CUET UG Biology: Genetics - Mendelian Genetics, Monohybrid, Dihybrid, Incomplete Dominance

Must-Know

• Gregor Johann Mendel conducted hybridization experiments on garden pea (Pisum sativum) between 1856 and 1863 at Brno, Austria (now Czech Republic).
• Mendel selected 7 pairs of contrasting traits in pea plants, such as tall vs dwarf (height), yellow vs green (seed colour), and round vs wrinkled (seed shape).
• In a monohybrid cross, Mendel studied the inheritance of one trait; e.g., cross between pure tall (TT) and pure dwarf (tt) pea plants produced all tall (Tt) F? progeny.
• The F? generation in a monohybrid cross shows a phenotypic ratio of 3:1 (e.g., 3 tall : 1 dwarf) and genotypic ratio of 1:2:1 (1 TT : 2 Tt : 1 tt).
• The Law of Segregation states that during gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.
• A test cross is performed by crossing an individual with a dominant phenotype (but unknown genotype) with a homozygous recessive individual to determine its genotype.
• In a dihybrid cross, Mendel studied two traits simultaneously; e.g., cross between round yellow (RRYY) and wrinkled green (rryy) seeds.
• The F? generation in a dihybrid cross is heterozygous for both traits (RrYy) and shows dominant phenotypes (round yellow).
• The F? generation in a dihybrid cross shows a phenotypic ratio of 9:3:3:1 — 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green.
• The Law of Independent Assortment states that alleles of different genes are distributed independently of one another during gamete formation, provided they are on non-homologous chromosomes.
• Incomplete dominance is a non-Mendelian inheritance pattern where the heterozygous phenotype is intermediate between the two homozygous phenotypes.
• In snapdragon (Antirrhinum majus), red (RR) × white (rr) produces pink (Rr) flowers in F? generation — an example of incomplete dominance.
• The F? generation in incomplete dominance shows a phenotypic ratio of 1:2:1, which matches the genotypic ratio (1 RR : 2 Rr : 1 rr).
• Unlike complete dominance, no single allele is completely dominant in incomplete dominance; both contribute to the phenotype.
• The phenotypic ratio in a monohybrid cross with complete dominance is 3:1, whereas in incomplete dominance it is 1:2:1.
• Mendel’s work remained unrecognized until 1900 when de Vries, Correns, and von Tschermak rediscovered his laws independently.
• Mendel used statistical analysis and large sample sizes, which gave credibility to his results; e.g., in one experiment, he counted 929 F? plants.
• The term "gene" was coined later by Wilhelm Johannsen; Mendel called factors "hereditary units".
• Alleles are alternative forms of a gene; e.g., T and t are alleles for height in pea plants.
• Homozygous individuals have identical alleles for a gene (e.g., TT or tt), while heterozygous have different alleles (e.g., Tt).

Difficulty Level

Intermediate — Requires understanding of ratios, crosses, and distinction between Mendelian and non-Mendelian patterns, but based entirely on NCERT examples.

Common CUET Traps

• Trap: Assuming the phenotypic ratio is always 3:1 in all monohybrid crosses.
  Avoid: Remember that in incomplete dominance, the phenotypic ratio is 1:2:1, same as genotypic ratio.

• Trap: Believing the Law of Independent Assortment applies to all genes regardless of location.
  Avoid: It applies only to genes located on different (non-homologous) chromosomes or far apart on the same chromosome.

• Trap: Confusing test cross with back cross.
  Avoid: A test cross is always with homozygous recessive; a back cross is with any parent, used in breeding programs.

Practice MCQs

1. In a monohybrid cross between two heterozygous tall pea plants (Tt), what proportion of the offspring is expected to be dwarf?
   A) 1/4
   B) 1/2
   C) 3/4
   D) 1/8
   Answer: A) 1/4
   Explanation: The cross Tt × Tt gives 1 TT : 2 Tt : 1 tt; tt (dwarf) is 1/4.
   Why others fail: B) 1/2 is the proportion of heterozygotes, not dwarfs.

2. Which of the following correctly represents the F? phenotypic ratio in a dihybrid cross?
   A) 3:1
   B) 1:2:1
   C) 9:3:3:1
   D) 1:1
   Answer: C) 9:3:3:1
   Explanation: This ratio results from independent assortment of two genes.
   Why others fail: A) 3:1 is monohybrid cross ratio; students may misapply it.

3. In a cross between red-flowered and white-flowered snapdragons, the F? progeny have pink flowers. What is the probability of getting red flowers in the F? generation?
   A) 100%
   B) 75%
   C) 50%
   D) 25%
   Answer: D) 25%
   Explanation: Incomplete dominance gives 1 red (RR) : 2 pink (Rr) : 1 white (rr), so red is 1/4.
   Why others fail: B) 75% is the dominant phenotype in complete dominance; not applicable here.

4. Which Mendelian law is verified by a test cross involving a single trait?
   A) Law of Dominance
   B) Law of Segregation
   C) Law of Independent Assortment
   D) Both A and B
   Answer: B) Law of Segregation
   Explanation: Test cross reveals segregation of alleles into gametes.
   Why others fail: D is tempting because dominance is observed, but segregation is being tested.

5. A plant with genotype RrYy is self-pollinated. How many types of genotypes are expected in the progeny?
   A) 4
   B) 9
   C) 16
   D) 8
   Answer: B) 9
   Explanation: For two genes, number of genotypes = 3? = 3² = 9 (e.g., RRYY, RRYy, RRyy, RrYY, RrYy, Rryy, rrYY, rrYy, rryy).
   Why others fail: C) 16 is the number of possible zygotes (4×4), not distinct genotypes.

Last-Minute Revision

• Mendel worked on Pisum sativum — not beans or maize.
• Monohybrid F? phenotypic ratio: 3:1 (complete dominance).
• Monohybrid F? phenotypic ratio: 1:2:1 (incomplete dominance).
• Dihybrid F? phenotypic ratio: 9:3:3:1 — must be memorized.
• Law of Segregation — applies to single gene; basis of test cross.
• Law of Independent Assortment — applies to two or more genes on different chromosomes.
• Test cross = cross with homozygous recessive.
• Incomplete dominance example: Snapdragon flower colour.
• Co-dominance example: ABO blood group — not part of this topic but often confused.
• F? generation in incomplete dominance shows intermediate phenotype.
• Genotypic ratio in monohybrid cross: 1:2:1 — same as incomplete dominance phenotype.
• Mendel presented his work in 1865; published in 1866 in Proceedings of the Natural History Society of Brünn.
• Rediscovery year: 1900 — by three scientists independently.
• Number of contrasting traits studied by Mendel: 7.
• Alleles segregate during meiosis I — basis of Law of Segregation.
• Phenotype = observable character; Genotype = genetic makeup.
• Homozygous = same alleles; Heterozygous = different alleles.
• Pure line = plants with stable trait inheritance through self-pollination.
• Cross between RR and rr gives all Rr — verify from NCERT.
• Dihybrid cross proves Law of Independent Assortment.