Gregor Mendel – 1822-1884
Mendelian Genetics
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
JohnMaynardSmith
Why Sex?
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 Iof meiosis
Nonsister chromatidsheld togetherduring synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughtercells
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.
Gregor Mendel – 1822-1884
Mendelian Genetics
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 garden at Brunn (Brno) Monastery
Mendel’s time Today
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
Parentalgeneration(P) Stamens
Carpel
First filialgenerationoffspring(F1)
TECHNIQUE
RESULTS
3
2
1
4
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
P Generation
EXPERIMENT
(true-breedingparents) Purple
flowersWhite
flowers
P Generation
EXPERIMENT
(true-breedingparents)
F1 Generation(hybrids)
Purpleflowers
Whiteflowers
All plants had purple flowers
Self- or cross-pollination
P Generation
EXPERIMENT
(true-breedingparents)
F1 Generation(hybrids)
F2 Generation
Purpleflowers
Whiteflowers
All plants had purple flowers
Self- or cross-pollination
705 purple-flowered
plants
224 whiteflowered
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 SegregationP Generation
Appearance:Genetic makeup:
Gametes:
Purple flowers White flowersPP pp
P p
Law of SegregationP Generation
F1 Generation
Appearance:Genetic makeup:
Gametes:
Appearance:Genetic makeup:
Gametes:
Purple flowers White flowers
Purple flowersPp
PP pp
P
P
p
p1/21/2
Law of SegregationP Generation
F1 Generation
F2 Generation
Appearance:Genetic makeup:
Gametes:
Appearance:Genetic makeup:
Gametes:
Purple flowers White flowers
Purple flowers
Sperm from F1 (Pp) plant
Pp
PP pp
P
P
P
P
p
p
p
p
Eggs from F1 (Pp) plant
PP
ppPp
Pp
1/21/2
3 : 1
Phenotype
Purple
Purple
Purple
White
3
1
1
1
2
Ratio 3:1 Ratio 1:2:1
Genotype
PP(homozygous)
Pp(heterozygous)
Pp(heterozygous)
pp(homozygous)
Test cross
Dominant phenotype,unknown genotype:
PP or Pp?
Recessive phenotype,known genotype:
pp
PredictionsIf purple-floweredparent is PP
If purple-floweredparent is Pp
or
Sperm Sperm
Eggs Eggs
or
All offspring purple 1/2 offspring purple and 1/2 offspring white
Pp Pp
Pp Pp
Pp Pp
pp pp
p p p p
P
P
P
p
TECHNIQUE
RESULTS
P Generation
F1 Generation
Predictions
Gametes
EXPERIMENT
RESULTS
YYRR yyrr
yrYR
YyRr
Hypothesis ofdependent assortment
Hypothesis ofindependent assortment
Predictedoffspring ofF2 generation
Sperm
Spermor
EggsEggs
Phenotypic ratio 3:1
Phenotypic ratio 9:3:3:1
Phenotypic ratio approximately 9:3:3:1315 108 101 32
1/21/2
1/2
1/2
1/41/4
1/41/4
1/4
1/4
1/4
1/4
9/163/16
3/161/16
YR
YR
YR
YRyr
yr
yr
yr
1/43/4
Yr
Yr
yR
yR
YYRR YyRr
YyRr yyrr
YYRR YYRr YyRR YyRr
YYRr YYrr YyRr Yyrr
YyRR YyRr yyRR yyRr
YyRr Yyrr yyRr yyrr
Segregation ofalleles into eggs
Segregation ofalleles into sperm
Sperm
Eggs
1/2
1/2
1/21/2
1/41/4
1/41/4
Rr Rr
R
R
RR
R
R
r
r
r
r r
r
Probability of YYRR
Probability of YyRR
1/4 (probability of YY)
1/2 (Yy)
1/4 (RR)
1/4 (RR)
1/16
1/8
Probability of YYRR
Probability of YyRR
1/4 (probability of YY)
1/2 (Yy)
1/4 (RR)
1/4 (RR)
1/16
1/8
Probability of yyrr = ?A. 1/8 B. 1/16 C. 1/32
Probability of YYRR
Probability of YyRR
1/4 (probability of YY)
1/2 (Yy)
1/4 (RR)
1/4 (RR)
1/16
1/8
Probability of YYrr = ?A. ¼ B. 1/8 C. 1/16
Probability of YYRR
Probability of YyRR
1/4 (probability of YY)
1/2 (Yy)
1/4 (RR)
1/4 (RR)
1/16
1/8
Probability of YxRr = ?(x can be Y or y)A. ½ B. 3/4 C. 3/8 D. 1/16
Chance of at least two recessive traits
ppyyRr
ppYyrr
Ppyyrr
PPyyrrppyyrr
1/4 (probability of pp) 1/2 (yy) 1/2 (Rr) 1/4 1/2 1/2 1/2 1/2 1/2 1/4 1/2 1/2 1/4 1/2 1/2
1/16
1/16 2/16
1/16
1/16
6/16 or 3/8