Mendelian Genetics

Nature of Science Focuses on the natural world Aims to explain the natural world Uses testable ideas Relies on evidence Involves the scientific community Leads to ongoing research

Scientific Method Make insightful observations Pose and clarify testable questions Formulate hypotheses Do experiments to gather data Quantify the data Test the hypotheses Refine hypotheses and re-test Answer questions and make conclusions

Gregor Mendel “Experiments in Plant Hybridization” – published in 1866, noticed in 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 “true- breeding lines” from breeders in which a trait was always seen Removed stamens from purple flower Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower Parental generation (P) Pollinated carpel matured into pod Carpel (female) Stamens (male) Planted seeds from pod Examined offspring: all purple flowers First generation offspring (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 ? ? F1 Parental

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 ? X ? F1 F2 Parental ? ?

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 ? X ? F1 F2 ? ? Parental

Mendel’s Experiments with a Single Trait Mendel let the F1 generation mature and self-pollinate. Monohybrid cross – between two parents that each carry two different genetic determinants for the same trait (each F1 individual had a smooth and wrinkly parent) Outcome: 5474 round, 1850 wrinkly! (~3:1 ratio) X ? X ? F1 F2 Parental

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 ? X ? F1 F2 Parental ? ?

Mendel’s Experiments with a Single Trait Repeated this experiment with other traits, had identical results, with a ratio of 3:1 in F2. 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 ? X ? F1 F2 Parental ? ?

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 chromosomes. SSxss S s Parental Genotype Parental Gametes F1 Genotype

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

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 What is the genotype of pea #2? Of pea #3? What possible gametes can pea #2 produce? Example x x? 1 2 3

Determine color and height ratios for corn plants Examine ears of corn with different color grains Record observations and determine the probable genotypes of the parents of each cross. Each kernel is a seed/individual. Cross involving two traits

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. YyRr x

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. YyRr x YYRR yyrr Parental F1

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, I A I B 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) XX - femaleXY - male fruit fly eye color

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 sickle- shaped red blood cells but also resistance to malaria

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