You are at the bench, looking down your own microscope. You will fertilize eggs, test the embryos and decide which ones to keep.
🏆 The challenge
Make two healthy embryos with a normal pair of sex chromosomes and none of the three inherited diseases. Put one in the growth medium for implantation and one in cold storage as a backup.
Sexual reproduction joins two gametes: an oocyte (egg) from the female and a spermatozoon (sperm) from the male.
Each gamete is haploid — one copy of every gene. Fertilization puts two copies back together and makes a diploid zygote.
Genotypes are written with the dominant allele first: Aa, never "aA", whichever parent supplied it. Aa is an unaffected carrier; only aa is affected.
Before an embryo is transferred it is screened for three recessive conditions.
| Gene | Alleles | Genotype of concern |
|---|
| HBB | A / a |
aa → sickle cell disease |
| HEXA | T / t |
tt → Tay-Sachs disease |
| CFTR | C / c |
cc → cystic fibrosis |
A typical embryo has XX or XY. Anything else — X, Y, YY, XXX, XXY, XYY, XXXX, XXYY, or none at all — has too many or too few.
That happens when a pair fails to separate during meiosis. It is called nondisjunction, and the gamete it makes carries an extra sex chromosome or none.
The embryo simply adds up what the two gametes bring, so two errors can even cancel each other out.
Pipette off → tap anything to read it in the readout panel.
Pipette on → tap a gamete to pick it up, then tap a destination to put it down.
Holding a sperm, tap an oocyte in the microscope field to inject it. That is ICSI.
Only fertilized embryos may go into the growth medium or cold storage.
New samples refills both tubes to six and keeps your embryos. Reset lab empties everything.
Keyboard: arrow keys move between things, Enter selects or places.
A teaching model of fertilization and inheritance, not a clinical simulator. Knowing where a model bends the truth is part of using one properly.
What it gets right
Each gamete carries one copy of every gene; fertilization restores the pair.
The three genes are inherited independently of each other and of the sex chromosomes.
One recessive allele is not enough: a carrier is unaffected, two copies are affected.
A gamete can end up with two sex chromosomes or none, and the embryo adds up whatever arrives.
Turned up on purpose
Nondisjunction is far commoner here than in real meiosis: about 3 in 5 oocytes and 2 in 5 sperm carry the wrong number of sex chromosomes.
At the real rate a whole class could work all lesson and never meet an abnormal embryo — the thing the lesson is about.
Every recessive allele sits at 50%, so carriers are everywhere.
Real carrier rates for these three conditions are low enough that an affected embryo would almost never turn up in one lesson.
Every injection works and every embryo survives to testing.
Real injections fertilize roughly 7 eggs in 10, and more stop developing before they can be tested.
These rates are teaching decisions, not measurements. The mechanism is real; the frequency is not.
What it leaves out
The mother's age, which strongly raises the real risk.
Chromosomes 1–22: only sex chromosomes can go wrong here, so Down syndrome never appears.
The biopsy itself — real testing removes a few cells and the answer can be mixed or unclear.
Linked genes. These three sit on different chromosomes, so nothing here depends on two genes traveling together — real inheritance often does.
Transfer is not pregnancy: implantation is never guaranteed.
The counselling, consent and ethics around choosing embryos.