Mendel's Laws and Punnett Squares | Six Minutes: Biology, Part 16

Six Minutes: Biology, Part 16: Mendel's Laws and Punnett Squares: Segregation and independent assortment; then Punnett Squares Mendel's law of segregation says the two alleles of a gene separate into different gametes, and independent assortment says different gene pairs sort on their own. A two-trait cross of double heterozygotes gives the classic 9:3:3:1 ratio, checked two ways. Then: Learn how alleles, dominance, and homozygous versus heterozygous pairs set up a monohybrid Punnett square. Solve crosses step by step and read the genotype and phenotype ratios they predict. Chapters 0:00 Introduction 0:19 Law of segregation 0:55 Law of independent assortment 1:21 Worked example 2:13 Trap 2:30 Recap 2:57 The idea 3:35 Setting up a cross 4:23 Worked example 5:18 Trap 5:39 Recap 5:56 Thanks for watching Practice problems 1. A plant is Tt for height (T = tall, dominant) and Rr for seed shape (R = round, dominant). List all gamete types it can make, and give the fraction of gametes carrying t. Answer: TR, Tr, tR, tr; one half of the gametes carry t. 2. Cross AaBb x AaBb (A and B dominant). What fraction of offspring shows both recessive traits (aabb)? Answer: one sixteenth. 3. In fireflies, glowing antennae G are dominant over dull antennae g. Cross two heterozygous glowing fireflies (Gg × Gg). What fraction of offspring has dull antennae? Answer: 1/4 (25%), from one gg box out of four. 4. A homozygous black rabbit (BB) is crossed with a white rabbit (bb), where black B is dominant. Two of their offspring are then crossed (Bb × Bb). What fraction of the grandchildren is black? Answer: 3/4 (75%), since BB and Bb offspring are black. Six Minutes @SixMinuteClass: One technique, about six minutes. The narrator's voice is synthetic (computer-generated). Independent educational video. © 2026 Six Minutes · @SixMinuteClass