Transcript PowerPoint Presentation - WELCOME TO BIOLOGY 2002

Figure 10.7
R = Dominant allele for seed shape (round)
r = Recessive allele for seed shape (wrinkled)
= Round-seeded phenotype
= Wrinkled-seeded phenotype
Mother
Rr
Female gametes
Father
Rr
Male gametes
R
r
R
RR
Rr
Rr
rr
r
Resulting genotypes: 1/4 RR : 1/2 Rr : 1/4 rr
Resulting phenotypes:
3/4
: 1/4
Crosses Between Plants that Differ in One Trait Identifies
A Fundamental Pattern in Genetics

Mendel develops Principle of Segregation
• The two alleles of each gene must segregate into different gamete cells
during the formation of eggs and sperm in the parent.
Figure 10.11a
Principle of segregation
Dominant allele
for seed shape
(round)
R
r
Recessive allele
for seed shape
(wrinkled)
Chromosomes replicate
R
R r
r
Meiosis I
R
R
r
r
Gametes
Meiosis II
R
R
r
r
Figure 10.8a
Cross peas that differ in two traits
R = Dominant allele for seed shape (round)
r = Recessive allele for seed shape (wrinkled)
Y = Dominant allele for seed color (yellow)
y = Recessive allele for seed color (green)
Father
Mother
Parental
generation
Meiosis
Gametes
Fertilization
F1 generation
Figure 10.8b
Self-fertilize F1 peas and count F2 offspring
F2 generation
phenotype
Number
Fraction of
progeny
315
9/16
101
3/16
108
3/16
32 = 556
1/16 = 1
Figure 10.8c
How does the 9 : 3 : 3 : 1 ratio observed for two traits
relate to the 3 : 1 ratio observed for one trait?
Round seeds : Wrinkled seeds
315 + 108 : 101 + 32
423 : 133
3:1
Yellow seeds : Green seeds
315 + 101 : 108 + 32
416 : 140
3:1
Figure 10.9
Parental
generation
RRYY
R = Dominant allele for seed shape (round)
r = Recessive allele for seed shape (wrinkled)
Y = Dominant allele for seed color (yellow)
y = Recessive allele for seed color (green)
rryy
RrYy
F1
ALL
generation
Female gametes
RrYy
1/4 RY
1/4
RrYy
Male gametes
1/4
1/4 Ry
1/4 rY
1/4 ry
RY
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
Ry
1/4 rY
1/4
Resulting genotypes:
Resulting phenotypes:
ry
9/16R-Y- :
3/16R-yy : 3/16rrY- : 1/16rryy
9/16
3/16
3/16
1/16
Crosses Between Plants that Differ in Two Traits
Identifies A Second Fundamental Pattern in Genetics

Mendel develops Principle of Independent Assortment
• Each pair of alleles segregates into gametes separately
from every other pair of alleles.
• A testcross to a homozygous recessive confirms
independent assortment of alleles. (Fig. 10.10)
Figure 10.10
Homozygous
recessive
parent
F1 parent
rryy
RrYy
1/4
All
RY
1/4
Ry
1/4
rY
1/4
ry
ry
1/4 RrYy
1/4 Rryy
1/4 rrYy
1/4 rryy
Chromosome Theory of Inheritance

Mendel's rules of inheritance can be explained
by independent assortment of chromosomes
during meiosis. (Fig. 10.11b)
Figure 10.11b
Principle of independent assortment
R
R
r
R
y
Y
R
r
Alleles for seed shape
Alleles for seed color
Y
r
y
Meiosis I
Meiosis I
y
Y
Gametes
r
y
Meiosis II
Y
Meiosis II
R
Y
r
y
1/4 RY
1/4 ry
R
y
1/4 Ry
r
Y
1/4 rY
Figure 10.12
Thomas Hunt Morgan
Thomas Hunt Morgan discovered a male fly with white
eyes in one of his Drosophila cultures

Mated true-breeding red-eyed flies with the white-eyed mutant male

Bred P flies to produce an F1 generation

F1 generation all had red eyes
• Concluded gene for white eyes is recessive

Bred F1 flies to produce an F2 generation
• Got ratio of 3:1 red-eyed flies to white-eyed flies
• However, all of the females had red eyes and half of the males had
white eyes

To explore this unexpected result, Morgan next bred some of
the F1 females (red-eyed) with white-eyed males
• Some of the female offspring had white eyes
• Now Morgan could do the reciprocal cross

Cross white-eyed females with wild type (red-eyed) males
• All of the female offspring had red eyes
• All of the male offspring had white eyes!!!
• Clearly, the reciprocal cross did not produce the same
results as the male white-eyed x female red-eyed cross