D. Genetic Processes · Genetic ProcessesWhy Siblings Differ: Meiosis, Variation and Gamete Formation
D3.1 D3.2 D2.2 D2.1 A1.8About 70 minutes
Learning goals- I can explain how crossing over, independent assortment and random fertilization create genetic variation.
- I can calculate the number of chromosome combinations produced by independent assortment.
- I can compare mitosis and meiosis.
- I can compare spermatogenesis and oogenesis.
Words to know
genetic variation
Differences in alleles and allele combinations among individuals of a species.
chiasma (plural chiasmata)
The X-shaped point where non-sister chromatids cross over and exchange segments.
recombinant chromatid
A chromatid with a new combination of alleles produced by crossing over.
independent assortment
The random orientation of each homologous pair at metaphase I, so the chromosomes of different pairs are sorted into gametes independently.
random fertilization
Any one of a huge number of possible sperm can join with any one of a huge number of possible eggs.
spermatogenesis
The formation of sperm by meiosis in the testes; four sperm from each cell that starts meiosis.
oogenesis
The formation of eggs by meiosis in the ovaries; one egg plus polar bodies.
polar body
A tiny cell produced by the unequal divisions of oogenesis; it usually breaks down.
No two the same
Siblings with the same two parents can look and act very differently. Except for identical twins, every person who has ever lived has a unique combination of alleles. Last lesson you learned the phases of meiosis. This lesson is about the payoff: meiosis is not just a way to halve the chromosome number, it is also a machine for shuffling genes. Genetic variation matters because it gives a population the raw material for natural selection, a key idea in the Evolution unit of this course.
Source 1: Crossing over
In prophase I, homologous chromosomes are held tightly together. Non-sister chromatids can break at the same point and swap matching segments. The crossing point looks like an X under a microscope and is called a chiasma.
No genes are gained or lost, but the combination of alleles on a chromatid can change. If one homologue carries A and B and the other carries a and b, crossing over between the two genes can produce chromatids carrying Ab and aB. These are recombinant chromatids. In humans, each pair of homologues usually crosses over at least once, often several times.
Source 2: Independent assortment
In metaphase I, each homologous pair lines up on its own. Whether the mother's or the father's chromosome faces a particular pole is random, like a coin flip, and it does not depend on what the other pairs are doing.
With n pairs, the number of possible combinations of chromosomes in a gamete is 2ⁿ:
- 2 pairs: 2² = 4
- 3 pairs: 2³ = 8
- Humans, 23 pairs: 2²³ = 8 388 608
And that is before counting crossing over.
Source 3: Random fertilization
Any sperm can fertilize any egg. If each parent can make about 8.4 million chromosome combinations, one couple can produce about 8.4 million × 8.4 million ≈ 70 trillion (7 × 10¹³) different combinations in a child, without even counting crossing over. With crossing over, the number is effectively unlimited.
Worked exampleExample: Counting combinations
A pea plant has 2n = 14.
(a) How many different chromosome combinations can its gametes have, from independent assortment alone? (b) How many combinations are possible in the offspring of two pea plants (ignore crossing over)? (c) Why is the real number much larger?
- 2n = 14, so n = 7 pairs.
- (a) Each pair can face either way: 2⁷ = 128 combinations.
- (b) Any of 128 pollen types can join any of 128 egg types: 128 × 128 = 16 384.
- (c) Crossing over creates new allele combinations within chromosomes, so many more gamete types are possible.
(a) 128; (b) 16 384; (c) crossing over makes even more combinations.
Mitosis and meiosis compared
| Feature | Mitosis | Meiosis |
|---|
| Purpose | growth, repair, asexual reproduction | making gametes (or spores in plants) for sexual reproduction |
| Number of divisions | 1 | 2 |
| Homologues pair up? | no | yes, in prophase I (synapsis) |
| Crossing over? | no (normally) | yes |
| Daughter cells | 2, diploid (2n) | 4, haploid (n) |
| Genetically identical? | yes, to the parent cell | no, all different |
| Where in humans | almost all body cells | only in testes and ovaries |
Making gametes: spermatogenesis and oogenesis
In human males, spermatogenesis happens in the testes from puberty onward. Each cell that completes meiosis produces four small sperm cells. Millions of sperm are produced every day.
In human females, oogenesis happens in the ovaries. It begins before birth, when cells start meiosis I and then pause in prophase I for years. From puberty on, usually one cell per menstrual cycle continues. The cytoplasm divides unequally: one cell keeps almost all the cytoplasm and nutrients, and the other is a tiny polar body. Meiosis II is completed only if a sperm enters the egg. The result is one large egg (plus polar bodies that break down). The egg's large supply of cytoplasm feeds the early embryo.
Just for funIf each parent can make about 8.4 million chromosome combinations, one couple could have about 70 trillion genetically different children (even before crossing over). That is thousands of times more than the number of people alive on Earth, which is why "you look just like your brother" is never exactly true.
Hands-on activityCoin-flip gametes
You need: 3 coins, paper and a pencil.
Imagine an organism with 3 pairs of chromosomes. Pair 1 carries A or a, pair 2 carries B or b, pair 3 carries C or c. Heads = the mother's chromosome (capital letter), tails = the father's (small letter).
- Flip all three coins. Write down the gamete (for example, heads-tails-heads = AbC).
- Repeat 24 times. Tally each different gamete.
- Predict: how many different gametes are possible? (2³ = 8.) Did you get all 8? About how often did each appear? (Expected: 24 ÷ 8 = 3 times each.)
- Now, with a partner (or a parent), each make a gamete and "fertilize": combine your gamete with theirs. How many different offspring combinations are possible? (8 × 8 = 64.)
What to notice: your tallies won't be exactly 3 each; small samples vary by chance. That is an important idea for the probability lessons coming up.
Your turn
1Name the three sources of genetic variation in sexual reproduction.
Crossing over, independent assortment and random fertilization.
2In which phase does independent assortment happen? Crossing over?
Independent assortment: metaphase I (random orientation of pairs; the chromosomes then separate in anaphase I). Crossing over: prophase I.
3An organism has 2n = 10. How many chromosome combinations can its gametes have (ignoring crossing over)?
4Give two differences between mitosis and meiosis.
Any two: mitosis has one division and meiosis two; mitosis makes 2 diploid cells and meiosis 4 haploid cells; mitosis daughter cells are identical and meiosis cells differ; homologues pair and cross over only in meiosis.
5Why does oogenesis produce only one egg, while spermatogenesis produces four sperm?
In oogenesis the cytoplasm divides unequally, so one cell gets almost all the cytoplasm (to feed an early embryo) and the others become tiny polar bodies that break down. Sperm divisions are equal.
6What is a recombinant chromatid?
A chromatid with a new combination of alleles made by crossing over (for example, Ab when the parent chromosomes were AB and ab).
More practice: warm-up
1What does a chiasma look like?
An X-shaped crossing point between two non-sister chromatids.
2Where does meiosis happen in human males?
3How many sperm are made from one cell going through meiosis?
4Does mitosis produce genetic variation between daughter cells?
No (normally); the daughter cells are identical.
More practice: on your own
1An organism has 2n = 8. How many kinds of gametes can it make by independent assortment, and how many offspring combinations are possible from two parents (ignore crossing over)?
n = 4: 2⁴ = 16 gamete types. Offspring: 16 × 16 = 256.
2Why do crossing over and independent assortment only matter if the homologues carry different alleles?
If both homologues carried identical alleles at every locus (homozygous everywhere), swapping or sorting them would give the same combination every time. Variation comes from shuffling different alleles.
3Explain why identical twins are genetically identical, but non-identical twins are not.
Identical twins come from one zygote that splits early, so both have the same DNA. Non-identical (fraternal) twins come from two different eggs fertilized by two different sperm, each made by meiosis, so they are as different as any siblings.
4A student says crossing over adds new genes to a chromosome. Correct the statement.
Crossing over swaps matching segments, so the same genes stay at the same loci. It changes which alleles are together on a chromatid, not which genes are present.
5Why is it useful for the egg to be large?
The egg's large store of cytoplasm, nutrients and organelles supports the zygote's first cell divisions before the embryo can get nutrients another way.
Challenge
1Two genes are far apart on the same chromosome, and two other genes are very close together. Which pair is more likely to be separated by crossing over? Explain.
The genes that are far apart. Crossing over can happen at many points along the chromosome; the more distance between two genes, the more likely a crossover falls between them and separates their alleles. (Genes very close together tend to be inherited together; they are called linked genes.)
2Show that 2²³ is about 8.4 million and estimate 2²³ × 2²³ in scientific notation.
2¹⁰ = 1024 ≈ 10³, so 2²⁰ ≈ 1.05 × 10⁶ and 2²³ = 2²⁰ × 8 ≈ 8.4 × 10⁶ (exactly 8 388 608). Then 2⁴⁶ ≈ (8.4 × 10⁶)² ≈ 7.0 × 10¹³.
3Asexual reproduction (by mitosis) is faster and needs only one parent. Use what you know about variation to explain one advantage of sexual reproduction when the environment changes.
Sexual reproduction produces offspring with many different allele combinations. If the environment changes (a new disease, a warmer climate), some offspring are more likely to have a combination that lets them survive and reproduce. Asexual offspring are copies, so a change that harms one may harm all of them.
Quiz
1. Which process happens in prophase I and produces recombinant chromatids?
2. How many kinds of gametes can an organism with 2n = 6 produce by independent assortment alone?
3. Which is TRUE of mitosis but NOT of meiosis?
4. In oogenesis, the polar bodies:
5. Independent assortment is caused by:
6. Fraternal (non-identical) twins:
Big idea: This completes D3.1 (crossing over and how chromosomes move) and links meiosis to inheritance (D3.2). Students should be able to explain why siblings differ in terms of meiosis.
How to help: Do the coin-flip activity together, then talk about family resemblance: which traits are shared among siblings, and which are different? Keep it light; some families include adopted or donor-conceived children, so focus on the science rather than on any one family's biology.
Common difficulties:
- Mixing up crossing over (within a pair, prophase I) and independent assortment (between pairs, metaphase I).
- Calculating 2 × n instead of 2ⁿ for combinations.
- Thinking polar bodies become eggs.