 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_________