SOLVING HYBRID CROSS PROBLEMS
 Punnett
Squares – used to predict the
percentages of genotypes and phenotypes
of crosses
 Ex. Widows Peak X Straight line
Widows Peak is dominant = W
Straight line is recessive = w
Phenotype v. Genotype
Phenotype = physical appearance
example: widows peak or straight line
Genotype = genetic makeup
example: Ww, WW, or ww
Genotypes can be homozygous (purebred)or
heterozygous (hybrid)
Homozygous Widows Peak = WW
Homozygous Straight line = ww
Heterozygous Widows Peak = Ww
What will the genotypes and phenotypes be
of a cross of heterozygous parents?
Ww X Ww (P1 = 1st Parents)
1st Law – ½
chance for
each
W
w
F1 = 1st Offspring
Phenotypes
W
WW
Ww
Straight line = 25%
2nd Law –
½X½=¼
w
Ww
3rd Law
½X½+½X½=½
Widows peak =75%
Genotypes
ww
WW =25%
ww=25%
Ww=50%
PUNNETT SQUARE PRACTICE

1.
2.
3.
Use a Punnett Square to determine the phenotype
and genotype ratios of a cross between:
Bb X bb
TT X tt
Tt X Tt
R1.ESULTS OF PUNNETT SQUARES
b
B
b
b
Bb
Bb
bb
bb
2.
T
T
Tt
Tt
Tt
Tt
t
t
PUNNETT SQUARE RESULTS
3.
T
t
TT
Tt
Tt
tt
T
t
DIHYBRID CROSSES – TESTS HOW
TWO DIFFERENT TRAITS ARE
INHERITED
Led to Principle of Independent
Assortment - genes for different traits
usually segregate into gametes
independently of one another.
TWO-FACTOR CROSS

Cross two plants that are heterozygous for both pod
color (G) and height (T)
TtGg X TtGg
Meiosis produces the following gametes:
TG, Tg, tG, tg
T = tall
t = short
G = green
g = yellow
TG
tG
Tg
tg
TG
TTGG
TtGG
TTGg
TtGg
tG
TtGG
ttGG
TtGg
ttGg
Tg
TTGg
TtGg
TTgg
Ttgg
tg
TtGg
ttGg
Ttgg
ttgg
Red =
tall
Black=
short
BEYOND DOMINANT AND
RECESSIVE ALLELES
 Incomplete
Dominance – Cases in which
one allele is not completely dominant over
another. The heterozygous phenotype is
somewhere in between the two
homozygous phenotypes.
Ex. Four o’clock plants:
Homozygous are white or red
Heterozygous are pink
BEYOND DOMINANT AND
RECESSIVE ALLELES
 Codominance
– Both alleles contribute to the
phenotype of the organism.
Ex. Hair color in cattle:
Homozygous are red or white
Heterozygous are roan (pinkish brown)
– a mixture of red and white hairs
CROSS 2 ROAN CATTLE
R
RR (red)
R
W
Rr (Roan)
W
Rr (Roan)
Rr (White)
BEYOND DOMINANT AND
RECESSIVE ALLELES
 Multiple
Alleles – Many genes have more
than two alleles. An individual organism
has only two alleles, but more than two
possible alleles exist in the population.
Ex. 1. Coat color in rabbits:
Four alleles exist for one gene. This
results in four possible coat colors.
Ex. 2. Genes for blood type in humans
ABO BLOOD GROUP
Blood type is an example of a trait with multiple
alleles: A, B, O
 A and B are co-dominant, O is recessive
 Type O is the Universal Donor – all blood types
can receive type O blood.
 Type AB is the Universal Recipient – can receive
blood from all other blood types.

GENOTYPES & PHENOTYPES
Genotype
Phenotype
AA or AO
Type A
BB or BO
Type B
AB
Type AB
OO
Type O
PRACTICE PUNNETT SQUARES FOR
BLOOD TYPE INHERITANCE
1.
2.
What is the probable genotype ratio among
children born to an AO mother and an AB
father?
One parent has type A blood and the other has
type B. What are their genotypes if they
produced children who were:
1.
2.
3.
All AB
½ AB and ½ A
¼ AB, ¼ A, ¼ B and ¼ O
BEYOND DOMINANT AND
RECESSIVE ALLELES

Polygenic Traits – Traits controlled by two or
more genes.
Ex. 1. Three genes control eye color in fruit flies.
Different combinations of alleles for these
genes produce different eye colors.
Ex. 2. Skin color in humans is probably
controlled by more than four genes.
Ex. 3. Eye color in humans
GENES AND THE ENVIRONMENT

Environmental conditions can affect gene
expression and influence genetically determined
traits

Ex. Western White Butterfly – those that hatch
earlier have more pigment in wings – increases body
temperature so they can fly
GENE LINKAGE
Alleles of different genes on the same chromosome
tend to be inherited together – linkage groups
 This was missed by Mendel – he happened to study
traits located on different chromosomes.
 Led to clarification that chromosomes assort
independently
 Alleles of genes far apart on chromosomes can assort
independently because of crossovers.
 Frequency of crossovers can be used to determine
location of genes on chromosome

The more frequent recombination is between two genes,
the more likely those genes are far apart on the chromosome.