Gregor Mendel’s Discoveries

Gregor Mendel’s Discoveries

Lecture 8: Genetics

• He was born in 1822 in Austria . النمسا

• In 1854, Mendel began his classic experiments with the garden pea plant نبات البازالء.

• He discovered the laws of heredity الوراثة قوانين in plants and animals.

• He died in 1884 by a kidney disorder.

• Each pea plant has male (stamens (الكرابل carpels) and female (األسديةsexual organs.

• In nature, pea plants typically self-fertilize ذاتي fertilizing ,تلقيحova البويضات with their own pollens . حبوب اللقاح

• However, Mendel could also move pollens from one plant to another to cross-pollinate خلطي تلقيح

plants.

Mendel’s experimental design

Three steps of Mendel’s experimental

• Mendel concluded that inherited characteristics are controlled by factors that occur in pairs.

• Mendel also referred to these F1 (first generation) individuals as hybrids )خليط( were aالنسل because the offspring هجين 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 صفة سائدة and white flower as the recessive trait متنحية .صفة

• 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.

Mendel’s Results and Conclusions

The laws of Mendel in genetics

1. First law: Segregation قانون انعزال الصفات

2. Second law: Independent assortment ق;;انون التوزي;;ع

for a األليالت First: Law of segregation: The two alleles الحرcharacteristic are isolated into separate gametes.

Peas exhibit a variety of contrasting traits )صفات متضادة )متباينة (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 .صفتين مختلفتين

For example, when Mendel crossed two true-breeding varieties, one produced round seeds م?ستديرة with plant بذور

produced wrinkled seeds م?جBعدة بذور , 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).

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

Mendel developed a hypothesis فرضية to explain these results.1. Alternative version of the same gene is called allele.

Different alleles vary somewhat ما E in the sequence of نوعاnucleotides at the specific locus عقمو of a gene.

2. For each characteristic, an organism inherits يرث two alleles, one fromeach 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.

• 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 بانيت مربع

predicts the results of a genetic

cross between individuals of

known genotype ال;جيني الطراز

• Mendel’s model accounts for the

3:1 ratio in the F2 generation

9

PPppPPpp

P p P p

PPPPPPPP PPppPPpp PPppPPpp pppp

PPPP pp

100% 100% Purple

F2 generation

3 3 Purple : 1 : 1 White

F1 generation

Recessive allele

Dominant allele

PPPPPPPP ppppXX PPppPPpp PPppPPppXXPea

plant

Homozygous Heterozygous

Genotype(Genetic make

up)

PPPP

Phenotype(Colour)

PPPPPPPP pppppppp1) Homozygous

PPppPPpp 2) Heterozygous

An organism with two identical alleles المتماثلة for a األليالت characteristic.

An organism with two different alleles المختلفة for a األليالت characteristic.

Phenotype: الطراز المظهري A description of an organism’s appearance مظهر

Genotype: الطراز الجينيA description of an organism’s genetic makeup الجيني .التركيب Genotype may be either:

متماثل الجينات

متباين الجينات

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 المظهرية

The expected numbers of different phenotypes

Phenotypic and genotypic ratios

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?

How could Mendel determine the F2 genotype as either homozygous dominant

or heterozygous?