Solving Crosses
Polleverywhere.com Bell-Ringer

http://www.polleverywhere.com/multiple
_choice_polls/LTExNDcxODUyMjI
Introduction to Punnett Squares

http://www.youtube.com/watch?v=Y1PC
wxUDTl8
Punnett squares - probability diagram illustrating the
possible offspring of a mating.
Ss X Ss
gametes
Steps for Solving a Genetics
Problem:
Trait – dominant = A (AA or Aa)
Trait – recessive = a (aa)
 ___________ x ___________
 Punnett Square

____ ____
____
____

Answer questions based on results from
Punnett Square
Monohybrid Crosses
Cross that involves one pair of
contrasting traits
 Solve using Punnett Square
 Sample problems:

– Rr x rr
– RR x rr
– Rr x Rr
– Rr x RR
Ratios

Genotypic Ratio: the probability that
you will see an individual with a certain
genotype
– Ex. PP : Pp : pp  What is genotypic
ratio?

Phenotypic Ratio: the probability that
you will see an individual with a certain
phenotype
– Ex. Purple Flowers : White Flowers 
What is the phenotypic ratio?
Let’s Solve Together

Short hair (L) is dominant to long hair (l)
in mice. What is the genotype and
phenotype ratio of a heterozygous
short-haired mouse crossed with a longhaired mouse?
Example 1: Monohybrid





Short hair = dominant = L (LL or Ll)
long hair = recssive = l
Ll x ll
(heterozygote parent = Ll)
L
l
Punnett Square:
l
Ll ll
l
Ll ll
Genotype ratio: ½ Ll: ½ ll
Phenotype ratio: ½ short hair: ½ long hair
Monohybrid Practice

In mice, Brown hair (B) is dominant to
black hair (b). Researchers crossed a
homozygous dominant brown-haired
mouse with a black-haired mouse.
What are the genotypic and phenotypic
ratios for this cross?
Dihybrid Crosses

Involves two pairs of contrasting traits
– Pea shape and pea color
– Coat length and coat color in rodents
– Plant height and flower color
Practice Determining Genotypes

SSBb x Ssbb
Let’s Solve Together

In guinea pigs, the allele for short hair
(H) is dominant to long hair (h), and the
allele for black hair (B) is dominant over
the allele for brown hair (b). What is the
probable offspring phenotype ratio for a
cross involving two parents that are
heterozygotes for both traits?
Determining Dihybrid Genotypes
Short hair = dominant = HH or Hh
Long Hair = recessive = hh
Black coat = dominant = BB or Bb
Brown coat = recessive = bb
 HhBb x HhBb (gametes done by the
FOIL method)

– HB, Hb, hB, hb and HB, Hb, hB, hb
Let’s Solve Together

In guinea pigs, the allele for short hair
(H) is dominant to long hair (h), and the
allele for black hair (B) is dominant over
the allele for brown hair (b). What is the
probable offspring phenotype ratio for a
cross involving two parents that are
heterozygotes for both traits?
Example 2: Punnett Square
HB
Hb
hB
hb
HB
HHBB HHBb HhBB HhBb
Hb
HHBb HHbb HhBb Hhbb
hB
HhBB HhBb hhBB hhBb
hb
HhBb Hhbb
hhBb
hhbb
Example 2: Answer the Question

What is the probable offspring
phenotype ratio for a cross involving two
parents that are heterozygotes for both
traits?
– 9/16 Black, short coats
– 3/16 Black, long coats
– 3/16 Brown, short coats
– 1/16 Brown, long coats
Practice Problem
What are the probable offspring for a
cross involving a heterozygous short,
brown haired guinea pig and a long,
brown haired guinea pig?
 Show your Punnett Square and the
percentage of individuals with the
different phenotypes.


Ssbb x ssbb
Sb
Sb
sb
Sb
sb
Ssbb
Ssbb
ssbb
Ssbb
sb
Ssbb
Ssbb
ssbb
Ssbb
sb
Ssbb
Ssbb
ssbb
Ssbb
sb
Ssbb
Ssbb
ssbb
Ssbb
Other Types of Heredity Patterns

Incomplete Dominance – blending of
traits in heterozygote.
– Pink flowers
• RR = red
• RW = pink
• WW = white

RW x RW Show Punnett Square
Other Types of Heredity Patterns

Codominance – can see both alleles at
the same time.
– Roan coats in horses
• RR= red hair
• RW= some red and some white hairs (roan)
• WW= white hair

RW x RW
Multiple Alleles

Blood Types in Humans
– Single gene, but four phenotypes
•
•
•
•
Type A  can be AA or Ao
Type B  can be BB or Bo
Type AB  only AB (codominant pattern here)
Type O  only oo (both recessive)
– All 3 blood types are dominant to O
Continuous Variation
Multiple genes are involved
 Examples

– Eye color
– Skin color
– Hair color
Sex-linked Genes
Present on the X chromosome
 More common in males
 When would a female have this
phenotype?
 Examples:

– Baldness
– Hemophilia
Some Human Genetic Disorders
Of Interest
Cystic Fibrosis
 Sickle-cell Anemia
 Tay-Sachs Disease
 Phenylketonuria (PKU)
 Hemophilia
 Huntington’s Disease
 Muscular Dystrophy
