MENDEL’S EXPERIMENTS Part I: Law of Dominance

Biology 12

Joke of the day:

Mendel’s experiments• Studied principles of inheritance in plants• Looked for general trends and created a basic set of rules

about the transmission of traits• Large populations, like his peas, are better to analyze

statistically

Mendel studied Pea Plants. Why?

The pea flower: Because……

• They were easy to grow• The male part of the

flower (stamen) was easily removed to prevent self-pollination.

• Easily identifiable traits• Can work with large

numbers of samples

Mendel’s Peas• He looked at 7 characteristics:

SO WHAT DID MENDEL FIND OUT ABOUT HIS PEA PLANTS?

Mendel’s first experiments• Mendel first selected certain traits (characteristic) and

bred those plants so that they would be purebred for that trait.• For example, he chose plants that were tall and bred them

together. He then took only the tall offspring and bred those together. He did this until only tall offspring were produced.

• Mendel called his purebred plants the parent generation or P generation.

Then what?• For each trait, he took two plants with opposite traits and

crossed them• Example: purple flower plant crossed with white flower plant

• This is called a monohybrid cross because only one trait, flower colour, is being tested.

• The offspring from this cross are called the first filial generation or the F1 generation

What did he find?• Mendel found that 100% percent of the F1’s had purple

flowers• This happened with all the traits he studied!• Mendel found that one trait always appeared and the other trait

vanish? The trait that appeared = dominant and he trait that disappeared = recessive

Dominant vs. recessive• This is where the terms dominant and recessive came

from. • Example: Mendel concluded that purple flowers (PP) were

dominant over white flowers (pp).

• Uppercase letters stand for dominant and lowercase letters for recessive• Two letters are used for each trait to represent the allele from

each parent• Dominant (AA)• Recessive (aa)

1. Law of Dominance• When there are two different alleles of a gene in an

individual, the allele for the dominant trait will be expressed not the recessive trait

or

• When individuals that are pure for contrasting traits are crossed, the offspring will express only the dominant trait

• For example, when a purebred tall plant is crossed with a purebred short plant, all the offspring will be tall

Example: What will happen when pure yellow peas are crossed with pure green peas?

• Yellow is dominant and green is recessive

To do:• Complete vocabulary list

using pages 130-135:A. GenesB. TraitC. GenomeD. DominantE. RecessiveF. Allele G. Genotype H. PhenotypeI. HomozygousJ. HeterozygousK. Monohybrid crossL. Punnett square

• Watch first 3 sections of “Greatest discoveries in Genetics” video