Chapter 10 Section 2 Mendelian Genetics Main Idea

Chapter 10 Section 2: Mendelian Genetics Main Idea: Mendel explained how a dominant allele can mask the presence of a recessive allele

Gregor Mendel: The Father of Modern Genetics • How Mendel Shaped the Field of Genetics • Austrian Monk with a passion for gardening • Began performing experiments on pea plants

How Genetics Began… • Heredity (inheritance) – passing of traits to the next generation • Genetics – the science of heredity • Mendel studied visible traits of pea plants, one at a time • Use math to recognize patterns

The Inheritance of Traits • Took plants that were truebreeding for one trait • Cross-pollinated them and expected a mixture of each trait in the offspring • Surprised at the results

The Results • After crossing plants true-breeding for flower color (P generation), all offspring were purple (F 1 generation) • Allowed F 1 to self-pollinate • F 2 generation had a mixture of purple and white in a 3: 1 ratio

Mendel’s Traits • Seed color • Seed shape • Seed pod color • Stem length • Flower color • Flower position Same 3: 1 ratio in F 2 generation with all traits

Mendel’s Conclusions • 2 factors that determine each trait • 1 from mother 1 from father • Alleles – alternative forms of traits • Gene = flower color; alleles = purple or white

Mendel’s Laws • Law of Dominance • Law of Segregation • Law of Independent Assortment

Law of Dominance • Certain traits are dominant to others • That trait will mask the recessive version • Recessive trait disappears in f 1 generation and reappears in f 2 generation • If the dominant allele is present, the dominant trait will be expressed

Law of Dominance • Dominant allele is represented with a capital letter (P); recessive allele with a lower case letter (p) • Homozygous – individual with 2 of the same allele for a trait (PP or pp) • Heterozygous – individual with 2 different alleles for a trait (Pp) • If the dominant allele is present, the dominant trait is expressed • The recessive allele is only expressed if 2 recessive alleles are present

Genotype vs. Phenotype • Genotype = genetic makeup; allele pairs • Phenotype = physical appearance; outward expression of allele pairs • PP = ? • Pp = ? • pp = ?

Law of Segregation • The two alleles for a trait segregate (separate) during sex cell formation • Alleles reunite in fertilization • So…each pea plant gets one allele from each parent

Law of Independent Assortment • Alleles located on different chromosomes assort independently of one another • In other words, pea color does not determine flower color, etc. • Assortment is random and each allele combination is equally likely to occur

Alleles and Genes

Punnett Squares T • A Punnett square is a tool used to predict all possible outcomes of a cross between two individuals focusing on a specific trait T t Tt Tt • The genotype of the mother is shown on the top of the square (TT) • The genotype of the father is shown on the side of the square (tt) • The various possible genotypes of the offspring are shown in the

Punnett Squares • What happens if you cross a heterozygous tall plant with a short plant? • Phenotypic Ratio = ? • Genotypic Ratio = ?

Punnett Squares • If deafness in dogs is recessive, show the possible offspring when 2 dogs heterozygous are mated. • Phenotypic ratio = ? • Genotypic ratio = ?

Test Cross • Farmer Dan breeds guinea pigs • There is a gene in guinea pigs for tooth length. Teeth can either be normal (T) or bucked (t) • Farmer Dan thinks he buys a homozygous normal (TT) guinea pig. How can he be sure that he has a “true-breeding” (TT) and not a heterozygous (Tt) guinea pig? • He performs a test cross – crossing an organism with an unknown genotype with one that is known

Test Cross • If it is TT, the result will be all normal teeth • If it is Tt, there will be a mixture of normal and bucked teeth T T t Tt Tt T t t Tt tt

PUNNETT SQUARE MADNESS!!!