1. Gregor Mendel’s Discoveries
Lecture 8
Gregor Mendel’s Discoveries

2. Mendel’s experimental design
He was born in 1822 in Austria.
In 1854, Mendel began his classic experiments with the garden pea plant.
He discovered the laws of heredity in plants and animals.
He died in 1884 by a kidney disorder.
Each pea plant has male (stamens) and female (carpels) sexual organs.
In nature, pea plants typically self-fertilize, fertilizing ova with their own pollens.
However, Mendel could also move pollens from one plant to another to cross-pollinate plants.

3. Why did Mendel use Pea Plants
peas grow easily.
peas have relatively short life spans.
peas have numerous and distinct characteristics
mating of peas can be controlled.
Note: good biological experimentation results from choosing good study organisms that allow experimenter to focus on particular questions

4. Three steps of Mendel’s experimental

5. Mendel’s Results and Conclusions
Mendel concluded that inherited characteristics are controlled by factors that occur in pairs.
Mendel also referred to these F1 (first generation) individuals as hybrids because the offspring were a mixture from parents with different traits.
We will refer to these offspring as monohybrids because they are hybrid for only one characteristic.
Because all the F1 plants were purples, Mendel referred to purple flower as the dominant trait and white flower as the recessive trait.
Mendel wondered what happened to the white traits in the F1 generation.
Therefore, self-fertilization was done to produce the second generation or F2.
This cross produced a 3 purple to 1 white ratio of traits in the F2 offspring.

6. The laws of Mendel in genetics
First law: Segregation قانون انعزال الصفات
Second law: Independent assortment قانون التوزيع الحر
First: Law of segregation: The two alleles for a characteristic are isolated into separate gametes.
Peas exhibit a variety of contrasting traits (seven traits).
Mendel found similar 3:1 ratio of two traits (dihybrids) among F2 offspring when he conducted crosses for six other characteristics, each represented by two different varieties.
For example, when Mendel crossed two true-breeding varieties, one produced round seeds with plant produced wrinkled seeds, all the F1 offspring had round seeds, but among the F2 plants, 75% of the seeds were round and 25% were wrinkled (see the second low in lecture 9).

7. The results of Mendel’s F1 crosses from seven characteristics in pea plants

8. Mendel developed a hypothesis to explain these results.
1. Alternative version of the same gene is called allele.
Different alleles vary somewhat in the sequence of nucleotides at the specific locus of a gene.
2. For each characteristic, an organism inherits two alleles, one from each parent.
In the flower-color example, the F1 plants inherited a purple-flower allele from one parent and a white-flower allele from the other.
If the two alleles differ, one of
them will be dominant, and the
other is recessive.
The two alleles for a characteristics
are separated (segregated) into
separate gametes and aggregated
again by fertilization.

9. The F1 hybrids will produce two classes of gametes, half with the purple-flower allele and half with the white-flower allele.
During self-pollination, the gametes of these two classes unite randomly.
This can produce four equally likely combinations of sperm and ovum.
A Punnett square predicts the results of a genetic cross between individuals of known genotype
Mendel’s model accounts for the 3:1 ratio in the F2 generation

10. Pp P Pp Pp Pp PP X X P PP pp p p pp F1 generation F2 generation
Pea plant Pp X P p
Homozygous
Heterozygous P p PP Pp Pp pp Pp
Recessive allele
Dominant allele
100% Purple
3 Purple : White
F1 generation
F2 generation

11. PP Pp PP Phenotype pp Genotype (Colour) (Genetic make up)
A description of an organism’s appearance
Phenotype
(Colour)
Genotype: A description of an organism’s genetic makeup. Genotype may be either: PP
Genotype
(Genetic make up) PP pp
1) Homozygous
An organism with two identical alleles for a characteristic. Pp
2) Heterozygous
An organism with two different alleles for a characteristic.

12. Phenotypic and genotypic ratios
For flower color in peas, both PP and Pp plants have the same phenotype (purple) but different genotypes (homozygous and heterozygous).
The only way to produce
a white phenotype is to
be homozygous recessive
(pp) for the flower-color
gene.
Genotypic ratio: النسبة الجينية
The expected numbers of
different genotypes.
Phenotypic ratio: النسبة لمظهرية
different phenotypes

13. How could Mendel determine the F2 genotype as either homozygous dominant or heterozygous?
By Test cross:
Crosses any organism with a recessive homozygote, can determine the identity of the unknown allele.
Question: What is the result of crossing between plants with purple and white flowers?