Unit 3 — Variation of Traits in Living Organisms

Science — Third Preparatory

1.11 Summary of Lesson 1

DNA → chromosome → gene; gametes carry one copy, zygote two.
Alleles, dominant/recessive, homozygous/heterozygous, genotype/phenotype.
Mendel’s first law segregation; 100% dominant in F₁ and 3 : 1 in F₂.
Mendel’s second law independent assortment; 9 : 3 : 3 : 1 in F₂.
The Punnett square predicts offspring.
Recessive genetic diseases appear when both alleles are the disease allele; carriers look healthy; relatives’ marriage raises risk; screening and counselling reduce it.

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