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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)
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
Biology Bell Ringer
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.
In some cattle, white is codominant with red
hair. Roan coats--Some white hairs, some
red hair.
In some chickens, black and white feathers
are codominant, so a chicken with the
heterozygous genotype with have speckled
feathers.
Multiple Alleles
Having
more than 2 alleles. Figure 1112 on pg. 273.
Blood Type
Polygenic Traits—alleles produced by the
interaction of several genes. Traits
controlled by 2 or more genes.
Skin Color in humans….
Some Human Genetic Disorders
Of Interest pg. 345 chart
Cystic Fibrosis
Sickle-cell Anemia
Tay-Sachs Disease
Phenylketonuria (PKU)
Hemophilia
Huntington’s Disease
Achondroplasia
Albinism
Bell Ringer
In a particular dog, short hair is dominant
over long, and dark hair is dominant over
light. One dog is heterozygous for hair length
and shows the recessive trait for hair color.
The other dog is homozygous for hair length
and heterozygous for hair color.
Identify offspring and their traits using
numbers and percentages. Show genotypes
and phenotypes using a Punnett Square.