Identifying Dominant
or Recessive Traits
You can determine some of the genotypes and all of the pheno-
types for human traits that are inherited as simple dominant or
recessive traits.
Materials
pencil, paper
Procedure
- Makea table like the one at
right. For each trait, circle the
phenotype that best matches
your own phenotype.
- Determinehow many
students in your class share
your phenotype by recording
your results in a table on the
chalkboard.
Analysis
- Summarizethe class
results for each trait.
- Calculatethe class
dominant:recessive ratio for
each trait.
3.Critical Thinking
Applying InformationFor
which phenotypes in the table
can you determine a person’s
genotype without ever having
seen his or her parents?
Explain.
Dominant trait Recessive trait
Cleft chin No cleft
Dimples No dimples
Hair above knuckles Hairless fingers
Freckles No freckles
168 CHAPTER 8Mendel and Heredity
In heterozygous individuals, only the dominant
allele is expressed; the recessive allele is present
but unexpressed. An example of a human trait
that is expressed in a heterozygous individual is
freckles. Freckles F, is a dominant allele. The
recessive allele is f, no freckles. The recessive
allele may be present but not expressed. As shown
in Figure 6, people who are heterozygous for
freckles (Ff) will have freckles even though they
also have the allele for no freckles, f.
The set of alleles that an individual has is
called its (JEE noh tiep).The physi-
cal appearance of a trait is called a
(FEE noh tiep). Phenotype is determined by
which alleles are present. For example, if Ppis
the genotype of a pea plant, its phenotype is
purple flowers. If pp is the genotype of a pea
plant, its phenotype is white flowers. When
considering seed color, if Yyis the genotype of
a pea plant, its phenotype is yellow seeds. If yy
is the genotype of a pea plant, its phenotype is
green seeds. Note that by convention, the dom-
inant form of the trait is written first, followed
by the lowercase letter for the recessive form of
the trait.
phenotype
genotype
Figure 6 Dominent alleles.In het-
erozygous individuals, freckles, F,is the
dominant allele. Similarly, the allele for a
cleft chin is dominant to the allele for a
chin without a cleft.
6A 6D
