Genetics
Lab 12
Genes & Alleles

Gene – Basic unit of heredity. A short
sequence of DNA that codes for a
polypeptide.


Example – the gene that codes for flower
color in peas.
Allele – one of two or more forms of a
gene.

Example – purple or white for flower color
in peas.
Dominant vs. Recessive

Dominant – allele that controls the
appearance of heterozygotes.


Example – an individual with alleles for
purple and white will appear purple.
Recessive – allele that is masked
in the presence of a dominant
allele.

Example – white flowers only appear
when only white alleles are present.
Homozygous vs. Heterozygous

Homozygous – both alleles are the
same.

Hom. Recessive – shows recessive trait.


Hom. Dominant – shows dominant trait.


pp – pea flowers appear white.
PP – pea flowers appear purple.
Heterozygous – two different
alleles.

Pp – pea flowers appear purple.
Genotype vs. Phenotype

Genotype – which ALLELES are
actually present.


PP, Pp, pp
Phenotype – what the organisms
LOOKS like.

Purple, white
Punnett Square

A purple flowered plant PP is
crossed with a white flowered plant
pp. PP x pp.


PP can pass on only one of the two
alleles – P or P.
pp can pass on only one of the two
alleles – p or p.
Punnett Square
PP x pp
P
P
p
Pp
Pp
p
Pp
Pp
Genotype –
All Pp
Phenotype – all Purple
Monohybrid Cross

Monohybrid cross – examines
how one trait is passed on when
both parents are heterozygous for
the trait.
Monohybrid Cross – Punnett
Square
Pp x Pp
P
P
PP
p
Pp
p
Pp Genotype –
1PP:2Pp:1pp
pp
Phenotype – 3 purple : 1 white
Dihybrid Cross


Dihybrid cross – examines how two
traits are passed on when both parents
are heterozygous for both traits.
Flower color in peas:



P allele is dominant for purple flowers.
p allele is recessive for white flowers.
Plant height in peas:


T allele is dominant for tall plants.
t allele is recessive for short plants.
Dihybrid Cross - PpTt x PpTt





Each parent is the same and can
donate the same alleles.
PpTt – P with T – PT
PpTt – P with t – Pt
PpTt – p with T – pT
PpTt – p with t – pt
Dihybrid Cross - PpTt x PpTt
PT
Pt
pT
pt
PT
PPTT PPTt
PpTT
PpTt
Pt
PPTt
PpTt
Pptt
PpTT PpTt
ppTT
ppTt
PpTt
ppTt
pptt
pT
pt
PPtt
Pptt
Genotypes
Dihybrid Cross - PpTt x PpTt





Phenotypes
9 purple, tall (P_T_)
3 purple, short (P_tt)
3 white, tall (ppT_)
1 white, short (pptt)