CLASS 10 SCIENCE · CHAPTER 8 · STUDY NOTES
Heredity
A comprehensive guide to the mechanisms of inheritance, exploring how traits are passed from parents to offspring through DNA, Mendel's laws of genetics, and the biological basis of sex determination in humans.
EXPLORE
Follow a difference through generations
Trace inherited differences separately from new copying changes.SEE THE INHERITANCE
A difference can be passed down the family tree.
Follow the green branch: its descendants inherit that difference. Coral marks show additional differences in later descendants.
- Heritable variation
- A difference in inherited information that can be passed to descendants.
DNA copying preserves a shared basic design while occasionally introducing differences. Later descendants can inherit an earlier difference and also acquire new ones.
Sexual reproduction combines inherited variants from two gametes. The environment affects which variants survive and reproduce; it does not invent a useful change on demand.
Go deeper: A family tree records ancestry, not a goal
A branch connects a parent to its descendants. In the illustration, the green variant remains in one branch, while coral marks distinguish additional copying changes. The colours are labels, not bacterial pigments.
A variant is not necessarily beneficial. Heat tolerance may help during heat stress but does not establish superiority in every environment. A common variant cannot, by frequency alone, be dated reliably.
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Question
Two descendants share a green marker; only one has an extra coral marker. Which difference could have been inherited from their shared parent?
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EXPLORE
Two copies can carry different versions
Locate a gene at the same position on a homologous chromosome pair.SEE THE INHERITANCE
One chromosome copy comes from each parent.
Alleles are versions of a gene. T and t occupy the same gene position on the two homologous chromosomes in this pea model.
- Allele
- An alternative version of a gene.
| Genotype | Allele combination | Pea phenotype |
|---|---|---|
| TT | Two T alleles | Tall |
| Tt | T and t | Tall |
| tt | Two t alleles | Short |
For the pea trait in this diploid model, one allele comes from each parent. T and t occupy the same gene position on the two homologous chromosomes.
An individual can therefore have TT, Tt or tt. The allele combination is its genotype; the observed tall or short form is its phenotype.
Go deeper: Inheritance is not a visual average
Mendel counted contrasting pea features across generations. Quantitative patterns helped him infer inherited factors, rather than assuming offspring blended the two parental appearances.
The chapter’s earlobe observation illustrates human variation, but human earlobe shape should not be treated as this simple T/t pea model. Many human characteristics involve several genes and environmental influences.
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Question
Two homologous chromosome rods carry T and t at the same marked position. Are these two unrelated genes, or two versions of one gene?
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EXPLORE
A recessive allele can stay hidden
Use gametes to explain why short pea plants reappear.SEE THE INHERITANCE
The hidden allele has not disappeared.
3 tall : 1 short · four equally likely fertilisation combinations
Tt is tall, but it still carries t. Change the parents to compare TT × tt, Tt × Tt and Tt × tt.
- Dominant allele
- An allele expressed in the heterozygote under the complete-dominance model.
Crossing true-breeding TT tall peas with tt short peas gives Tt offspring, all tall. The t allele is inherited even though its short phenotype is not expressed.
When Tt plants self-pollinate, their T and t gametes combine as TT, Tt, Tt and tt. Expected genotype proportions are 1:2:1; expected tall:short proportions are 3:1.
Go deeper: Count probabilities, not compulsory families
Each gamete receives one allele from the pair. A Punnett square lists equally likely fertilisation combinations under the model; a family of four is not required to match its four cells.
TT and Tt have the same tall phenotype but different genotypes. Crossing a tall plant with tt can help distinguish them: TT predicts only tall offspring, whereas Tt predicts tall and short possibilities. Small samples can differ from expected ratios.
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Question
For Tt × tt, a learner reports three tall possibilities and one short. Use the gametes to correct the expected proportions.
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