SME
Grade 6 Science · Life Science
Reproduction and traits
Use this for “An egg and a sperm each carry 23 chromosomes. How many does the offspring’s body cell have?”
Diagram
Half from each parent: 23 + 23 = 46.
This is an example. Type your own values to start a new problem.
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Formulas
The sperm carries 23, the same as the egg’s 23
Chromosomes in a sperm = chromosomes in an egg
23 + 23 = 46
Chromosomes from the egg + chromosomes from the sperm = chromosomes in a body cell
46 ÷ 2 = 23
Chromosomes in a body cell ÷ 2 = chromosomes in an egg
Assumptions
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In sexual reproduction each parent gives half of its chromosomes, so the offspring gets a mix of both.
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A body cell has chromosomes in pairs. An egg or sperm has one from each pair: half as many.
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Egg + sperm = the full set again. People: 23 + 23 = 46. Dogs: 39 + 39 = 78.
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Asexual reproduction copies one parent: every chromosome is the parent’s, so the offspring is a genetic match.
Step-by-step
WE KNOW
Chromosomes in an egg: 23
FIND
chromosomes in a sperm, chromosomes in a body cell
Step 1 · Find chromosomes in a sperm
The sperm carries ?, the same as the egg’s 23
Chromosomes in a sperm = chromosomes in an egg
Egg and sperm each carry one chromosome from every pair: the same number.
Chromosomes in a sperm = 23
Step 2 · Find chromosomes in a body cell
23 + 23 = ?
Chromosomes from the egg + chromosomes from the sperm = chromosomes in a body cell
The egg’s set and the sperm’s set join: one of each pair from each parent.
Chromosomes in a body cell = 46
Check
Put the numbers back in. Both sides should match.
The sperm carries 23, the same as the egg’s 23 ✓
23 + 23 = 46 ✓
46 ÷ 2 = 23 ✓
Topic check
See what you know: 5 questions on this topic
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