Genetics and HeredityKEY VOCABULARY

Gene

Unit or section of a chromosome that determines a trait

Passed on from parent to offspring

Contains the DNA code for traits

Allele

One of two (or more) different forms of the same trait

Specific version of a gene

Gene is for eye color

Allele is for blue or brown eyes

Found at the same place on a chromosome

Genotype and Phenotype

You get one version of a gene from each parent

One allele from mother, One allele from father

The interaction between these two versions gives you

your unique characteristics or traits

Genotype – Alleles of a gene an organism contains

“What you get.”

Allele from mother and allele from father

Phentotype – Form of a trait an organism displays

“What you see.”

Different Genotypes

Dominant vs. Recessive

Dominant alleles – an allele that masks/overpowers the effect of

the other allele

Written as an UPPER CASE letter (Example - “T”)

Recessive alleles – an allele that is masked or overpowered by a

dominant allele

Written as a LOWER CASE letter (Example - “t”)

Different Genotypes

Homozygous – Genotype where both alleles are the same type

“Homo” means “same”

Homozygous Dominant – “TT”; Homozygous Recessive – “tt”

Heterozygous – Genotype where alleles are different

“Hetero” means “different”

One dominant allele, one recessive allele – “Tt”

How alleles interact

Dominant allele matched with dominant allele – Genotype “TT”

Dominant trait will be displayed/visible

Dominant Phenotype

Dominant allele matched with recessive allele – Genotype “Tt”

Dominant trait will be displayed/visible **most of the time, some exceptions**

Dominant Phenotype

Recessive allele matched with recessive allele – Genotype “tt”

Recessive trait will be displayed/visible

Recessive Phenotype

Question?

There are always two alleles interacting for every trait.

Why is that?

Question?

What is the only way a recessive trait can be visibly displayed by an

organism (Recessive Phenotype)?

Different types of Dominance

Sometimes when a trait has a Heterozygous Genotype (“Tt”) the

phenotype (what you see) isn’t completely dominant in presentation

Three types of Dominance

Complete Dominance – dominant allele is fully displayed

No presence of the recessive allele

Example – Red Flower (dominant); White Flower (recessive)

Incomplete Dominance – dominant and recessive allele are displayed as a MIX

Example – Pink Flower (MIX of Red and White)

Codominance – dominant and recessive allele are BOTH displayed

Example – Red and White Flower (BOTH Red and White displayed)

Determining the Probability of a Trait

Geneticists use Punnett Squares to determine the possible

outcomes of a specific cross of two parent organisms

For example:

One parent is Homozygous Dominant for a trait: “TT”

Other parent is Heterozygous for a trait: “Tt”

TT TT

Tt Tt

One parent alleles go here

One parent alleles go here

T

t

T

T

Sections inside table show possible

offspring genotypes:

• TT = 50% probability

• Tt = 50% probability