Compare Mitosis and Meiosis (Russell fig 3.7 and 3.12)
Significance of meiosis:
a) generates haploid cells so that fusion of the haploid nuclei
restores the diploid number,
thus maintaining the chromosome number in
sexually reproducing organisms
b) generates genetic variability:
1) each maternally derived and paternally
derived chromosome has an equal chance of
aligning on one or the other side of the metaphase plate
2) in prophase I: crossing over between
maternal and paternal chromatid pairs
In class we have already discussed that:
1) Hereditary traits are characteristics of an individual which
are transmitted from one
generation to the next. Traits are under
the control of DNA segments called genes.
2) Genes are on chromosomes.
3) We know how chromosomes behave during meiosis.
**Therefore, we already know the mechanism of how hereditary traits are passed from one generation to the next.
This is summed up in Mendel's two laws:
Law of Segregation: (during meiosis) the two members of
a gene pair (alleles) segregate (separate) from each other in the formation
of gametes (Russell fig 3.30)
Law of Independent Assortment: (during meiosis) alleles for different traits can assort independently of one another. Another way to say this: Genes on different chromosomes behave independently in the production of gametes (Russell fig 3.30).
Remarkably, these laws were described in 1866 by Gregor Mendel who at that time knew nothing about genes, chromosomes and meiosis. In light of this, he is regarded as the father of genetics!
(Looking ahead: Today we will look at some experiments which lead Mendel to develope his first law. Wednesday and Friday, we will learn how to apply Mendel's first law to solve a variety of genetics problems, including monohybrid crosses, complete and incomplete dominance. Next week we will apply Mendel's second law to solving genetic problems which involve dihybrid crosses.)
Gregor Mendel
Augustinian monk in Czechoslovakia
garden pea, Pisum sativum
published findings in 1866 - not understood until 1900
Features of Mendel's experiments which aided in his discovery:
1. He chose an organism that was: easy to grow and crossbreed;
took up little space; short
generation time; and many offspring
were produced in each generation.
2. He followed a single set of traits with obvious differences
3. He interpreted his data using small whole number ratios.
4. He worked with pure-breeding plants
The pea normally reproduces by self-fertilization, ie the male (stamen) and female (pistil) reproductive organs are both located in the same flower (draw). So, male gametes (pollen) land on the female reproductive organ and combine with the female eggs within the same flower to fertilize the plant.
To cross-fertilize, Mendel would remove the stamens from the developing flower before they produced mature pollen (emasculation). Then pollen from the stamen of another flower could be dusted onto the pistil of the emasculated flower to fertilize it (fig 2.5 Russell).
Mendel allowed each plant to self-fertilize for many generations to obtain pea strains in which the trait remained unchanged from generation to generation. These strains are pure-breeding, where the alleles for a given trait are the same for the paternal and maternal chromosomes.
Mendel studied the following 7 pairs of traits (Tamerin fig 2.3)
One of Mendel's crosses (monohybrid)
round seeds X wrinkled seeds --------------->
all round seeds
crossed progeny to themselves, & in next generation, found:
round seeded plants
5474 (2.96)
wrinkled seeded plants 1850
(1.00)
Very close to a 3:1 ratio. When Mendel analyzed the other 6 pairs of traits, he found the same thing: a 3:1 ratio
Conclusions:
1. The traits were determined by particulate factors (we now
know as genes) which occur in pairs.
2. Each factor existed in alternate forms (we now call
alleles), each specified one form of the trait.
3. When both alleles are present one (dominant) can mask the
expression of the other(recessive) allele.
Terminology in breeding experiments:
1. P generation=parents
2. first filial generation (F1)=progeny of the parental
mating
3. F2 generation=interbreeding of the brothers and
sisters of the F1 progeny
4. monohybrid crosses=crosses where one trait is looked
at
5. dihybrid crosses=crosses where two traits are looked
at
6. Alleles=alternate forms of a gene
-dominant=in a heterozygous condition, this
allele is expressed
-recessive=in a heterozygous condition, this
allele is masked
7. genotype = the genetic constitution (or the different
kinds of genes) of an organism
(RR, Rr, ect...)
-homozygous (pure-breeding)= having identical
alleles (RR, rr)
-heterozygous = having different alleles (Rr)
8. phenotype = The observable properties of an organism,
produced by the genotype in
association with
the environment and other influences,
is called the phenotype (round or wrinkled,...)
9. Types of crosses:
a. self cross:male & female
gametes from same individual
self fertilization
in plants
b. inbreeding: brother-sister crosses
c. backcross: cross to one of
the parents or parental types
d. testcross: cross to homozygous
recessive (a kind of backcross)
Let's look at the above cross more carefully, using the right terms:
R = round (capital letter for dominant allele)
r = wrinkled (lowercase letter for recessive allele)
parental generation
round x
wrinkled
(each was
pure-breeding)
RR (homozygous dominant) x rr (homozygous recessive)
gametes ------------->
R and
r
F1 (1st filial)
Rr (heterozygous, all smooth)
F1 (Rr) X F1 (Rr) F2
To find out the proportion of F2 offspring, you can do a Punnett
square to show all the possible fusions of gametes. The hardest part
of any genetic cross is determining what gametes are produced by each parent,
once determined, put the different possible male gametes on one side of
square and the female on the other side of the square, then combine the
gametes, as would happen during fertilization.
F1
gametes R
r
------------------------------
R RR
Rr
r
Rr
rr
F2 geneotype: 1 (RR): 2 (Rr): 1 (rr)
F2 phenotype: 3 (round) : 1 (wrinkle)
Mendel hypothesized that=Alleles separate from each other in the formation of the gametes (Mendel's 1st Law: Law of Segregation)
Testing Mendel's first law: If Mendel's hypothesis is correct, then he could make the following predictions:
1. the F1 plants are all heterozygous, Rr; How did he test this? did
a backcross:
two possibilities:
1) F1 X round
parent, what do you expect? (no good, why?)
2) F1 X
wrinkled parent (also called a test-cross), expect?
Mendel's backcross:
F1 X wrinkled parent ------>
106 round seeded plants
102 wrinkled seeded plants
2. of the F2 plants with round peas, 1/3 should be RR (homozygous)
and 2/3 Rr (heterozygous)
How did Mendel test this?
did a self-cross, to generate an F3.
If homozygous (RR),
then when do a self-cross (RR X RR) what would you expect?
all round seeds
If heterozygous (Rr), then
when do a self-cross (Rr X Rr), what would you expect?
both round and wrinkle seeds (3:1)
tested
565 round F2 plants, found
193 - round offspring only (34 %=1/3 of F2)
372 - both round and wrinkled (66 %=2/3 of F2)
What other type of cross could be used to distinguish heterozygous round
pea plants from homozygous round pea plants? test cross