Mendel's Two Laws

Mendel’s Two Laws

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Heredity is the passing of traits from parents to offspring through genes, which are segments of DNA located on chromosomes. Each gene has two forms called alleles — one inherited from each parent. A dominant allele (written with a capital letter, e.g., A) masks a recessive allele (lowercase, a). The genotype is the genetic makeup (AA, Aa, aa); the phenotype is the observable trait. Homozygous individuals carry two identical alleles (AA or aa); heterozygous individuals carry two different alleles (Aa). Sex-linked traits sit on the X chromosome, so they appear more often in males (XY) than females (XX). Memorize: incomplete dominance blends traits (red × white → pink), while codominance shows both traits at once (AB blood type).

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Mendel’s Two Laws

Gregor Mendel established two foundational principles still used today. The Law of Segregation states that during meiosis, the two alleles for a trait separate so that each gamete (sperm or egg) carries only one allele. The Law of Independent Assortment states that alleles of different genes are distributed to gametes independently of one another — which is why a Punnett square for two traits (dihybrid cross) yields the classic 9:3:3:1 phenotypic ratio.

Dominance Patterns

Three patterns govern how alleles express:

• Complete dominance: the dominant allele fully masks the recessive one. A Tt (heterozygous) tall pea plant looks identical to a TT plant.
• Incomplete dominance: heterozygotes show a blended intermediate phenotype — crossing a red (RR) snapdragon with a white (WW) one gives pink (RW) offspring.
• Codominance: both alleles are fully and simultaneously expressed in the heterozygote, as in the ABO blood group system where I^A I^B produces type AB blood with both A and B antigens on red blood cells.

DNA, RNA, and Gene Expression

DNA (deoxyribonucleic acid) is the double-helix molecule that stores genetic information using four bases: adenine (A), thymine (T), guanine (G), and cytosine (C). A pairs with T; G pairs with C. RNA is single-stranded and uses uracil (U) instead of thymine. Protein synthesis occurs in two stages: transcription (DNA is copied into messenger RNA in the nucleus) and translation (the mRNA is read at the ribosome to assemble amino acids into a protein using transfer RNA).

Sex-Linked Inheritance

Traits carried on the X chromosome — such as hemophilia and red-green color blindness — are called sex-linked. Because males have only one X chromosome (XY), a single recessive allele on it will be expressed. Females (XX) need two recessive copies to show the trait, which is why these conditions predominantly affect males. Punnett squares for sex-linked crosses must track X and Y chromosomes separately.

Mutations

A mutation is any change in the DNA sequence. Mutations in germ cells (sperm/egg) can be inherited by offspring; mutations in somatic cells affect only the individual. Outcomes vary — some mutations are harmful (e.g., sickle-cell anemia), some neutral, and rarely beneficial.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Worked Mini-Example: Monohybrid Cross

Cross a homozygous tall pea plant (TT) with a homozygous short one (tt). All F₁ offspring are Tt — phenotypically tall but heterozygous. Self-pollinating the F₁ (Tt × Tt) gives the F₂ generation: 1 TT : 2 Tt : 1 tt, producing a 3:1 phenotypic ratio (tall : short) and a 1:2:1 genotypic ratio. This is the classic Mendelian outcome when complete dominance applies.

Dihybrid Cross Tip

For a cross involving two genes on different chromosomes (e.g., seed shape Rr and seed color Yy), the F₂ phenotypic ratio from a dihybrid cross of double heterozygotes RrYy × RrYy is 9:3:3:1 — 9 showing both dominants, 3 each for the two single-dominant classes, and 1 showing both recessives.

Common Traps in UPCAT

1. Genotype vs. phenotype confusion: Two plants with the same phenotype (both tall) can have different genotypes (TT or Tt) — the Punnett square reveals this.
2. Dominance ≠ commonness: A dominant allele is not necessarily more frequent in the population; it simply masks recessive alleles when present.
3. Incomplete dominance vs. codominance: Blending (pink) is not the same as both traits appearing fully (AB blood type).
4. Sex-linked squares: Always label gametes as X or Y and remember that an X-linked recessive allele in a male is automatically expressed.

Connection to Other Science Topics

Heredity links directly to cell division (mitosis and meiosis determine how alleles segregate) and molecular biology (DNA replication errors cause mutations). Understanding the Central Dogma (DNA → RNA → protein) prepares you for biotechnology and genetics-engineering questions that occasionally surface in UPCAT.

Practice Prompts

1. A man with hemophilia (X-linked recessive) has children with a non-carrier woman. What fraction of their sons will have the disease? (Answer: zero — sons receive the Y from the father; daughters become carriers.)
2. In snapdragons, red (RR) crossed with white (WW) yields 100% pink. Two pinks are crossed — what phenotypic ratio is expected? (Answer: 1 red : 2 pink : 1 white.)

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