1 Section A: Gregor Mendel’s Discoveries CHAPTER 14 MENDEL AND THE GENE IDEA

2 Mendel brought an experimental and quantitative approach to genetics Around 1857, Mendel began breeding garden peas to study inheritance وراثة. Because they are available in many varieties with distinct heritable مُتوارث characters صفات with different traits (genes).Around 1857, Mendel began breeding garden peas to study inheritance وراثة. Because they are available in many varieties with distinct heritable مُتوارث characters صفات with different traits (genes). Each pea plant has male (stamens) and female (carpal) sexual organs.Each pea plant has male (stamens) and female (carpal) sexual organs. In nature, pea plants typically self-fertilize, fertilizing ova with their own pollens.In nature, pea plants typically self-fertilize تلقيح ذاتى, fertilizing ova with their own pollens. However, Mendel could also move pollens from one plant to another to cross-pollinate يُـلقح plants.However, Mendel could also move pollens حبوب اللقاح from one plant to another to cross-pollinate يُـلقح plants.

3 In a breeding experiment, Mendel would cross-pollinate تلقيح خلطى (hybridize هَجن ) two contrasting متباينين, true- breeding pea varieties أنواع.In a breeding experiment, Mendel would cross-pollinate تلقيح خلطى (hybridize هَجن ) two contrasting متباينين, true- breeding pea varieties أنواع. –The true-breeding parents are the P generation and their hybrid offspring النسل المُهجن are the F 1 generation. Mendel would then allow the F 1 hybrids to self-pollinate to produce an F 2 generation.Mendel would then allow the F 1 hybrids to self-pollinate to produce an F 2 generation. It was mainly Mendel’s quantitative analysis تحليل كمى of F 2 plants that revealed the two fundamental lows of heredity:It was mainly Mendel’s quantitative analysis تحليل كمى of F 2 plants that revealed the two fundamental lows of heredity: A)- The law of segregation. B)- The law of independent assortment. A)- The law of segregation. B)- The law of independent assortment.

4 The F 1 hybrids from a cross تلقيح between purple-flowered and white- flowered pea plants would have pale purple flowers.The F 1 hybrids from a cross تلقيح between purple-flowered and white- flowered pea plants would have pale purple flowers بنفسجى باهت. Instead, ولكن the F 1 hybrids all have purple flowers, just a purple like their parents.Instead, ولكن the F 1 hybrids all have purple flowers, just a purple like their parents. A)- Law of segregation: the two alleles for a character are isolated into separate gametes A)- Law of segregation: the two alleles ﭽـينات for a character are isolated into separate gametes Fig. 14.2, page 249 This cross produced a 3 purple to 1 white ratio of traits in the F 2 offspring,This cross produced a 3 purple to 1 white ratio of traits in the F 2 offspring, Mendel reasoned that the heritable factor for white flowers was present in the F 1 plants, but it did not affect flower color.Mendel reasoned that the heritable factor for white flowers was present in the F 1 plants, but it did not affect flower color. Thus, purple flower is a dominant color (صفة سائدة) and white flower is a recessive one(صفة مُتنحية).Thus, purple flower is a dominant color (صفة سائدة) and white flower is a recessive one(صفة مُتنحية).

5 Mendel found similar 3 to 1 ratios of two traits among F 2 offspring when he conducted crosses for six other characters, each represented by two different varieties صفتين مختلفتين.Mendel found similar 3 to 1 ratios of two traits among F 2 offspring when he conducted crosses for six other characters, each represented by two different varieties صفتين مختلفتين. For example, when Mendel crossed two true-breeding varieties, one of which produced round seeds بذور مُستديرة, the other of which produced wrinkled seeds بذور مُجَعدة, all the F 1 offspring had round seeds, but among the F 2 plants, 75% of the seeds were round and 25% were wrinkled ().For example, when Mendel crossed two true-breeding varieties, one of which produced round seeds بذور مُستديرة, the other of which produced wrinkled seeds بذور مُجَعدة, 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 second low in the next lecture ).

6 Table 14.1, Page 250

7 Mendel developed a hypothesis to explain these results that consisted of four related ideas.Mendel developed a hypothesis إفتراض to explain these results that consisted of four related ideas. 1. Alternative version of genes (different alleles) account for variations in inherited characters. –Different alleles vary somewhat in the sequence of nucleotides at the specific locus of a gene. –Different alleles vary somewhat in the sequence of nucleotides at the specific locus موضع of a gene. Fig. 14.3, Page For each character, an organism inherits two alleles, one from each parent. 2. For each character, an organism inherits يرث two alleles, one from each parent. –These homologous loci مَوقِعُه على الكروموسوم may be differ –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.

8 3. If two alleles differ, then, the dominant allele is fully expressed in the organism’s appearance and the recessive allele has no effect on the organism’s appearance. on the organism’s appearance. 4. The two alleles for each character segregate (separate) ينفصل during gamete production. –This segregation of alleles because of the distribution of homologous chromosomes to different gametes in meiosis. –If an organism has identical allele for a particular character, then that allele exists as a single copy in all gametes. –If different alleles are present, then 50% of the gametes will receive one allele and 50% will receive the other. The separation of alleles into separate gametes is summarized as Mendel’s law of segregation.The separation of alleles into separate gametes is summarized as Mendel’s law of segregation.

9 Summary The two alleles (genes) for a character are separated (segregated) into separate gametes and aggregated again by fertilization. Mendelian inheritance reflects rules of probability for the behaviour of genes. alleles For each character, an organism inherit two alleles (one from each parent). Dominant Recessive If the two alleles differ, one of them will be Dominant, and the other is Recessive. Fig. 14.3, Page 247 Red colour gene ( allele ) allele White colour gene ( allele ) Homologous chromosomes

10 Fig A Punnett square analysis of the flower-color example demonstrates Mendel’s model.A Punnett square analysis of the flower-color example demonstrates Mendel’s model. Mendel’s model accounts for the 3:1 ratio in the F 2 generationMendel’s model accounts for the 3:1 ratio in the F 2 generation Mendel’s law of segregation accounts for the 3:1 ratio in the F 2 generation.Mendel’s law of segregation accounts for the 3:1 ratio in the F 2 generation. The F 1 hybrids will produce two classes of gametes, half with the purple-flower allele and half with the white-flower allele.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.During self-pollination, the gametes of these two classes unite randomly. This can produce four equally likely combinations of sperm and ovum.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 الطرز الـﭽينى.A Punnett square predicts the results of a genetic cross between individuals of known genotype الطرز الـﭽينى.

11 PpPpPpPp PpPpPpPp P p P p PPPP PpPpPpPp PpPpPpPp PpPpPpPp PpPpPpPp pp PP p p 100% Purple F 2 generation 3 Purple : 1 White F 1 generation Recessive allele Dominant allele PPPP pp X PpPpPpPp PpPpPpPp PpPpPpPp PpPpPpPpX Pea plant Homozygous Heterozygous

12 Genotype (Genetic make up) PPPhenotype(Colour) PPPPpppp Homozygous PpPpPpPp PpPpPpPp Heterozygous An organism having a pair of identical alleles An organism having a pair of two different alleles Phenotype: Is t he organism’s appearance. Genotype: Is t he organism’s genetic makeup.

13 Dominant character (allele)Dominant character (allele) الصفة السائدة Is fully expressed in the organism’s appearance. Is fully expressed in the organism’s appearance. Recessive character (allele)Recessive character (allele) الصفة المُتنحية Has no noticeable effect تأثير غير ملحوظ on the organism’s appearance. Has no noticeable effect تأثير غير ملحوظ on the organism’s appearance. Homozygous مُتماثل الجيناتHomozygous مُتماثل الجينات An organism with two identical alleles for a character. An organism with two identical alleles for a character. Heterozygous مُختلف الجيناتHeterozygous مُختلف الجينات An organism with two different alleles for a character. An organism with two different alleles for a character. Karyotype الطرز الكروموسومىKaryotype الطرز الكروموسومى The display of an organism’s chromosomal pattern Phenotype الطرز المظهرىPhenotype الطرز المظهرى A description of an organism’s traits (feature مظهر). A description of an organism’s traits (feature مظهر). Genotype الطرز الجينىGenotype الطرز الجينى A description of an organism’s genetic makeup. A description of an organism’s genetic makeup.

14 For flower color in peas, both PP and Pp plants have the same phenotype (purple) but different genotypes (homozygous and heterozygous).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.The only way to produce a white phenotype is to be homozygous recessive ( pp ) for the flower- color gene. Fig. 14.5

15 It is not possible to predict the genotype of an organism with a dominant phenotype.It is not possible to predict the genotype of an organism with a dominant phenotype. –The organism must have one dominant allele, but it could be homozygous dominant or heterozygous. A test cross, breeding a homozygous recessive with dominant phenotype, but unknown genotype, can determine the identity of the unknown allele.A test cross, breeding a homozygous recessive with dominant phenotype, but unknown genotype, can determine the identity of the unknown allele. Fig Q: What is the result of Cross hybridization of purple X white colored flowers ?