Gregor Mendel’s Discoveries

1. Lecture 8: Genetics Gregor Mendel’s Discoveries

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

3. Three steps of Mendel’s experimental

4. 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 هجين (خليط)because the offspring النسلwere 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 dominanttrait صفة سائدةand white flower as the recessive trait صفة متنحية. • 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.

5. The laws of Mendel in genetics First: Law of segregation:The two alleles الأليلاتfor a characteristic are isolated into separate gametes. First law: Segregation قانون انعزال الصفات Second law: Independent assortmentقانون التوزيع الحر • 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 بذور مُستديرةwith plant produced wrinkled seedsبذورمُجَعدة, 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).

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

7. Mendel developed a hypothesis فرضيةto explain these results. 1. Alternative version of the same gene is called allele. • Different alleles vary somewhat نوعاً ماin the sequence of nucleotides at the specific locus موقع of a gene. 2. For each characteristic, an organism inherits يرثtwo 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.

8. 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 مربع بانيتpredicts the results of a genetic cross between individuals of known genotype الطرازالـجيني • Mendel’s model accounts for the 3:1 ratio in the F2 generation

9. Pp Pp PP pp X X P p P p P p Dominant allele Recessive allele Pea plant Heterozygous Homozygous Pp PP Pp pp Pp 100% Purple 3 Purple : 1 White F2generation F1generation

10. pp PP 1) Homozygous Phenotype (Colour) Pp 2) Heterozygous Genotype (Genetic make up) Phenotype:الطراز المظهري A description of an organism’s appearance مظهر Genotype:الطراز الجيني A description of an organism’s genetic makeup التركيب الجيني. Genotype may be either: PP متماثل الجينات An organism with two identical allelesالأليلات المتماثلةfor a characteristic. متباين الجينات An organism with two different allelesالأليلات المختلفةfor a characteristic.

11. 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 المظهرية The expected numbers of different phenotypes

12. 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?