Chapter 14. Mendel and the Gene Idea. Gregor Mendel. Monk Pea Plants many varieties, easy to reproduce and control, tracked traits that were “either-or”, started with true breeding plants. P Generation (true-breeding parents). . Purple flowers. White flowers. F 1 Generation
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Mendel and the Gene Idea
Pea Plants many varieties, easy to reproduce and control, tracked traits that were “either-or”, started with true breeding plants
All plants had
Allele for purple flowers
Locus for flower-color gene
Allele for white flowers
Each trait is controlled by 2 alleles – one contributed from each parent
PP or Pp?
APPLICATION An organism that exhibits a dominant trait,
such as purple flowers in pea plants, can be either homozygous forthe dominant allele or heterozygous. To determine the organism’s
genotype, geneticists can perform a testcross.
TECHNIQUE In a testcross, the individual with the
unknown genotype is crossed with a homozygous individual
expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent.
then all offspring
then 1⁄2 offspring purple
and 1⁄2 offspring white:
Typical Outcome for 2 heterozygous individuals: 9:3:3:1
Fraction of progeny
Ex: Hair, Skin, Eye Color
Outcome differs from 9:3:3:1
Controlled by genetics and environment
Ex: Skin color in humans, hydrangea color
Nature vs. Nurture
Circles = Females
Squares = Males
Shaded = Expresses phenotype
Half-Shaded = Carriers
Parents joined by horizontal lines
Offspring listed below parents in birth order