Biology 1001A Lecture Notes - Lecture 4: Zygosity
7 May 2018
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Cycle 5 GSA
.: The Begiigs of Geetis: Medel’s Garde Peas
- Before Mendel, people believed in the blending theory of inheritance and acquired characteristics.
However, the basic laws that governed inheritance were discovered by Mendel through his work with
garden peas. In his work with peas, Mendel studied a variety of heritable characteristics (characters) like
flower colour, height and seed shape. A variation in the character (like purple or white flower colour) is
called a trait.
Note: Although ay people had studied iheritae patters efore Medel, Medel’s study as
innovative because he took a quantitative approach to science. Mendel repeated his experiments
countless times and applied statistical analysis in a time when purely qualitative and descriptive science
was the accepted practice.
10.1a: Mendel Chose True-Breeding Garden Peas for His Experiments
- To begin his experiments, Mendel chose pea plants that were known to be true-breeding (i.e. plants,
that when self-fertilized, produce offspring that had a trait exactly like the parent plant). This way the
parents were "knowns" and nothing surprising popped up (using true-breeding plants was one of the
main reasons why Mendel was able to figure out the basic rules and laws that govern genetics)
.: Medel’s First Experiet Ivolved Sigle-Character Crosses
- Mendel crossed a true-breeding purple plant with a true-breeding white plant. According the prevailing
theory of blending, the parental generation (P) should produce offspring (F1) with pink flowers, however,
the F1 generation all had purple flowers, as if the trait for white flowers had disappeared.
From the result of this test, and many more like it using two true-breeding forms of the contrasting
traits, Mendel formulated the Law of Dominance - of a idiidual’s pair of gees osists of differet
alleles, one allele is dominant over the other, recessive allele. Thus, the dominant allele determines the
appearae of a orgais, ut it does’t diretly ihiit reessie alleles.
Medel’s Seod Experiet
- Mendel then crossed the all purple F1 generation in a monohybrid cross. What he found was that the F2
generation was comprised of both purple and white flowers in a 3:1 ratio even though none of the F1
flowers were white. (Note: In a heterozygous monohybrid cross involving completely dominant and
recessive alleles, this 3:1 ratio is always observed).
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