Meiosis & Genetic Variability Tournament
Two onboarding diagrams orient you in meiosis. Then eight MCAT-DoK quiz rounds: crossing over in pachytene, maternal MI nondisjunction in Down syndrome, the canonical S-phase -> MI -> MII sequence, the three sources of genetic variation, the magnitude of independent assortment (2^23), oocyte arrest + sex-specific gametogenesis, mitosis vs. meiosis, and Prader-Willi / Angelman imprinting.
Where the Meiosis fits in Cell Cycle
Meiosis is the specialized cell division that generates haploid gametes and is the molecular source of nearly all genetic variability between siblings. Reactome groups it under the broader Cell Cycle map alongside mitotic cell cycle and chromosome maintenance. Click the highlighted Meiosis box to enter the tournament.
Click the highlighted Meiosis box to continue.
What this tournament tests
Each task maps to a distinct MCAT cognitive demand. The first two orient you in the broader topology; the next eight test the high-yield mechanism, regulation, sequence and quantitative reasoning that consistently appear on test day.
The Bigger Picture
Anchor meiosis within Reactome's Cell Cycle overview alongside mitotic cell cycle.
Whole-Pathway Overview
Pan and zoom the curated WikiPathways meiosis figure before you start answering.
Fill in the Blank
Recall pachytene of prophase I as the stage of crossing over.
Disruptor
Recognize maternal MI nondisjunction as the dominant mechanism of trisomy 21 + the maternal-age effect.
Sequence Ordering
Trace meiosis from S phase through MI (homologs) and MII (sister chromatids).
Match the Pairs
Pair each variability source / aneuploidy / Robertsonian translocation with its molecular signature.
Numeric Input
Compute 2^23 distinct chromosome combinations from independent assortment.
Select All That Apply
Identify TRUE facts about reductional vs equational division, oocyte arrest, and male vs female gametogenesis.
Odd One Out
Distinguish mitosis from meiotic mechanisms of variability.
Imprinting Disruptor
Recognize Prader-Willi / Angelman as the textbook parent-of-origin (imprinting) syndromes.
Public leaderboard
Your score posts to a global, persistent leaderboard scored by points first, time as tiebreaker.
Meiosis & genetic variability in 60 seconds
Meiosis = one round of replication followed by two rounds of division, producing four haploid gametes per primary cell. Meiosis I is REDUCTIONAL (homologs separate); meiosis II is EQUATIONAL (sister chromatids separate, mitosis-like).
The three sources of genetic variability: crossing over (pachytene recombination at chiasmata), independent assortment (2^23 = 8.4 million combinations from 23 random bivalent orientations at metaphase I), and random fertilization of any sperm with any egg.
Sex differences: spermatogenesis is continuous (puberty-life) and produces 4 viable sperm per primary spermatocyte; oogenesis arrests in prophase I from fetal life, completes MII only at fertilization, and produces 1 oocyte + 3 polar bodies. The long oocyte arrest underlies the maternal-age effect on aneuploidy.
Aneuploidies: trisomy 21 (Down) = most common viable autosomal aneuploidy, ~95% maternal MI nondisjunction. 47,XXY (Klinefelter), 45,X (Turner). Imprinting disorders (Prader-Willi vs. Angelman at 15q11-q13) demonstrate that parent-of-origin matters - same DNA, different methylation, different disease.
FAQ
Why is the maternal-age effect so strong in Down syndrome?
Oocytes arrest in prophase I from fetal life (so a 40-year-old's eggs have been arrested ~40 years). The cohesin protein complexes that hold homologs and sister chromatids together degrade slowly, increasing the probability of homolog mis-segregation at MI. By contrast, spermatogenesis is continuous, so paternal age effects are smaller and different in nature (point mutations rather than aneuploidies).
What's the difference between Prader-Willi and Angelman if they involve the same chromosomal region?
Genomic imprinting. At 15q11-q13, certain genes are expressed only from the paternal allele (e.g. SNRPN), others only from the maternal allele (e.g. UBE3A). Paternal deletion -> Prader-Willi (loss of paternally-expressed genes). Maternal deletion -> Angelman (loss of UBE3A specifically). Same DNA sequence, opposite imprint, different phenotype.
How does crossing over differ from independent assortment in producing variability?
Crossing over recombines maternal + paternal segments WITHIN a single chromosome (mixing alleles at linked loci). Independent assortment shuffles WHOLE chromosomes (which of each homolog pair goes to a daughter cell). Together they produce essentially limitless combinations - which is why siblings (other than identical twins) are so genetically distinct.
Do I need an account to play?
No. The tournament is fully public. You get a randomized handle and your score posts to the public leaderboard at the bottom of this page.
Keep going
The mitotic counterpart - cyclin-CDK regulation, checkpoints, and chromosomal segregation in somatic cells.
S-phase replication is the prerequisite for both mitotic and meiotic chromosomal segregation.
Overview diagram: Reactome Pathway R-HSA-1640170, licensed CC BY 4.0.