In the 19th century, the Austrian monk Mendel discovered the rules of heredity through experiments using pea plants. These are important laws that became the starting point of genetics.
Basic Knowledge
Traits Mendel focused on included seed shape(round and wrinkled), cotyledon color(yellow and green), and flower color(purple and white)—seven pairs of contrasting traits.
Contrasting traits: Two opposing characteristics of the same trait(for example, round vs. wrinkled).
Pure line: An individual that produces only the same trait even after being crossed for several generations(the genotype is the same).
Notation of genes: Capital letters(A) representdominant traits, and lowercase letters(a) representrecessive traits.
For example, in pea seed shape, round(A) is dominant and wrinkled(a) is recessive.
Three important laws:
① Law of dominance: In the offspring(first filial generation F₁) of two pure lines, only the dominant trait appears. Aa→round.
② Law of segregation: During meiosis, paired genes(Aa) separate and enter different reproductive cells. This determines the ratio in the grandchildren.
③ Law of independent assortment: Genes for different traits are inherited independently(advanced topic).
In the grandchildren(F₂), the dominant and recessive traits separate in a ratio of3:1.
Mendel(An Austrian monk who discovered the laws of heredity. Announced in 1865)
Contrasting traits(Two opposing traits, such as round and wrinkled)
Pure line(An individual whose same trait is inherited through generations. AA or aa)
Dominant trait(The trait that appears in the first filial generation. Its symbol is the capital letter A)
Recessive trait(The trait hidden in the first filial generation. Its symbol is the lowercase letter a)
Law of segregation(Paired genes separate during meiosis and enter reproductive cells)
Further Exploration (Background and Significance)
Example: Round seeds(AA) × wrinkled seeds(aa)
① Parents(P): AA × aa
② Offspring(F₁): all Aa → round(law of dominance)
③ Crossing F₁(Aa) with each other producesgrandchildren(F₂):
Combinations of Aa × Aa(Punnett square):
・AA: 1
・Aa: 2
・aa: 1
→ AA・Aa(round):aa(wrinkled) = 3:1
This is the 3:1 ratio discovered by Mendel.
Relationship to chromosomes: One gene A and one gene a are located at the same position on a pair of homologous chromosomes. During meiosis, A and a enter different reproductive cells, so an Aa individual can pass A to offspring with a probability of 1/2 and a with a probability of 1/2. This is the basis of the law of segregation.
Historical significance: Mendel published his paper in 1865, but DNA and chromosomes were not well understood at the time, so his work was ignored for a long time. It was rediscovered in 1900 and became the starting point of genetics. His greatness lay in organizing complex hereditary phenomena as “particle-like units(genes)” and describing them mathematically.
Dominant and recessive do not mean “strong” and “weak”: Dominant means “more likely to appear,” while recessive means “more likely to be hidden.” When the combination is aa, the recessive trait always appears.
- Mendel = discovered the laws of heredity using pea plants(1865)
- Law of dominance = only the dominant trait appears in F₁(Aa→round)
- Law of segregation = paired genes separate during meiosis
- Law of independent assortment = different traits are inherited independently
- In F₂, dominant:recessive = 3:1
- Pure AA × pure aa → all F₁ are Aa → 3:1 in F₂
- Genotype ratio AA・Aa・aa = 1:2:1
- Dominant and recessive describe “how traits appear,” not “strength”
Points to Note (Often Confused)
① Dominant means more likely to appear, and recessive means less likely to appear. They do not mean “strong and weak” or “superior and inferior”(because the terms can cause misunderstandings, “dominant” and “recessive” are increasingly being replaced by “expressed” and “masked” terminology). ② F₁(offspring) all show the dominant trait, while F₂(grandchildren) shows a 3:1 ratio. Do not confuse F₁ and F₂. ③ Distinguish between genotype(AA・Aa・aa) and phenotype(round・wrinkled). AA and Aa have different genotypes but the same phenotype, “round.” ④ The physical basis of the law of segregation is meiosis.
Practice
- What plant did Mendel use in his research on heredity?
- When pure round(AA) and wrinkled(aa) pea plants are crossed, what kind of seeds will the offspring generation(F₁) have? Explain why.
- When F₁ individuals are crossed with each other, what is the ratio of round to wrinkled seeds in the grandchild generation(F₂)?