Mendel The experiments The results The interpretation Aim: to learn

Mendel
The experiments
The results
The interpretation
Aim: to learn as from well done experiments we
understood how traits are inherited.
•1799 Knight: crosses with pisum sativum –
observes flower colours either white or red.
• 1824 Goss: pure lines by self cross in
pisum sativum – seeds either yellow or
green
• 1866 Mendel quantitative analysis of the results
for the same traits, and for new traits.
• 1900 Correns, De Vries, Tschermak and
others, repeat previous experiments.
……..step by step
G. Mendel (1822-1884)
1866
Gregor Mendel
1866
1900
In 1877 Fleming identifies some structures
(later named chromosomes by Waldeyer in 1888 )
within cell nucleous
The mechanisms of cell division (mitosis and
meiosis) are clarified at the end of 19th and at
the beginning of the 20th century
4
In thinking about an experimental design it is of the
utmost relevance the choice of the biological
sample to be studied:
Mendel, why peas?
5
Peas
-have a short time between one generation and
the next,
-many varieties are available,
-they are easy to cross, easy to grow, cheap.
-some
characters
show
well
defined
differences, (like seed color, green or yellow),
but for each plant only one character is present
and no intermediate forms are observed
- “pure lines” i.e. plants showing the same
character in all subsequent generations are also
known
6
Crosses
Pollen granules, (male cells) can be
transferred wit a brush
Mendel studied 7 couples of traits … …
…and decided to start crossing only “pure lines”
8
Mendel’s Experiments
Mendel noticed that some plants always produced offspring that had a form of a
trait exactly like the parent plant. He called these plants “purebred” plants. For
instance, purebred short plants always produced short offspring and purebred tall
plants always produced tall offspring.
X
Purebred Short Parents
Short Offspring
X
Purebred Tall Parents
Tall Offspring
Mendel’s First Experiment
Mendel crossed purebred plants with opposite forms of a trait. He called these plants
the parental generation , or P generation. For instance, purebred tall plants were
crossed with purebred short plants.
X
Parent Tall
P generation
Parent Short
P generation
Offspring Tall
F1 generation
Mendel observed that all of the offspring grew to be tall plants. None resembled
the short short parent. He called this generation of offspring the first filial , or F1
generation, (The word filial means “son” in Latin.)
P
Parental cross between two pure lines.
F1
The two forms of the caracters studied are determined by factors
transmitted by parental strain to progeny theough gametes.
11
Mendel’s Law of Segregation
Mendel’s first law, the Law of Segregation, has three
parts. From his experiments, Mendel concluded that:
1. Plant traits are handed down through “hereditary factors”
in the sperm and egg.
2. Because offspring obtain hereditary factors from both
parents, each plant must contain two factors for every
trait.
3. The factors in a pair segregate (separate) during the
formation of sex cells, and each sperm or egg receives
only one member of the pair.
He named
“parental plants” (> P) the plants to be crossed
“first generation” (>F1) the plants obtained from crossing.
P
yellow seeds X green seeds (pure lines)
F1
yellow seeds
From this kind of experiments, he derived the
“Principle of uniformity of F1”
¾all the progenies obtained from crossing two pure
lines are alike and show only one of the two
parental characters.
¾He also named dominant the trait that appears in
the F1, while the one that does not appear is called
recessive.
13
Are F1 plants pure lines? How to check it?
F1
Parental cross
1st generation
Self-cross
F2
Green
recessive
2nd generation
14
Results of Mendel experiments:
Parental cross
F1
F2 (self cross of F1)
ratio
Seeds,
yellow x green
yellow
6022 yellow
2001 green
3.01:1
Pod,
Green x yellow
green
428 green
152 yellow
2.82:1
Seeds,
Spherical x wrinkled
spherical
5474 spherical
1850 wrinkled
2.96:1
Flower color
Purple x white
purple
705 purple
224 white
3.15:1
More experiments, based on other traits, gave similar results; all the
ratios were about 3:1
Similar results obtained exchanging parental caracters
15
Law of Independent Assortment
Mendel’s second law, the Law of Independent Assortment, states that each pair
of genes separate independently of each other in the production of sex cells.
For instance, consider an example of the following gene pairs:
According to Mendels’ Law of Independent Assortment, the gene
pairs will separate during the formation of egg or sperm cells.
The plant will donate one allele from each pair. The plant will
donate either a yellow or green seed allele, either a yellow or
green pod allele, and a wrinkled or round seed allele. It will
always donate a wrinkled pod shape. The donation of one allele
from each pair is independent of any other pair. For example, if
the plant donates the yellow seed allele it does not mean that it
will also donate the yellow pod allele.
Mendel correctly interpreted his results as follows: The various traits are controlled by pairs of factors (now called genes) one factor derived from male parent, the other from the female.
The reapperance of some traits in F2, indicates that they are neither modified nor lost, but are indipendently transmitted
17
Y
G
Y
YY
YG
G
GY
GG
Punnett square
P
spherical
wrinkled
AA
aa
gametes
gametes
A
a
F1 (self cross)
Aa
1
A
1
a
pollen
ovules
1
A
AA
1 spherical, pure
F2
1
a
Aa
1 spherical, hybrid
Aa
aa
1 spherical, hybrid
Total:
3 spherical
(2 hybrids, 1
pure)
1 wrinkled, pure
:
1 wrinkled
21
Heterozygous for
two traits
27
Heterozygous
for
two traits
28
For each gene two
alleles are present in
eukaryiotes.
Gene
Locus
Many alleles exist in the
population.
One allele codes for
“spherical seeds”.
The second allele codes
for “wrinkled seeds”.
Dominant
Recessive
The two alleles are found
at the same locus on
homologous chromosomes
29
GENE
LOCUS
ALLELES
GENOTYPE‐PHENOTYPE
DOMINANT ‐ RECESSIVE
HOMOZIGOUS ‐ HETEROZIGOUS
PURE LINE ‐ HYBRID
30
LOCUS
1 Gene
GENE
2 alleles A ; a
ALLELES 3 genotypes AA; Aa; aa
GENOTYPE
31
Meiosis I
Metaphase II
Gametes