--- title: "Quick check: mitosis, fertilisation and sex-linked traits" subtitle: "Year 10 Biology · Genetics · Lesson 6" --- # Quick check: mitosis, fertilisation and sex-linked traits **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Siblings share both parents but are not copies of each other. And why do some conditions show up more in boys? :::teacher-note **In one sentence:** A self-test on mitosis, how fertilisation and meiosis give variation, and sex-linked inheritance (AC9S10U01, nodes 2, 5 and 6, taught in class; node 6 a hinge). **Students should be able to say:** I can say what mitosis produces and why. I can explain why siblings differ. I can predict an X-linked trait with a Punnett square. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Mitosis :::keep-together ### The outcome **A body cell with 46 chromosomes divides by mitosis. What does each daughter cell have?** ☐ 23 chromosomes, different from the parent ☐ 92 chromosomes ☐ 46 chromosomes, all different from each other ☐ 46 chromosomes, genetically identical to the parent cell ::: :::teacher-note **Answer:** 46 chromosomes, genetically identical to the parent cell Mitosis copies the chromosomes and shares them equally, so each daughter cell is a genetic copy with the full number. ::: :::keep-together ### Mitosis or meiosis? **Is it mitosis or meiosis?** Groups: Mitosis, Meiosis - Makes two identical cells: {{field-15}} - Makes gametes: {{field-15}} - Used for growth and repair: {{field-15}} - Halves the chromosome number: {{field-15}} - Keeps the chromosome number the same: {{field-15}} ::: :::teacher-note **Answer:** Makes two identical cells → Mitosis; Makes gametes → Meiosis; Used for growth and repair → Mitosis; Halves the chromosome number → Meiosis; Keeps the chromosome number the same → Mitosis ::: ## Fertilisation and heredity :::keep-together ### Putting it together **Each human gamete has 23 chromosomes. How many does a fertilised egg have?** ☐ 92: two copies from each parent ☐ 23: the same as a gamete ☐ 46: one set from each parent ☐ Any number ::: :::teacher-note **Answer:** 46: one set from each parent Fertilisation joins two gametes, restoring the full number in the next generation. ::: :::keep-together ### Why siblings differ **Why can two children of the same parents have different genes?** ☐ Younger children get more of their genes from the mother ☐ Siblings always get the same genes ☐ Meiosis shuffles the genes into different gametes, and which sperm meets which egg is random ☐ Each child gets an exact blend of both parents ::: :::teacher-note **Answer:** Meiosis shuffles the genes into different gametes, and which sperm meets which egg is random Each gamete is a different mix, and fertilisation combines two at random, so every child gets a different combination (except identical twins). ::: :::keep-together ### Punnett ratios **Two parents are both Bb (B dominant). What is the chance of a child with genotype bb?** ☐ 0 ☐ 3 in 4 (75%) ☐ 1 in 4 (25%) ☐ 1 in 2 (50%) ::: :::teacher-note **Answer:** 1 in 4 (25%) Each parent passes B or b with equal chance, so the four combinations BB, Bb, Bb and bb are equally likely: bb is one of the four. ::: :::keep-together ### From parent to gamete **A parent has the genotype Bb. What gametes can they make?** ☐ B and b in a 3 to 1 ratio ☐ All carry B, because it is dominant ☐ All carry Bb ☐ Half carry B and half carry b ::: :::teacher-note **Answer:** Half carry B and half carry b Meiosis separates the pair, so each gamete gets one allele: B or b, equally likely. ::: ## Sex-linked inheritance :::keep-together ### Sex chromosomes **Which chromosome pair makes a person female or male?** ☐ XY is female, XX is male ☐ Another chromosome pair decides; X and Y don't matter ☐ XX is female, XY is male ☐ The mother's chromosomes alone decide ::: :::teacher-note **Answer:** XX is female, XY is male The Y chromosome carries the gene that starts male development. ::: :::keep-together ### A Punnett square **A mother is a carrier of an X-linked recessive condition (XN Xn). The father does not have it (XN Y). What fraction of their sons is expected to have the condition?** ☐ Half ☐ One quarter ☐ None ☐ All ::: :::teacher-note **Answer:** Half Sons get the father's Y and one of the mother's X chromosomes: half get Xn, so half have the condition. Daughters get XN from the father, so none have it, though half are carriers. ::: :::keep-together ### Why more in males? **Why are X-linked recessive conditions more common in males?** ☐ Males have only one X, so a single recessive allele is expressed; females need two ☐ The Y chromosome carries the condition ☐ Males have two X chromosomes ☐ Females never carry the allele ::: :::teacher-note **Answer:** Males have only one X, so a single recessive allele is expressed; females need two A female can have a normal second X that masks the recessive allele. A male has no second X. ::: :::keep-together ### Daughters **The same parents (XN Xn and XN Y). What about their daughters?** ☐ All are carriers ☐ Half have the condition ☐ None are expected to have the condition, and half are expected to be carriers ☐ One quarter have the condition ::: :::teacher-note **Answer:** None are expected to have the condition, and half are expected to be carriers Daughters get the father's XN and one of the mother's X chromosomes: XN XN or XN Xn. None have it; half carry it. ::: ## Exit ticket :::keep-together **Why is an X-linked recessive condition more common in males?** {{lines-3}} ::: :::teacher-note **Answer:** Males have only one X, so one recessive allele is enough; females need two. :::