Genetics • • • • • • • • • • • • • • • Genetics Gene Heredity Traits Allele Dominant Recessive Genotype Phenotype Characteristics Gamete Fertilization Zygote Homozygous Heterozygous Genetics is the science that studies how genes and their traits are transmitted from one generation to the next. Trait: A specific characteristic that varies from one individual to another. Examples include hair color, eye color, height, and skin color. Traits are controlled by genes. _________ Different forms of a gene are called alleles ________________ Gregor Mendel • An Austrian monk • Born in 1822. • Known as the Father of Genetics • Spent 14 years experimenting in genetics on pea plants • Discovered 3 Laws of Genetics still used today. • Mendel's great contribution was to demonstrate that inherited characteristics are carried by genes. The three basic laws of inheritance that Mendel discovered are: 1. The Principle of Dominance and Recessiveness 2. The Principle of Segregation 3. The Principle of Independent Assortment If the peas self pollinate: • Tall plants produce tall offspring plants. • Plants with yellow seeds produce offspring plants with yellow seeds. • We call this “TRUE BREEDING” Because these plants self pollinate and produce identical offspring, Mendel was able to control his experiments ALLELE A capital letter shows a A lower case letter shows a DOMINANT recessive characteristic. characteristic. This displays the DOMINANT characteristic. (True-breeding) • Homozygous• The two alleles inherited by the offspring are the same • TT Homozygous DOMINANT Homozygous recessive • tt • Heterozygous• The two alleles inherited by the offspring are different • Tt Before we proceed, the following genetic Generation: vocabulary: 1. P generation: Parental generation 2. F1 generation: First generation of offspring 3. F2 generation: Second generation of offspring 4. Hybrids: The offspring of parents with different traits. All the organisms belonging to the same time period/parents. Tt DNA Replication (You could also think of this as Homologous chromosomes lined up) The formation of gametes (MEIOSIS) TT tt Meiosis I TT tt Meiosis II T T t t 4 Haploid gametes each containing only one Alelle • Every individual carries 2 alleles for each trait. • The two alleles segregate (separate) during meiosis. Tt Replication of DNA TTtt Meiosis I tt TT Meiosis II T T t t Using his protocol, Mendel crossed tall pea plants with dwarf pea plants. Children = F1 P = Tall x dwarf ALL tall offspring • The F1 hybrids were all tall. • All of the offspring had the appearance of only one of the parents. • The trait of the second parent seemed to have disappeared. • Mendel wondered if the dwarf trait had been lost. Mendel allowed the hybrid tall offspring from the first generation (F1) to self pollinate Those children = F1 = new parents New children = F2 = the grandchildren F1 Tall x F1 Tall offspring: ¾ tall and ¼ dwarf Dominant allele: The allele whose trait will always be represented in the offspring if its allele is present. Recessive allele: The allele whose trait will only show up in the offspring if the dominant allele is NOT present. Phenotype: Genotype: The genetic The physical makeup of characteristics an organism. of an organism Genotype Phenotype TT Tt tt Tall Tall Dwarf For example, in Mendel’s pea plants, the tall allele was DOMINANT over the dwarf allele: So they all look Tall Why did the recessive allele seem to disappear in the F1 generation and then reappear in the F2 generation? TT • • tt Tt Tt TT Tt Tt tt Each parent passed on 1 allele to each offspring. In the F1 generation: • Each F1 plant contributed one tall allele or one dwarf allele. • Therefore the F2 generation got either two tall alleles, two dwarf alleles or one tall and one dwarf allele. • Two tall = F2 tall • Two dwarf alleles = F2 dwarf • One tall and one dwarf = F2 Tall = heterozygous Using Probability and Punnett Squares to Work Genetics Problems Punnett Square Diagram showing the allele combinations that might result from a genetic cross between two parents. Punnett Square T t T T T Tt 1. Using letters, place the alleles of the first parent at the top. T T T Tt 2. Place the alleles of the second parent on the left side. T=dominant Parent 1 = Heterozygous Tt Parent 2 = Homozygous dominant TT 3. Fill in the square with the possible gene combinations of the offspring. 4. Indicate your genotypic and phenotypic ratios for the offspring. G P 2TT:2Tt = 1:1 = 50%, 50% 4 Tall 100% Having dimples is DOMINANT over the absence of dimples. Create a punnett square for crossing a heterozygous dimpled man with a woman who does not have dimples. What is the genotype of the man? What is the genotype of the woman? D d d Dd d d d Dd d d 2 Dd 2 dd 2 with Dimples 2 without Dimples
