Cross workshop
About this app
From chromosome to Punnett square: sort chromosomes into gametes in meiosis, drag the gametes into the square and count the peas. Monohybrid, dihybrid with 9 : 3 : 3 : 1, test cross, linkage and crossing over. Column width and row height show gamete frequency, and the exam task asks for the recombination frequency.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
Why Mendel’s laws follow from meiosis
Homologous chromosomes separate in meiosis I, so each gamete receives exactly one allele of a gene. That is the law of segregation. If two genes lie on different chromosome pairs, the pairs line up independently in metaphase I. Two equally likely arrangements give four gamete types at one quarter each. For YyRr × YyRr this produces the ratio 9 : 3 : 3 : 1. Genes on the same chromosome are linked and move together until a crossover separates them.
What the app shows
On the left are two parent cells with glowing chromosomes of two sister chromatids each. They produce gametes that move to the top and left of the Punnett square. Column width and row height match the gamete frequencies, and the area of a square matches the share of offspring. This makes it visible that the square counts gametes. With crossing over the recombinant columns widen the further apart the genes lie. A switch tests the assumption that traits blend.
Try it yourself
Drag a chromosome into a gamete and its homologue moves to the opposite side. Find all gamete types, drag them into the square and tap the peas to count the phenotypes. Answer the prediction question for the cross first. In crossing over you set a chiasma and drag a gene. The exam task gives you test-cross numbers: you identify the recombinant classes, calculate the recombination frequency and set it in the workshop.
Frequently asked questions
- How does the 9 : 3 : 3 : 1 ratio arise?
- In YyRr × YyRr each parent forms four gamete types at one quarter each if the genes lie on different chromosomes. That gives 16 equally likely combinations. 9 show both dominant traits, 3 each show one of them, and 1 shows neither. As a calculation this is ¾ · ¾, ¾ · ¼, ¼ · ¾ and ¼ · ¼.
- How do you calculate the recombination frequency?
- Divide the number of recombinant offspring of a test cross by all offspring and multiply by 100%. The recombinant classes are the two rare ones. If 61 and 59 of 1,000 offspring are recombinant, the recombination frequency is 12%, which equals 12 centimorgans.
- Do parental traits blend?
- No. Alleles stay intact as units on the chromosomes. In the F₁ of yellow and green all seeds are yellow, and in the F₂ green reappears unchanged. Mendel counted 6,022 yellow and 2,001 green seeds.
- What can I do in the app myself?
- You sort chromosomes into gametes, drag gametes into the Punnett square, count phenotypes, answer prediction questions, set a chiasma, move a gene and work through an exam task on the recombination frequency. The explain mode guides you through monohybrid and dihybrid crosses, linkage and crossing over.
Subject: Biology (Gymnasium)
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