Ch 2: Mendel and Law of Segregation

How did Mendel design his pea plant experiments and what did he discover?

Mendel used pure-breeding (homozygous) parents (P generation) and crossed them to produce F1 hybrids (heterozygotes). Crossing the F1 produced the F2 generation, where he observed characteristic ratios.

He found that traits never blended—instead, recessive traits hidden in heterozygotes reappeared in F2, establishing the principles of inheritance.

How did monohybrid crosses lead to Mendel’s Law of Segregation? (explain what Mendel discvered, identify zygosity, and dominance.)

Monohybrid crosses showed that hereditary determinants are particulate and occur in pairs (alleles), which can be homozygous or heterozygous. In heterozygotes, one allele is dominant (visible in F1), while the other is recessive (masked but reappears in F2). These alleles segregate into haploid gametes during Meiosis I, supporting the Law of Segregation.

• Notation: Dominant = uppercase or “+” (YY, Yy, +/+, +/-).

• Recessive: lowercase or “–” (yy, -/-).

• Traits are named for their mutant condition; the standard form is the wildtype.

What is dominance, and how are haplosufficiency, haploinsufficiency, and mutation types related?

Dominance = phenotype of a heterozygote.

• Haplosufficient (dominant): one allele is enough.

• Haploinsufficient (recessive): one allele not enough.

• Dominance is not absolute (ex: ABO blood).

• Mutations:

  • Pseudofunctionalization: loss of function (LOF).

  • Neofunctionalization: new function (GOF).

  • Subfunctionalization: shared function.

Ratios of 3:1, 1:2:1, 1:1 indicate?

indicate a single gene

How can you tell if a yellow pea (Y_) is YY or Yy?

By doing a test cross with a homozygous recessive (yy).

• If all offspring are yellow → the parent was YY.

• If offspring are 50% yellow, 50% green → the parent was Yy.
  This is how Mendel distinguished dominant/recessive, phenotype/genotype, and homozygous/heterozygous

What is a test cross, and how do its outcomes compare to selfing a heterozygote?

• Test cross: breed with a homozygous recessive (dd).

  • DD × dd → 100% Dd (all dominant phenotype).

  • Dd × dd → 50% Dd, 50% dd (1:1).

  • dd × dd → 100% dd (all recessive).

• If no tester available: self the heterozygote (Aa × Aa).

  • Result = 3:1 phenotype and 1:2:1 genotype ratio.

What are Mendel’s key monogenic and digenic ratios?

Back:
Monogenic (1 trait):

• 4:0 → F1 from crossing pure homozygous parents

• 3:1 phenotype → F2 from selfing F1 heterozygotes

• 1:2:1 genotype → F2 from selfing F1 heterozygotes

• 1:1 → F2 tester cross (Aa × aa)

Digenic (2 traits):

• 9:3:3:1 phenotype → F2 from selfing dihybrid F1s (AaBb × AaBb)

• 1:2:1:2:4:2:1:2:1 genotype → F2 from selfing dihybrid F1s

• 1:1:1:1 → F2 tester cross (AaBb × aabb)