16.1 Passage of Information from Parents to Offspring

A haploid cell has one complete set of chromosomes, written n. A diploid cell has two complete sets, written 2n, so its chromosomes occur as homologous pairs: one chromosome of each pair was inherited through each parent.

• Gametes, such as egg and sperm cells, are haploid and contain one chromosome from each homologous pair.
• Most body cells are diploid and contain both homologues of each pair.
• Haploid and diploid describe the number of chromosome sets, not the total number of chromosomes; n does not mean one chromosome in every species.

During fertilisation, two haploid gamete nuclei fuse: n + n → 2n. The zygote is therefore diploid, restoring the two chromosome sets needed for the next generation. Keeping the gametes haploid prevents chromosome-set number from doubling at each fertilisation.

Chromosome-set number is not the same as DNA amount or chromatid number. This card defines ploidy and the gamete/body-cell boundary; it does not explain the stages of meiosis or sources of genetic variation.

A homologous pair contains two chromosomes with the same genes in the same order at corresponding loci. They have the same characteristic shape, length and centromere position, but the allele at a locus may differ between the two homologues.

• In a diploid zygote, one chromosome of each homologous pair came from the female gamete and the other came from the male gamete.
• The homologues therefore carry corresponding genetic information, but they are not necessarily genetically identical because their alleles may differ.
• A locus is the position of a gene on a chromosome; the allele is the particular form found at that locus.

Matching genes in the same order allows homologous chromosomes to line up alongside one another during meiosis. This pairing relationship is different from sister chromatids: sister chromatids are replicated copies of one chromosome joined at a centromere, whereas homologues are the maternal and paternal members of a pair.

Homologous chromosomes are not the same as sister chromatids and do not have to carry the same alleles. This card stops at homologue identity and pairing; crossing over and independent assortment are covered separately.

Meiosis is a form of nuclear division in plants and animals that produces haploid cells for sexual reproduction. It starts from a diploid cell and includes one round of DNA replication followed by two nuclear divisions: meiosis I and meiosis II.

1. DNA is replicated once, so each chromosome consists of two sister chromatids joined at a centromere.
2. In meiosis I, homologous chromosomes pair and whole homologues are separated, reducing the chromosome set number; the centromeres do not divide at this division.
3. There is no further DNA replication before meiosis II. In meiosis II, centromeres divide and sister chromatids separate.
4. Cytokinesis after the two divisions produces four haploid cells. In animals these develop as gametes; in plants the products contribute to sexual reproduction.

Meiosis is a reduction division because the chromosome number changes from diploid to haploid before fertilisation. This allows two haploid gamete nuclei to fuse and restore the diploid number. Unlike mitosis, meiosis has two successive nuclear divisions and reduces chromosome number.

DNA replication happens once, not before each division. Meiosis I separates homologues; meiosis II separates sister chromatids. The detailed PMAT image-reading method and the mechanisms that generate genetic variation are covered separately.

Identify a meiosis image by combining chromosome behaviour, spindle arrangement, nuclear-envelope change and cell division evidence. The key distinction is whether homologous pairs are present and whether whole homologues or sister chromatids are separating.

1. Decide whether the image shows meiosis I or meiosis II. Paired homologues indicate meiosis I; single chromosomes and the developing cell count provide supporting evidence. Two new cells indicate the first division, while four products indicate the second division.
2. Assign the PMAT stage only after checking more than one feature. In prophase, chromosomes condense; in metaphase, they align at the spindle equator; in anaphase, chromosomes or chromatids move towards opposite poles; in telophase, nuclei reform and cytokinesis begins.
3. For meiosis I, check for homologous pairs at prophase/metaphase and whole chromosomes moving with centromeres intact at anaphase. For meiosis II, check single-file chromosomes and centromere division followed by chromatid separation.
4. Cross-check the label against spindle direction, nuclear membranes and cell-surface division. Do not assign a stage from one shape or position alone.

The most diagnostic evidence is pairing versus single chromosomes, the alignment pattern, centromere behaviour and the number of forming cells. Uneven staining, rotation or a crowded photomicrograph can hide one clue, so the final identification should be justified by converging visible features.

A pair of dark chromosome shapes is not automatically a homologous pair. This card teaches image/diagram identification of meiosis stages; it does not explain crossing over, independent assortment or other sources of genetic variation.

Meiosis generates genetic variation in gametes through crossing over and independent assortment, and fertilisation adds further variation when gametes fuse at random. These processes change allele combinations without changing the basic haploid chromosome-set number of a gamete.

• During meiosis I, homologous chromosomes pair and non-sister chromatids can exchange corresponding sections at chiasmata. The exchanged DNA creates new combinations of alleles on the homologues.
• Crossing over is recombination between homologous non-sister chromatids; it is not exchange between sister chromatids or a change in chromosome number.

• At metaphase I, each homologous pair can align in either orientation, independently of the other pairs. Which maternal or paternal homologue moves to each pole therefore changes the whole-chromosome combinations in the resulting gametes.
• During fertilisation, any male gamete can fuse with any female gamete. Combining two independently varied haploid gamete pools gives zygotes with different allele combinations.

Crossing over changes chromosome segments, whereas independent assortment changes which whole homologues enter a gamete; random fertilisation combines two gametes. These are sources of variation associated with sexual reproduction, not the clonal outcome of ordinary mitosis, and no probability value is implied.

Interpret a meiosis photomicrograph or diagram by matching visible chromosome behaviour to the division and stage. Use pairing, alignment, centromere behaviour and cell number together rather than relying on one shape or position.

1. Decide whether meiosis I or meiosis II is shown. Homologous chromosomes paired side by side indicate meiosis I; single chromosomes suggest meiosis II. Two forming cells support meiosis I, while four forming haploid cells support meiosis II.
2. Identify the PMAT evidence: condensed chromosomes in prophase; chromosomes aligned at the spindle equator in metaphase; chromosomes or chromatids moving to opposite poles in anaphase; nuclei reforming and cytokinesis beginning in telophase.
3. Check the separation event. In anaphase I, whole homologous chromosomes move while centromeres remain intact. In anaphase II, centromeres divide and sister chromatids move apart.
4. Cross-check spindle direction, nuclear-envelope change and cell-surface division, then state the stage only when multiple visible clues agree.

A crowded, rotated or unevenly stained image may hide a clue. Record only visible evidence, explain how the evidence supports the stage label, and avoid inventing chromosome details that the image does not show.

A pair of dark shapes is not automatically a homologous pair, and anaphase I is not the same as anaphase II. This card is an image/diagram interpretation method; it does not add a separate mechanism for genetic variation.

Meiosis generates genetically varied gametes through two distinct chromosome processes: crossing over changes allele combinations within homologues, while independent assortment changes which whole homologues enter each gamete. Random fertilisation adds variation when gametes fuse.

• During meiosis I, homologous chromosomes pair and non-sister chromatids can exchange corresponding sections at chiasmata. A section can break and rejoin with the other chromatid, creating new combinations of alleles.
• Crossing over is recombination between homologous non-sister chromatids; it is not exchange between sister chromatids and does not change chromosome number.

• At metaphase I, each homologous pair can align in either orientation independently of the other pairs. Which maternal or paternal homologue moves to each pole therefore changes the whole-chromosome combinations in the resulting gametes.
• Independent assortment changes combinations of whole chromosomes, whereas crossing over changes combinations within chromosome segments.

During fertilisation, any male gamete can fuse with any female gamete. Combining two independently varied haploid gamete pools produces zygotes with different allele combinations.

Crossing over, independent assortment and random fertilisation are distinct sources of variation. They do not imply a fixed probability for one family, and this card does not cover image-stage identification or detailed inheritance calculations.

Meiosis produces haploid gametes that already differ because of chromosome behaviour. Random fertilisation adds a further source of variation: any genetically different male gamete may fuse with any genetically different female gamete, so the zygote receives a new combination of alleles from the two parents.

• Each parent contributes one gamete, but the particular sperm and egg that fuse are not predetermined. Their random fusion samples from two varied gamete pools.
• The resulting zygote is diploid and combines one haploid chromosome set from each gamete; different pairings can therefore produce different allele combinations in offspring.

Meiosis creates variation within each parent through crossing over and independent assortment. Random fertilisation combines that parental variation, so offspring from the same parents can differ even when no new mutation is being considered.

Random fertilisation describes the genetic sampling principle, not a claim that every pairing has equal population frequency. This card completes the meiosis-to-offspring variation chain; it does not replace the neighbouring cards’ mechanisms for crossing over, independent assortment or image-stage identification.