--- marp: true theme: default html: true --- # Quick check: mitosis, fertilisation and sex-linked traits Year 10 Biology · Genetics --- ## Today **We are learning to** a few quick questions on mitosis, how fertilisation and meiosis give variation among siblings, and sex-linked inheritance. Taught in class; use this to check yourself. - 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. --- ## Why it matters > Siblings share both parents but are not copies of each other. And why do some conditions show up more in boys? --- ✅ Check ## 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 * Answer: 46 chromosomes, genetically identical to the parent cell --- ✅ Check ## Mitosis or meiosis? **Is it mitosis or meiosis?** Groups: Mitosis · Meiosis Makes two identical cells · Makes gametes · Used for growth and repair · Halves the chromosome number · Keeps the chromosome number the same * 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 --- ✅ Check ## 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 * Answer: 46: one set from each parent --- ✅ Check ## 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 * Answer: Meiosis shuffles the genes into different gametes, and which sperm meets which egg is random --- ✅ Check ## 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%) * Answer: 1 in 4 (25%) --- ✅ Check ## 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 * Answer: Half carry B and half carry b --- ✅ Check ## 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 * Answer: XX is female, XY is male --- ✅ Check ## 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 * Answer: Half --- ✅ Check ## 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 * Answer: Males have only one X, so a single recessive allele is expressed; females need two --- ✅ Check ## 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 * Answer: None are expected to have the condition, and half are expected to be carriers --- ✅ Check ## Exit ticket **Why is an X-linked recessive condition more common in males?** * Males have only one X, so one recessive allele is enough; females need two.