Ch 14 Mendel and The Gene Idea 1 Chapter 14 Mendel and the Gene Idea Drawing From the Deck of Genes o o o generation offspring The explanation of heredity most widely in favor during the 1800s was the blending hypothesis the idea that genetic material contributed by the two parents mixes in a manner The blending hypothesis also fails to explain other phenomena of inheritance such as traits reappearing after skipping a The gene idea is the idea that parents pass on discrete heritable units genes that retain their separate identities in Concept 14 1 Mendel used the scientific approach to identify two laws of inheritance o Mendel s experimental quantitative approach o The law of segregation Around 1857 Mendel began breeding garden peas in the abbey garden to study inheritance Peas were chosen because they were available in many varieties and they have short generation time with large offspring A heritable feature that varies among individuals such as flower color is called a character Each variant for a character such as purple or white color for flowers is called a trait To achieve cross pollination fertilization between different plants Mendel removed the immature stamens of a plant before the produced pollen and then dusted pollen from another plant onto the altered flower Mendel also made sure that he started his experiments with varieties that over many generations of self pollination had produced only the same variety as the parent plant Such plants are said to be true breeding This mating or crossing of two true breeding varieties is called hybridization The true breeding parents are referred to as the P generation parental generation and their hybrid offspring are the F1 generation second filial generation Mendel s quantitative analysis of the F2 plants from thousands of genetic crosses like these allowed him to deduce two fundamental principles of heredity the law of segregation and the law of independent assortment In Mendel s terminology purple flower color is a dominant trait and white flower color is a recessive trait Mendel developed a model to explain the 3 1 inheritance pattern that he consistently observed among the F2 offspring in his pea experiments Alternative versions of genes account for variations in inherited characters The gene flow for flower color in pea plants for example exists in two versions one for purple flowers and the other for white flowers These alternative genes are called alleles Each gene is a sequence of nucleotides at a specific place or locus along a particular For each character an organism inherits two copies of a gene one from each parent These are also chromosome called alleles of that gene each parent pair of chromosomes o o o o Each somatic cell in a diploid organism has two sets of chromosomes one set inherited by A genetic locus is actually represented twice in a diploid cell once on each homolog of specific 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 Accordingly Mendel s F1 plants had purple flowers because the allele for that trait is dominant and the allele for white flowers is recessive Ch 14 Mendel and The Gene Idea 2 The law of segregation The two alleles for a heritable character segregate separate from each other during gamete formation and end up in different gametes o o o In terms of chromosomes this segregation corresponds to the distribution of the two members of a pair of homologous chromosomes to different gametes in meiosis If an organism has identical alleles for a particular character that is the organism is true breeding for that character then that allele is present in all gametes If different alleles are present then 50 of the gametes receive the dominant allele and 50 receive the recessive allele The Punnett Square is a handy diagrammatic device for predicting the allele composition of offspring from a cross between individuals of known genetic makeup Capital letters represent the dominant allele and the lowercase letters for the recessive allele An organism that has a pair of identical alleles for a character is said to be homozygous for the gene controlling that character Homozygous plants breed true because all of their gametes contain the same allele An organism that has two different alleles for a gene is said to be heterozygous for that gene Heterozygotes produce gametes with different alleles so they are not true breeding Because of the different effects of dominant and recessive alleles an organism s traits do not always reveal its genetic composition An organism s appearance or observable traits is called its phenotype and its genetic makeup its genotype Breeding an organism of unknown genotype with a recessive homozygote is called a testcross because it can reveal the genotype of that organism The offspring of a Pp x pp cross will be expected to have a 1 1 phenotypic ratio o The Law of independent assortment flower color Mendel derived the law of segregation from experiments in which he followed only a single character such as All the F1 progeny produced in his crosses of true breeding parents were monohybrids meaning that they were heterozygous for the one particular character being followed in the cross A cross between such a heterozygotes is a monohybrid cross Mendel identified his second law of inheritance by following two characters at the same time such as seed color and seed shape cross YyRr The F1 plants will be dihybrids individuals heterozygous for the two characters being followed in the A dihybrid cross determines which of the two hypotheses are correct This dependent assortment hypothesis predicts that the phenotypic ratio of the F2 generation will be 3 1 just as in a monohybrid cross The result of Mendel s dihybrid experiments are the basis for what we now call the law of independent assortment which states that each pair of alleles segregates independently of each other pair of alleles during gamete formation This law applies only to genes allele pairs located on different chromosomes that is on chromosomes that are not homologous or very far apart on the same chromosome 14 1 Summary In the 1860s Gregor Mendel formulated the theory of inheritance based on experiments with garden peas proposing that parents pass on to their offspring discrete genes that retain their identity through generations This theory includes two laws The law of