Mendelian Genetics Gregor Mendel Experiments in Plant Hybridization

Mendelian Genetics

Gregor Mendel • “Experiments in Plant Hybridization” – published in 1866, noticed in 1900. • Replaced “blending theory” of inheritance (herencia mezcladora) with “particulate theory” (unidades de herencia)

Gregor Mendel – Experimental Design • Used the garden pea, Pisum sativum as a model organism – Easily recognizable traits: e. g. , flower color, seed shape – Could control which individuals were mating (removed male reproductive organ before pollen formed) • Obtained “pure lines” or “truebreeding lines” from breeders in which a trait was always seen 1 Removed stamens from purple flower 2 Transferred sperm- bearing pollen from stamens of white flower to eggbearing carpel of purple flower Parental generation (P) 3 Pollinated carpel Stamens Carpel (male) (female) matured into pod 4 Planted seeds from pod 5 Examined First generation offspring (F 1) offspring: all purple flowers

Mendel’s Experiments with a Single Trait • Blending inheritance hypothesis: claimed that traits observed in a mother and father blend together to form the traits observed in their offspring. As a result, traits are an intermediate between the mother’s and father’s. X Parental ? ? F 1

Mendel’s Experiments with a Single Trait • Blending inheritance hypothesis: claimed that traits observed in a mother and father blend together to form the traits observed in their offspring. As a result, traits are an intermediate between the mother’s and father’s. X Parental ? X? F 1 ? ? F 2

Mendel’s Experiments with a Single Trait • Blending inheritance hypothesis: claimed that traits observed in a mother and father blend together to form the traits observed in their offspring. As a result, traits are an intermediate between the mother’s and father’s. Parental X ? X? ? ? F 1 F 2

Mendel’s Experiments with a Single Trait • Mendel let the F 1 generation mature and self-pollinate. • Monohybrid cross – between two parents that each carry two different genetic determinants for the same trait (each F 1 individual had a smooth and wrinkly parent) • Outcome: 5474 round, 1850 wrinkly! (~3: 1 ratio) X Parental ? X? F 1 F 2

Mendel’s Experiments with a Single Trait • Outcome: 5474 round, 1850 wrinkly! (~3: 1 ratio) • Mendel coined these terms: – Recessive – the wrinkled-seed shape seemed to recede or become hidden. – Dominant – the round seeds appeared to dominate. X Parental ? X? ? ? F 1 F 2

Mendel’s Experiments with a Single Trait • Repeated this experiment with other traits, had identical results, with a ratio of 3: 1 in F 2. • Mendel formulated the Particulate Inheritance Hypothesis – Hereditary determinants maintain their integrity from generation to generation. Instead of blending together, they act as discrete entities or particles. X Parental ? X? ? ? F 1 F 2

Particulate Theory • Inherited characters are determined by particular factors (now called genes). • These factors occur in pairs (i. e. , genes occur on maternal and paternal homologous chromosomes). • When gametes form, these genes segregate so that only one of the homologous pair is contained in a particular gamete.

Mendel’s Laws • Mendel’s First Law, Law of Segregation – Members of each gene pair must separate into different gamete cells during the formation of eggs and sperm. Each gamete has an equal chance of possessing either member of a pair of homologous Parental Genotype SS chromosomes. x ss Parental Gametes F 1 Genotype S s Ss

Punnett Squares • • • Monohybrid cross A = Yellow a = Green Genotype ratio: Phenotype ratio:

Punnett Squares • • • Monohybrid cross A = Yellow a = Green Genotype ratio: 1: 2: 1 Phenotype ratio: 3: 1

Example • Say we have a yellow pea plant that had a green pea plant as the mother. We cross the yellow pea plant to a green pea plant. • A = Yellow • a = Green 1 x? • What is the genotype of pea #2? • Of pea #3? • What possible gametes can pea #2 produce? 2 x 3

Experiments with Two Traits • Do different genes segregate together or independently? • The Dihybrid Cross – Crossing two individuals heterozygous for two traits. • Possibilities: – Independent assortment: seed shape and color will separate from each other and be transmitted independently. – Allele for seed shape and seed color present always as seen in parent plants, meaning genes are transmitted together. x Yy. Rr

Experiments with Two Traits • Do different genes segregate together or independently? • The Dihybrid Cross YYRR – Crossing two individuals heterozygous for two traits. • Possibilities: – Independent assortment: seed shape and color will separate from each other and be transmitted independently. – Allele for seed shape and seed color present always as seen in parent plants, meaning genes are transmitted together. Parental yyrr F 1 x Yy. Rr

Mendel’s Laws • Mendel’s Second Law, Law of Independent Assortment – Genes on nonhomologous or different chromosomes will be distributed randomly into gametes. • Anaphase of Meiosis I – Chromosomes segregate randomly

Codominance - Blood Type • Phenotypes of both alleles expressed at the same time. • Example: AB blood phenotype, IAIB genotype • Today: Test blood for phenotype, determine genotype and possible paternity

Incomplete Dominance

Sex Linkage and Chromosome Theory • Sex-linked traits are located on the sex chromosomes (X or Y) fruit fly eye color XX - female XY - male

Sex Linkage and Chromosome Theory • Sex-linked traits are located on the sex chromosomes (X or Y)

Polygenic Traits • Trait influenced by more than one gene • Examples: Blood type (e. g. , O-); fur color in many species

Pleiotrophy • One gene influences more than one trait • Example: Sickle-cell anemia causes sickleshaped red blood cells but also resistance to malaria • (La anemia de células falciformes)

Environmental Influence on Gene Expression • Phenotypes of most genes strongly influenced by the environment • PKU (Phenylketonuria) • Winter coats

Mendelian Genetics in Real Life • Most traits are controlled by many genes, so Punnett squares may oversimplify • Commonly used in selective breeding on simple traits • May be used in genetics counselling