Mendel Used the Scientific Approach to Identify Two Laws of Inheritance Mendel s Experimental Quantitative Approach Character a heritable feature that varies among individuals Trait each variant for a character Mendel used peas to test traits b c of short generation time many varieties large number of offspring and easy to control True Breeding after years of self pollination same variety parent plant is produced Hybridization mating or crossing of two true breeding varieties P generation true breeding parents F1 generation first filial generation produces F2 generation The Law of Segregation If blending model was correct F1 of white flowered and purple flowered plants would have pale purple flowers Heritable trait for white flowers did not disappear but stay somewhat hidden Purple flowers are dominant white flowers are recessive Mendel s Model Alternative versions of genes account for variations in inherited characters Alleles alternate versions of genes Each genes is a sequence of nucleotides at a specific place or locus along particular chromosome DNA at locus can vary in information For each character an organism inherits two copies of a gene one from each parent A genetic locus is represented 2x in diploid cell 1x on each homolog If the two alleles at a locus differ then one the dominant allele determines the organism s appearance the other the recessive allele has no noticeable effect on the organism s appearance The two alleles for a heritable character segregate during gamete formation and end up in different gametes The Law of Segregation An egg or sperm only gets one of the two alleles present in the somatic cells of the organism making the gamete Segregation corresponds to the distribution of the two members of a pair of homologous chromosomes to different gametes in meiosis Punnett Square illustrates combinations of alleles Useful Genetic Vocabulary Homozygous organism that has a pair of identical alleles for a character PP or pp Breed true Heterozygous organism that has two different alleles for a gene Produce gametes with different alleles Phenotype organism s appearance physiological or observable traits Genotype organism s genetic makeup PP and Pp have same phenotype but different genotype Mendel Used the Scientific Approach to Identify Two Laws of Inheritance The Law of Segregation cont Test Cross Test cross with recessive homozygous to determine mystery organism s genotype Reveals organism s genotype The Law of Independent Assortment Monohybrids heterozygous for the particular character being followed in the cross Mendel identified second law by following two characters at the same time Dihybrids individual heterozygous for 2 characters being followed in cross YyRr Are alleles packaged together and passed on Dependent assortment hypothesis Independent assortment hypothesis alleles segregate independently of each other Law of Independent Assortment each pair of alleles segregates independently of each other pair of alleles during gamete formation Applies to genes on different chromosomes not homologous The Laws of Probability Govern Mendelian Inheritance The Multiplication and Addition Rules Applied to Monohybrid Crosses Multiplication Rule to determine the probability of two or more independent events will occur together we multiply the probability of one event by the probability of the other event Addition Rule the probability that any one or two mutually exclusive events will occur is calculated by adding their individual probabilities Solving Complex Genetics Problems with the Rules of Probability A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously Knowing probabilities from Punnett Square we can use multiplication rule to determine probability of each genotype Rules of probability give the chance of various outcomes Inheritance Patterns are Often More Complex than Predicted by Simple Mendelian Genetics Relationship between phenotype genotype is rarely as straightforward as peas Extending Mendelian Genetics for a Single Gene Inheritance of characters deviates from simple Mendelian patterns when alleles are not completely dominant or recessive when particular gene has 2 alleles or when a single gene produces multiple phenotypes Degrees of Dominance Alleles can show different degrees of dominance and recessiveness in relation to each other Complete Dominance phenotypes of the heterozygote and dominant homozygote are indistinguishable Incomplete Dominance neither allele is dominant blending hypothesis Codominance the two alleles can affect the phenotype in separate distinguishable ways Inheritance Patterns are Often More Complex than Predicted by Simple Mendelian Genetics Extending Mendelian Genetics for a Single Gene Degrees of Dominance cont The Relationship Between Dominance and Phenotype Dominant because it shows in phenotype not because it subdues recessive The observed dominant recessive relationship of alleles depends on the level at which we examine phenotype Tay Sachs Disease inherited disorder in humans Frequency of Dominant Alleles Dominant gene might not show often as it is may not be found so frequently Multiple Alleles Blood type determined by 3 alleles IA IB i A B AB O Pleiotropy Most genes have multiple phenotypes Responsible for multiple symptoms associated with diseases Single gene can affect multiple characteristics Extending Mendelian Genetics for Two or More Genes Epistasis Phenotypic expression of a gene at one locus alters that of a gene at a second locus BB Bb Black fur bb brown fur EE Ee black brown fur ee yellow fur 9 3 3 1 Polygenic Inheritance Quantitative characters characters that vary in the population in gradations along a continuum Indicates polygenic inheritance Additive effect of two or more genes on a single phenotypic character Converse of pleiotropy Skin variation from AABBCC dark to aabbcc light Can be affected by environmental factors Nature and Nurture The Environmental Impact on Phenotype Phenotype depends on genotype and environment Norm of reaction range of phenotypic possibilities due to environmental influences Integrating a Mendelian View on Heredity and Variation Phenotype special characteristics or organism in entirety physical appearance anatomy physiology and behavior Genotype individual alleles or entire genetic makeup Mendel s laws explain heritable variations based on genes Many Human Traits Follow Mendelian Patterns of Inheritance