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1Biology Chapter 10 Study Guide TraitA trait is a variation of a particular character (e.g. color, height). Traits are passed from parents to offspring through genes. GenesGenes are located on chromosomes and consist of DNA. They are passed from parents to their offspring through reproduction. AllelesAlleles are alternative forms of a gene (e.g. a flower color may be white, another flower may be purple). This means that the gene for color is expressed in two different forms, either white or purple. HomozygousThe two alleles representing the trait are identical (e.g. PP for purple color, pp for white color). HeterozygousThe two alleles representing the trait are different (e.g. Pp for purple color). Although the pea plant appears purple in color (its phenotype is purple), its genotype (genetic make-up) is a mixture of a dominant “P” and a recessive ‘p” trait. Recessive Characteristics:These are the traits that are masked by the presence of the more dominant traits. The recessive allele is inherited unchanged and can reappear.
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2Gregor Mendel (1822-1884)Gregor Mendel was an Austrian Monk who discovered the basic rules or principals of heredity. Mendel chose the garden pea plant for his experiments and observations. He chose the pea plant for various reasons: a.easily grown b.easily manipulated c.self pollination (the pea plant has both the male and female reproductive organs “stamen”, and “carpel” within one plant). Mendel experimented with 7 pea plant traits (each trait had 2 forms or two alleles): 1.flower color (purple or white) 2.flower position (axial “near the middle of flower” or terminal “near the end of stem”) 3.pod color (yellow or green) 4.pod shape (inflated or constricted) 5.seed color (yellow or green) 6.stem length (long or short) 7.seed shape (round or wrinkled) In some of his first experiments (FIRST CROSS EXPERIMENTS), Mendel crossed a pure breeding purple flowered pea plant (PP) with a pure breeding white flowered pea plant (pp): PP X pp (Parents-true breeding and Homozygous) Pp Pp Pp Pp (F1 generation-offspring are Heterozygous) Results:All the 2ndgeneration pea plants (F1) were purple in color. No blending of colors. Again, Mendel crossed a pure breeding round seeded variety (RR) with a pure breeding wrinkled seeded pea plant (rr): RR X rr (Parents) Rr Rr Rr Rr (Offspring-F1) Results: All 2ndgeneration pea plants (F1) were round seeded plants.
3Mendel discovered that certain traits were masked (recessive characters, as the white flower color, and the wrinkled seed) while others appeared in the offspring without any blending of parent characteristics. This meant that F1 pea plants were either purple or white-no intermediate colors. Mendel then allowed the 2ndgeneration plants (F1) to self pollinate (SECOND CROSS EXPERIMENTS), and they produced: PP X pp (Parents) Pp Pp Pp Pp (F1) X PP Pp Pp pp (F2) Results: The white color trait reappeared in 25% (homozygous-pp) of the 3rdgeneration (F2). 50% of the flowers were purple (homozygous-PP) and 25% were purple (heterozygous-Pp).