Page 140 - Science Course 3 (Book 1)

Mo4-L2a: What Controls Traits?

Red
RR

R R

White r
rr

Rr Rr

r
Geneticists call how a trait appears, or is expressed,
Rr Rr the trait’s phenotype.

The two alleles that control the phenotype of a trait
are called the trait’s genotype.
A gene is a section on a chromosome that has
genetic information for one trait.
Genotype AA Aa aa
The different forms of a gene are called alleles.
Each chromosome has one allele for every gene on it.
Phenotype A:
The two chromosomes in an offspring cell may have dominant/a
the same or different alleles. recessive

Allele for
red flowers
Phenotype
A and a
co-dominant
Flower Homologous
color gene pair of
chromosome
When the two alleles of a gene are the same, the
genotype is homozygous.
Allele for
white flowers
If the two alleles of a gene are different, the
genotype is heterozygous.

Homozygous Heterozygous

Axial flower Bumpy Short stem RR Genotype rr
Allele pod Allele Allele
Phenotype

Rr Rr
X

Rr Rr
Axial flower Smooth Tall stem
Allele pod Allele Allele

RR Rr rR rr

Scientists use uppercase and lowercase letters as
symbols to represent the alleles in a genotype.
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