MendelianGenetics (I) Foundations of Genetics (A) Heredity Traits (characteristics) are passed down from the parent to the offspring. Traits (characteristics) are passed down from the parent to the offspring. Chromosomes of each gamete (sperm and ova) contain the traits of the parent. Chromosomes of each gamete (sperm and ova) contain the traits of the parent. The branch of science that is concerned with the ways in which hereditary information is transmitted to offspring is known as Genetics. The branch of science that is concerned with the ways in which hereditary information is transmitted to offspring is known as Genetics. (B) Gregor Mendel 1. An Austrian monk who performed a series of experiments with sweet peas ( ). 2. He crossed different pea plants displaying opposite traits, like tallness vs. shortness. 3. From this he wanted to see how these traits were inherited by the offspring plants. 4. Based on his findings, Mendel proposed that certain traits were inherited as a result of the transmission of hereditary factors. 5. Mendels hereditary factors, called genes, are located on the chromosomes. 6. Two genes (one from each parent) called alleles determine the trait of the offspring. 7. In the early 1900s, T.H. Morgan carried out breeding experiments (crosses) with the fruit fly, Drosophila melanogaster that supported Mendels findings. Background Information 1. Genes = alleles (found on chromosomes) 2. Capital letter = dominance Lower-case letter = recessive Lower-case letter = recessive Ex: T = tallness which is dominant Ex: T = tallness which is dominant t = shortness which is recessive t = shortness which is recessive Homozygous Pure Pure Both alleles are the same Both alleles are the same TT = homozygous tall TT = homozygous tall tt = homozygous short tt = homozygous short Heterozygous Also known as Hybrid Also known as Hybrid Both alleles are different Both alleles are different Tt = heterozygous or hybrid Tt = heterozygous or hybrid (II) Major Concepts Proposed by Mendel Mendel 1. Principle of Dominance Also referred to as Law of Dominance. Also referred to as Law of Dominance. States that the dominant allele will mask (cover) the recessive allele if the pair is hybrid. States that the dominant allele will mask (cover) the recessive allele if the pair is hybrid. Tt = tall, since T is dominant for tallness. Tt = tall, since T is dominant for tallness. The only time that the recessive allele is shown is when its homozygous recessive. The only time that the recessive allele is shown is when its homozygous recessive. tt = short tt = short 2. Principle of Segregation States that the alleles of an allelic pair will separate and then recombine to form a new trait. States that the alleles of an allelic pair will separate and then recombine to form a new trait. (In other wordsthe gene from each parent will combine to produce the traits in the offspring.) Ex: Punnett square Ex: Punnett square Drawing Punnet Squares Ex: Tt x tt 12 34 56 7 Punnet Square Genotype vs. Phenotype Genotype- genetic makeup (is it homozygous or heterozygous). Genotype- genetic makeup (is it homozygous or heterozygous). Phenotype- the appearance of the offspring due to the genotype. Phenotype- the appearance of the offspring due to the genotype. Y = the dominant allele for yellow y = the recessive allele for green Y = the dominant allele for yellow y = the recessive allele for green GENOTYPES 1. Homozygous Dominant (YY) 2. Heterozygous (Yy) 3. Homozygous Recessive (yy) GENOTYPES 1. Homozygous Dominant (YY) 2. Heterozygous (Yy) 3. Homozygous Recessive (yy) RESULTING PHENOTYPE 1. Yellow 2. Yellow 3. Green RESULTING PHENOTYPE 1. Yellow 2. Yellow 3. Green (III) Generations P 1 F 1 F 2 P 1 F 1 F 2 (parents) (offspring) (grandchildren) (IV) Intermediate Inheritance The hybrid offspring are phenotypically different (look different) than their homozygous parents. The hybrid offspring are phenotypically different (look different) than their homozygous parents. Two types: Two types: 1. Codominance 1. Codominance 2. Incomplete Dominance 2. Incomplete Dominance Codominance Both alleles are dominant. Both alleles are dominant. Mixture of the 2 traits. Mixture of the 2 traits. F 1 generation = roan cattle. Cattle that is roan will have both white hairs and red hairs on it (mixture). F 1 generation = roan cattle. Cattle that is roan will have both white hairs and red hairs on it (mixture). R = allele for red cattle W = allele for white cattle red x white ---> roan RR x WW ---> 100% RW (roan) Incomplete Dominance Think pink roses. Think pink roses. Both alleles are dominant. Both alleles are dominant. Blending of traits. Blending of traits. F 1 generation = pink roses. The roses will be pink due to a blending of traits. F 1 generation = pink roses. The roses will be pink due to a blending of traits. R = allele for red flowers W = allele for white flowers red x white ---> pink RR x WW ---> 100% RW In codominance and incomplete dominance, the F 2 generation is always In codominance and incomplete dominance, the F 2 generation is always 1:2:1 1:2:1 (V) Multiple Alleles More than two alleles for a given trait may be present within each cell. More than two alleles for a given trait may be present within each cell. Ex: blood types Ex: blood types Blood Types I = dominant i = recessive GENOTYPES I A I A I A i RESULTING PHENOTYPES Type A Type A IBIBIBiIBIBIBiType B IAIBIAIB Type AB iiType O