Biology (Holt)

or a “white” version. Today the different versions of a gene

are called its .As shown in Figure 4, an individual

receives one allele from each parent. Each allele can be

passed on when the individual reproduces.

3.When two different alleles occur together, one of them may

be completely expressed, while the other may have no

observable effect on the organism’s appearance. Mendel

described the expressed form of the trait as.

The trait that was not expressed when the dominant

form of the trait was present was described as.

For every pair of contrasting forms of a trait that

Mendel studied, the allele for one form of the trait was

always dominant and the allele for the other form of the

trait was always recessive. For example, if a plant has

both purple and white alleles for flower color but

blooms purple flowers, then purple is the dominant

form of the trait; white is the recessive form. This is

shown inFigure 5.

4.When gametes are formed, the alleles for each gene in an

individual separate independently of one another. Thus,

gametes carry only one allele for each inherited trait.

When gametes unite during fertilization, each gamete

contributes one allele.As shown in Figure 4, each parent

can contribute only one of the alleles because of the way

gametes are produced during the process of meiosis.

Mendel’s Findings in Modern Terms
Geneticists have developed specific terms and ways of representing
an individual’s genetic makeup. For example, letters are often used
to represent alleles. Dominant alleles are indicated by writing the
first letter of the trait as a capital letter. For instance, in pea plants,
purple flower color is a dominant trait and is written as P.Recessive
alleles are also indicated by writing the first letter of the dominant
trait, but the letter is lowercase. For example, white flower color is
recessive and is written as p.
If the two alleles of a particular gene present in an individual are
the same, the individual is said to be (hoh moh ZIE
guhs)for that trait. For example, a plant with two white flower alleles
is homozygous for flower color, as shown in Figure 5. The allele for
yellow peas, Y, is dominant to the allele for green peas, y.A plant
with two yellow-pea alleles, YY,is homozygous for seed color.
If the alleles of a particular gene present in an individual are dif-
ferent, the individual is (heht uhr oh ZIE guhs)for
that trait. As shown in Figure 5, a plant with one “purple flower”
allele and one “white flower” allele is heterozygous for flower color.
A plant with one “yellow pea” allele and one “green pea” allele is
heterozygous for seed color.

heterozygous

homozygous

recessive

dominant

alleles

SECTION 2Mendel’s Theory 167

Figure 5 Recessive alleles.

Alleles can be present but not

expressed. The allele for purple

flowers, P, is dominant to the

recessive allele, p.

pp

White flowers,

homozygous

recessive

PP

Purple flowers,

homozygous

dominant

Pp

Purple flowers,

heterozygous