example 3 — a dihybrid cross with two homozygous parents
Cross a pea plant with round, green seeds () with one having wrinkled, yellow seeds (). Find the F genotype and phenotype.
- gametes: all . gametes: all .
- F offspring genotype: (all identical, since both parents are homozygous).
- (round) is dominant over (wrinkled); (yellow) is dominant over (green) — so both dominant traits show.
Answer: F: all , round yellow seeds
Example 4 — the classic dihybrid F ratio
Cross two heterozygous F plants (). Find the expected phenotypic ratio in the F generation.
- By independent assortment, each parent produces four gamete types in equal proportion: .
- Combining these in a Punnett square gives equally likely combinations.
- Sorting by phenotype: round-yellow, round-green, wrinkled-yellow, wrinkled-green.
Answer: round yellow : round green : wrinkled yellow : wrinkled green
Quick Recap
- ●Chromosome (eukaryote): DNA histones (chromatin); centromere joins sister chromatids. Prokaryote: single circular DNA, no histones/nucleus
- ●Gene specific alleles (variants) at a fixed locus; DNA/RNA differ in strands, sugar, and bases (T vs U)
- ●Central dogma: DNA RNA (transcription, makes mRNA) Protein — genes control traits by coding for proteins
- ●Dominant allele expressed over recessive; genotype (alleles) vs phenotype (observed trait); homozygous (same alleles) vs heterozygous (different)
- ●Mendel used peas: easy to grow, clear traits, controllable pollination; true-breeding lines via self-pollination
- ●Law of Segregation: alleles separate into gametes. Law of Independent Assortment: different genes' alleles inherited independently
- ●Monohybrid cross (1 trait, e.g. or ratios). Dihybrid cross (2 traits): classic F ratio
