Download Mendel`s Work (pages 76–81)

Mendel’s Work (pages 76–81)
Key Concept: In all of Mendel’s crosses, only one form of the trait appeared
in the F1 generation. However, in the F2 generation, the “lost” form of the
trait reappeared in about one fourth of the plants.
• In the mid 1800s, Gregor Mendel used garden peas to study how traits were
passed from parents to offspring. A trait is a characteristic, such as seed color.
• The passing of traits from parents to offspring is called heredity. The study of
heredity is called genetics.
• Mendel started his experiments with purebred pea plants. A purebred always
produces offspring that have the same trait.
• In one experiment, Mendel crossed purebred tall plants with purebred short
plants. When Mendel crossed the plants, he took pollen from a flower on the tall
plant and used it to pollinate a flower on a short plant. He collected the seeds
that formed and grew them. All the offspring plants were tall. The trait for
shortness seemed to disappear.
• Mendel allowed the offspring plants to pollinate themselves. When he planted
the seeds, he saw some short plants. About one fourth of the plants were short.
Dominant and Recessive Alleles (pages 79–81)
Key Concept: An organism’s traits are controlled by the alleles it inherits
from its parents. Some alleles are dominant, while other alleles are
recessive.
• Mendel concluded that separate factors control how traits are inherited. These
factors are in pairs, with one factor from the mother and one from the father.
• Today, scientists call the factors that control traits genes. The different forms of
a gene are called alleles (uh LEELZ). For example, the gene for stem height in
pea plants has two alleles—one for tall stems and one for short stems.
• A dominant allele always shows up in an organism, even when the other allele
is present. A recessive allele is hidden whenever the dominant allele is present.
• In Mendel’s crosses, the purebred parent plants had two alleles for tall stems.
The purebred short plants had two alleles for short stems. The offspring plants
had one allele for tall stems from the tall parent and one allele for short stems
from the short parent.
• The offspring plants from Mendel’s crosses are called hybrids. A hybrid
(HY brid) has two different alleles for a trait. All the offspring plants were tall
because the dominant allele for tall stems covers up the recessive allele for short
stems.
• Geneticists use letters to represent alleles. A dominant allele has a capital letter.
The allele for tall plants is T. A recessive allele has the lowercase letter. The
allele for short plants is t. A purebred tall plant is TT. A purebred short plant is tt.
A hybrid tall plant is Tt.