Genetics and DNA
📖Definition— Key terminology
DNA (deoxyribonucleic acid): a double helix molecule made of nucleotides. It carries the genetic code for all life.
Chromosome: a long strand of DNA wrapped around proteins. Humans have 46 chromosomes in 23 pairs. Each pair of chromosomes carries genes for the same characteristics (homologous pairs).
Gene: a section of DNA that codes for a specific protein (and therefore a specific characteristic). Humans have approximately 20,000-25,000 genes.
Allele: a version of a gene. Most genes have two or more alleles. Example: the gene for ABO blood group has three alleles (A, B, O).
Genotype: the combination of alleles an organism has (e.g. Bb, BB, or bb).
Phenotype: the physical characteristic shown (e.g. brown eyes, blue eyes).
Dominant and recessive alleles
Dominant allele: expressed in the phenotype whether the organism is homozygous dominant BB or heterozygous (Bb). Written with a capital letter.
Recessive allele: only expressed in the phenotype when the organism is homozygous recessive (bb). Written with a lower case letter.
Homozygous: both alleles are the same (BB or bb).
Heterozygous (carrier): two different alleles (Bb).
Punnett squares
Punnett squares predict the probability of offspring genotypes and phenotypes.
Example: two heterozygous parents for eye colour (Bb × Bb):
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
- 1 BB : 2 Bb : 1 bb (genotype ratio)
- 3 brown : 1 blue (phenotype ratio, if B = brown, dominant)
Sex determination
Humans have 23 pairs of chromosomes. The 23rd pair are the sex chromosomes:
- Males: XY
- Females: XX
A Punnett square for sex determination:
| X | Y | |
|---|---|---|
| X | XX | XY |
| X | XX | XY |
50% chance of male, 50% chance of female.
Inherited disorders
Cystic fibrosis (CF): caused by a recessive allele (c). Only cc individuals have CF. Carriers (Cc) appear healthy. CF causes thick mucus in the lungs and digestive system.
Huntington's disease: caused by a dominant allele (H). Only ONE copy is needed to develop the disease. Hh or HH individuals are affected. Late onset (typically 30-50 years). No cure.
Contrast: CF can be carried silently; Huntington's cannot (every person with H develops the disease).
AI-generated · claude-opus-4-7 · v3-ccea-combined-science