BASICS OF GENETICS

Every person has two copies (alleles) for each gene. How those two alleles interact determines what you see for an organism, their phenotype.

Dominant Alleles

Represented by a CAPITAL LETTER

Dominant alleles hide recessive alleles when it is present in a genotype with one

It is expressed when an organism has 1 OR 2 copies of this allele

Recessive Alleles

Represented by a lower case letter

Recessive alleles are hidden by dominant alleles when present in a genotype with one May not be expressed, but is still present

and can be passed onto offspring

It is expressed only when organisms have 2 copies of a recessive alleles

The majority of alleles interact in a dominant/recessive relationship.

Some interact in a co-dominance or incomplete dominance relationship.

Co-Dominance

“Co”—think of cooperation, they share the spotlight, you see both

2 dominant alleles—dominant trait is seen 2 recessive alleles—recessive trait is seen

1 dominant and 1 recessive allele—you see BOTH the dominant and recessive phenotype

Ex.: If R=red and r=white, Rr would be a red flower with white spots

Incomplete Dominance

2 dominant alleles—dominant trait is seen

2 recessive alleles—recessive trait is seen

1 dominant and 1 recessive allele—you see a combination of the dominant and recessive

Ex: If R=red and r=white, Rr would have a blend of the two, a pink flower

Genotype

The genetic make-up of an organism, what genes (letter) combination the organism has.

Represented by 2 letter pairs

Ex: HH, Hh, hh

Phenotype

How the genotype is expressed, the actual physical characteristics you see

Ex: long hair, brown hair

How are genotypes related to phenotypes?

GENOTYPE DETERMINES PHENOTYPE The genes you have determine the

traits you express.

Homozygous An organism that has 2 of the same

alleles for a trait. Homozygous dominant: 2 capital letters Homozygous recessive: 2 lower case

letters

Prefix “homo” means same AKA—pure, or purebred Ex: HH, aa, GG, bb

Heterozygous

An organism that has 1 dominant and 1 recessive allele for a trait

Prefix “hetero” means different

AKA—hybrid

Ex: Aa, Hh, Gg, Bb

Examples

GG=homozygous dominant Same, and 2 dominant

gg=homozygous recessive Same, and 2 recessive

Gg= heterozygous

Practice Questions

1. Aa_______________________________

2. cc_______________________________ 3. RR

______________________________ 4. Bb

______________________________

Practice Questions

1. Aa ___Heterozygous________ 2. cc_Homozygous

recessive_________ 3. RR __Homozygous

dominant________ 4. Bb ___Heterozygous_________