Biol 114 Week 3: Mendelian Inheritance

Mendel’s 2 guiding questions

1. Why do offspring resemble parents?

2. Why are most offspring not blends of parents?

Are offspring blends of parents?

1. Prediction: If blending occurs, then a cross of pure-breeding wrinkled and round peas (P
   generation) would produce all mildly wrinkled F1 offspring. And self- fertilizing of the F1
   offspring would produce all mildly wrinkled offspring in the F2 generation.

2. Results: All F1 peas were round and indistinguishable from those of the round parent,
   discounting blending. F2 offspring had a ratio of 3:1 round to wrinkled, further evidence
   that there was no blending.

3. Interpretation 1: No blending. Mendel proposed that both the round and wrinkled traits
   were present in the F1 generation and that the wrinkled trait was not expressed in the F1
   generation because the round trait dominated it. Hence dominant and recessive traits.

4. Interpretation 2: Particles occur in pairs, and in order to occur in the 3:1 ratio in F2, the
   particles must segregate into the gametes (egg and sperm) before they leave the
   reproductive structures.

5. Mendel’s Law of Segregation: Particles (alleles) separate, without dilution, into gametes
   where each gamete contains one particle (allele) of each trait (gene)

Are different traits inherited together or seperately?

1. Prediction: If the 2 traits sorted dependently (ie, occurred together only), he would get one
   set of results in F2 (round yellow OR wrinkled green). If they assorted independently, he
   would get an entirely different result in the F2.

2. Results: Round, yellow 315 (9/16), wrinkled, yellow 101 (3/16), Round, green 108 (3/16),
   and wrinkled, green 32 (1/16).

3. Interpretation: Particles (alleles) of one particular trait (gene) are passed on to offspring
   independently of each other- Mendel’s Law of Independent Assortment.
   a) Only works for genes occurring on different chromosomes.
   b) Remember, Mendel still didn’t know about chromosomes. He thought about his
   “particles” as independent little bodies floating around in the cells. Almost as if each
   gene had its own chromosome, not many, many genes on a few chromosomes.
   c) Those genes on the same chromosome we’ll talk about later

The Test Cross

• used to determine the genotype of an individual showing dominant traits

• Test Cross is when this individual is crossed with a homozygous
  recessive individual. The resulting offspring phenotype distribution tells you what the unknown
  parent is.