BIO10& ch&10&&Foundations&of&Genetics& &&66&66& GENETICS and Heredity Mendel and the Garden Pea • Heredity: the tendency for traits to be passed from parent to offspring • heritable features: characters • traits are alternative forms of a character o Genes o Alleles o loci Gregor Mendel solved the puzzle of heredity • performed experiments with garden peas • Why study the garden pea? o many varieties/ easily distinguishable traits that can be quantified o small, easy to grow, and produce large numbers of offspring quickly o their reproductive organs easily manipulated so that pollination can be controlled o self-fertilize Mendel had a specific experimental design • 1st: establish true-breeding varieties • Plants self-fertilize for several generations, (each variety had only 1 type of trait) • pure lines: P generation • Mating P generation è F1 • F1 generation self-fertilize • F2 generationBIO10& ch&10&&Foundations&of&Genetics& &&67&67& What Mendel Observed • for each pair of contrasting varieties that he crossed, one of the traits disappeared in the F1 gen but reappeared in the F2 gen • trait expressed in F1 gen: dominant • trait not expressed in F1 gen: recessive trait • Mendel counted # of each type of plant in the F2 generation • 3/4of F2 individuals expressed the dominant trait while 1/4 expressed the recessive trait • the dominant:recessive ratio among the F2 plants was always close to 3:1 • recessive trait hidden? in F1 gen & not expressed • He allowed the F2 to self-fertilize and form the F3 generation • he found that one-fourth of the plants from the F2 that were recessive were true-breeding in the F3 • he found that of the three-fourths of the plants from the F2 • only one-third were true breeding in the F3 • the remaining half showed both traits • He determined that the ratio of 3:1 ratio that he observed in the F2 generation was in fact a disguised 1:2:1 ratio  1: true breeding dominant  2: not true breeding  1: true breeding recessive • The F2 gen is a disguised 1:2:1 ratioBIO10& ch&10&&Foundations&of&Genetics& &&68&68& Mendel’s 5 Hypothesis Theory Hypothesis 1 parents do not transmit traits directly to offspring parents transmit information about the trait in the form of what Mendel called factors Now called genes Hypothesis 2 each parent contains 2 copies of factor governing each trait the 2 copies of the factor may or may not be same Homozygous: two of the same copies Heterozygous: two different copies: Hypothesis 3 alternative forms of a factor lead to alt. traits Alleles: alternative forms of a factor appearance is determined by the alleles a plant receives from its parents (genotype) expression of the alleles =appearance (phenotype) Hypothesis 4: 2 alleles do not affect each other Hypothesis 5 —presence of allele does not ensure expression of trait —in heterozygotes, only dominant allele is expressed By convention, genetic traits are assigned a letter symbol referring to their more common form —dominant traits: capitalized —recessive trait: lower-case —Ie: flower color in peas is represented as follows —P signifies purple —p signifies whiteBIO10& ch&10&&Foundations&of&Genetics& &&69&69&Punnet Square —Testcross: determine the genotype of unknown individuals in the F2 gen • unknown individual is crossed with a homozygous recessive individual • if the unknown is homozygous, then all of the offspring will express dominant traits • if the unknown is heterozygous, then one-half of the offspring will express recessive traits • Mendel used the testcross to detect heterozygotes Mendel’s 1st Law: Segregation • the two alleles of a trait separate from each other during the formation of gametes, so that half of the gametes will carry one copy and half will carry the other copy • Mendel also investigated the inheritance pattern for more than one factor • when crossing individuals who are true-breeding for 2 different characters, the F1 individual that results is a dihybrid BBFf X BbFF • after the dihybrid individuals self-fertilize, there are 16 possible genotypes of offspring —Conclusion: the inheritance of one trait does not influence the inheritance of the other traitBIO10& ch&10&&Foundations&of&Genetics& &&70&70& Mendel’s 2nd Law: INDEPENDENT ASSORTMENT • genes located on different chromosomes are inherited independently of one another The journey from DNA to phenotype Some Traits Don’t Show Mendelian Inheritance Often the expression of phenotype is not straightforward  Continuous variation o characters can show a range of small differences when multiple genes act jointly to influence a character o this type of inheritance is called polygenic o Height is a continuously varying character  Pleiotropic effects o an allele that has more than one effect on a phenotype is considered pleiotropic o these effects are characteristic of many inherited disorders, such as cystic fibrosis and sickle-cell anemia  Incomplete dominance o not all alternative alleles are dominant or recessive in heterozygotes o some alleles exhibit incomplete dominance: produce a heterozygous phenotype (intermediate between 2 parents)  Environmental effects o Expression of some alleles depends on environment  ie: some alleles are heat-sensitive  arctic foxes only produce fur pigment when temperatures are warm  Codominance: o often, in heterozygotes, there is not a dominant allele but, instead, both alleles are expressed o these alleles are said to be codominantBIO10& ch&10&&Foundations&of&Genetics& &&71&71&Chromosomal theory of inheritance was first proposed in 1902 by Walter Sutton  supported by several pieces of evidence  reproduction involves union of only eggs & sperm  each gamete contains only 1 copy of the genetic information  since sperm have little cytoplasm, the material contributed must reside in the nucleus  chromosomes both segregate and assort independently during meiosis Linkage: the tendency of close-together genes to segregate together  the further two genes are from each other on the same chromosome, the more likely crossing over is to occur between them  this would lead to independent segregation  the closer that two genes are to each other on the same