Download Unit 2

Grade 11 University Biology – Unit 2 Genetics
Genetic Crosses
Section 5.2 Pages 208-218
with additional information from
http://www.biology.arizona.edu/mendelian_genetics/mendelian_genetics.html
http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html
Punnett Square
Every cell has two alleles for each gene, and as such, there are two possible genetic outcomes arising
from meiosis (i.e., when a haploid cell is formed).
What happens when the female gamete from one parent is crossed with a male gamete of a different
parent? Can the outcomes of the crosses be predicted? Developed by British geneticist Reginald
Punnett, a grid, called a Punnett square, is used to illustrate the possible genotypes and phenotypes of
offspring from genetic crosses
Y
Y
y
YY
Yy

y

Yy

yy

The genotypes of the female
gamete are written across the top
of the Punnett square.
The genotypes of the male
gamete are written along the left
column of the Punnett square.
Inside each square, identify the
combinations
There are four possible outcomes,
and as shown, the probability of a
particular combination of alleles must be 25 percent.
Using Figure 5.6 on Page 209, explain the outcome of Mendel’s experiment in your own words.
Codominance will be learned in detail in future chapters…but it is fun to play with the idea to predict the
genotype of an offspring. Codominance occurs when both alleles in a heterozygous individual are
expressed equally (i.e., when two traits are equal). Let’s explore blood typing. In humans, blood type is
determined by alleles that are condominant (IA and IB) and the allele i which is recessive to IA and IB. If a
mother is blood type AB (genotype IA IB) and the father is blood type B (IBi), what percentage of offspring
will be blood type O (ii)? Use a Punnett square to estimate the percent of each genotype.
Your answers –
 Percent offspring with AB blood type
 Percent offspring with A blood type
Percent offspring with B blood type
 Percent offspring with O blood type
Test Crosses (see Figure 5.7 on Page 209)
A test cross is used to determine whether an individual is heterozygous or homozygous. A test cross is a
cross between a parent of unknown genotype and a homozygous recessive parent (...in the example tt).
The unknown can be either homozygous dominant (TT) or heterozygous (Tt).
T
T
T
t

t
Using the two Punnett squares, how would a breeder
know if the unknown parent is homozygous dominant
or heterozygous?
t
t
t

Read the Sample Problems on Pages 210-211 (i.e., understand the process / technique) and
complete Practice Problems 1-10 on Page 212.
Monohybrid Cross
 When only one trait is monitored and hybrid offspring are produced. A cross of two individual that
differ by one trait
Let’s put codominance and monohybrid cross together for a challenge.
 Hemophilia is a sex-related, recessive disorder carried on the X chromosome. If a hemophilic
man (XhY) marries a non-carrier female (XX), what percentage of the daughters will be carriers of
hemophilia?
X
X
Xh
XXh
XXh
Y
XY
XY



Two sons are born (XY)
Two daughters are born (XXh)
Therefore, 100% of the daughters born will be
carriers of hemophilia because they receive the
recessive hemophilia allele from their father.
Challenger Question (use idea presented above)
 In humans, the allele for colour blindness is a recessive, sex-related trait located on the X
chromosome. It is represented as Xc. The dominant gene for normal colour vision is XC. The
phenotypic expressions for colour vision are (1) normal, (2) colour blind and (3) carrier.
 If a female is homozygous dominant for colour vision, she is X CXC. If a male has the XcY
genotype, what are the phenotypic expression for colour vision for any sons or daughters they
may have?

Do Questions – Page 212 Learning Check – Questions 7-12
Dihybrid Crosses – Inheriting Two Traits
 A cross of two individuals that differ in two traits due to two different genes (e.g., AABB x aabb)
 Dihybrid is an individual heterozygous for two pairs of alleles (AaBb). NOTE: A Dihybrid Cross is
NOT a cross between two dihybrids.
 See Page 213
 When Mendel crossed true-breeding plants with two traits (yellow, round seed (YYRR) and green,
wrinkled seed (yyrr), the F1 generation displayed the dominated form of each trait (i.e., yellow,
round seeds).


When the F1 generation self-fertilized, the recessive forms of each trait appeared in F2 (i.e.,
yellow-round, yellow-wrinkled, green-round and green-wrinkled) in a ratio of 9:3:3:1.
GW
Gw
gW
gw
GW
GGWW
GGWw
GgWW
GgWw
Gw
GGWw
GGww
GgWw
Ggww
gW
GgWW
GgWw
ggWW
ggWw
gw
GgWw
Ggww
ggWw
ggww
The ratio is constant regardless of trait. It is always nine offspring with the dominant form of both
traits, three offspring with one dominant trait and one recessive trait, three offspring with the
opposite dominant/recessive traits, and one offspring with both recessive traits.
Example
 In this example, each parent has the genotype SsYy. The dominant S represents a sphere shape
while recessive s is a square shape. The dominant Y is yellow while the recessive y is green.
Thus, each parent produces four different allele combinations in the gamete.
SY
Sy
sY
sy

The Punnett square would be represented as such…
SY
Sy
sY
sy
SY
Sy
sY
sy

Complete the Punnet square and determine the number of each phenotype (e.g., sphere and
yellow, square and green)
SY
Sy
sY
sy
SY
Sy
sY
sy

Number of
1. Sphere and yellow
2. Sphere and green
3. Square and yellow
4. Square and green

“I get it “Questions
The gametes of a plant of genotype AaBb should have the genotypes...
a. Aa and Bb
b. AB and ab
c. Aa, Bb, AB, and ab
d. AB, Ab, aB and ab
e. AA, aa, BB and bb
Which of the following genotypes would you NOT expect to find among the offspring when there
is a cross between CcZz x cczz?
a. cczz
b. CcZz
c. Cczz
d. ccZz
e. CcZZ
What is the expected phenotypic ratio of the progeny of a SsYy x ssyy test cross?
a. 9:3:3:1
b. 3:1
c. 1:1:1:1
d. 1:2:1:2
e. 3:1:1:3
For more practice with monohybrid and dihybrid crosses, go to the Biology Project – Mendelian Genetics
at http://www.biology.arizona.edu/mendelian_genetics/mendelian_genetics.html and follow the links.
Law of Independent Assortment
 During gamete formation, the two alleles for one gene segregate or assort independently of the
allele for other genes
 To watch the process, go to Independent Assortment at
http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html
 See Figure 5.9 on Page 214 and answer Questions 13-16 on Page 215
 Read the Sample Problem on Page 215 and complete Practice Problems 11-20
Chromosome Theory of Inheritance
 Traits determined by genes are inherited through the movement of chromosomes during meiosis
 See Figure 5.10 on Page 217