Classical Genetics

The Legacy of Gregor Mendel

Or

The Monk with the Missing “Peas”

The Big Question Why do children

look like their parents?

Who was Gregor Mendel? “Father of Genetics” Studied pea plants

for eight years Published his

results in 1865 Grew over 10,000

pea plants, keeping track of progeny number and type.

Father of Genetics 1st person to

succeed in predicting how traits are passed from one generation to next

Why is he so important? Studied one trait at

a time Analyzed his data

mathematically Looked at multiple

traits Used multiple trials

Why peas? Quick growing Lots of different traits

Step 1: Start with pure-bred plants

Pure-bred plants only produce one type of offspring– Green pea plants only make green

offspring– Yellow pea plants only make yellow

offspring

Step 2: The First Generation

Mendel chose true-breeding pea plants as his parental generation (when self-pollinated, always produced the same type of offspring)

He crossed a true-breeding tall plant with a true-breeding short plant

All of the offspring were tall!

Step 3: The Second Generation

Next, he crossed two tall offspring plants with each other

¾ of the offspring in the second generation were tall; ¼ were short

So how does this pea thing work?

DNA from the Beginning

The Second Generation, cont.

Mendel did similar monohybrid crosses with the other traits as well.

In every case, he found that one trait seemed to disappear in the F1 generation and reappear in ¼ of the F2 plants

This is where “dominant” and “recessive” come from

Conclusion: Law of Segregation

• Each parent has 2 genes that determine each trait

• Each parent can only give one of its two genes for each trait

• Therefore, these two genes must randomly separate to the sex cells so that sex cells contain only one gene of the pair.

Step 4: Mendel’s Dihybrid Crosses

Performed another set of crosses where he used peas that differed from each other in two traits rather than just one

The first generation Took true-breeding pea

plants that had round yellow seeds (RRYY) and crossed them with true-breeding pea plants that had wrinkled green seeds (rryy).

The F1 plants all had round yellow seeds

Step 5: The second generation

F1 plants self-pollinated

F2 plants:

9 round yellow

3 round green

3 wrinkled yellow

1 wrinkled green

Conclusion: The Law of Independent Assortment

Genes for different traits are inherited independently of each other

When a pea plant with the genotype RrYy produces gametes, the alleles R and r will separate from each other (the law of segregation) as well as from the alleles Y and y and vice versa

Alleles can then recombine in four different ways (see next slide)

We now know that this is only true if genes are located on different chromosomes or are far apart on the same chromosome

Testcross A cross of an individual of unknown genotype

with an individual of a known genotype (usually homozygous recessive)

Unknown R_ x rr

If any offspring show the recessive phenotype, then the unknown parent must have been heterozygous

Pedigrees A graphic representation of an

individual’s family tree, which permits patterns of inheritance to be recognized.

When are pedigrees used?

When testcrosses cannot be made When number of offspring is too small Or if results of testcross would take too

long