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If two brown-eyed parents have four children, why doesn't biology guarantee that exactly three will have brown eyes and one will have blue eyes?

In the 1860s, Austrian monk Gregor Mendel counted over 28,000 pea plants to prove that inheritance is not a random blend, but follows strict mathematical proportions.

To calculate these proportions, we first need to look at how alleles pair up during fertilization.

An **allele** is a specific version of a gene, represented by letters like $T$ for tall or $t$ for short. An organism's **genotype** is its genetic code ($TT$, $Tt$, or $tt$), while its **phenotype** is the physical trait you actually observe.

When both alleles are identical ($TT$ or $tt$), the genotype is **homozygous**; when the two alleles differ ($Tt$), it is **heterozygous**.

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="4:3">
  <title>Genotype vs Phenotype</title>
  <style>
    * {
      box-sizing: border-box;
      margin: 0;
      padding: 0;
      user-select: none;
      -webkit-user-select: none;
    }

    body {
      width: 100%;
      height: 100%;
      background: #f8f9fa;
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
      color: #1e2945;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      padding: 12px;
    }

    .diagram {
      width: 100%;
      max-width: 620px;
      position: relative;
    }

    .header-row {
      display: grid;
      grid-template-columns: 1fr 60px 1fr;
      gap: 8px;
      margin-bottom: 12px;
      text-align: center;
    }

    .col-title {
      font-size: 15px;
      font-weight: 700;
      letter-spacing: 0.5px;
      text-transform: uppercase;
    }

    .col-title.geno {
      color: #2563eb;
    }

    .col-title.pheno {
      color: #059669;
    }

    .main-grid {
      position: relative;
      display: grid;
      grid-template-columns: 1fr 80px 1fr;
      gap: 8px;
      align-items: center;
    }

    .card-col {
      display: flex;
      flex-direction: column;
      gap: 12px;
      z-index: 2;
    }

    .card-col.pheno-col {
      justify-content: space-around;
      height: 100%;
    }

    .item-card {
      background: #ffffff;
      border: 2px solid #e2e8f0;
      border-radius: 12px;
      padding: 10px 12px;
      cursor: pointer;
      transition: all 0.25s ease;
      box-shadow: 0 1px 3px rgba(0, 0, 0, 0.04);
      display: flex;
      align-items: center;
      gap: 10px;
    }

    .item-card:hover {
      box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
      border-color: #cbd5e1;
    }

    .item-card.active {
      border-color: #3b82f6;
      box-shadow: 0 0 0 3px rgba(59, 130, 246, 0.2);
    }

    .item-card.pheno-card.active {
      border-color: #10b981;
      box-shadow: 0 0 0 3px rgba(16, 185, 129, 0.2);
    }

    .item-card.dimmed {
      opacity: 0.35;
    }

    .badge-code {
      font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace;
      font-size: 18px;
      font-weight: 800;
      padding: 4px 8px;
      border-radius: 6px;
      background: #eff6ff;
      color: #1d4ed8;
      min-width: 38px;
      text-align: center;
    }

    .badge-code.recessive {
      background: #f1f5f9;
      color: #475569;
    }

    .label-group {
      display: flex;
      flex-direction: column;
    }

    .item-name {
      font-size: 13px;
      font-weight: 700;
      color: #1e2945;
      line-height: 1.2;
    }

    .item-sub {
      font-size: 11px;
      color: #64748b;
      font-weight: 500;
    }

    .plant-icon {
      font-size: 24px;
      line-height: 1;
      display: flex;
      align-items: center;
      justify-content: center;
      width: 36px;
      height: 36px;
      border-radius: 8px;
      background: #ecfdf5;
    }

    .plant-icon.short {
      font-size: 18px;
      background: #f8fafc;
    }

    .svg-overlay {
      position: absolute;
      top: 0;
      left: 0;
      width: 100%;
      height: 100%;
      pointer-events: none;
      z-index: 1;
    }

    .conn-path {
      fill: none;
      stroke: #cbd5e1;
      stroke-width: 2.5;
      stroke-linecap: round;
      transition: stroke 0.25s ease, stroke-width 0.25s ease, opacity 0.25s ease;
    }

    .conn-path.active {
      stroke: #10b981;
      stroke-width: 3.5;
      opacity: 1;
    }

    .conn-path.dimmed {
      opacity: 0.2;
    }

    .instructions {
      text-align: center;
      font-size: 12px;
      color: #64748b;
      margin-top: 10px;
      font-weight: 500;
    }

    @media (max-width: 420px) {
      .item-card {
        padding: 8px 8px;
        gap: 6px;
      }
      .badge-code {
        font-size: 16px;
        padding: 3px 6px;
        min-width: 32px;
      }
      .item-name {
        font-size: 12px;
      }
      .item-sub {
        font-size: 10px;
      }
      .col-title {
        font-size: 13px;
      }
    }
  </style>
</head>
<body>

  <div class="diagram">
    <div class="header-row">
      <div class="col-title geno">Genotype</div>
      <div></div>
      <div class="col-title pheno">Phenotype</div>
    </div>

    <div class="main-grid" id="gridArea">
      
      <div class="card-col">
        <div class="item-card" id="card-TT" data-geno="TT" data-pheno="tall">
          <div class="badge-code">TT</div>
          <div class="label-group">
            <span class="item-name">Homozygous</span>
            <span class="item-sub">Dominant</span>
          </div>
        </div>

        <div class="item-card" id="card-Tt" data-geno="Tt" data-pheno="tall">
          <div class="badge-code">Tt</div>
          <div class="label-group">
            <span class="item-name">Heterozygous</span>
            <span class="item-sub">Carrier of 't'</span>
          </div>
        </div>

        <div class="item-card" id="card-tt" data-geno="tt" data-pheno="short">
          <div class="badge-code recessive">tt</div>
          <div class="label-group">
            <span class="item-name">Homozygous</span>
            <span class="item-sub">Recessive</span>
          </div>
        </div>
      </div>

      
      <div></div>

      
      <div class="card-col pheno-col">
        <div class="item-card pheno-card" id="card-tall" data-pheno="tall">
          <div class="plant-icon">🌿</div>
          <div class="label-group">
            <span class="item-name" style="color: #059669;">Tall Plant</span>
            <span class="item-sub">Dominant Trait</span>
          </div>
        </div>

        <div class="item-card pheno-card" id="card-short" data-pheno="short">
          <div class="plant-icon short">🌱</div>
          <div class="label-group">
            <span class="item-name" style="color: #475569;">Short Plant</span>
            <span class="item-sub">Recessive Trait</span>
          </div>
        </div>
      </div>

      
      <svg class="svg-overlay" id="svgConnections">
        <path id="path-TT" class="conn-path" />
        <path id="path-Tt" class="conn-path" />
        <path id="path-tt" class="conn-path" />
      </svg>
    </div>

    <div class="instructions" id="hintText">Tap any box to highlight genetic expression</div>
  </div>

  <script>
    const svg = document.getElementById('svgConnections');
    const grid = document.getElementById('gridArea');
    const pathTT = document.getElementById('path-TT');
    const pathTt = document.getElementById('path-Tt');
    const pathtt = document.getElementById('path-tt');

    const cardTT = document.getElementById('card-TT');
    const cardTt = document.getElementById('card-Tt');
    const cardtt = document.getElementById('card-tt');
    const cardTall = document.getElementById('card-tall');
    const cardShort = document.getElementById('card-short');

    let selected = null;

    function updatePaths() {
      const gridRect = grid.getBoundingClientRect();

      function getRightMid(el) {
        const r = el.getBoundingClientRect();
        return {
          x: r.right - gridRect.left,
          y: r.top + r.height / 2 - gridRect.top
        };
      }

      function getLeftMid(el) {
        const r = el.getBoundingClientRect();
        return {
          x: r.left - gridRect.left,
          y: r.top + r.height / 2 - gridRect.top
        };
      }

      const pTT = getRightMid(cardTT);
      const pTt = getRightMid(cardTt);
      const ptt = getRightMid(cardtt);

      const pTall = getLeftMid(cardTall);
      const pShort = getLeftMid(cardShort);

      function makeBezier(p1, p2) {
        const dx = (p2.x - p1.x) * 0.55;
        return `M ${p1.x} ${p1.y} C ${p1.x + dx} ${p1.y}, ${p2.x - dx} ${p2.y}, ${p2.x} ${p2.y}`;
      }

      pathTT.setAttribute('d', makeBezier(pTT, pTall));
      pathTt.setAttribute('d', makeBezier(pTt, pTall));
      pathtt.setAttribute('d', makeBezier(ptt, pShort));
    }

    function applySelection(targetType, targetValue) {
      if (selected && selected.type === targetType && selected.value === targetValue) {
        // Reset to all active
        selected = null;
      } else {
        selected = { type: targetType, value: targetValue };
      }

      const allCards = [cardTT, cardTt, cardtt, cardTall, cardShort];
      const allPaths = [
        { el: pathTT, geno: 'TT', pheno: 'tall' },
        { el: pathTt, geno: 'Tt', pheno: 'tall' },
        { el: pathtt, geno: 'tt', pheno: 'short' }
      ];

      if (!selected) {
        allCards.forEach(c => {
          c.classList.remove('active', 'dimmed');
        });
        allPaths.forEach(p => {
          p.el.classList.remove('active', 'dimmed');
        });
        return;
      }

      allCards.forEach(card => {
        let match = false;
        if (selected.type === 'geno') {
          if (card.dataset.geno === selected.value) match = true;
          if (card.dataset.pheno && card.id.includes(cardTT.dataset.pheno) && selected.value === 'TT') match = true;
          if (card.dataset.pheno && card.id.includes(cardTt.dataset.pheno) && selected.value === 'Tt') match = true;
          if (card.dataset.pheno && card.id.includes(cardtt.dataset.pheno) && selected.value === 'tt') match = true;
        } else if (selected.type === 'pheno') {
          if (card.dataset.pheno === selected.value) match = true;
        }

        if (match) {
          card.classList.add('active');
          card.classList.remove('dimmed');
        } else {
          card.classList.remove('active');
          card.classList.add('dimmed');
        }
      });

      allPaths.forEach(p => {
        let match = false;
        if (selected.type === 'geno' && p.geno === selected.value) match = true;
        if (selected.type === 'pheno' && p.pheno === selected.value) match = true;

        if (match) {
          p.el.classList.add('active');
          p.el.classList.remove('dimmed');
        } else {
          p.el.classList.remove('active');
          p.el.classList.add('dimmed');
        }
      });
    }

    [cardTT, cardTt, cardtt].forEach(card => {
      card.addEventListener('click', () => {
        applySelection('geno', card.dataset.geno);
      });
    });

    [cardTall, cardShort].forEach(card => {
      card.addEventListener('click', () => {
        applySelection('pheno', card.dataset.pheno);
      });
    });

    window.addEventListener('resize', updatePaths);
    window.addEventListener('DOMContentLoaded', updatePaths);
    setTimeout(updatePaths, 50);
  </script>
</body>
</html>

What happens when two heterozygous parents reproduce in a **monohybrid cross** (a breeding experiment tracking a single trait)?

A Punnett square maps out all four equally likely fertilization combinations by pairing one allele from each parent.

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="1:1">
  <style>
    * {
      box-sizing: border-box;
      margin: 0;
      padding: 0;
      font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
      -webkit-tap-highlight-color: transparent;
    }

    body {
      width: 100%;
      min-height: 100%;
      background: #ffffff;
      color: #1e2945;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      padding: 12px;
    }

    .diagram {
      width: 100%;
      max-width: 360px;
      display: flex;
      flex-direction: column;
      align-items: center;
      gap: 14px;
    }

    .grid-container {
      display: grid;
      grid-template-columns: 36px 1fr 1fr;
      grid-template-rows: 36px 1fr 1fr;
      gap: 6px;
      width: 100%;
      aspect-ratio: 1 / 1;
    }

    .cell {
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      border-radius: 10px;
      transition: all 0.25s ease;
      user-select: none;
    }

    .corner-label {
      font-size: 13px;
      font-weight: 700;
      color: #64748b;
      display: flex;
      align-items: center;
      justify-content: center;
    }

    .gamete {
      background: #f1f5f9;
      font-size: 20px;
      font-weight: 800;
      color: #0284c7;
    }

    .gamete.female {
      color: #0284c7;
    }

    .gamete.male {
      color: #0284c7;
    }

    .box {
      border: 2px solid #e2e8f0;
      background: #ffffff;
      cursor: pointer;
      padding: 4px;
    }

    .box .alleles {
      font-size: 24px;
      font-weight: 800;
      letter-spacing: 1px;
      line-height: 1.1;
    }

    .box .trait {
      font-size: 13px;
      font-weight: 600;
      margin-top: 2px;
    }

    .box.tall {
      background: #f0fdf4;
      border-color: #bbf7d0;
      color: #166534;
    }

    .box.short {
      background: #fefce8;
      border-color: #fef08a;
      color: #854d0e;
    }

    .box.highlight {
      box-shadow: 0 0 0 3px #0284c7;
      transform: scale(1.02);
      z-index: 2;
    }

    .box.dimmed {
      opacity: 0.35;
    }

    .cards-row {
      display: grid;
      grid-template-columns: 1fr 1fr;
      gap: 8px;
      width: 100%;
    }

    .ratio-card {
      background: #f8fafc;
      border: 1.5px solid #e2e8f0;
      border-radius: 10px;
      padding: 8px 6px;
      text-align: center;
      cursor: pointer;
      transition: all 0.25s ease;
      user-select: none;
    }

    .ratio-card.active {
      border-color: #0284c7;
      background: #f0f9ff;
      box-shadow: 0 0 0 1px #0284c7;
    }

    .card-title {
      font-size: 11px;
      font-weight: 700;
      text-transform: uppercase;
      letter-spacing: 0.5px;
      color: #64748b;
      margin-bottom: 4px;
    }

    .card-val {
      font-size: 13px;
      font-weight: 800;
      color: #1e2945;
      white-space: nowrap;
    }

    .badge-t {
      color: #166534;
    }
    .badge-s {
      color: #854d0e;
    }
  </style>
</head>
<body>

  <div class="diagram">
    <div class="grid-container">
      
      <div class="cell corner-label">♀ \ ♂</div>

      
      <div class="cell gamete female">T</div>
      <div class="cell gamete female">t</div>

      
      <div class="cell gamete male">T</div>
      <div class="cell box tall" data-geno="TT" data-pheno="tall">
        <span class="alleles">TT</span>
        <span class="trait">Tall</span>
      </div>
      <div class="cell box tall" data-geno="Tt" data-pheno="tall">
        <span class="alleles">Tt</span>
        <span class="trait">Tall</span>
      </div>

      
      <div class="cell gamete male">t</div>
      <div class="cell box tall" data-geno="Tt" data-pheno="tall">
        <span class="alleles">Tt</span>
        <span class="trait">Tall</span>
      </div>
      <div class="cell box short" data-geno="tt" data-pheno="short">
        <span class="alleles">tt</span>
        <span class="trait">Short</span>
      </div>
    </div>

    
    <div class="cards-row">
      <div class="ratio-card" id="cardGeno">
        <div class="card-title">Genotypic Ratio</div>
        <div class="card-val">1 TT : 2 Tt : 1 tt</div>
      </div>
      <div class="ratio-card" id="cardPheno">
        <div class="card-title">Phenotypic Ratio</div>
        <div class="card-val"><span class="badge-t">3 Tall</span> : <span class="badge-s">1 Short</span></div>
      </div>
    </div>
  </div>

  <script>
    const boxes = document.querySelectorAll('.box');
    const cardGeno = document.getElementById('cardGeno');
    const cardPheno = document.getElementById('cardPheno');
    let activeFilter = null;

    function resetHighlights() {
      boxes.forEach(b => {
        b.classList.remove('highlight', 'dimmed');
      });
      cardGeno.classList.remove('active');
      cardPheno.classList.remove('active');
    }

    cardGeno.addEventListener('click', () => {
      if (activeFilter === 'geno') {
        activeFilter = null;
        resetHighlights();
      } else {
        activeFilter = 'geno';
        resetHighlights();
        cardGeno.classList.add('active');
      }
    });

    cardPheno.addEventListener('click', () => {
      if (activeFilter === 'pheno') {
        activeFilter = null;
        resetHighlights();
      } else {
        activeFilter = 'pheno';
        resetHighlights();
        cardPheno.classList.add('active');
      }
    });

    boxes.forEach(box => {
      box.addEventListener('click', () => {
        const geno = box.dataset.geno;
        if (activeFilter === geno) {
          activeFilter = null;
          resetHighlights();
        } else {
          activeFilter = geno;
          resetHighlights();
          boxes.forEach(b => {
            if (b.dataset.geno === geno) {
              b.classList.add('highlight');
            } else {
              b.classList.add('dimmed');
            }
          });
        }
      });
    });
  </script>
</body>
</html>

The **genotypic ratio** counts the exact genetic combinations ($1\,TT : 2\,Tt : 1\,tt$), while the **phenotypic ratio** groups them by visible traits ($3\text{ Tall} : 1\text{ Short}$).

Always simplify ratios by dividing all parts by the smallest number. For example, a result of $600\text{ Tall} : 200\text{ Short}$ simplifies to $3:1$.

How do exam questions use these ratios to make predictions about large real-world populations?

A phenotypic ratio represents fractional parts of a whole. In a $3:1$ ratio, the total number of parts is $3 + 1 = 4$, meaning the **proportion** of tall plants is $\frac{3}{4}$ ($75\%$) and short plants is $\frac{1}{4}$ ($25\%$).

To find the expected number of individuals with a trait, multiply the fraction by the total population size: $\text{Expected Number} = \text{Fraction} \times \text{Total}$.

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="4:3">
  <style>
    * { box-sizing: border-box; margin: 0; padding: 0; }
    body {
      font-family: system-ui, -apple-system, sans-serif;
      color: #1e2945;
      background: #ffffff;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: space-between;
      padding: 12px;
      min-height: 100%;
      user-select: none;
    }

    /* Problem Header */
    .problem-bar {
      width: 100%;
      max-width: 460px;
      background: #f1f5f9;
      border: 1px solid #cbd5e1;
      border-radius: 8px;
      padding: 8px 12px;
      display: flex;
      align-items: center;
      justify-content: space-between;
      font-size: 14px;
      font-weight: 600;
    }
    .badge-cross {
      background: #1e2945;
      color: #ffffff;
      padding: 3px 8px;
      border-radius: 6px;
      font-size: 13px;
      letter-spacing: 0.5px;
    }
    .total-badge {
      background: #e2e8f0;
      color: #334155;
      padding: 3px 8px;
      border-radius: 6px;
      font-size: 13px;
    }

    /* Stepper Navigation */
    .nav-bar {
      display: flex;
      gap: 6px;
      width: 100%;
      max-width: 460px;
      margin: 8px 0;
    }
    .step-btn {
      flex: 1;
      border: 1px solid #cbd5e1;
      background: #f8fafc;
      color: #64748b;
      padding: 6px 4px;
      border-radius: 6px;
      font-size: 13px;
      font-weight: 600;
      cursor: pointer;
      display: flex;
      align-items: center;
      justify-content: center;
      gap: 4px;
      transition: background 0.2s, color 0.2s, border-color 0.2s;
    }
    .step-btn.active {
      background: #2563eb;
      color: #ffffff;
      border-color: #2563eb;
    }

    /* Stage Container (Fixed Height to prevent CLS) */
    .stage-container {
      position: relative;
      width: 100%;
      max-width: 460px;
      height: 220px;
      overflow: hidden;
    }

    .step-panel {
      position: absolute;
      inset: 0;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      opacity: 0;
      pointer-events: none;
      transition: opacity 0.3s ease;
    }
    .step-panel.active {
      opacity: 1;
      pointer-events: auto;
    }

    /* Step 1: Punnett & Ratio */
    .punnett-wrap {
      display: flex;
      align-items: center;
      gap: 16px;
    }
    .punnett-grid {
      display: grid;
      grid-template-columns: 24px 44px 44px;
      grid-template-rows: 24px 44px 44px;
      gap: 3px;
      align-items: center;
      justify-items: center;
      font-weight: 700;
      font-size: 14px;
    }
    .cell-allele {
      color: #64748b;
    }
    .cell-box {
      width: 100%;
      height: 100%;
      display: flex;
      align-items: center;
      justify-content: center;
      border-radius: 6px;
      font-size: 15px;
      border: 1px solid #cbd5e1;
    }
    .cell-black {
      background: #1e2945;
      color: #ffffff;
      border-color: #1e2945;
    }
    .cell-white {
      background: #ffffff;
      color: #1e2945;
      border-color: #94a3b8;
    }
    .ratio-result {
      display: flex;
      flex-direction: column;
      gap: 8px;
    }
    .ratio-pill {
      background: #f8fafc;
      border: 1px solid #e2e8f0;
      border-radius: 6px;
      padding: 6px 10px;
      font-size: 13px;
      font-weight: 600;
      display: flex;
      align-items: center;
      gap: 6px;
    }
    .ratio-pill.highlight {
      background: #eff6ff;
      border-color: #bfdbfe;
      color: #1d4ed8;
    }

    /* Step 2: Fraction */
    .math-card {
      display: flex;
      flex-direction: column;
      align-items: center;
      gap: 14px;
      width: 100%;
    }
    .equation-row {
      display: flex;
      align-items: center;
      gap: 8px;
      font-size: 18px;
      font-weight: 700;
    }
    .frac {
      display: inline-flex;
      flex-direction: column;
      align-items: center;
      vertical-align: middle;
      padding: 0 4px;
      font-size: 17px;
    }
    .frac-num {
      border-bottom: 2px solid #1e2945;
      padding-bottom: 2px;
      width: 100%;
      text-align: center;
    }
    .frac-den {
      padding-top: 2px;
      width: 100%;
      text-align: center;
    }
    .fraction-bar {
      display: flex;
      width: 80%;
      max-width: 280px;
      height: 32px;
      border-radius: 8px;
      overflow: hidden;
      border: 2px solid #cbd5e1;
      font-weight: 700;
      font-size: 13px;
    }
    .bar-seg {
      flex: 1;
      display: flex;
      align-items: center;
      justify-content: center;
    }
    .seg-black {
      background: #1e2945;
      color: #ffffff;
    }
    .seg-white {
      background: #f8fafc;
      color: #1e2945;
      border-left: 1px solid #cbd5e1;
    }

    /* Step 3: Total Calculation */
    .calc-row {
      display: flex;
      align-items: center;
      gap: 10px;
      font-size: 20px;
      font-weight: 700;
      background: #f8fafc;
      border: 1px solid #e2e8f0;
      padding: 12px 18px;
      border-radius: 10px;
    }
    .result-badge {
      display: inline-flex;
      align-items: center;
      gap: 6px;
      background: #22c55e;
      color: #ffffff;
      padding: 6px 14px;
      border-radius: 8px;
      font-size: 18px;
      font-weight: 700;
    }

    /* Footer Controls */
    .controls {
      display: flex;
      align-items: center;
      justify-content: center;
      gap: 12px;
      width: 100%;
      max-width: 460px;
      margin-top: 4px;
    }
    .ctrl-btn {
      background: #ffffff;
      border: 1px solid #cbd5e1;
      color: #1e2945;
      border-radius: 6px;
      padding: 4px 10px;
      font-size: 13px;
      font-weight: 600;
      cursor: pointer;
      display: flex;
      align-items: center;
      gap: 4px;
    }
    .ctrl-btn:hover {
      background: #f1f5f9;
    }
  </style>
</head>
<body>

  
  <div class="problem-bar">
    <div style="display:flex; align-items:center; gap:6px;">
      <span class="badge-cross">Bb × bb</span>
      <span>Cross</span>
    </div>
    <span class="total-badge">Total = 480 mice</span>
  </div>

  
  <div class="nav-bar">
    <button class="step-btn active" onclick="setStep(0)">1. Ratio</button>
    <button class="step-btn" onclick="setStep(1)">2. Fraction</button>
    <button class="step-btn" onclick="setStep(2)">3. Calculate</button>
  </div>

  
  <div class="stage-container">
    
    <div class="step-panel active" id="panel-0">
      <div class="punnett-wrap">
        <div class="punnett-grid">
          <div></div>
          <div class="cell-allele">b</div>
          <div class="cell-allele">b</div>
          <div class="cell-allele">B</div>
          <div class="cell-box cell-black">Bb</div>
          <div class="cell-box cell-black">Bb</div>
          <div class="cell-allele">b</div>
          <div class="cell-box cell-white">bb</div>
          <div class="cell-box cell-white">bb</div>
        </div>
        <div class="ratio-result">
          <div class="ratio-pill">2 Bb : 2 bb</div>
          <div class="ratio-pill highlight">➔ 1 Black : 1 White</div>
        </div>
      </div>
    </div>

    
    <div class="step-panel" id="panel-1">
      <div class="math-card">
        <div class="equation-row">
          <span>White Fraction =</span>
          <div class="frac">
            <span class="frac-num">1</span>
            <span class="frac-den">1 + 1</span>
          </div>
          <span>=</span>
          <div class="frac">
            <span class="frac-num">1</span>
            <span class="frac-den">2</span>
          </div>
        </div>
        <div class="fraction-bar">
          <div class="bar-seg seg-black">½ Black</div>
          <div class="bar-seg seg-white">½ White</div>
        </div>
      </div>
    </div>

    
    <div class="step-panel" id="panel-2">
      <div class="math-card">
        <div class="calc-row">
          <div class="frac">
            <span class="frac-num">1</span>
            <span class="frac-den">2</span>
          </div>
          <span>× 480 =</span>
          <span class="result-badge">240 White 🐭</span>
        </div>
      </div>
    </div>
  </div>

  
  <div class="controls">
    <button class="ctrl-btn" onclick="prevStep()">◀</button>
    <button class="ctrl-btn" id="playBtn" onclick="togglePlay()">⏸ Pause</button>
    <button class="ctrl-btn" onclick="nextStep()">▶</button>
  </div>

  <script>
    let current = 0;
    let isPlaying = true;
    let timer = null;
    const totalSteps = 3;
    const intervalTime = 4000;

    function render() {
      document.querySelectorAll('.step-btn').forEach((btn, idx) => {
        btn.classList.toggle('active', idx === current);
      });
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      });
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    function setStep(idx) {
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      render();
      resetTimer();
    }

    function nextStep() {
      current = (current + 1) % totalSteps;
      render();
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    function prevStep() {
      current = (current - 1 + totalSteps) % totalSteps;
      render();
      resetTimer();
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      isPlaying = !isPlaying;
      document.getElementById('playBtn').textContent = isPlaying ? '⏸ Pause' : '▶ Play';
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      } else {
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      clearInterval(timer);
      timer = setInterval(() => {
        current = (current + 1) % totalSteps;
        render();
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        startTimer();
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    }

    startTimer();
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</body>
</html>

A common mistake is assuming that a $3:1$ ratio guarantees exact counts in small families, like expecting every 4 offspring to have exactly 3 dominant and 1 recessive individuals.

Each fertilization is an independent event with its own probability, meaning real-world data only matches exact theoretical ratios when sample sizes are very large.

science

Using Ratios and Proportions in Genetic Crosses. Calculate and simplify phenotypic and genotypic ratios from monohybrid genetic crosses and determine direct proportions.

Models of Single-Gene Inheritance: Punnett Squares (KS4 Combined Science / Biology)

GCSE Biology: Genetic Inheritance and Punnett Squares

Probabilities and Punnett Squares in Monohybrid Inheritance

To achieve mastery in GCSE quantitative genetics (Standard 1c), instruction must explicitly bridge biological mechanisms (meiosis, gamete formation, random fertilisation) with core proportional reasoning. Emphasise that ratios must always be stated with explicit phenotypic labels (e.g., '3 tall : 1 dwarf' rather than just '3:1') to prevent mark loss in UK exam boards (AQA, Edexcel, OCR). Use dual bar modelling to teach students how to convert a part-to-part ratio ($a:b$) into a part-to-whole proportion $\frac{a}{a+b}$ before scaling to large population counts.

Using Ratios and Proportions in Genetic Crosses

Alleles and Monohybrid Crosses

Genotypic vs Phenotypic Ratios