Biology 101: Writing Phenotypes and Genotypes


Phenotype: An organism's physical appearance, or how it looks.


Genotype: An organism's actual (real) genetics, the genes it carries.


How to Write a Phenotype

Just objectively describe the organism.  Is it white, green, wrinkled, short?


How to Write a Genotype

Every organism has two copies of every gene, one from each parent.  So there are pairs for traits.  These two copies may NOT be exactly alike (like owning two sets of encyclopedias).  These different versions of a trait (gene) are called alleles.  Rules for writing genotypes:


1.     A gene is represented by a letter of the alphabet; usually one picked that has a relationship with the trait.  Ex. eye color.  Two alleles for same trait = blue and brown.  So letter picked is B.

2.     There are always two letters, one for each copy from each parent.

3.     Dominant alleles are capitalized.  Recessive alleles are lower case.

4.     The dominant allele, when present, is written first.


Ex. Brown eye color allele dominant = B

Blue eye color allele recessive = b


Different genotype possibilities: BB, Bb, bb.


5.     To read a genotype, always go by the dominant allele, in this case there are only two phenotypes.  NOTE!! There are fewer phenotypes than there are genotypes.

6.     To have a dominant phenotype, you only need one dominant allele.

7.     To have a recessive phenotype, you need both recessive alleles.

8.     One half of an offspring's genes come from each parent.

9.     Offspring can only inherit alleles that the parents have to donate.


Terms for Genotype Combinations

Homozygous dominant = BB

Homozygous recessive = bb

Heterozygous = Bb