Today’s Objectives
TSW employ the four primary rules for
solving genetics problems.
 TSW successfully solve genetics
crosses involving one and two alleles.
 TSW successfully solve monohybrid
and dihybrid crosses using the Punnett
Square.

Solving Crosses
Steps for Solving a Genetics
Problem:
Trait – dominant = A (AA or Aa)
– recessive = a (aa)
 ___________ x ___________
 Punnett Square

Trait
____ ____
____
____

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
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
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
Let’s Solve Together

In guinea pigs, the allele for short hair
(S) is dominant to long hair (s), 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: Dihybrid
Short hair = dominant = SS or Ss
Long Hair = recessive = ss
Black coat = dominant = BB or Bb
Brown coat = recessive = bb
 SsBb x SsBb (gametes done by the
FOIL method)

– SB, Sb, sB, sb and SB, Sb, sB, sb
Example 2: Punnett Square
SB
Sb
sB
sb
SB
SSBB SSBb SsBB SsBb
Sb
SSBb SSbb SsBb
Ssbb
sB
SsBB SsBb
ssBB
ssBb
sb
SsBb
ssBb
ssbb
Ssbb
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
Other Types of Heredity Patterns

Incomplete Dominance – blending of
traits in heterozygote.
– Pink flowers
• RR = red
• Rr = pink
• rr = white
Other Types of Heredity Patterns

Codominance – can see both alleles at
the same time.
– Roan coats in horses
• Some white hairs, some red hairs
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
