AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/5/5-2)
Unit 5 · Topic 5.2
5.2 Meiosis and Genetic Diversity
Meiosis doesn't just halve the chromosome number; it also shuffles genes so that almost every gamete is unique. Crossing over, the random lineup of homologous pairs and random fertilization all add variation, while mistakes in separating chromosomes (nondisjunction) give gametes the wrong number of chromosomes.
Key terms
- crossing over
- recombination
- independent assortment
- random fertilization
- nondisjunction
Why variation matters
Natural selection can only act on differences between individuals. Sexual reproduction is a major source of those differences, because each offspring gets a new combination of alleles from two parents. Three steps create the new combinations: crossing over, independent assortment and random fertilization. The first two happen during meiosis; the third happens when gametes meet.
Crossing over (recombination)
In prophase I, homologous chromosomes pair up tightly. Non-sister chromatids (one from the maternal homolog and one from the paternal homolog) can break and swap matching segments. This is crossing over, and the result is called recombination: a chromatid that now carries some alleles from one parent and some from the other.
Example: suppose your chromosome from your mother carries alleles A and B, and the matching chromosome from your father carries a and b. Without crossing over, your gametes would get either A B or a b. A crossover between the two genes can produce chromatids with A b or a B, combinations that didn't exist on either chromosome you inherited. The swap that matters is between non-sister chromatids; a swap between sister chromatids wouldn't change anything, because sisters are identical copies.
Independent assortment of chromosomes
In metaphase I, each homologous pair lines up at the middle of the cell, and which side the maternal or paternal chromosome faces is random. Each pair's orientation doesn't affect any other pair's. So a gamete can get the maternal copy of chromosome 1, the paternal copy of chromosome 2, and so on, in any mix.
The number of possible combinations is 2ⁿ, where n is the haploid number. For humans, n = 23, so independent assortment alone can produce 2²³ = 8,388,608 different chromosome combinations in gametes, before crossing over adds even more.
Random fertilization
Any sperm can fertilize any egg. If each parent can make about 8.4 million chromosome combinations, one couple could produce about 8.4 million × 8.4 million ≈ 7.0 × 10¹³ different zygotes from independent assortment alone. Add crossing over and the real number is effectively unlimited, which is why siblings (other than identical twins) are essentially never genetically identical.
Nondisjunction
Normal meiosis gives each gamete exactly one copy of each chromosome. Nondisjunction is a failure of chromosomes to separate properly: either a homologous pair doesn't separate in anaphase I, or sister chromatids don't separate in anaphase II. The resulting gametes are no longer properly haploid; some have an extra chromosome (n + 1) and some are missing one (n − 1).
If an n + 1 gamete is fertilized by a normal gamete, the zygote has three copies of that chromosome (trisomy, 2n + 1). An n − 1 gamete gives a zygote with one copy (monosomy, 2n − 1). Having the wrong number of individual chromosomes is called aneuploidy. Its effects on phenotype are covered in 6.7.
You won't be tested on the details of life cycles in specific plants and animals, such as alternation of generations; focus on how meiosis itself creates variation.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Counting combinations from independent assortment
A species has a diploid number of 8 (2n = 8). Ignoring crossing over, how many genetically different gametes can one individual make? How many different zygotes could two parents of this species produce?
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- Step 1: The haploid number is n = 8 ÷ 2 = 4, so there are 4 homologous pairs.
- Step 2: Each pair can orient two ways in metaphase I, independently of the others, so the number of gamete combinations is 2⁴ = 16.
- Step 3: Each parent can make 16 kinds of gametes, and any sperm can meet any egg, so the number of zygote combinations is 16 × 16 = 256.
Answer: 16 kinds of gametes per individual; 256 possible zygote combinations (more once crossing over is included).
- Example 2
Nondisjunction in meiosis I vs. meiosis II
In a species with n = 3, nondisjunction happens once in a cell undergoing meiosis. Compare the four gametes produced if the error happens (a) in anaphase I, for one homologous pair, or (b) in anaphase II, in just one of the two cells.
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- Step 1: (a) If one homologous pair fails to separate in anaphase I, both homologs go to the same daughter cell. That cell has 4 chromosomes and the other has 2. Meiosis II then divides each normally.
- Step 2: So the cell with 4 makes two gametes with 4 chromosomes (n + 1), and the cell with 2 makes two gametes with 2 chromosomes (n − 1). All four gametes are abnormal.
- Step 3: (b) If meiosis I is normal, both cells have 3 chromosomes. If sister chromatids of one chromosome fail to separate in one cell during anaphase II, that cell makes one gamete with 4 (n + 1) and one with 2 (n − 1).
- Step 4: The other cell divides normally, making two gametes with 3 chromosomes.
Answer: (a) Meiosis I error: 2 gametes with n + 1 (4 chromosomes) and 2 with n − 1 (2 chromosomes), so all four are abnormal. (b) Meiosis II error: 1 gamete with n + 1, 1 with n − 1, and 2 normal gametes with 3 chromosomes.
Common mistakes
- Saying crossing over happens between sister chromatids. It happens between non-sister chromatids of homologous chromosomes, which is why it creates new allele combinations.
- Placing crossing over or independent assortment in the wrong phase. Crossing over is in prophase I; random orientation of pairs is in metaphase I and the separation that follows in anaphase I.
- Thinking mutations are the main reason siblings differ. New mutations do add variation, but most of the difference between siblings comes from shuffling existing alleles through meiosis and fertilization.
- Forgetting that nondisjunction in meiosis I affects all four gametes, while a meiosis II error affects only the two gametes from the cell where it happened.
On the exam
- Questions often ask how sexual reproduction increases genetic variation. Name the specific process (crossing over, independent assortment, random fertilization) and say when it happens and what it does.
- Be ready to predict gamete chromosome numbers after nondisjunction from a diagram, and to say whether the error happened in meiosis I or II from the pattern of gametes.
Connected topics
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Check yourself
4 questions on 5.2 Meiosis and Genetic Diversity. Pick an answer to see if you got it, and why.
An organism has a diploid number of 2n = 8. Counting only the random alignment of homologous pairs in meiosis I, and ignoring crossing over, how many genetically different gametes can it make?
Two siblings with the same biological parents (who are not identical twins) differ in many traits. Which of the following best explains this?
During meiosis I in a human, one pair of homologous chromosomes fails to separate, but meiosis II proceeds normally. What is the chromosome makeup of the four resulting gametes?
During meiosis in a human, meiosis I is normal. In meiosis II, the sister chromatids of one chromosome fail to separate in one of the two cells; the other cell divides normally. What is the chromosome makeup of the four resulting gametes?
0 of 4 answered