Genes, Cell Division & Heredity — Print · Life Science & Scientific Inquiry, Grade 7

1. Correct the nesting: DNA is the molecule, a gene is a section of it, and a chromosome is one very long DNA molecule carrying many genes — so a gene sits inside a chromosome, not the reverse.

2. Accept the premise that is right: every body cell does carry the same DNA, and that part of the draft is correct.

3. Reject the conclusion: cells differ because different genes are switched on in each type, so the same instructions produce different products.

4. Give an example: a muscle cell builds contractile proteins while a gut lining cell builds digestive enzymes and absorbing structures, from the same genome.

The nesting in the draft is inverted. DNA is the molecule whose sequence carries information; a gene is a section of that sequence specifying a product, usually a protein; and a chromosome is one very long DNA molecule packaged with proteins, carrying hundreds or thousands of genes along its length. So a chromosome contains genes, not the other way about. Human cells carry 46 chromosomes in 23 pairs, with roughly 20,000 genes distributed among them. The draft's premise is actually correct — body cells do carry the same DNA — and the conclusion drawn from it is not. Cells differ because they use different parts of the same instructions: in each cell type particular genes are switched on and read while others are kept silent, so a muscle cell produces large amounts of contractile protein and a gut lining cell produces digestive enzymes and absorbing structures, from an identical genome. During development, cells take different paths in response to signals and position, and settle into using a particular subset of what is available. No separate inheritance is needed, and none occurs. Two apparent exceptions confirm rather than undermine this: gametes carry half a set because of meiosis, and mature mammalian red blood cells have discarded their nucleus as they specialised. Both are things that happen to cells during development, not evidence of different instructions being inherited.

1. Start with the count: six chromosomes before replication.

2. Apply replication: the DNA is copied, so each chromosome now consists of two identical sister chromatids joined together — still six chromosomes, with twice the DNA.

3. Apply separation: mitosis pulls the sister chromatids apart, sending one copy of each chromosome to each end of the cell.

4. State the result: each daughter cell receives six chromosomes, the same number the original had.

Each daughter cell has six chromosomes, the same as the cell that produced them. The sequence explains why. The cell begins with six. Replication copies the DNA so that each chromosome consists of two identical sister chromatids joined at a centromere: under the standard convention, structures attached at their own centromere are counted, so there are still six chromosomes, now carrying twice the DNA. Mitosis then separates the sister chromatids and sends one copy of each chromosome to each end of the cell, so each end has six complete chromosomes, and the cell divides. The distinction the task asks for is between the amount of DNA and the number of chromosomes in a set. Replication doubles the DNA temporarily, and that doubling is undone by the separation — the whole point of copying first is that both daughters end up with a complete set rather than half of one. A permanent doubling of the chromosome set would mean the daughters had twelve, which would be a different event entirely and would change what the cells were. Stating the counting convention is part of a correct answer, because "twelve" would be the result of counting chromatids rather than chromosomes at the replicated stage. It is also worth noting that this account describes a normal division; the cycle has checkpoints that can halt it, and a diagram of the process freezes something continuous into stages.

1. Apply meiosis: four pairs means a gamete receives one chromosome from each pair, so a gamete has four chromosomes.

2. Apply fertilisation: two haploid gametes fuse, so the zygote has 4 + 4 = 8 chromosomes, with the pairs reformed.

3. Explain independent assortment: which member of each pair enters a given gamete is independent for each pair, giving 2 to the power of 4 = 16 possible combinations of whole chromosomes.

4. Add recombination: paired chromosomes exchange corresponding segments beforehand, so the chromosomes in a gamete can be mixtures rather than whole parental ones, raising the number of possibilities far beyond sixteen.

A gamete has four chromosomes and the zygote has eight. Meiosis takes the diploid cell with its four pairs and produces haploid gametes carrying one chromosome from each pair — four in total, which is one complete set rather than any four chromosomes. Fertilisation then fuses two gametes, so the zygote receives 4 + 4 = 8, with the pairs reformed and one member of each pair coming from each parent. Halving and combining are matched operations, which is why the number is stable from generation to generation instead of doubling. Different gametes carry different combinations for two reasons acting together. Independent assortment: when the pairs line up before the first division, which member of each pair goes to which side is decided independently for each pair, so with four pairs there are 2 × 2 × 2 × 2 = 16 possible combinations of whole chromosomes. Recombination: before that division, the members of a pair come together and exchange corresponding segments, so a chromosome entering a gamete may be part one parental chromosome and part the other. That makes the possibilities far greater than sixteen, because the combinations are no longer limited to the whole chromosomes the parent inherited. The model simplifies in ways worth naming: four pairs rather than a real species' number, no account of how often recombination occurs or where, and the assumption that every gamete is equally likely to take part in fertilisation.

1. List the gametes: the Aa parent gives A or a in equal proportions; the aa parent gives a only.

2. Fill the square: the four combinations are Aa, aa, Aa, aa.

3. Apply the dominance rule: with A completely dominant, Aa shows the dominant phenotype and aa the recessive one, so the recessive phenotype occupies two of four cells — a probability of 50%.

4. Explain small samples: each fertilisation is independent, so four offspring can fall three and one or four and none, exactly as four coin tosses often do.

The possible genotypes are Aa and aa. The heterozygous parent produces gametes carrying A and gametes carrying a in equal proportions, because meiosis separates the pair; the homozygous recessive parent produces only gametes carrying a. Combining one allele from each parent gives four equally likely cells: Aa, aa, Aa and aa. Applying the stated rule of complete dominance, any genotype containing an A shows the dominant phenotype, so the two Aa cells are dominant and the two aa cells are recessive. The probability of the recessive phenotype is therefore 2 in 4, which is one half or 50%, and the dominant phenotype has the same probability in this particular cross. Four offspring need not split two and two because each fertilisation is an independent event. Nothing records what the previous offspring was, and no mechanism corrects an imbalance, so four offspring can easily come out three and one or even four and none — in exactly the way four coin tosses frequently are not two heads and two tails. The 50% figure describes the proportion expected over many offspring, and the observed proportion approaches it as numbers grow, which is why Mendel's conclusions rested on tallies across thousands of plants rather than on a single pod. Two model assumptions are worth stating: that the two gamete types are equally likely, and that every combination is equally likely to survive to be counted.

1. Identify what was varied and what was held constant: light differed, genotype and all other stated conditions did not.

2. State the supported conclusion: the 3 cm difference between group averages, from 8 cm to 5 cm, is attributable to the difference in light under these conditions.

3. Correct the mutation claim: no sequence change is recorded, and the difference is already explained by the factor the comparison deliberately varied.

4. Add the transmission point: even a mutation in a leaf or stem cell would not be heritable, since only changes in cells producing gametes can reach offspring.

The comparison supports an environmental explanation. Genotype and all other stated conditions were matched, and light was the one factor varied, so the 3 cm difference between the group averages — 8 cm with adequate light against 5 cm with low light — is attributable to the light under these conditions. That is a well-structured comparison, and the claim it supports is appropriately narrow: it does not establish how much light matters for other species, at other times, or over longer periods, and it says nothing about which factor matters more in general. The mutation claim should be rejected on two separate grounds. First, it is unnecessary: the difference already has a cause, created deliberately by the design, and no change in DNA sequence is recorded for either group. Adding an unevidenced cause to explain something already accounted for is the error. Second, even if some plants had acquired a mutation, it would not follow that it was heritable — transmission depends on the cell in which the change occurred, and a mutation in a leaf or stem cell is passed only to that cell's descendants within the same plant. Only a change in cells that give rise to gametes could reach offspring. What would be needed to support a mutation claim is direct evidence of a sequence change, and ideally evidence that offspring of the shorter plants were also shorter when grown in good light — which would test heritability rather than assume it.