TEAS Ready🧬 Science · Biology · Lesson 5 of 7 · 7 min
Mendel's laws and Punnett squares
🌱 Two brown-eyed parents can have a blue-eyed child. A Punnett square shows exactly how often, and the TEAS asks you to draw one in your head.
💡 Key idea
Bb × Bb gives 3:1 phenotypes (1:2:1 genotypes); AaBb × AaBb gives 9:3:3:1.
🧠 The idea
Genes come in versions called ALLELES. Your GENOTYPE is the allele pair (BB, Bb, bb); your PHENOTYPE is the trait you show. A dominant allele (B) shows whenever present; a recessive allele (b) shows only in bb. Homozygous means two matching alleles; heterozygous means two different ones. Mendel's law of segregation says each parent passes on one allele of each gene; the law of independent assortment says genes on different chromosomes sort separately. A PUNNETT SQUARE crosses the parents' gametes. Bb × Bb gives 1 BB : 2 Bb : 1 bb, a 3:1 phenotype ratio. A dihybrid cross of two double heterozygotes (AaBb × AaBb) gives 9:3:3:1. In incomplete dominance the heterozygote is a blend (red × white gives pink); in codominance both show fully (AB blood type). X-linked recessive traits such as hemophilia show more often in males, who have only one X.
✏️ Worked example
A carrier mother (XᴴXʰ) and an unaffected father (XᴴY) have a son. What is the chance he has hemophilia? Step 1: sons get Y from Dad and one X from Mom. Step 2: Mom gives Xᴴ or Xʰ with equal odds. Step 3: so each son has a 1/2 (50%) chance of XʰY, which is affected. Daughters get Dad's Xᴴ, so none are affected, but half are carriers.
📌 Takeaways
- Genotype = letters; phenotype = the visible trait.
- Monohybrid heterozygous cross: 3:1. Dihybrid heterozygous cross: 9:3:3:1.
- Incomplete dominance blends; codominance shows both; X-linked recessive traits hit males more.
📖 Terms in this lesson
Genotype and phenotype: The genotype is an organism's allele combination, and the phenotype is the trait that shows; for example, genotype Bb, phenotype brown eyes.
Dominant allele: An allele whose trait shows even when only one copy is present (a recessive allele shows only with two copies); for example, B in Bb.
✅ Check yourself
Question 1 of 3
In pea plants, tall (T) is dominant over short (t). What fraction of offspring from Tt × Tt are expected to be short?
- 1/2
- 3/4
- 0
- 1/4
Show the answer and rationale
Answer: D
Tt × Tt gives TT, Tt, Tt and tt; only tt (1 of 4) is short, so 1/4. Three-fourths is the fraction that is tall, not short.
Question 2 of 3
A person with type AB blood shows both A and B antigens. This is an example of what?
- A recessive trait
- Sex-linked inheritance
- Codominance
- Incomplete dominance
Show the answer and rationale
Answer: C
In codominance both alleles are fully expressed, so A and B antigens both appear. Incomplete dominance would produce a blended, in-between trait instead.
Question 3 of 3
Which term describes the genotype rr?
- Homozygous dominant
- Codominant
- Homozygous recessive
- Heterozygous
Show the answer and rationale
Answer: C
Two copies of the recessive allele, written as two lowercase letters, is homozygous recessive. Homozygous dominant would be RR, heterozygous would be Rr, and codominance describes how alleles are expressed, not a genotype.
The full lesson with XP and a streak in the app. No account needed.