Mendelian Genetics

Vocabulary Dominant Recessive Hybrid Phenotype Genotype Homozygous Heredity Genetics Trait Gamete Pollination Fertilization Law of segregation Law of independent assortment Dominant Recessive Hybrid Phenotype Genotype Homozygous Heterozygous Allele

Genetics Scientific study of heredity Heredity-passing of traits from parents to offspring Trait -characteristic that is inherited Many had realized family resemblance but didn’t know how

Gregor Mendel Austrian monk 1822-1884 Came up with the laws of heredity Importance of work not realized until 1900

Mendel’s Work First to succeed in predicting how traits would be transferred from one generation to the next Used garden peas (reproduced sexually via gametes) Pollination -transfer of male pollen grains to the pistil of flower (female)

Pollination Results in fertilization (fusion of male and female gametes) Pea plants normally went through self-pollination (gametes from self) Mendel needed to cross pollinate Taking pollen from one plant and using it to fertilize another plant

Monohybrid Crosses Crosses where the parents only differed by one trait (mono) Hybrid -offspring formed by parents having different forms of a trait First trait he looked at was height

First Generation Six-foot-tall pea plant that came from a population of pea plants, all of which were over six feet tall Short pea plant that was less than two feet tall and which came from a population of pea plants that were all short Cross-pollinated the two plants

Result All of the offspring grew to be as tall as the taller parent It was as if the shorter parent had never existed Ratio of 4:0

Second Generation Allowed the tall plants in this first generation to self-pollinate Counted more than 1000 plants in this second generation ¾ of the plants were tall ¼ of the plants were short Ratio of approximately three tall plants to one short plant (3:1)

Generations Original parents, the true-breeding tall and short plants, are known as the P1 generation (P- parent) Cross of P1 are known as the F1 generation (F- filial or son/daughter) Cross two F1 plants with each other equals F2 generation

Further Experimentation Did similar monohybrid crosses with a total of seven pairs of traits Studying one pair of traits at a time In every case, he found that one trait of a pair seemed to disappear in the F1 generation, only to reappear unchanged in one-fourth of the F2 plants

Genes Located on chromosomes, units of heredity The rule of unit factors Each organism has two factors that control each of its traits Genes exists in alternative forms

Alleles Alternative forms of a gene for each variation of a trait of an organism Could have 2 alleles for tallness 2 alleles for shortness 1 allele for tallness and 1 for shortness Receive 1 allele from each parent

The Rule of Dominance One gene in a pair masks another Dominant -observed trait of an organism that masks the recessive form of a trait Recessive -trait of an organism that can be masked by the dominant form of a trait

How to Write the Alleles Each gene is expressed as a letter Dominant allele is expressed as a uppercase letter Recessive allele is expressed as a lower case letter Previous example: T for tall, t for short

Law of Segregation 1. There are alternative forms for genes. 2. For each characteristic or trait organisms inherit two alternative forms of that gene, one from each parent. These alternative forms of a gene are called alleles.

Law of Segregation 3. When gametes (sex cells) are produced, allele pairs separate or segregate leaving them with a single allele for each trait. 4. When the two alleles of a pair are different, one is dominant and the other is recessive.

Phenotypes and Genotypes What was the difference between the tall plants in P1 and the tall plants in F1? Two organisms, therefore, can look alike but have different underlying gene combinations

Phenotype Outward appearance of an organism, regardless of its genes How the organism looks A tall TT plant and a tall Tt plant look what? Their phenotype would be tall

Genotype Combination of genes in an organism Does the tall TT plant and the tall Tt plant have the same genotype? Can’t always tell an organism’s genotype by looks

Homozygous vs. Heterozygous Homozygous- when there are two identical alleles for a trait TT and tt Heterozygous- when there are two different alleles for a trait Tt

Law of Independent Assortment Genes for different traits are inherited independently of each other For example, seed shape and seed color

Punnett Squares Shorthand way of finding the expected proportions of possible genotypes in the offspring of a cross

How to Work Punnett Square Step #1: Determine the genotypes of the parent organisms Step #2: Write down your "cross" (mating).  Write the genotypes of the parents in the form of letters (ex: Tt x tt)

Step #3: Draw a p-square T is the allele for being tall and t is the allele for being short, cross one plant that is homozygous recessive (tt) with another that is heterozygous tall (Tt).

Step #4: "Split" the letters of the genotype for each parent & put them "outside" the p-square

Step #5: Determine the possible genotypes of the offspring by filling in the p-square