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  1 st 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 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