Genes located on chromosomes encode proteins that produce traits. A gene has alleles that are alternative forms influencing a trait. Genes determine the genotype, while the phenotype is the observable expression of a trait. Dominant alleles are usually uppercase and recessive lowercase. The document discusses Mendel's laws of inheritance including dominance, where dominant traits hide recessive ones, and provides examples of complete dominance where the dominant phenotype always appears and incomplete dominance where traits are mixed. It also covers codominance where both alleles are expressed.

1. “Traits &
Genes”

2. 6.4 Traits, Genes, and Alleles
KEY CONCEPT
Genes encode proteins that produce a
diverse range of traits

3. 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

4. 6.4 Traits, Genes, and Alleles
• For shape, there is a
wrinkled allele and a
round allele. They
receive one allele from
each parent.
An allele is an alternative form of a gene
-Each parent donates one allele for every
gene

5. 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. 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

7. 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.

8. 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

9. 6.4 Traits, Genes, and Alleles
Phenotype :The appearance.
• The genotype determines the phenotype.

10. 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.

11. 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

12. 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.

13. 6.4 Traits, Genes, and Alleles
• Most traits occur
in a range and do
not follow simple
dominant-
recessive
patterns