What you'll learn
Heredity is the passing of traits from parents to offspring, and genetics is the science that explains it. In this lesson you will learn:
- how DNA, genes and chromosomes store hereditary information;
- what alleles are, and how dominant and recessive alleles combine;
- how to use a Punnett square to predict the probability of a trait;
- special patterns: incomplete dominance, codominance and sex-linked traits;
- how to read a pedigree (family chart);
- what mutations are and how they change proteins.
How it shows up on the GED: genetics is one of the most frequently tested Life Science topics. Expect a Punnett square to complete or interpret, a pedigree to analyze, a probability to calculate (often as a fill-in-the-blank percent or fraction), and a short passage about DNA or a mutation.
Key terms
- DNA — the molecule that carries hereditary instructions; shaped like a twisted ladder (double helix).
- gene — a section of DNA that codes for a protein, which affects a trait (eye color gene).
- chromosome — a long, coiled DNA molecule carrying many genes (humans: 23 pairs = 46).
- allele — one version of a gene (the purple allele or the white allele of the flower-color gene).
- dominant allele — shows its trait even if only one copy is present; written with a capital letter (P).
- recessive allele — shows its trait only with two copies; written lowercase (p).
- genotype — the pair of alleles an organism has (Pp).
- phenotype — the trait you can observe (purple flowers).
- homozygous — two identical alleles (PP or pp).
- heterozygous — two different alleles (Pp); also called a hybrid.
- Punnett square — a grid that shows all possible allele combinations from two parents.
- incomplete dominance — the heterozygote shows a blend (red × white gives pink).
- codominance — both alleles show fully (type AB blood).
- sex-linked trait — controlled by a gene on the X chromosome (red-green color blindness).
- pedigree — a chart that tracks a trait through a family.
- mutation — a change in the DNA sequence (one base swapped).
- codon — three DNA bases that code for one amino acid.
1. DNA, genes and chromosomes
DNA (deoxyribonucleic acid) is a double helix: two strands twisted around each other like a spiral ladder. The "rungs" are pairs of bases: A pairs with T, and C pairs with G. If one strand reads ATCG, the matching strand reads TAGC.
The order of bases is a code. A gene is a stretch of DNA whose bases spell out the instructions for one protein. Proteins (enzymes, hemoglobin, pigments) produce our traits.
From gene to protein: a copy of the gene (RNA) goes to a ribosome, which reads it three bases (one codon) at a time; each codon adds one amino acid to the protein chain.
Packaging: nucleotide → gene → chromosome → nucleus. Human body cells have 46 chromosomes in 23 pairs. One chromosome of each pair comes from the mother and one from the father, so you have two copies of most genes. Those two copies may be the same or different versions, called alleles.
Pair 23 is the sex chromosomes: females are usually XX and males XY.
2. Dominant and recessive alleles and Punnett squares
Gregor Mendel, working with pea plants in the 1860s, discovered that traits are passed on in units (now called genes) and that each parent gives one allele for each gene to each offspring.
- A dominant allele (capital letter) shows up whenever at least one copy is present.
- A recessive allele (lowercase) shows up only when there are two copies.
| Genotype | Name | Phenotype (P = purple dominant) |
|---|---|---|
| PP | homozygous dominant | purple |
| Pp | heterozygous | purple (carrier of white) |
| pp | homozygous recessive | white |
Making a Punnett square
- Write one parent's two alleles across the top and the other parent's down the side.
- Fill each box with one allele from its column and one from its row.
- Each of the 4 boxes has a 1/4 (25%) chance.
- Count boxes to find genotype and phenotype probabilities.
The three crosses to recognize
| Cross | Genotype result | Phenotype ratio |
|---|---|---|
| PP × pp | all Pp | all dominant |
| Pp × Pp | 1 PP : 2 Pp : 1 pp | 3 dominant : 1 recessive |
| Pp × pp | 2 Pp : 2 pp | 1 : 1 |
Example: two heterozygous brown-eyed parents (Bb × Bb). The chance of a blue-eyed (bb) child is 1 box of 4, or 25%; the chance of a brown-eyed child is 75%. (Real eye color involves several genes; textbooks simplify it to one.)
3. Beyond simple dominance: blends, both, and the X chromosome
Incomplete dominance: neither allele fully hides the other, and the heterozygote is in between. Red snapdragons (RR) × white (WW) give all pink (RW). Pink × pink gives 1 red : 2 pink : 1 white.
Codominance: both alleles show fully at the same time. Example: human ABO blood type. Alleles I^A and I^B are codominant, and i is recessive. A person with I^A I^B has type AB blood: both A and B markers appear, not a blend.
| Genotype | Blood type |
|---|---|
| I^AI^A or I^Ai | A |
| I^BI^B or I^Bi | B |
| I^AI^B | AB |
| ii | O |
Sex-linked (X-linked) traits: some genes are on the X chromosome, and the Y chromosome does not carry them. Males (XY) have only one X, so a single recessive allele shows up in a male. That is why red-green color blindness and hemophilia are much more common in males.
Example: a carrier mother (X^BX^b) and a father with normal vision (X^BY). Sons get the mother's X and the father's Y, so each son has a 50% chance of being color-blind (X^bY). Daughters get the father's X^B, so none are color-blind, but half are carriers.
Genes and environment: phenotype also depends on the environment; height depends on genes and nutrition.
4. Pedigrees: tracing traits through a family
A pedigree is a family tree for one trait.
- Square = male; circle = female; shaded = has the trait.
- A horizontal line between a male and a female = a couple; a vertical line down = their children.
- Roman numerals (I, II, III) label generations; numbers label individuals (II-3).
Deciding the inheritance pattern
| Clue in the pedigree | What it means |
|---|---|
| Two unaffected parents have an affected child | Trait is recessive; both parents are carriers (Aa) |
| Two affected parents have an unaffected child | Trait is dominant; both parents are Aa |
| Trait skips a generation | Usually recessive |
| Mostly males affected; passed through unaffected mothers | Likely X-linked recessive |
Working out genotypes (for a recessive trait)
- Every affected person is aa.
- An unaffected parent of an affected child must be Aa.
- An unaffected child of an affected (aa) parent must be Aa.
- An unaffected child of two carriers may be AA or Aa: not certain.
Example: cystic fibrosis is recessive. Each child of two carriers (Aa × Aa) has a 25% chance of CF and a 50% chance of being a carrier.
5. Mutations
A mutation is a permanent change in the DNA sequence. Mutations can happen by chance when DNA is copied, or be caused by mutagens such as UV light, X-rays, tobacco smoke and some chemicals.
Types of gene mutation
| Type | What happens | Example (codons) | Usual effect |
|---|---|---|---|
| Substitution (point) | One base is swapped for another | GAG → GTG | Changes one amino acid, or none (silent) |
| Insertion | A base is added | CTG ACT → CTG CAC T… | Frameshift: every later codon changes |
| Deletion | A base is removed | CTG ACT CCT → CTG CTC CT… | Frameshift: every later codon changes |
Effects can be:
- harmful: sickle cell disease comes from one substitution in the hemoglobin gene;
- neutral (silent): the new codon codes for the same amino acid (GAG and GAA both code for glutamic acid);
- helpful: a mutation may give an advantage, such as bacteria resistant to an antibiotic. This is the raw material for evolution.
Only mutations in gametes are passed to offspring. A sunburn mutation in a skin cell can cause cancer but is not inherited.
Probability with Punnett squares
P(\text{outcome}) = \frac{\text{number of boxes with that outcome}}{\text{total boxes (4)}} \text{expected number} = P(\text{outcome}) \times \text{total offspring}Convert to a percent by multiplying by 100: \tfrac{1}{4} = 25\%, \tfrac{2}{4} = 50\%, \tfrac{3}{4} = 75\%.
The Science test has no formula sheet, but they are just counting. On the test, give the answer in the form asked: fraction, percent or ratio. A ratio compares parts (3 : 1), while a probability compares a part to the whole (3/4).
Worked example 1: A basic Punnett square
Stimulus: In a species of beetle, shiny shells (S) are dominant over dull shells (s). Two heterozygous shiny beetles are crossed.
| S | s | |
|---|---|---|
| S | SS | Ss |
| s | Ss | ss |
Question: What fraction of the offspring are expected to be shiny?
Think-aloud:
- Shiny needs at least one S: SS, Ss, Ss, which is 3 boxes.
- Probability = \tfrac{3}{4} = 75%.
- Trap: \tfrac{1}{4} counts only SS, forgetting that Ss is also shiny.
Answer: 3/4 (75%).
Worked example 2: Incomplete dominance
Stimulus: In snapdragons, RR = red, RW = pink, WW = white. Two pink plants are crossed.
Question: Out of 80 seeds, how many are expected to grow into pink plants?
Think-aloud:
- RW boxes: 2 of 4, so P(\text{pink}) = \tfrac{1}{2}.
- Expected number: \tfrac{1}{2} \times 80 = 40.
- Check: red = \tfrac{1}{4} \times 80 = 20 and white = 20, so 20 + 40 + 20 = 80 ✓
- Trap: answering 60 (using a 3 : 1 ratio). With incomplete dominance there are three phenotypes, not two.
Answer: 40 plants.
Worked example 3: Reading a pedigree
Stimulus: A pedigree shows a father and a mother who both have a trait. Of their three children, two have the trait and one daughter does not.
Question: Is the trait dominant or recessive, and what are the parents' genotypes?
Think-aloud:
- Find the key family: two affected parents have an unaffected child.
- If the trait were recessive, both parents would be aa and could pass only a, so every child would be affected. That contradicts the unaffected daughter.
- So the trait is dominant, and each parent must carry a hidden recessive allele to pass to the daughter: both are Aa.
Answer: dominant; both parents are Aa.
Worked example 4: Classifying a mutation
Stimulus: A normal gene section reads ATG GCA TTC. A mutated version reads ATG CAT TC (the first G of codon 2 is missing).
Question: What type of mutation is this, and why is it likely to be harmful?
Think-aloud:
- Count bases: normal 9, mutated 8. One base is missing, so it is a deletion.
- Regroup by threes from the start: ATG | CAT | TC… Codon 2 changed from GCA to CAT, and everything after it is shifted.
- A shifted reading frame changes every amino acid after the mutation (and often creates an early stop codon), so the protein usually cannot work.
Answer: a deletion that causes a frameshift; it changes all codons after the mutation.
Common mistakes to avoid
- Using 3 : 1 for every cross. Wrong: 3 : 1 comes only from Aa × Aa. Aa × aa gives 1 : 1. Always build the square from the given parents.
- Thinking dominant means common. Wrong: "dominant" only means one copy is enough to show the trait. Some dominant alleles (such as the one for Huntington's disease) are rare.
- Confusing genotype and phenotype. Pp is a genotype; "purple" is a phenotype. Read which one the question asks for.
- Thinking past children change the odds. Wrong: after three bb children, the fourth still has a 25% chance. Each child is independent.
- Assuming every unaffected relative is AA. Unaffected people with carrier parents may be AA or Aa. Only evidence such as an affected child makes it certain.
- Treating every mutation as harmful, or as changing one amino acid. Silent mutations change nothing, and frameshifts change everything after the mutation. Count the bases and regroup by threes.
GED strategy for genetics questions
- Write down the key first. Which letter is dominant? What phenotype goes with each genotype? Many errors come from mixing these up.
- Draw the Punnett square on your erasable note board, even when you think you know the ratio. It takes 20 seconds and prevents the 3 : 1 trap.
- Answer in the form asked: percent, fraction, ratio or number of offspring. "Expected number" = probability × total.
- For pedigrees, find the key family first (unaffected parents with an affected child means recessive; affected parents with an unaffected child means dominant). Then label genotypes, starting with the people you are sure of.
- For DNA and mutation items, count bases and regroup in threes. Use the codon table that is provided. You never need to memorize the genetic code.
Check yourself
Flower Color in Pea Plants
In pea plants, flower color is controlled by one gene with two alleles. The allele for purple flowers (P) is dominant over the allele for white flowers (p). A plant shows white flowers only if it has two copies of the recessive allele.
A gardener crossed a purple-flowered plant that has the genotype Pp with a white-flowered plant. The Punnett square below shows the possible combinations of alleles in the offspring. Each of the four boxes is equally likely.
Based on the Punnett square, what percent of the offspring are expected to have white flowers?
Hint
Which genotype gives white flowers? Count how many of the 4 boxes have that genotype.
Step-by-step solution
Where the answer comes from: the passage says "a plant shows white flowers only if it has two copies of the recessive allele," that is, pp. The Punnett square has four equally likely boxes: Pp, pp, Pp, pp.
Step by step:
- Identify the white genotype: pp.
- Count pp boxes: 2 out of 4.
- Convert: \dfrac{2}{4} = \dfrac{1}{2} = 50\%.
- The other 2 boxes (Pp) have one dominant P allele, so those plants are purple.
Answer: 50%.
Why 25% is tempting: many students memorize "1 in 4 recessive," but that is the result of crossing two heterozygous parents (Pp × Pp). Here one parent is pp, so half the offspring get two recessive alleles.
Tip: never apply a memorized ratio without checking the parents' genotypes. Count the boxes in the square that is actually given.
Check yourself
Flower Color in Pea Plants
In pea plants, flower color is controlled by one gene with two alleles. The allele for purple flowers (P) is dominant over the allele for white flowers (p). A plant shows white flowers only if it has two copies of the recessive allele.
A gardener crossed a purple-flowered plant that has the genotype Pp with a white-flowered plant. The Punnett square below shows the possible combinations of alleles in the offspring. Each of the four boxes is equally likely.
Select the words that correctly complete each sentence.
The purple-flowered parent is for the flower-color gene. The white-flowered offspring are . The expected ratio of purple-flowered to white-flowered offspring is .
Choose an answer from each menu.
Hint
Homo- means "same" and hetero- means "different." Then count purple boxes and white boxes in the square.
Step-by-step solution
Where the answer comes from: the passage gives the purple parent's genotype as Pp, and the Punnett square shows two Pp boxes and two pp boxes.
Step by step:
- Pp has two different alleles, so the purple parent is heterozygous.
- White offspring are pp: two copies of the same recessive allele, so they are homozygous recessive.
- Purple boxes (Pp) = 2, white boxes (pp) = 2. Ratio purple : white = 2 : 2 = 1 : 1.
Answer: heterozygous; homozygous recessive; 1 : 1.
Vocabulary check:
- Homozygous dominant: PP (purple).
- Heterozygous: Pp (purple, but carries p).
- Homozygous recessive: pp (white).
- Genotype is the allele pair (Pp). Phenotype is the visible trait (purple).
Tip: a 3 : 1 ratio comes from Aa × Aa; a 1 : 1 ratio comes from Aa × aa. Recognizing these two patterns lets you work backward from offspring counts to parent genotypes.
Summary
- DNA bases (A–T, C–G) spell out genes; genes sit on chromosomes; humans have 46 chromosomes in 23 pairs, so you get one allele of each gene from each parent.
- Dominant alleles show with one copy; recessive alleles need two. Genotype = alleles (Pp); phenotype = trait (purple).
- A Punnett square gives probabilities: each box = 1/4. Aa × Aa → 3 : 1; Aa × aa → 1 : 1; incomplete dominance → 1 : 2 : 1.
- Codominance shows both alleles (AB blood); X-linked recessive traits are more common in males.
- Pedigrees: unaffected parents with an affected child means recessive; an affected child proves that both unaffected parents are carriers.
- Mutations: substitutions change one codon (or none, if silent); insertions and deletions shift the reading frame. Only gamete mutations are inherited.