the production of haploid gametes from diploid cells by meiosis, including the significance of crossing over of chromatids in prophase I and independent assortment of homologous chromosomes in metaphase I for the generation of genetic diversity

What this dot point is asking

VCAA wants you to describe the two divisions of meiosis that produce haploid gametes from diploid parent cells, and explain the two main sources of genetic diversity in those gametes: crossing over in prophase I and independent assortment in metaphase I.

The answer

Why meiosis exists

Sexual reproduction requires fusion of two gametes to make a zygote. To keep the chromosome number constant across generations, the gametes must contain half the normal chromosome number. Meiosis is the cell division that produces these haploid gametes from diploid cells.

In humans, a diploid germ cell (2n = 46) undergoes meiosis to produce four haploid gametes (n = 23). When sperm meets egg, the zygote is again 2n = 46.

Meiosis also generates genetic variation, the raw material of evolution.

The two meiotic divisions

Meiosis consists of two consecutive divisions after one round of DNA replication. It produces four haploid daughter cells from one diploid parent cell.

Interphase (before meiosis). DNA is replicated in S phase. Each chromosome now consists of two identical sister chromatids joined at the centromere.

Meiosis I (reduction division). Separates homologous chromosomes (one homologue to each daughter cell). Halves the chromosome number from 2n to n.

• Prophase I. Chromosomes condense. Homologous chromosomes pair up (synapsis) to form bivalents (also called tetrads, because each contains 4 chromatids). Crossing over occurs: non-sister chromatids exchange segments at points called chiasmata.
• Metaphase I. Bivalents line up at the equator. Each bivalent is oriented independently: maternal homologue may face either pole. This is independent assortment.
• Anaphase I. Spindle fibres pull each homologue to opposite poles. Sister chromatids remain joined at the centromere.
• Telophase I and cytokinesis. Two haploid daughter cells form. Each has n chromosomes, each chromosome still consisting of two sister chromatids.

Meiosis II (equational division, like mitosis). Separates sister chromatids. Does not change chromosome number.

• Prophase II. Chromosomes recondense. New spindles form.
• Metaphase II. Chromosomes line up at the equator of each cell.
• Anaphase II. Centromeres divide; sister chromatids are pulled to opposite poles.
• Telophase II and cytokinesis. Four haploid daughter cells in total, each with n chromosomes, each chromosome now a single chromatid.

End result: four genetically unique haploid gametes from one diploid parent.

Crossing over (prophase I)

During prophase I, homologous chromosomes pair up tightly (synapsis). Non-sister chromatids cross each other at chiasmata and exchange DNA segments.

The consequence: each chromatid ends up with a mosaic of maternal and paternal DNA. Genes that were linked on one parental chromosome can be recombined with alleles from the other parent.

Crossing over creates new allele combinations on each chromatid. Without crossing over, only two combinations would exist (pure maternal or pure paternal) for any whole chromosome.

The frequency of crossing over between two genes is roughly proportional to the distance between them on the chromosome. This is the basis for genetic mapping (the Unit 2 linked/unlinked genes dot point).

Independent assortment (metaphase I)

At metaphase I, each homologous pair lines up at the equator independently of every other pair. The maternal homologue may face the "top" pole or the "bottom" pole, with equal probability, and each pair makes that choice independently.

For one pair, two possible orientations gives 2 combinations. For two pairs: 2 × 2 = 4. For three pairs: 8.

For humans (23 pairs): 2 to the power of 23 = 8,388,608 possible combinations of maternal and paternal chromosomes per gamete from independent assortment alone.

Genetic diversity: the three sources

1. Crossing over in prophase I (within chromosomes).
2. Independent assortment at metaphase I (between chromosomes).
3. Random fertilisation of any one of millions of possible sperm with any one of hundreds of possible eggs.

For humans: 8.4 million × 8.4 million × crossing over variation = effectively infinite combinations. Every human (except identical twins) is genetically unique.

This variation is the raw material on which natural selection acts.

Meiosis vs mitosis

Errors in meiosis

Non-disjunction: failure of homologous chromosomes (meiosis I) or sister chromatids (meiosis II) to separate. Produces gametes with the wrong chromosome number (n+1 or n-1). After fertilisation, this leads to aneuploidies like trisomy 21 (Down syndrome), XO (Turner) or XXY (Klinefelter).

The risk rises with maternal age because eggs are arrested in prophase I from before birth until ovulation, accumulating damage over decades.

Examples in context

Example 1. Orange-bellied parrot breeding at Healesville Sanctuary. The orange-bellied parrot (Neophema chrysogaster) is critically endangered with fewer than 70 wild birds. Healesville Sanctuary and Moonlit Sanctuary run captive breeding programs that maximise genetic diversity by pairing distantly related individuals. Each parental pair generates gametes by meiosis, with two sources of variation: independent assortment of the bird's roughly 40 homologous chromosome pairs (giving 2 to the 40th, around a trillion, possible combinations) and crossing over during prophase I. Even though the founder population is small, careful pairing combined with meiotic recombination keeps allelic diversity higher than predicted by census numbers alone. Studbook software at Zoos Victoria calculates "mean kinship" to choose the optimal pair each season.

Example 2. Crossing-over and the Vitis vinifera genome on the Mornington Peninsula. Mornington Peninsula vineyards grow Pinot Noir, a clonally propagated cultivar of Vitis vinifera. When breeders at Australian Wine Research Institute attempt to create disease-resistant lines, they cross Pinot Noir with a wild grape carrying mildew resistance, then rely on crossing over during meiosis I to recombine alleles. After several generations of recombination, lines carry the resistance allele on a small chunk of wild chromosome embedded in an otherwise Pinot Noir genome. Without meiotic crossing over, every chromosome would be inherited as one block from a single grandparent, and breeders could not separate desirable from undesirable alleles.

Try this

Q1. State two mechanisms that increase genetic variation during meiosis, and identify the phase at which each occurs. [2 marks]

• Cue. Crossing over in prophase I; independent assortment of homologous chromosomes in metaphase I.

Q2. A cell has $2n = 6$ chromosomes. Calculate the number of distinct gametes possible from independent assortment alone (ignore crossing over) and explain your method. [2 marks]

• Cue. $2^n = 2^3 = 8$ distinct gametes from independent assortment.

Q3. Refer to meiosis in mammals. (a) Distinguish meiosis I from meiosis II in terms of what separates. (b) Explain how non-disjunction at meiosis I can produce a trisomy. (c) Predict the chromosomal composition of the four daughter cells if non-disjunction occurs at meiosis II in one pair. [2+2+2 marks]

• Cue. (a) Meiosis I separates homologues; meiosis II separates sister chromatids. (b) Both homologues end up in one gamete (n+1) and none in the other (n-1); fertilisation gives 2n+1 or 2n-1. (c) Two normal (n), one (n+1), one (n-1).