BASIC GENETICS TERMS
Terms you’ve gotta know
GENE
Gene: a section of DNA
that codes for a trait
 A chromosome is a
chunk of DNA and
genes are parts of
chromosomes

ALLELE
Allele: the different options for a gene
 Example: attached or unattached earlobes are
the alleles for the gene for earlobe shape

DOMINANT/RECESSIVE
Dominant: an allele that blocks or hides a
recessive allele
 Recessive: an allele that is blocked by or hidden
by a dominant allele
 Question: Using purebred crosses, how did
Mendel figure out that genes are either dominant
or recessive?

HOMOZYGOUS/HETEROZYGOUS
Homozygous: When a person’s two alleles for a
gene are the same
 Heterozygous: When a person’s two alleles for a
gene are different
 You get one allele from your mom and one from
your dad.
 If you get the same alleles from your mom and
dad, you are homozygous for that trait.
 If your mom gave you a different allele than your
dad, you are heterozygous for that trait

GENOTYPE
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Genotype: A person’s set of alleles (gene options)
 Genotype is written in letters
 It is usually the first letter of the dominant allele
to make it easier
 Choose a letter for each of these traits. Write
both the dominant and the recessive genotype
allele letters

Attached Earlobes
 Freckles
 Dimples
 Tongue-roller

GENOTYPE
Now, using your letters from before write a
genotype for a person without freckles (Freckles
are dominant)
 Write a genotype for a person with freckles

GENOTYPE-HETEROZYGOUS AND
HOMOZYGOUS
A Heterozygous genotype means an organism
has both the dominant and the recessive allele
(e.g. Aa)
 Homozygous recessive means an organism has
2 recessive alleles (e.g. aa)
 Homozygous dominant means an organism
has 2 dominant alleles (e.g. AA)
 Write a heterozygous genotype for handedness
 Write a homozygous dominant and a homozygous
recessive genotype for handedness

PHENOTYPE
Phenotype: A person’s physical features because
of their genotype
 What you look like (your phenotype) is based on
what your genotype is (your genes)
 What will the phenotype of these genotypes be:

WW, Ww, ww (Widow’s Peak dominant over straight hair
line)
 HH, Hh, hh (Hitch hiker’s thumb is dominant over a
straight thumb)
 CC, Cc, cc (Curly hair is dominant over straight hair)

You’ve got it!! Now you are a genetics super star 