Biology Chapter 11 – Introduction to Genetics Notes The Work of Gregor Mendel * Genetics – the scientific study of heredity A.) Gregor Mendel – Austrian monk; developed the foundation for the science of genetics 1.) Used garden peas in his experiments 2.) Understood that fertilization between two different plants produces a new cell a.) Fertilization – the joining of male and female reproductive cells 3.) He started his experiment with true-breeding pea plants a.) True-Breeding – describes organisms that produce offspring identical to themselves if allowed to self-pollinate b.) Each true-breeding pea plant had a different characteristic (tall plants, short plants, green seeds, yellow seeds, etc) 4.) He cross-pollinated plants with different traits and studied their offspring a.) Trait – a specific characteristic that varies from one individual to another. b.) He studied seven (7) different traits, each trait had two contrasting characters c.) Each original pair of plants are called the P (parental) generation d.) Offspring are called the F1 (“first filial”) generation. (Filius and filia are the Latin words for “son” and “daughter”) e.) Hybrids – offspring of crosses between parents with different traits TRAITS P generation F1 generation Seed Shape Round OR Wrinkled Seed Color Yellow OR Green Seed Coat Color Gray OR White Pod Shape Smooth OR Constricted Pod Color Green OR Yellow Flower Position Axial OR Terminal Plant Height Tall OR Short Round Yellow Gray Smooth Green Axial Tall 5.) All offspring from the P generation had the trait of only one of the parents (the Dominant Trait) 6.) Two Conclusions formed by Mendel a.) Biological inheritance is determined by factors that are passed from one generation to the next. - Genes – chemical factors that determine traits - Each trait is controlled by one gene that occurred in two contrasting forms - These contrasting forms produced the different characters of each trait EX: Plant height occurs in one form that produces tall plants and another form that produces short plants - Alleles – the different forms of a gene b.) The principle of dominance states that some alleles are dominant and others are recessive. - The dominant allele for a particular form of a trait will always exhibit that form of the trait. 7.) Mendel questioned if the recessive alleles had disappeared or were they hidden in the F1 generation a.) He allowed the F1 generation to self-pollinate, which produced an F2 generation. b.) One out of four plants produced in the F2 generation had the trait for the recessive allele c.) The recessive allele was segregated from the dominant allele in the F1 generation during the formation of sex cells - Segregation – separation of alleles during gamete formation - Gamete – specialized cell involved in sexual reproduction 8.) When each F1 plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene. Therefore, each F1 plant produces two types of gametes – those with the allele for tallness and those with allele for shortness EX: Tallness – T = dominant allele t = recessive allele Probability and Punnett Squares A.) Probability – the likelihood that a particular event will occur * The principles of probability can be used to predict the outcomes of genetic crosses B.) Punnett Square – diagram showing the gene combinations that might result from a genetic cross * Punnett squares can be used to predict and compare the genetic variations that will result from a cross 1.) Homozygous – organisms that have two identical alleles for a particular trait (True-breeding for a particular trait) EX: Homozygous Dominant TT or Homozygous Recessive tt 2.) Heterozygous – organisms that have two different alleles for the same trait (Hybrid for a particular trait) EX: Tt 3.) Phenotype – physical characteristic EX: Tall pea plants – TT or Tt – alleles do not matter just the physical appearance 4.) Genotype – genetic makeup EX: Tall plant – TT Tall plant – Tt Phenotype ratio – 3:1 T t T TT Tt t Tt tt Exploring Mendelian Genetics * Does the segregation of one pair of alleles affect the segregation of another pair of alleles? For example, does the gene that determines seed shape (round or wrinkled) have anything to do with the gene for seed color (yellow or green)? A.) The Two-Factor Cross: F1 1.) Crossed homozygous dominant plants that produced only round yellow peas (genotype RRYY) with homozygous recessive plants that produced wrinkled green peas (genotype rryy). rryy ry ry ry ry RY RrYy RrYy RrYy RrYy RY RrYy RrYy RrYy RrYy RY RrYy RrYy RrYy RrYy RY RrYy RrYy RrYy RrYy RRYY 2.) All of the F1 offspring were heterozygous dominant for round yellow peas (RrYy) 3.) This cross does not indicate whether genes assort, or segregate, independently. B.) The Two-Factor Cross: F2 1.) Crossed two plants that were heterozygous dominant for round yellow peas (RrYy) RrYy RY RrYy Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy 2.) The F2 resulted in a 9 : 3 : 3 : 1 ratio. a.) 9 – round yellow (like the parents) b.) 3 – round green c.) 3 – wrinkled yellow d.) 1 – wrinkled green 3.) This proved that alleles for seed shape segregated independently of those for seed color a.) Independent assortment – states that genes for different traits can segregate independently during the formation of gametes. * Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms. C.) Beyond Dominant and Recessive Alleles * Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes. 1.) Incomplete dominance – one allele is not completely dominant over another (the heterozygous phenotype is somewhere in between the two heterozygous phenotypes) a.) EX: Four o’clock plants RR = red flowered (homozygous dominant) WW = white flowered (homozygous dominant) R R W RW RW W RW RW b.) The F1 generation consists of pink flowers 2.) Codominance – both alleles contribute to the phenotype a.) Ex: The allele for black feathers is codominant with the allele for white feathers in certain varieties of chicken. Feathers are speckled with black and white feathers. (Black and white colors appear separate) 3.) Multiple Alleles – three or more alleles for the same gene a.) Ex: Coat color in rabbits - It is controlled by four different alleles C = full color; dominant to all other alleles cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles ch = Himalayan; color in certain parts of body; dominant to c allele c = albino; no color; recessive to all other alleles 4.) Polygenic Traits – traits controlled by two or more genes a.) Ex: - Fruit flies - three genes control eye color (Aa,Bb,Cc) - Humans – four genes control skin color (Aa, Bb, Cc, Dd) D.) Thomas Morgan – American geneticist; performed experiments with Fruit flies by cross breeding them for different phenotypes
