Mendelian Genetics Gregor Mendel’s research

Mendel proposed the first theory about the units of inheritance (what we now call genes) and described two fundamental rules governing how traits are inherited. Mendel’s work is recognized today as one of the greatest breakthroughs in the history of science.

In one experiment he made a cross between two true-breeding strains, one with wrinkled seeds and another with round seeds. This is called a monohybrid cross because the two plants involved in the cross differ only in one character (in this case, seed shape).

The plants involved in the original cross are called the parental or P generation. In this experiment, Mendel found that all the progeny plants, the first filial or F1 generation, produced only round seeds. The wrinkle trait seemed to have been masked, or “dominated,” by the round trait in the F1 seeds.

Mendel called the round trait dominant and the wrinkled trait recessive. All seven monohybrid crosses behaved in a similar manner; they produced F1 progeny processing the trait of one of the parents. Mendel then planted the F1 seeds for each cross, raised the plants, and allowed them to self pollinate to produce the second filial or F2 generation. He found that both dominant and recessive types appeared in the F2 generation in a numerical ratio of 3 dominant to 1 recessive.

Mendel proposed that genes exist in different forms and can consequently give different traits. Today we call different forms of one gene an allele.

Autosomal Dominant If the trait were dominant, we would use the following designations: A = the trait (a genetic disease or abnormality, dominant) a = normal (recessive)

Autosomal Recessive If the trait were recessive, we would use the following designations: A = normal (dominant) a = the trait (a genetic disease or abnormality, recessive)

X-linked Recessive We use the following symbols for X-linked recessive: X A = normal X a = the trait (a genetic disease or abnormality) Y = Y chromosome (males only)

Rr rr Rr rr

XA XA Xa XA Y Xa Y* * = trait is expressed

Assignment Pg. 178, Section 1 Review 1-7 Due on Wednesday (1/15)