Biology 212 General Genetics
Fall 2004
Lecture 5: "Mendelian Genetics II"
Reading: Chap. 3 pp. 43-49
Lecture outline:
1. Dihybrid cross
2. Mendel’s law of independent assortment
3. Chromosomal basis of heredity
Lecture:
1. Dihybrid cross
Cross between individuals varying in two characters
Illustrate using two Mendelian characters
Trait 1: Seed color
Yellow vs. green
Trait 2: Seed shape
Round vs.wrinkled
Fig. 3-5
Cross: P1 true-breeding yellow, round x
true-breeding green, wrinkled
F1 yellow, round x F1 yellow, round
F2
Yellow round
Yellow wrinkled
Green round
Green wrinkled
#
315
101
108
32
Yellow is dominant to green (seed color)
Symbols for seed color gene:
Let G=Yellow
g=green
1
fraction
9/16
3/16
3/16
1/16
ratio
9
3
3
1
Round is dominant to wrinkled (seed shape)
Let W=round
w=wrinkled
P1
GGWW
x
Yellow, round
ggww
Green, wrinkled
F1
GgWw
x
Yellow, round
GgWw
Yellow, round
F2 generation:
GW
GGWW
GGWw
GgWW
GgWw
GW
Gw
gW
gw
Gw
GGWw
GGww
GgWw
Ggww
gW
GgWW
GgWw
ggWW
ggWW
gw
GgWw
Ggww
ggWw
ggww
F2 progeny:
Genotypes
1/16
2/16
2/16
4/16
GGWW
GgWW
GGWw
GgWw
Phenotypes
9/16 round yellow
1/16 GGww
2/16 Ggww
3/16 wrinkled yellow
1/16 ggWW
2/16 ggWw
3/16 round green
1/16 ggww
1/16 wrinkled green
2. Mendel’s Law of Independent Assortment: During gamete formation,
segregating pairs of factors (genes) assort independently of each other.
Behavior of G/g gene is independent of W/w gene.
Predict combination of alleles in gametes based on principles of probability.
Predict offspring genotypes using Punnett square of “forked line” method.
Predict phenotype ratios based on probabilities. Can use the outcomes expected from two
monohybrid crosses to predict outcome of dihybrid cross.
2
Monohybrid cross I
Yellow
x
yellow
Gg
Gg
Monohybrid cross II
Round x round
Ww
Ww
Predicts
¾ G—
¼ gg
Predicts
¾ W—
¼ ww
Predict phenotypes based on product of probabilities
¾ G—
¾ G—
¼ gg
¼ gg
x
x
x
x
¾ W— = 9/16 G—W—
¼ ww
= 3/16 G—ww
¾ W— = 3/16 ggW—
¼ ww
= 3/16 ggww
Yellow round
Yellow wrinkled
Green round
Green wrinkled
3. Chromosomal basis of heredity
Rediscovery of Mendel’s work early 1900s
Hugo de Vries
Karl Correns
Erich Tschermak
Cytology (study of cells) was united with Mendelian genetics by Walter Sutton and
Theodor Bovari in 1902.
Fig. 3-10
1) Genes exist in pairs on homologous chromosomes.
2) When homologs separate during meiosis, each pair of factors separate (Mendel’s
law of segregation).
3) Nonhomologous chromosomes with different genes assort independently.
All possible genetic combinations are formed with equal probability
3