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_________