Lecture 8 Genetics Gregor Mendels Discoveries Mendels experimental

Lecture 8: Genetics Gregor Mendel’s Discoveries

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 ﻗﻮﺍﻧﻴﻦ ﺍﻟﻮﺭﺍﺛﺔ in 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 ﺍﻟﺒﻮﻳﻀﺎﺕ with their own pollens ﺣﺒﻮﺏ ﺍﻟﻠﻘﺎﺡ. However, Mendel could also move pollens from one plant to another to cross-pollinate ﺗﻠﻘﻴﺢ ﺧﻠﻄﻲ plants.

Three steps of Mendel’s experimental

Mendel’s Results and Conclusions • • • Mendel concluded that inherited characteristics are controlled by factors that occur in pairs. Mendel also referred to these F 1 (first generation) individuals as hybrids ( ﻫﺠﻴﻦ )ﺧﻠﻴﻂ because the offspring ﺍﻟﻨﺴﻞ 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 F 1 plants were purples, Mendel referred to purple flower as the dominant trait ﺻﻔﺔ ﺳﺎﺋﺪﺓ and white flower as the recessive trait ﺻﻔﺔ ﻣﺘﻨﺤﻴﺔ. Mendel wondered what happened to the white traits in the F 1 generation. Therefore, self-fertilization was done to produce the second generation or F 2. This cross produced a 3 purple to 1 white ratio of traits in the F 2 offspring.

The laws of Mendel in genetics 1. First law: Segregation ﻗﺎﻧﻮﻥ ﺍﻧﻌﺰﺍﻝ ﺍﻟﺼﻔﺎﺕ 2. Second law: Independent assortment ﻗﺎﻧﻮﻥ ﺍﻟﺘﻮﺯﻳﻊ ﺍﻟﺤﺮ First: Law of segregation: The two alleles ﺍﻷﻠﻴﻼﺕ for a characteristic 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 F 2 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 ﺳﺘﺪﻳﺮﺓ with plant produced wrinkled seeds ﺑﺬﻭﺭ ﻋﺪﺓ , all the F 1 offspring had round seeds, but among the F 2 plants, 75% of the seeds were round and 25% were wrinkled (see the second low in lecture 9).

The results of Mendel’s F 1 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. v Different alleles vary somewhat ﻧﻮﻋ ﻣﺎ in the sequence of nucleotides at the specific locus ﻣﻮﻗﻊ of a gene. 2. For each characteristic, an organism inherits ﻳﺮﺙ two alleles, one from each parent. v In the flower-color example, the F 1 plants inherited a purple-flower allele from one parent and a white-flower allele from the other. v If the two alleles differ, one of them will be dominant, and the other is recessive. v The two alleles for a characteristics are separated (segregated) into separate gametes and aggregated again by fertilization.

• The F 1 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 ﺑﺎﻧﻴﺖ predicts the results of a genetic cross between individuals of known genotype ﺍﻟﻄﺮﺍﺯ ﺍﻟـﺠﻴﻨﻲ • Mendel’s model accounts for the 3: 1 ratio in the F 2 generation

PP X pp Pea plant Pp X Heterozygous Homozygous P Pp P p PP Pp P p p Dominant allele Pp 100% Purple F 1 generation Recessive allele 3 Purple Pp : pp 1 White F 2 generation 9

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

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

How could Mendel determine the F 2 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?