B.6.F predict possible outcomes of various genetic combinations such as monohybrid crosses, dihybrid crosses and non Mendelian inheritance Gregor Mendel Austrian monk * Studied science and mathematics at University of Vienna * Conducted breeding experiments with the garden pea Pisum sativum * Carefully gathered and documented mathematical data from his experiments Formulated fundamental laws of heredity in early 1860s Fruit and Flower of the Garden Pea Blending Inheritance Theories of inheritance in Mendel s time: * Based on blending * Parents of contrasting appearance produce offspring of intermediate appearance Mendel s findings were in contrast with this * He formulated the particulate theory of inheritance * Inheritance involves reshuffling of genes from generation to generation
One-Trait Inheritance Mendel performed cross-breeding experiments * Used true-breeding (homozygous) plants * Chose varieties that differed in only one trait (monohybrid cross) * Performed reciprocal crosses o Parental generation = P o First filial generation offspring = F1 o Second filial generation offspring = F2 * Formulated the Law of Segregation Mendel s Monohybrid Crosses: An Example Law of Segregation Each individual has a pair of factors (alleles) for each trait The factors (alleles) segregate (separate) during gamete (sperm & egg) formation Each gamete contains only one factor (allele) from each pair Fertilization gives the offspring two factors for each trait Modern Genetics View Each trait in a pea plant is controlled by two alleles (alternate forms of a gene) Dominant allele (capital letter) masks the expression of the recessive allele (lowercase) Alleles occur on a homologous pair of chromosomes at a particular gene locus Homozygous = identical alleles Heterozygous = different alleles
Homologous Chromosomes Genotype Versus Phenotype Genotype * Refers to the two alleles an individual has for a specific trait * If identical, genotype is homozygous * If different, genotype is heterozygous Phenotype * Refers to the physical appearance of the individual Punnett Square Table listing all possible genotypes resulting from a cross * All possible sperm genotypes are lined up on one side * All possible egg genotypes are lined up on the other side * Every possible zygote genotypes are placed within the squares Punnett Square Showing Earlobe Inheritance Patterns
Monohybrid Testcross Individuals with recessive phenotype always have the homozygous recessive genotype However, Individuals with dominant phenotype have indeterminate genotype * May be homozygous dominant, or * Heterozygous Test cross determines genotype of individual having dominant phenotype One-Trait Test Cross: Unknown is Heterozygous One-Trait Test Cross: Unknown is Homozygous Dominant
Predicting genotype and phenotype Punnett squares You can predict the genotypes and phenotypes of offspring if you know the genotypes of the parents. A Punnett square shows all of the possible combinations of alleles from the parents. The figure below shows how a Punnett square is made. You can predict the possible genotypes and phenotypes of offspring if you know the genotypes of the parents. A Punnett square of Mendel s first cross. You can use a punnett square to show Mendel s first cross. He crossed a true-breeding, purple-flowered plant with a true-breeding, white-flowered plant. Since the purple flowered plant is true breeding, it has two dominant alleles. The genotype of the purple flowered plant is PP. Since white flowers are recessive, the only possible genotype for a white-flowered plant is pp. Analyzing the Punnett square As you can see, all of the offspring in Mendel s first cross had a genotype of Pp. That s why all of the plants in the first generation had purple flowers. Using a Punnett square, you can predict the possible genotypes and phenotypes of the offspring. In the example above, the only possible genotype is Pp and the only possible phenotype is purple flowers.
Two-Trait Inheritance Dihybrid cross uses true-breeding plants differing in two traits * Observed phenotypes among F2 plants * Formulated Law of Independent Assortment o The pair of factors for one trait segregate independently of the factors for other traits o All possible combinations of factors can occur in the gametes Two-Trait (Dihybrid) Cross Two-Trait Test Cross
Dihybrid Crosses: Crosses that involve 2 traits. For these crosses your punnet square can be a 4x4 In any case where the parents are heterozygous for both traits (AaBb x AaBb) you will get a 9:3:3:1 ratio.
Non-Mendelian Heredity Incomplete and Codominance When Mendel studied pea plants, he happened to select traits that were determined by two alleles where one allele was completely dominant over the other allele. For instance, for flower color in peas, purple flowers are dominant and white flowers are recessive. But some patterns of inheritance are different than the ones Mendel discovered. In this skill sheet, you will get some practice with two other patterns of inheritance called incomplete dominance and codominance. When a red snapdragon is crossed with a white snapdragon, the next generation will have all pink flowers! Because red and white blend, this is an example of a pattern of inheritance called incomplete dominance (see example below). In incomplete dominance, the phenotypes of the two alleles blend just like mixing paints.
In codominance, an organism that has both alleles of a gene displays both phenotypes at the same time. For example, a cross between a black cat and a tan cat results in a tabby cat. Codominant genes are BOTH expressed in the phenotype!
Sex Linked Traits The genes for these traits are on the X chromosome, because boys only receive one X chromosome they are more likely to inherit disorders passed to them from their mother who would be a carrier. Hemophilia and Colorblindness are sex linked traits, the punnet square below shows how a woman who is a carrier passes the trait to her son, but not her daughters. Muliple Allele Traits Traits that are controlled by more than two alleles. Blood type in humans is controlled by three alleles: A, B, and O Phenotype A B AB O Genotype AA or AO BB or BO AB only OO only Examples of Blood type crosses
Blood Transfusions Blood can only be transferred to a body of a person who's immune system will "recognize" the blood. A and B are antigens on the blood that will be recognized. If the antigen is unfamiliar to the body, your body will attack and destroy the transfused blood as if it were a hostile invader (which can cause death). O is like a blank, it has no antigens. O is called the universal donor because a person can receive a transfusion from O blood without having an immune response AB is the universal acceptor, because a person with AB blood has both the A and B antigens already in the body, A and B blood can be transfused to the person (as well as O) and the body will recognize it and not attack. Multiple Allelic Traits Example Some traits controlled by multiple alleles The gene exists in several allelic forms (but each individual only has two) ABO blood types The alleles: I A = A antigen on red cells, anti-b antibody in plasma I B = B antigen on red cells, anti-ab antibody in plasma i = Neither A nor B antigens, both antibodies Phenotype Genotype (Blood Type) A (actually AA or AO) I A I A or I A i B (actually BB or BO) I B I B or I B i AB I A I B O (actually OO) ii Inheritance of Blood Type
Terminology Incomplete Dominance Heterozygote has phenotype intermediate between that of either homozygote Codominance More than one allele is fully expressed ABO blood type (multiple allelic traits) Epistasis A gene at one locus interferes with the expression of a gene at a different locus Human skin color (polygenic inheritance) Environment and Phenotype: Himalayan Rabbits