Mendelian Genetics and Plant Sexual
Reproduction
R. N. Trigiano, Professor EPP
15 January 2014
Alternation of Generations
Mitosis and Meiosis
Mendelian Genetics
Sexual Reproduction in Plants
DEFINITIONS:
Diploid: Having two sets of chromomosomes or 2N –
the sporophyte phase of plants. 2N = 2X = 22 (Flowering
dogwood)
Haploid: Having one set of chromosomes or N – the
gametophyte phase of plants. N = X=11 (Flowering
dogwood)
Polyploid: Having more than two sets of chromosomes
– 2N = 6X = 54 sporophyte; gametophyte N = 3X= 27
(Chrysanthemum)
How is Alternation of Generations Accomplished?
Mitosis (1 cell
2 cells) and
Meiosis (1 cell
4 cells)
and Sexual Reproduction where 1 +1 = 2 = 1!
Gametophyte (N) mitosis
Gametes (N) fuse
(sexual reproduction) to form zygote (2N) mitosis
Mature Sporophyte (2N) meiosis
Spores (n)
ALTERNATION OF GENERATIONS
Lower Plants
Higher Plants
SHADED AREA REPRESENT RELATIVE TIME IN DIPLOID OR
SPOROPHYTIC PHASE; OPEN = RELATIVE TIME IN HAPLOID
OR GAMETOPHYTIC PHASE.
1 MOSS
FERN
A
www.calflora.net
silviahartmann.com
D
E
A
B
D
C
C
B
Mitosis and Meiosis
http://www.vcbio.science.ru.nl/images/cellcycle/mmitosis_onion_zoom.jpg
http://www.phschool.com/science/biology_place/biocoach/index.html
Figure 6.1
Structure of DNA
Nitrogenous bases
Adenine
Thymine
Guanine
Cytosine
Phosphate (PO4)
Deoxyribose sugar
Figure 2.2
A. Chromosome Physical States
Nucleus
Nucleus
Relaxed Chromosome
Condensed Chromosome
B. Chromosome Conformations
Chromatid
Centromere
C. Homologous Chromosomes
Sister Chromatids
DNA
Synthesis
Locus 2
Centromere
Locus 1
Chromosome 1
Chromosome 1
Chromosome 1
Chromosome 2
http://nobelprize.org/nobel_prizes/medicine/laureates/2001/cellcycle_eng.jpg
STAGES OF MITOSIS
PROPHASE: Shortening and thickening of chromosomes (actually sister chromatids:
Movement to the metaphase plate; nucleous disappears; nuclear
membrane disappears.
METAPHASE: Chromosomes lie on the “ cell plate”.
ANAPHASE: Sister chromatids separate and are pulled toward the poles.
TELOPHASE: Last phase of mitosis; chromosomes become reorganized into nuclei,
nucleolus reappears and nuclear membrane is reformed.
CYTOKINESIS: Literally division of the cytoplasm (including organelles such mitochondria, chloroplasts, etc.
Essentially, mitosis is “asexual division (formation of somatic cells
and builds the plants). Theoretically, all cells resulting from
mitosis are “clones” – not quite true, but close enough for our
purposes.
http://tainano.com/Molecular%20Biology%20Glossary.files/image011.gif
1 = Interphase, 2-5 = Prophase; 6 = Early Metaphase; 7 = Late
Metaphase; 8 = Anaphase; 9 = Telophase; 10 = Completion of
Cytokinesis
http://www.vcbio.science.ru.nl/images/cellcycle/mmitosis_onion_zoom.jpg
OH NO! MORE DEFINITIONS
Meiosis:
Two successive divisions (reduction followed by an equal) that
reduces the ploidy level from 2n to n or diploid to haploid.
Homologous Chromosomes: Chromosomes that associate in pairs in
the first division of meiosis: each member of the pair was derived from a
different parent.
Crossing Over: The exchange of corresponding segments of genetic
material between chromatids of homologous chromosomes. See also
Synapse and Chiasma
Recombination: The process by which offspring can gain combination
of genes different from the combinations in either of their parents. Leads
to genetic diversity.
Figure 2.9. Crossing- over
B.
A.
Synapse –
pairing
of homologous
Chromosomes
C.
Chiasma - Exchange between
homologous chromosomes. Actually sister
chromatids during Prophase I.
Results in recombination and increased diversity
Stages of Meiosis
Prophase: Same as in mitosis except that crossing over occurs here.
Metaphase I: Homologous chromosomes (each chromosome consists of 2
sister chromatids) line up “opposite” each other on the plate.
Anaphase I: One of each of the homologous pair of chromosomes are
“drawn” to the poles. This essentially reduces the number of chromosomes
that are in each cell by half. Diploid
Haploid.
Telophase I: Haploid cells are established; chromosomes “disappear”.
Prophase II: Chromosome condense (sister chromatids are not identical to
one another because of crossing over).
Metaphase II: Chromosomes (actually sister chromatids) align on the plate.
Anaphase II: One sister chromatid (now chromosome) are drawn to each
pole.
Teleophase II and Cytokinesis: Essentially the same as in mitosis.
Meiosis is essentially a “reduction” division followed by an “equal” division.
“Tetrad” (4) of cells or spores are formed.
Figure 2.8.
Mitosis
Metaphase
Meiosis I
Metaphase I
Anaphase I
Metaphase Plate
Anaphase II
Cytokinesis
Cytokinesis
N
N
Metaphase Plate
Metaphase Plate
Anaphase
Metaphase II
2N
2N
Meiosis II
N
Cytokinesis
N
Figure 2. 3.
Gregor Mendel, working with peas, made two innovations to the science of genetics:
1) developed pure lines
2) counted his results and kept statistical notes
SOME MORE DEFINITIONS
Pure Line - a population that breeds true for a particular trait
[this was an important innovation because any non-pure (segregating)
generation would and did confuse the results of genetic experiments]
Allele - one alternative form of a given allelic pair; tall and dwarf are the alleles for the
height of a pea plant; more than two alleles can exist for any specific gene, but only two
of them will be found within any individual
Dominant - the allele that expresses itself at the expense of an alternate allele;
the phenotype that is expressed in the F1 generation from the cross of two pure lines.
Recessive - an allele whose expression is suppressed in the presence of a dominant
allele; the phenotype that disappears in the F1 generation from the cross of two pure
lines and reappears in the F2 generation
Co-dominance: a relationship among alleles where both alleles contribute to the
phenotype of the heterozygote.
Allelic pair - the combination of two alleles that comprise the gene pair
http://www.ndsu.edu/instruct/mcclean/plsc431/mendel/mendel1.htm
•
•
•
•
•
•
•
•
•
Homozygote - an individual which contains only one allele at the allelic
pair; for example DD is homozygous dominant and dd is homozygous
recessive; pure lines are homozygous for the gene of interest
Heterozygote - an individual which contains one of each member of the
gene pair; for example the Dd heterozygote
Genotype - the specific allelic combination for a certain gene or set of
genes
Phenotype - literally means "the form that is shown"; it is the
outward, physical appearance of a particular trait
Mendel's pea plants exhibited the following phenotypes:
- round or wrinkled seed phenotype
- yellow or green seed phenotype
- red or white flower phenotype
- tall or dwarf plant phenotype
http://www.ndsu.edu/instruct/mcclean/plsc431/mendel/mendel1.htm
Mendel’s Laws
Mendel's First Law - the law of segregation; during gamete
formation each member of the allelic pair separates from the
other member to form the genetic constitution of the gamete
e.g.
Aa
yields gametes with A
a
Mendel’s Second Law -- the law of independent assortment;
during gamete formation the segregation of the alleles of
one allelic pair is independent of the segregation of the
alleles of another allelic pair (assume no close linkage)
Figure 2.5. Mendel's First Law - the law of segregation; during gamete formation each member
of the allelic pair separates from the other member to form the genetic constitution of the gamete
A. Monohybrid Cross
B. F1 Self Fertilization
Parent 1
Parent 2
X
X
YY
Gametes:
Y
yy
Y
Parent 2
Parent 1
y
Yy
Yy
y
F1 Fertilization:
Gametes:
Y
F2 Fertilization:
y
Y
Parent 1
Parent 1
Y
Y
Y
y
y
Yy
Yy
YY
Yy
y
Yy
Y
YY & Yy
Parent 2
Parent 2
Yy
F1 Hybrid Plants: 100% yellow
Yy
y
Y
Yy
yy
F2 Plants: 75% yellow
25% green
yy
ty
TY
TY
TtYy
TtYy
ty
TtYy
TtYy
F1 Generation of Dihybid cross
Figure 2.6. Independent Assortment – Two or more traits (color and height).
A. Dihybrid Cross
Parent 1
TTYY
Parent 2
X
F1 Fertilization:
ttyy
TtYy
F1 Plants: 100% tall/yellow
B. F1 Self Fertilization
TtYy
X
TtYy
Parent 1
F2 Fertilization:
Ratios of
F2 Plants:
TY
Ty
tY
ty
TY
TTYY
TTYy
TtYY
TtYy
T_Y_ = 9/16
tall/yellow
Ty
TTYy
TTyy
TtYy
Ttyy
T_yy = 3/16
tall/green
tY
TyYY
TtYy
ttYY
ttYy
ttY_ = 3/16
short/yellow
ty
TtYy
Ttyy
ttYy
ttyy
ttyy = 1/16
short/green
Parent 2
Co-dominance –Variation to Mendel “laws”
Another Monohybrid Cross
RR = Red Flower Parent
r
R
Rr
-- rr = White Flower Parent
r
Rr
F1
R
Rr
Rr
If Dominance (R) then all should be RED
r
r
R
Rr
Rr
R
Rr
Rr
F1
R
R
But co-dominance of R and r
produces all pink flowers
RR
r
Rr
F2
r
rR
rr
1 Red; 2 Pink; 1 White
Flowers and Sexual Reproduction
Flower Parts – Only Angiosperms!
5
PIST
SEP
PET
ST
4
AN
ST
FL
ST
STL
PIST
OVY
PET
OVL
SEP
REC
RAF
STY
6
OVY
SEP
PET
OVL
FL
CAR
A
OVL
OVY
LOC
OVL
FIL
PET
SEP
B
REC
8
T
MMC
B
C
A
GN
S
PTN
PTN
PT
D
E
RAF
7
A
V
I
M
B
MSMC
HN
N
C
DN
FMSN
D
M
AN
PN
E
S
RAF
You’re Right!!! More Definitions
Self Incompatibility: Plant that cannot fertilize its female gametes with its
own male gametes. Basically two types: Gametophytic in which the pollen
grain will not grow when it shares the same incompatibility gene (s) and
Sporophytic in which the diploid parent hinders germination and growth of
the pollen tube. Both types prevent double fertilization.
Outcrossing Species: basically self-incompatible; requires a partner of a
different genotype.
Pollination: The process in which the pollen is carried from the anther to
the stigma.Basically two types: Cross (between different genotypes of
plants) and Self (same plant).
Double Fertilization: one sperm nucleus unites with egg to form the zygote
and restore the sporophytic phase (2N) of the plant and the other sperm
nucleus uniting with the polar nuclei of the female gametophyte to form the
primary endosperm nucleus.
Figure 2.12
Pollen parent genotype
S1S2
(a)
S2
S1
S2
S1
S1S3 pistil
S1S2 pistil
Gametophytic
Incompatibility
ES
S2
S1
ES
S3S4 pistil
ES
Pollen parent genotype
S1S2
(b)
S1
Sporophytic
Incompatibility
S1
S3
S1S2 pistil
ES
S1
S3
S2S3 pistil
ES
S3
S2S4 pistil
ES
Double Fertilization
9
AN
PN
E
SP
PT
B
N
PG
PN
E
PT
2
1
PT
C
A
OVY
PEN
OVL
ZY
D
RAF
Zygotic Embryo Ontogeny -- Dicot
Figure 4.4
10
YGE
I
EN
E
S
A
Zygotic embryogenesis
Cercis canadensis
(Eastern Redbud)
D
GE
I
EN
S
C
B
C
SM
H
RM
VT
C
E
11
Somatic Embryogenesis
EN
SC
CL
SM
SN
RM
Orchardgrass – a monocot
CR
A
I
B
EN
C
SM
C
H
SM
H
C
D
Eastern Redbud – a dicot
Summary Slide 1
• All plants (and animals) exhibit alternation
of generations
• For higher plants, the sporophytic (2N)
generation is the dominant dominant phase
• Meiosis is the driving biological process
behind alternation of generations.
Summary Slide 2
• Mitosis in an equal division in which the ploidy of the
mother cell is maintained
• The phases of mitosis are Prophase, Metaphase,
Anaphase, and Telophase followed by Cytokinesis.
• Meiosis is a reduction division followed by an equal
division similar to mitosis. Results in four cells of
reduced (haploid) ploidy
• Crossing over (exchange of sister chromatid
segments occurs in Prophase I and results in genetic
recombination.
Summary Slide 3
• Mendel's First Law - the law of segregation; during
gamete formation each member of the allelic pair
separates from the other member to form the genetic
constitution of the gamete
• Mendel’s Second Law -- the law of independent
assortment; during gamete formation the segregation of
the alleles of one allelic pair is independent of the
segregation of the alleles of another allelic pair
(assumes no close linkage)
• Expression of alleles can be dominant, recessive or
codominant.
Summary Slide 4
• The four floral organs are pistils, anthers,
sepals and petals.
• Higher plants do NOT require water for
fertilization to occur
• Male gametophyte is 3 cells, whereas the
female gametophyte is 8 cells (nuclei)
• The process of double fertilization in higher
plants produces a zygote (2N) and primary
endosperm nucleus (3 or 5 N)