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Monohybrid Cross - Cross Involving a Single Trait E.G Mendelelian Genetics copyright cmassengale 1 Gregor Mendel (1822-1884) Responsible for the Laws governing Inheritance of Traits 2 • 1822 - Brunn in Austria, now, Brno (Czechoslovakia) • Financial and Health Problem • Unable to continue studies • Joined St. Augustinian Monastery • 1847 – Priest • 1851 – Higher studies at Vienna • University • 1854- Science teacher 3 Gregor Johann Mendel .Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants .He found that the plants' offspring retained traits of the parents 4 Gregor Johann Mendel .Austrian monk .Developed the laws of inheritance .Presented his findings in the Natural History Society of Brunn – 1865 .Paper entitled, “Experiments in Plant Hybridization” – 1866 .German language. 5 . Mendel's work was not recognized until the turn of the 20th century . 1900 - Carl Correns, Hugo deVries, and Erich von Tschermak rediscover and confirm . Called the “Father of Genetics“ 6 Mendel’s MendelsworkplacMendelwork’s place 7 Fig. 2.5 Site of Gregor Mendel’s experimental garden in the Czech Republic 8 Genes and Environment Determine Characteristics copyright cmassengale 9 Particulate Inheritance .Mendel stated that physical traits are inherited as “particles” .Mendel did not know that the “particles” were actually Chromosomes & DNA copyright cmassengale 10 Genetic Terminology . Trait - any characteristic that can be passed from parent to offspring . Heredity - passing of traits from parent to offspring . Genetics - study of heredity copyright cmassengale 11 Designer “Genes” . Alleles - two forms of a gene (dominant & recessive) . Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R) . Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r) copyright cmassengale 12 Mendelian genetics • Character (heritable feature, i.e., fur color) • Trait (variant for a character, i.e., brown) • True-bred (all offspring of same variety) • Hybridization (crossing of 2 different true-breds) • P generation (parents) • F1 generation (first filial generation) Genetic vocabulary……. • Punnett square: • Gene: point on a chromosome that controls the trait • Allele: an alternate form of a gene A or a • Homozygous: identical alleles for a character • Heterozygous: different alleles for a gene • Phenotype: physical traits • Genotype: genetic makeup • Testcross: breeding of a recessive homozygote X dominate phenotype (but unknown genotype) Genotype & Phenotype in Flowers Genotype of alleles: R = red flower r = yellow flower All genes occur in pairs, so 2 alleles affect a characteristic Possible combinations are: Genotypes RR Rr rr Phenotypes RED RED YELLOW copyright cmassengale 15 Genotypes . Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr); also called pure . Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Rr); also called hybrid copyright cmassengale 16 Punnett Square Used to help solve genetics problems copyright cmassengale 17 copyright cmassengale 18 Mendel’s Pea Plant Experiments copyright cmassengale 19 copyright cmassengale 21 copyright cmassengale 22 The Reality of “Round and Wrinkled” – Two Alternative Traits of the Seed Shape Character Note that each of seed is a new individual of a different generation – seeds are not of the same generation as the plant that bears them. Reproduction in Flowering Plants •Pollen contains sperm –Produced by the stamen •Ovary contains eggs –Found inside the flower Pollen carries sperm to the eggs for fertilization Self-fertilization can occur in the same flower Cross-fertilization can copyrightoccur cmassengale between flowers 24 How Mendel Began Mendel produced pure strains by allowing the plants to self- pollinate for several generations copyright cmassengale 26 Cross- Pollination Mendel’s Experimental Results copyright cmassengale 29 Law of dominance States that on crossing homozygous organisms for single pair of contrasting characters, only one characters make its appearance in F1 generation and is name as Dominant character. 30 Ear lobes • Free earlobes (dominant trait) hang below the point of attachment to the head. • Attached ear lobes (recessive trait) are attached directly to the side of the head. Thumbs • Straight thumbs (dominant trait) can be seen as nearly a straight line and may contain a slight arch when viewed from the side as in the illustrations. • Curved thumbs (recessive trait) can be seen as part of a circle. Pinky Bent pinky (dominant trait) vs. Straight pinky (recessive trait): 1. Hold your hands together as if you are covering your face. 2. If the tips of the pinkies (or baby fingers) point away from one another, the pinkies are bent (recessive trait). Simple Dominant/Recessive Traits • Having a bent little finger is a dominant trait Thumb Crossing • In a relaxed interlocking of the fingers, left thumb over right results from having 1 or 2 copies of the dominant version of the gene. People with 2 recessive places right thumb over left Tongue rolling • People with the DOMINANT allele can roll their tongues into a tube shape. • People with 2 recessive versions are non-rollers Incomplete Dominance • Incomplete Dominance- a cross between 2 different phenotypes that produces a third phenotype. – A Blending of the parents’ traits – Ex: Red flower x White flower = Pink flower – Punnett Square- Both letters are capital (because neither trait dominates the other) R = allele red flowers R1 = allele white flowers ** Same letter is used with different superscripts Incomplete Dominance Red (RR) x White (R1R1) R R R1 RR1 R R1 R1 R R1 R R1 Phenotype – 4 Pink (RR1) Incomplete Dominance Pink (RR1) x Pink (RR1) R R1 R RR RR1 R1 RR1 R1R1 Phenotype- 1 Red (RR) 2 Pink (RR1) 1 White (R1R1) Incomplete Dominance Codominance • Codominance- 1 allele is not dominant over the other, but both are expressed equally – You see both parents’ traits – Ex: Red flower x White flower = Red & White spotted flower R = allele red flower W = allele white flower ** Both letters are capital and different letter for each trait Codominance Red (RR) x White (WW) R R W RW RW W RW RW Phenotype- 4 Red & White Spotted (RW) Codominance Red & White Spot (RW) x Red & White Spot (RW) R W R RR RW W RW WW Phenotype- 1 Red (RR) 2 Red & White Spot (RW) 1 White (WW) Co-dominance • A cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together. 2. Codominance • Both alleles contribute to the phenotype. – Example: In come chickens Black Chicken x White Speckled Chicken YOU tell me which type of dominance… Codominance! Codominance Polygenic Traits • Traits controlled by two or more genes • Show a wide range of phenotypes • The phenotype is produced by the interaction of more than ________1 pair of genes. Polygenic Traits • These are called polygenic___________ traits, which means “having _______many genes”. • In humans, eye_____ color, • skin_____ color, Types of Genetic Crosses . Monohybrid cross - cross involving a single trait e.g. flower color . Dihybrid cross - cross involving two traits e.g. flower color & plant height copyright cmassengale 51 Monohybrid Crosses 52 P1 Monohybrid Cross • Trait: Seed Shape • Alleles: R – Round r – Wrinkled • Cross: Round seeds x Wrinkled seeds • RR x rr r r Genotype: Rr Phenotype: Round R Rr Rr Genotypic Ratio: All alike R Rr Rr Phenotypic Ratio: All alike 55 P1 Monohybrid Cross Review . Homozygous dominant x Homozygous recessive . Offspring all Heterozygous (hybrids) . Offspring called F1 generation . Genotypic & Phenotypic ratio is ALL ALIKE 56 F1 Monohybrid Cross • Trait: Seed Shape • Alleles: R – Round r – Wrinkled • Cross: Round seeds x Round seeds • Rr x Rr R r Genotype: RR, Rr, rr Phenotype: Round & R RR Rr wrinkled G.Ratio: 1:2:1 r Rr rr P.Ratio: 3:1 57 F1 Monohybrid Cross Review . Heterozygous x heterozygous . Offspring: 25% Homozygous dominant RR 50% Heterozygous Rr 25% Homozygous Recessive rr . Offspring called F2 generation . Genotypic ratio is 1:2:1 . Phenotypic Ratio is 3:1 58 What Do the Peas Look Like? 59 …And Now the Test Cross • Mendel then crossed a pure & a hybrid from his F2 generation • This is known as an F2 or test cross • There are two possible testcrosses: Homozygous dominant x Hybrid Homozygous recessive x Hybrid 60 st F2 Monohybrid Cross (1 ) • Trait: Seed Shape • Alleles: R – Round r – Wrinkled • Cross: Round seeds x Round seeds • RR x Rr R r Genotype: RR, Rr Phenotype: Round R RR Rr Genotypic Ratio: 1:1 R RR Rr Phenotypic Ratio: All alike 61 F2 Monohybrid Cross (2nd) • Trait: Seed Shape • Alleles: R – Round r – Wrinkled • Cross: Wrinkled seeds x Round seeds • rr x Rr R r Genotype: Rr, rr Phenotype: Round & r Rr rr Wrinkled G. Ratio: 1:1 r Rr rr P.Ratio: 1:1 62 F2 Monohybrid Cross Review . Homozygous x heterozygous(hybrid) . Offspring: 50% Homozygous RR or rr 50% Heterozygous Rr . Phenotypic Ratio is 1:1 . Called Test Cross because the offspring have SAME genotype as parents 63 Results of Monohybrid Crosses • Inheritable factors or genes are responsible for all heritable characteristics • Phenotype is based on Genotype • Each trait is based on two genes, one from the mother and the other from the father • True-breeding individuals are homozygous ( both alleles) are the same 64 Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. All the offspring will be heterozygous and express only the dominant
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