Mendelian Genetics

Mendelian Genetics

Vocabulary 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 t t T t

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

Mendelian Genetics