“Traits &
Genes”
6.4 Traits, Genes, and Alleles
KEY CONCEPT
Genes encode proteins that produce a
diverse range of traits
6.4 Traits, Genes, and Alleles
A gene is a piece of DNA that
tells a cell to make a certain
protein.
• Each gene has a locus,
a specific position
on a pair of
homologous chromosomes
6.4 Traits, Genes, and Alleles
An allele is an alternative form of a gene
-Each parent donates one allele for every
gene
• For shape, there is a
wrinkled allele and a
round allele. They
receive one allele from
each parent.
6.4 Traits, Genes, and Alleles
Genotype
• Homozygous (pure)
•
when both letters are the same
•
ex: TT, tt
• Heterozygous
(hybrid)
•
when letters are different
•
always write the big letter first
•
ex: Tt , Xx , Bb
• Dominant Alleles are always upper case and
recessive alleles are always lower case.
6.4 Traits, Genes, and Alleles
E. Mendel’s observed traits
1. Rule of Dominance: When a
dominant trait is present the
recessive trait is hidden
6.4 Traits, Genes, and Alleles
Genes influence the development of traits.
• All of an organism’s genetic
material is called the genome.
• A genotype refers to the actual
genes.
• A phenotype is the physical
expression of a trait.
6.4 Traits, Genes, and Alleles
C. Genotype:
1. The gene combination of an
organism
• It consists of 2 alleles
• For example:
– PP
– Pp
– pp
6.4 Traits, Genes, and Alleles
Phenotype :The appearance.
• The genotype determines the phenotype.
6.4 Traits, Genes, and Alleles
Probability: The likelihood of an event.
• A probability is expressed as a percentage or a
fraction.
• Genotypic Ratio: The ratio of genotypes that will
appear.
• Phenotypic Ratio: The ratio of phenotypes that will
appear.
• We can find the probability of a certain genotype or
phenotype by constructing a Punnett Square.
6.4 Traits, Genes, and Alleles
• When a dominant allele always gets its way (the
dominant phenotype is present no matter its partner)
it is called COMPLETE DOMINANCE.
– Example:
Pp = Purple Flower
PP = Purple Flower
• Some times dominant traits don’t completely get
their way……the alleles compromise. This results in
a phenotype that is a mix of both parents. This is
called INCOMPLETE DOMINANCE.
– Example
Pp = light purple flower
6.4 Traits, Genes, and Alleles
• In a few instances both alleles need to be expressed.
This is called Codominance.
– Example
A mom chicken is black and dad chicken is white, but the baby
chick comes out black and white.
6.4 Traits, Genes, and Alleles
• Most traits occur
in a range and do
not follow simple
dominantrecessive
patterns