# Genetics and Heredity Review
Genetics passages are common on the ACT Science section. They may involve pedigree charts, Punnett squares, DNA experiments, or data on trait inheritance. This review covers the essential concepts you should know.
1. DNA and Genes
Structure of DNA
- Double helix made of nucleotides
- Each nucleotide: phosphate + deoxyribose sugar + nitrogenous base
- Base pairing: A–T (adenine–thymine), C–G (cytosine–guanine)
- In RNA: A–U (adenine–uracil replaces thymine)
Gene Expression
- Transcription: DNA → mRNA (in the nucleus)
- Translation: mRNA → Protein (at ribosomes)
- Codon: 3-base sequence on mRNA coding for one amino acid
- 64 possible codons; 20 amino acids (genetic code is degenerate/redundant)
2. Inheritance Patterns
Mendelian Genetics
- Dominant allele (A) masks recessive allele (a)
- Homozygous: AA or aa | Heterozygous: Aa
- Genotype: genetic makeup | Phenotype: observable trait
Punnett Squares
Cross: Aa × Aa
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
- Genotype ratio: 1 AA : 2 Aa : 1 aa
- Phenotype ratio: 3 dominant : 1 recessive
Beyond Simple Dominance
- Incomplete dominance: Heterozygote is intermediate (e.g., red × white → pink)
- Codominance: Both alleles expressed (e.g., blood type AB)
- Sex-linked traits: Genes on X chromosome (e.g., colour blindness, haemophilia)
- Males (XY) more likely to express recessive X-linked traits
3. Genetic Variation
Sources of Variation
- Mutations: Changes in DNA sequence (point mutations, insertions, deletions)
- Crossing over: Exchange of DNA between homologous chromosomes during meiosis
- Independent assortment: Random orientation of chromosome pairs during meiosis I
- Random fertilisation: Any sperm can fertilise any egg
Natural Selection
- Variation exists in a population
- Some variations are more advantageous in the environment
- Organisms with advantageous traits survive and reproduce more
- These traits become more common over generations
4. Worked Example
Q: In a pedigree chart, two unaffected parents have a child with cystic fibrosis (autosomal recessive disorder). What are the parents' genotypes? What is the probability their next child will be affected?
A:
- Cystic fibrosis is recessive (ff). Affected child is ff.
- Each parent must carry one recessive allele: both parents are Ff (carriers).
- Punnett square for Ff × Ff:
| F | f | |
|---|---|---|
| F | FF | Ff |
| f | Ff | ff |
- Probability of affected child (ff) = \(\frac{1}{4} = 25%\)
5. Practice Questions
Q1. A DNA strand reads 3'-TACGGA-5'. What is the mRNA sequence?
A1. mRNA is complementary to the template strand (with U replacing T):
- Template: TAC GGA
- mRNA: AUG CCU
- AUG is the start codon (codes for methionine).
Q2. Colour blindness is X-linked recessive. A carrier mother (X^C X^c) and a normal father (X^C Y) have children. What fraction of their sons will be colour-blind?
A2.
- Sons get Y from father and X from mother
- Mother's X chromosomes: X^C and X^c (50% chance of each)
- \(\frac{1}{2}\) of sons will be X^c Y = colour-blind (50%)
Want to check your answers and get step-by-step solutions?
Summary
- Know DNA structure, base pairing, and the central dogma (DNA → RNA → Protein)
- Use Punnett squares for mono-hybrid crosses and calculate probabilities
- Understand sex-linked inheritance and why males are more often affected
- Sources of genetic variation: mutations, crossing over, independent assortment