 Genetics is the study of how traits
are inherited in offspring.
 Gene: Segment of
chromosome (DNA)
that codes for a trait
 Allele: Different form
of the same gene.
 Example: Skin Color
(Color, Albino)
 Humans normally have
two alleles for each
gene.
 Dominant: Trait that
gets expressed from
dominant allele.
 Phenotype: Observable physical or
behavioral trait
 Genotype: Genes (Alleles) that code
for a trait
 Homozygous: having the same
forms (alleles) for a gene on both
chromosomes.
 Also said to be purebred.
 Homozygous Dominant = AA
 Homozygous Recessive = aa
 Heterozygous: having different
forms (alleles) for a gene on both
chromosomes.
 Also said to be hybrid.
Heterozygous = Aa, or aA
 Monohybrid Crosses
 Gene: Albinism
 Dominant Allele: A = normal skin
color
 Recessive Allele: a = albino
Heterozygous = Aa, or aA
 Cross a heterozygous individual with
a heterozygous individual: Aa X Aa
A
a
A
AA
Aa
a
Aa
aa
Heterozygous = Aa, or aA
 Cross a homozygous dominant
individual with a homozygous
recessive individual: AA X aa
a
a
A
Aa
Aa
A
Aa
Aa
Heterozygous = Aa, or aA
 Test Cross: Cross a homozygous
recessive individual with an
unknown (homozygous dominant or
heterozygous): aa X A?
a
a
A
Aa
Aa
?
A?
A?
Heterozygous = Aa, or aA
 The outcome determines the
offspring. If half of the offspring are
albino, then the unknown is
heterozygous (Aa).
a
a
A
Aa
Aa
a
aa
aa
Heterozygous = Aa, or aA
 If all of the offspring have normal
skin color, then the unknown is
homozygous dominant.
a
a
A
Aa
Aa
A
Aa
Aa