Question

Phenomenon:
Two purple-flowered plants produce offspring with white flowers, even though neither parent has white flowers
Scientists discover that a new mutation is present in the parents' germ cells but not in their somatic (body) cells.
Claim:
How can a new flower color appear in the offspring even though neither parent plant shows that trait?
Evidence:
Provide at least three pieces of evidence.
Reasoning:
Explain why your evidence supports your claim.
Checklist:
■ Clear, testable claim
■ Three pieces of relevant evidence
■ Reasoning that connects evidence to claim
■ Scientific vocabulary used correctly

Answer 1

Claim (clear and testable)
A de novo germline mutation in a gene required for purple pigment production arose in the parents’ germ cells (but not in their somatic cells) and was transmitted in the parents’ gametes; offspring that inherit the mutant allele from both parents are homozygous for a loss-of-function allele and therefore produce white flowers. This claim can be tested by sequencing parents’ germ cells and offspring, by checking segregation ratios, and by complementation/rescue of the mutant allele.

Evidence (three pieces)
1. Direct detection of the mutation in parental germ cells but not in parental somatic tissue.
- Example method: DNA sequencing (or PCR + allele-specific assay) showing the new nucleotide change in pollen/ovules or isolated germ-line tissue, but absence of that change in leaves/stems/flowers of the parents.

2. Offspring genotype–phenotype correlation showing the white-flowered progeny are homozygous for the mutant allele.
- Example method: sequencing the offspring that have white flowers and finding the mutation on both homologous chromosomes (homozygous), while purple siblings are heterozygous or wild-type.

3. Functional evidence linking the mutated gene to pigment production.
- Example methods: the mutation maps to a known anthocyanin biosynthesis gene (e.g., CHS, DFR, ANS) and is predicted to be loss-of-function (nonsense, frameshift, or disruption of catalytic residue); or a complementation/rescue experiment where introducing a wild-type copy of the gene into white offspring restores purple pigment.

Reasoning (how the evidence supports the claim)
- If the mutation is present in parental germ cells but absent from their somatic cells, the parents will appear phenotypically purple (because their somatic tissues express the wild-type allele) while still producing some mutant gametes. Sequencing evidence (evidence 1) demonstrates this germline-specific presence, explaining why the trait is not visible in parents.
- When both parents contribute a gamete carrying the new mutant allele, an offspring can inherit two mutant copies and be homozygous for the loss-of-function allele. Finding homozygosity for the mutation specifically in white offspring (evidence 2) directly links inheritance of the mutant allele to the white-flower phenotype and matches Mendelian expectations for a recessive loss-of-function allele.
- Identifying the mutated gene as one required for anthocyanin (purple pigment) biosynthesis, and showing that the mutation abolishes gene function or that a wild-type copy rescues purple color (evidence 3), provides mechanistic support: the genotype change causes the biochemical inability to make pigment, producing the white phenotype.
- Together these pieces show causation rather than coincidence: germline-restricted mutation explains parental phenotype, transmission explains occurrence in offspring, and gene function/rescue experiments explain how the allele produces white flowers.

Checklist
- Claim is clear and testable (sequencing, crosses, complementation).
- At least three relevant pieces of evidence provided.
- Reasoning connects each piece of evidence to the claim using correct scientific vocabulary (germline mutation, somatic cells, allele, homozygous/heterozygous, loss-of-function, genotype–phenotype correlation, complementation).