MENDEL’S EXPERIMENTS Part II: Law of Segregation

Biology 12

Joke of the day:

Review: experiment 1• Mendel found that 100% percent of the F1’s had purple

flowers• Mendel found that one trait always appeared and the other trait

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

Mendel’s Second Experiment• Crossed the F1 generation with itself to produce the F2 generation

What happened?• Mendel found that the recessive trait appeared again• Mendel repeated this experiment many times and each

time found that the F2 generation had the dominant trait 75% of the time and the recessive trait 25% of the time• This 3:1 ratio is known as the Mendelian ratio

Again: tall and short…

Again: yellow and green….

Why did this happen?• Mendel realized that there must be two sets of instructions for

each characteristic and that each parent would donate one set of instructions to offspring• Now we know that the sets of instructions are genes and that a fertilized

egg would have two forms of the same gene for every characteristics and we call these two forms alleles.

Why did this happen?• During meiosis the alleles (one on each chromosome) for

a trait separate into different gametes • Because one chromosomes of each pair goes to a different gamete

• Offspring inherit one allele from sperm and one from egg

• If the dominant allele is present it will be expressed (PP and Pp)• The recessive allele will only be expressed if both alleles are

recessive (pp)

2. Law of Segregation• The two alleles of a gene pair are separated from each

other during gamete formation

Mendel’s other principle:• 3. Principle of Independent Assortment

• When more then one trait is studied in the same cross, the genes for each trait sort into gametes independently of the genes of the other traits

But today…..• We now know that Mendel’s principles of Dominance and

Independent Assortment are not always true!• Some genes have incomplete dominance.• Some genes are linked on the same chromosomes so they do not

sort independently.

To do:• Complete Personal Profile and Questions