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Mendelian Genetics
Gregor Mendel – 1822-1884
Asexual Reproduction
• Bacteria can reproduce as often as every 12
minutes – and may go through 120 generations
in one day
• Thus capable of producing 6 x 1035 offspring
per day
• Bacteria often produce 1 mutation per 1000
replications of DNA
• So for fast-growing species, mutation is a good
way to respond to a changing environment
Why Sex?
John
Maynard
Smith
Sexual reproduction leads to
genetic variation via:
• Independent assortment
during meiosis
• Crossing over during
meiosis
• Random mixing of
gametes (sperm and egg)
Independent Assortment
Prophase I
of meiosis
Nonsister chromatids
held together
during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughter
cells
Recombinant chromosomes
• The random nature of fertilization adds to the
genetic variation arising from meiosis.
• Any sperm can fuse with any egg.
– A zygote produced by a mating of a woman and
man has a unique genetic identity.
– An ovum is one of approximately 8,388,608
possible chromosome combinations (223).
– The successful sperm represents one of
8,388,608 different possibilities (223).
– The resulting zygote is composed of 1 in 70
trillion (223 x 223) possible combinations of
chromosomes.
– Crossing over adds even more variation to this.
Mendelian Genetics
Gregor Mendel – 1822-1884
Two possible types of inheritance
• One possible explanation of heredity is a “blending”
hypothesis
– The idea that genetic material contributed by two
parents mixes in a manner analogous to the way
blue and yellow paints blend to make green
• An alternative to the blending model is the
“particulate” hypothesis of inheritance: the gene idea
– Parents pass on discrete heritable units, later
known as genes
Mendel’s time
Today
Mendel’s garden at Brunn (Brno) Monastery
Some genetic vocabulary
– Character: a heritable
feature, such as
flower color
– Trait: a variant of a
character, such as
purple or white
flowers
Garden Pea
Flower Structure
TECHNIQUE
1
2
Parental
generation
(P)
3
Stamens
Carpel
4
RESULTS
First filial
generation
offspring
(F1)
5
In Mendel’s Experiments:
• Mendel chose to track
– Only those characters that varied in an “either-or”
manner
• Mendel also made sure that
– He started his experiments with varieties that were
“true-breeding”
• In a typical breeding experiment
– Mendel mated two contrasting, true-breeding
varieties, a process called hybridization
Breeding Terminology
• The true-breeding parents
– Are called the P (parental) generation
• The hybrid offspring of the P generation
– Are called the F1 (filial) generation
• When F1 individuals self-pollinate
– The F2 generation is produced
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
EXPERIMENT
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)
Purple
flowers
White
flowers
All plants had purple flowers
Self- or cross-pollination
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had purple flowers
Self- or cross-pollination
F2 Generation
705 purpleflowered
plants
224 white
flowered
plants
Mendel developed a hypothesis to explain his
results that consisted of four ideas
• Alternative versions of genes (different alleles)
account for variations in inherited characters
• For each character, an organism inherits two alleles,
one from each parent
• If two alleles differ, then one, the dominant allele, is
fully expressed in the organism’s appearance. The
other, recessive allele has no effect on a hybrid
organism’s appearance
• The two alleles for each character segregate
(separate) during gamete formation
Law of Segregation
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
pp
PP
p
Gametes:
P
Law of Segregation
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
pp
PP
p
Gametes:
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
1/
2 p
2 P
Law of Segregation
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
pp
PP
p
Gametes:
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
1/
2 p
2 P
Sperm from F1 (Pp) plant
F2 Generation
P
p
PP
Pp
Pp
pp
P
Eggs from
F1 (Pp) plant
p
3
:1
3
Phenotype
Genotype
Purple
PP
(homozygous)
Purple
Pp
(heterozygous)
1
2
1
Purple
Pp
(heterozygous)
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
1
Test cross
TECHNIQUE
Dominant phenotype,
unknown genotype:
PP or Pp?
Predictions
If purple-flowered
parent is PP
Sperm
p
p
Recessive phenotype,
known genotype:
pp
or
If purple-flowered
parent is Pp
Sperm
p
p
P
P
Pp
Eggs
Pp
Eggs
P
Pp
Pp
pp
pp
p
Pp
Pp
RESULTS
or
All offspring purple
1/
2
offspring purple and
1/ offspring white
2
EXPERIMENT
YYRR
P Generation
yyrr
Gametes YR
yr
F1 Generation
Predictions
YyRr
Hypothesis of
dependent assortment
Hypothesis of
independent assortment
Sperm
or
Predicted
offspring of
F2 generation
1/
Sperm
1/
2
YR
1/
2
2
YyRr
YYRR
2
1/
4
1/
Yr
4
yR
1/
4
yr
4
YR
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
Eggs
1/
YR
yr
1/
1/
4
1/
4
Yr
Eggs
yr
YyRr
3/
yyrr
1/
4
1/
yR
4
4
Phenotypic ratio 3:1
1/
yr
4
9/
16
3/
16
3/
16
1/
16
Phenotypic ratio 9:3:3:1
RESULTS
315
108
101
32
Phenotypic ratio approximately 9:3:3:1

Rr
Segregation of
alleles into eggs
Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
2
Eggs
4
r
2
r
R
R
1/
1/
r
2
R
R
1/
1/
1/
4
r
r
R
r
1/
4
1/
4
Probability of YYRR  1/4 (probability of YY)  1/4 (RR)  1/16
Probability of YyRR  1/2 (Yy)
 1/4 (RR)  1/8
Probability of YYRR  1/4 (probability of YY)  1/4 (RR)  1/16
Probability of YyRR  1/2 (Yy)
Probability of yyrr = ?
A. 1/8
B. 1/16
C. 1/32
 1/4 (RR)  1/8
Probability of YYRR  1/4 (probability of YY)  1/4 (RR)  1/16
Probability of YyRR  1/2 (Yy)
Probability of YYrr = ?
A. ¼
B. 1/8
C. 1/16
 1/4 (RR)  1/8
Probability of YYRR  1/4 (probability of YY)  1/4 (RR)  1/16
Probability of YyRR  1/2 (Yy)
Probability of YxRr = ?
(x can be Y or y)
A. ½
B. 3/4
C. 3/8
 1/4 (RR)  1/8
D. 1/16
ppyyRr
ppYyrr
Ppyyrr
PPyyrr
ppyyrr
1/ (yy)  1/ (Rr)
(probability
of
pp)

4
2
2
1/  1/  1/
4
2
2
1/  1/  1/
2
2
2
1/  1/  1/
4
2
2
1/  1/  1/
4
2
2
1/
Chance of at least two recessive traits
 1/16
 1/16
 2/16
 1/16
 1/16
 6/16 or 3/8