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CHAPTER 12
Sexual
Reproduction
Meiosis
The significance of Meiosis
Meiosis has three significant results:
1. Haploid cells are produced because two rounds
of division follow only one round of
chromosome replication.
2. Crossing-over between maternal and paternal
chromatids pairs (or homologous pairs) during
meiosis I provides still more variation, making
the number of possible progeny nuclei extremely
large.
3.
Alignment of paternally and maternally
derived chromosomes (homologous pairs) is
random in metaphase I, resulting in random
combinations of chromosomes in each
nucleus of the daughter cells. The number of
possible chromosome arrangements at the
meiosis I metaphase plate is 2n-1 (n is the
number of chromosome pairs).
Conclusion:
Due to the differences between the paternally
and maternally derived chromosomes
(homologous), crossing over during Prophase
I and the random alignment of homologous
pairs during metaphase I, the nuclei produced
by meiosis will be genetically distinct from
parental cells, and from one another.
Meiosis Duration
The duration of Meiosis is constant for each species as it
is the duration of their stages, however variation in
environmental conditions may affect the duration
Drosophila melanogaster 1-2 days
Ovis aries17 days
Mus musculus 12 days
Petunia hybrida 24 hs.
Trillium sp 3 months
Lillium sp 10.5 days
Humans male = 74 hs.
Human female = up to 50 years!
Epididymis
Spermatogonium/a
Testis
Scrotum
Penis
Diploid cell
2n
Differentiation and
onset of Meiosis Ι
Primary spermatocyte
(in prophase of Meiosis Ι
Meiosis Ιcompleted
2n
Cross section of
seminiferous
tubule
Secondary spermatocyte
(haploid; double chromatids)
n
n
Testis
Seminiferous tubule
Meiosis ΙΙ
n
n
n
n
Developing sperm cells
(haploid; single chromatids)
Differentiation
n
n
n
n
Sperm cells
(haploid)
Center of
seminiferous tubule
Spermatogenesis
Diploid cell
2n
In embryo
Differentiation and
onset of Meiosis Ι
Primary oocyte
(arrested in prophase
of Meiosis Ι)
2n
Present at birth
Completion of Meiosis Ι
and onset of Meiosis ΙΙ
Secondary oocyte
(arrested at metaphase of Meiosis ΙΙ;
released from ovary)
n
n
First
polar body
Entry of sperm triggers
completion of Meiosis ΙΙ
Ovum
(haploid)
n
n
Second
polar body
Oogenesis
Degenerating
corpus luteum
Start:
Primary oocyte
within follicle
Corpus luteum
Growing
follicles
Mature follicle
Secondary
oocyte
Ovary
Ovulation
Ruptured follicle
Estrous
The estrous cycle can be divided into four stages:
Proestrus: Pre-ovulatory follicle undergoes it final
growth phase. (Attraction)
Estrus: Increase in estradiol (estrogen) and
ovulation. Mating.
Diestrus: Pregnancy and/or intercycles period of
time (60-90 days in dogs)
Anestrus: Period of Resting between cycles
Estrous Cycle
Pro/Estrus
O
Anestrus or
Interestrus
vu
lat
io
n/
Ac
Fe
Ov ce
o
ula pta n rtil
i za
wh tion nce
ti
en ta of M
LH kes
ate
p
pe lac
ak es
s
Pregnancy
Diestrus
Species
Mouse, rat
Hamster
Guinea pig
Sheep
Goat
Cattle
Pig
Horse
Elephant
Red kangaroo
Lion
Dog
Estrus
0.5
1
0.5
2
3
0.5
2
5
4
3
9
7
Cycle
4
4
16
17
20
21
21
21
22
35
55
60
The Pollen Grain
Different positions of the
ovary respect to the rest of
the floral parts.
Gametophytic Generation
Sporophytic Generation
Alternation of Generations
Figure 38.3ax1 Lily
Figure 38.3ex Begonia, a monoecious species
What is pollination?
„
Pollination: The transfer of pollen from the
male anther to the female stigma.
Strategies to avoid self-pollination
„
„
„
Perfect flowers have both male and female organs, so
plants have strategies to prevent self-pollination:
1. Timing – male and female structures mature at different
times
2. Morphological – structure of male and female organs
prevents self-pollination (imperfect flower)
„
. Biochemical – recognition/signaliung on
surface of pollen and stigma/style that prevent
pollen tube germination on the same flower
(incompatible)
CHAPTER 11
Mendelian
Genetics
Classic Genetics
From Mendel to the
Central Dogma of
Genetics (1866-1941).
Mendel’s published
work, Experiments in
Plant Hybridization
(1865), languished with
no discernable impact
until in 1900 three other
investigators
independently
discovered the same
genetic principles.
Mendelism
„
„
1. Gregor Mendel (1822–1884) laid the
foundation for our current understanding of
heredity.
2. Mendel did not know about chromosomes
or genes, which were discovered after his
lifetime
Petal
Stamen
Carpel
Terminology
Self-fertilization: fertilization of eggs by sperm-carrying
pollen of the same flower
Cross-fertilization (cross): fertilization of one plant by
pollen from a different plant
P generation: true-breeding parents
G1 generation: The reproductive cells or “gametes”
produced by the parents.
F1 generation: hybrid offspring of true-breeding parents
F2 generation: offspring of self-fertilizing F1 parents
„
„
Mendel first grew strains of peas using selffertilization to be certain that the traits of
interest were unchanged in subsequent
generations (true-breeding or pure-breeding
strains).
Mendel then looked at inheritance of traits
selected because they have only two distinct
possibilities for phenotype. The traits he studied
are listed below, and the dominant phenotype is
indicated by an asterisk:
Monohybrid
Crosses
•For example, when Mendel
crossed two true-breeding varieties,
one of which produced round
seeds, the other of which produced
wrinkled seeds, all the F1 offspring
had round seeds.
When Mendel cross two of the F1
pea plants, 75% of F2 plant seeds
were round and 25% were wrinkled.
Details
„
„
Terminology
When Mendel had conducted experiments for
the seven different traits in garden peas (he
made these conclusions:
a. Results of reciprocal crosses are always the
same.
b. The F1 resembled only one of the parents.
c. The F2 always shows a 3:1 proportion of
dominant phenotype vs. recessive phenotype.
Mendel developed a hypothesis to explain these
results that consisted of four related ideas.
1. Alternative version of genes (different alleles)
account for variations in inherited characters.
Different alleles vary somewhat in the sequence of
nucleotides at the specific locus of a gene. The
purple-flower
allele and white-flower
allele are two DNA
variations at the
flower-color locus.
Fig. 14.3
2. For each character, an organism inherits
two alleles, one from each parent.
A diploid organism inherits one set of chromosomes
from each parent. Each diploid organism has a pair of
homologous chromosomes and therefore two copies of
each locus (gene location).
These homologous loci may be identical (identical
alleles) as in the true-breeding plants of the P generation
(homozygous). Alternatively, the two alleles may differ
(heterozygous).
3. Dominance and recessiveness
If two alleles differ, the dominant allele, is
fully expressed in the organism’s appearance.
The other, the recessive allele, has no
noticeable effect on the organism’s
appearance.
„
Mendel’s F1 plants had purple flowers because the
purple-flower allele is dominant and the whiteflower allele is recessive.
„
„
4. The two alleles for each character segregate
(separate) during gamete production.
This segregation of alleles corresponds to the
distribution of homologous chromosomes to
different gametes in meiosis.
If an organism has identical allele for a particular
character, then that allele exists as a single copy in all
gametes.
„ If different alleles are present, then 50% of the
gametes will receive one allele and 50% will receive
the other.
„
„
The separation of alleles into separate gametes is
summarized as Mendel’s law of segregation.
The Principle of Segregation
„
„
The first Mendelian law, the principle of segregation,
states: “Recessive characters, which are masked in the
F1 from a cross between two true-breeding strains,
reappear in a specific proportion in the F2.”
This is because alleles segregate during anaphase I of
meiosis, and progeny are then produced by random
combination of the gametes. (During anaphase of
meiosis homologous chromosomes separate. Each
homologous chromosome carries an allele. Which cell
will receive each one of the alleles is random)
Representing Crosses with a Branch
Diagram
„
The branch diagram is an alternative approach to
predicting the outcome of crosses and the results from a
branch diagram will be identical to those obtained with a
Punnett square.
Dihybrid
Crosses
The principle of independent
assortment.
„
„
Using these results Mendel formulated his or
principle of independent assortment known
today as Mendel’s second law . It states that the
factors for different traits assort independently
of one another. This allows for new
combinations of the traits in the offspring.
A dihybrid cross that follows this principle will
produce four possible phenotypic classes, in a
9:3:3:1 ratio.
Using the branch diagram
Mendel’s Garden Pea Pisum sativum has become the symbol
of classic genetics.
What’s wrong with Mendel?
Doesn’t he like cooked peas?
„ In pea plants, flower color is determined
by a single gene with two alleles: Red (R,
dominant) and white (r). Stem length is
also controlled by a single gene with two
alleles: long (L- dominant) and short (l).
What are the expected genotype and
phenotype ratios of the offspring of a cross
between two double heterozygous plants?
„
In sesame plants, the one-pod condition (P) is
dominant to the three-pod condition (p), and normal
leaf (L) is dominant to wrinkled leaf (l). Pod type and
leaf type are inherited independently. Determine the
genotypes for the two parents for all possible matings
producing the following offspring:
„
„
„
„
„
318 one-pod normal, 98 one-pod wrinkled
323 three-pod normal, 106 three-pod wrinkled
401 one-pod normal
150 one-pod normal, 147 one-pod wrinkled, 51 three-pod
normal, 48 three-pod wrinkled
223 one-pod normal, 72 one-pod wrinkled, 76 three-pod
normal, 27 three-pod wrinkled