CDS Biology · Formula sheet
Genetics and Evolution formulas
1 formula, 6 reference tables and 12 common traps for CDS Biology Genetics and Evolution, grouped by subtopic.
Heredity and DNA
Learn this subtopic in the notesMendel's monohybrid cross
Monohybrid cross
- dominant copy (tall)
- recessive copy (short)
- first and second generations
Chromosomes, DNA and genes
| Term | Fact |
|---|---|
| Histones | Basic proteins in the nucleus, bound to DNAQ CDS 2019 (I): histones are proteins present in the nucleus in association with DNA. |
| Photo 51 | X-ray picture of DNA by Rosalind FranklinQ CDS 2024 (I), HARD: Photo 51 is an image of DNA molecules. |
| Double helix | Watson and Crick, 1953 |
| Base pairs | A with T, G with C |
| Insulin gene | One functional gene in adult humansQ CDS 2017 (II), HARD: adult humans have one functional insulin gene. |
| Mutation | A change in the genetic materialQ CDS 2022 (I): a mutation in a virus is a change in its genetic material. |
| Human chromosome number | 46, in 23 pairs |
How sex is decided
| Egg from mother | Sperm from father | Child |
|---|---|---|
| X | X | XX: girlQ CDS 2020 (II): a normal girl gets one X from the mother and one X from the father. |
| X | Y | XY: boy |
Genetic engineering
| Tool or product | What it is or does |
|---|---|
| Restriction endonuclease | Cuts DNA at a specific sequence; used to form recombinant DNAQ CDS 2026 (II): exonucleases remove end nucleotides AND restriction endonucleases are used to form recombinant DNA; both statements are correct. |
| Exonuclease | Removes nucleotides from the ends of DNA |
| DNA ligase | Joins pieces of DNA |
| Plasmid | Small ring of bacterial DNA used as a vector |
| Human insulin | Made by genetically engineered bacteria |
| Bt cotton | Carries an insect-killing gene from Bacillus thuringiensisQ CDS 2022 (I): the false statement is that genetic modification is used only in plants. |
Common traps
Histones live in the nucleus
Histones are proteins wound up with DNA in the nucleus. They are not membrane proteins of the mitochondria, and they are not joined to lipids or carbohydrates in the cytosol.
Photo 51 is DNA, not a virus
Photo 51 is Rosalind Franklin's X-ray picture of DNA. It has nothing to do with the COVID-19 virus, the common cold or the Moon.
Pure tall means TT only
A pure tall plant is TT. The Tt plants are tall too, but they are hybrids. So in the F2 of TT × tt, 'tall' is 3 in 4, while 'pure tall' is only 1 in 4.
3 : 1 is what you see, 1 : 2 : 1 is what they carry
3 : 1 is the F2 phenotype ratio (tall : short). 1 : 2 : 1 is the genotype ratio (TT : Tt : tt). Read which one the question asks for.
The mother can only give X
A woman is XX, so every egg carries one X. Options that give a child two X chromosomes from the mother, or an X and a Y from the father, cannot happen.
Genetic engineering is not only for plants
It is used in bacteria (human insulin), plants (Bt cotton) and animals (transgenic animals). A statement limiting it to plants is false.
Scissors and glue
Restriction enzymes cut DNA; ligase joins it. Exonucleases nibble from the ends, endonucleases cut inside.
Evolution
Learn this subtopic in the notesInherited and acquired traits, and the theories
| Person or principle | Key idea |
|---|---|
| Lamarck | Inheritance of acquired characters (rejected) |
| Darwin | Branching descent and natural selection |
| Hugo de Vries | Evolution by mutation |
| Hardy-Weinberg | Allele frequencies stay stable unless disturbedQ CDS 2026 (II): the wrong statement says Hardy-Weinberg makes allele frequencies unstable. |
| Weismann | Cut the tails of mice for generations; the young still had tails |
Variation and natural selection
| Source or process | What happens |
|---|---|
| Mutation | A change in the DNA creates a new variant |
| Sexual reproduction | Genes of two parents mix in new combinations |
| Asexual reproduction | Very little variation |
| Natural selection | Variants best suited to the environment survive and reproduce moreQ CDS 2021 (II): the false statement is that all variations in a species have equal chances of survival. |
| Genetic drift | Chance change in gene frequency in a small population |
Homologous and analogous organs
| Kind | Origin and function | Examples |
|---|---|---|
| Homologous | Same origin, different function | Forelimbs of human, whale and bat; thorn of Bougainvillea and tendril of Cucurbita |
| Analogous | Different origin, same function | Wings of bird and bat; wings of butterfly and bird; potato and sweet potatoQ CDS 2020 (II): bird and bat wings are analogous because they have a different origin and a common function. |
| Vestigial | Reduced, no longer used | Human appendix, wisdom teeth |
Common traps
Learnt skills are not in the DNA
A skill such as chess or swimming is acquired, so children do not inherit it. Hair type, earlobes and freckles are inherited because genes decide them.
Hardy-Weinberg means stable
The Hardy-Weinberg principle says allele frequencies stay constant in a population. Mutation, gene flow, drift, recombination and selection are what disturb that balance.
Variations are not equal in survival
Some variants suit the environment better than others, so they have better chances of survival. That inequality is exactly what natural selection works on.
Analogous: different origin, same job
Analogous organs have a different origin and a common function. Homologous organs have a common origin and may do different jobs. Swapping the two halves is the trap in every option set.
Homologous shows divergence, analogous convergence
Homologous organs show divergent evolution (one plan, many uses). Analogous organs show convergent evolution (many plans, one use).