10 16 13 Chapter 12 Early work Josef kolreuter 1760 tobacco TA KNIGHT 1823 garden pea Gregor Mendel 1850s garden pea 2 many pea varieties were available 3 easy to grow 4 peas self fertilize 1 other research showed that pea hybrids could be produced 1 Produce true breeding strains for each trait 2 Cross fertilize true breeding strains having alternate forms of a trait 3 Allow the hybrid offspring to self fertilize for several generations Monohybrid crosses Cross to study only 2 variations of a single trait Mendel produced true breeding pea strains for 7 different traits Each trait had 2 variants F1 gen Offspring produced by crossing 2 true breeding strains All Mendel s F1 plants resembled only 1 parent called dominate alternative recessive No plants with intermediate characteristics F2 offspring from fertilization of F1 plants although hidden in F1 gen the recessive trait appereared among some f2 indivuals counted proportions of traits always 3 1 F2 plants dominant plants recessive Mendel discovered the ratio is actually 1 true breeding dominant palnt 2 non true 1 true breeding recessive plant Principle of segregation 5 element model 1 Parents transmit discrete factors 3 Not all copies of a gene is identical alles alternative form a gene Homo 2 of the same alle heterozygous different alles 4 alles remain discrete no blending Two alleles for a gene seg During gamete formation And are rejoined at random during fertilization Physical basis for allele seg is the behavior chromosomes during meiosis 2 Each individual receives one copy of a gene from each parent 5 Presensce of alles does not guarantee expression dominant expressed recessive hidden by dominant Genotype total set of alles phenotype Punnett squares predict outcomes PP Pp Pp pp true breedig Pp f2 generation PP Pp pP pp Human traits Some human trait are controlled by a single gene Pedigree analysis is used to track inheritances patterns in families Dominant pedigree juvenile glaucoma dominant trait appears in every generation Recessive pedigree Example albinism Condition in which the pigment melanin is not produced most affected individuals have unaffected parents Dihybrid crosses exam 2 separate traits in a single cross True breeding lines for 2 traits F1 generation all dominant phenotypes for each trait RrYy F2 shows all 4 possible phenotypes in a set rato 9 3 3 1 Round yellow round green wrinkled yellow wrinkled green Principle of independent assortment Di hybrid the alles assort independently The segregation of diferent alle pairs is independ Chromosomes assort inpendently into each gametes Testcross reveal unknown genotypes of an individual with dominant phenotype Mendels model of inheritance assumes that each trait is controlled by a single gene Each gene has only 2 alles there is a clear dominant recessive relationship between the alles most genes do not meet these Polygenic inheritance Occurs when muiltiple genes are involved in controlling the phenotype of a tait These traits show continuous variation and are referred to as quantitative traits Pleiotropy refers to an allele which has more than one effect on the phenotype pleiotropic effects are difficult to predict because a gene that affects one tait often performs other unknown functions this can be seen in human diseases like cystic fibrosis or sickle cell anemia miltiple systoms can traced to one alle Multiple alles anything more than 2 alleles for a gene 3 alles 2 dominat and 1 is recessive Each individual can only have 2 alleles Dominance is not always complete incomplete dominance heterozygotes is intermediate in phenotype between the 2 homozygotes Heterozygote shows some aspect of the phenotypes of both homozygotes type AB blood both mult Allele codominant Human ABO blood group demonstrates multiple alleles 3 alleles of the I gene Codominace 1A and I B dominant Environmental influence diet nutrition sun on color coat color in Himalayan rabbits and Siamese cats Epistatsis interactions of gene products affect predicted ratios crossed of 2 white varitities of corn F1 was all purple F2 was 9 purple 7 white not expected two genes interact to produce purple grain color if both present no pigmentation