3—5 Why can offspring differ from their parents?
Genes and Traits Gregor Mendel hypothesized that inherited “factors” produced certain traits. These
factors are now called genes. Mendel based his hypothesis on the pea plants he was studying. Some
of the plants were tall. Others were short. Mendel decided there must be a gene for tallness and
another gene for shortness. The trait of height for a pea plant is determined by two genes, one from
each parent. Each trait of an organism is determined by at least one gene from each parent.
Homozygous Plants Mendel found that one kind of pea plant always had tall offspring. These plants
had two tall genes, one from each parent. Mendel called these plants homozygous tall. An organism
is homozygous if it has two like genes for a given trait. Mendel found that some short pea plants
always have short offspring. These plants have two short genes. Mendel called these plants
homozygous short.
Heterozygous Plants Mendel wondered what the offspring of one homozygous tall parent and one
homozygous short parent would look like. He crossed a homozygous tall pea plant with a homozygous
short pea plant. All of the offspring were tall. They had received a tall gene from the tall parent and a
short gene from the short parent. They had one tall gene and one shrot gene but grew up tall. Mendel
called these plants heterozygous. Organismes that have two unlike genes for a trait are heterozygous
for that trait.
Dominant Genes Mendel found that in a heterozygous organism, one gene always shows its traits.
The other gene is hidden. Mendel called the gene that always shows itself the dominant gene. He
called the hidden gene a recessive gene. The tall gene is dominant in pea plants. The short gene is
recessive. That is why plants with one tall gene and one short gene are always tall. Only plants with
two short genes will be short.
CHECKING CONCEPTS
1. A pea plant with two tall genes is called
.
2. A heterozygous tall pea plant has one tall gene and one
gene.
3. The gene whose trait always show itself is called the
gene.
4. The recessive gene for height in pea plants is
5. A homozygous short pea plant has two
.
genes.