Bài giảng Biology - Chapter 12: Patterns of Inheritance

Patterns of InheritanceChapter 121Early Ideas of HeredityBefore the 20th century, 2 concepts were the basis for ideas about heredity: -heredity occurs within species -traits are transmitted directly from parent to offspringThis led to the belief that inheritance is a matter of blending traits from the parents.2Early Ideas of HeredityBotanists in the 18th and 19th centuries produced hybrid plants.When the hybrids were crossed with each other, some of the offspring resembled the original strains, rather than the hybrid strains.This evidence contradicted the idea that traits are directly passed from parent to offspring.3Early Ideas of HeredityGregor Mendel -chose to study pea plants because:1. other research showed that pea hybrids could be produced2. many pea varieties were available3. peas are small plants and easy to grow4. peas can self-fertilize or be cross-fertilized45Early Ideas of HeredityMendel’s experimental method:1. produce true-breeding strains for each trait he was studying2. cross-fertilize true-breeding strains having alternate forms of a trait -perform reciprocal crosses as well3. allow the hybrid offspring to self-fertilize and count the number of offspring showing each form of the trait67Monohybrid CrossesMonohybrid cross: a cross to study only 2 variations of a single traitMendel produced true-breeding pea strains for 7 different traits-each trait had 2 alternate forms (variations)-Mendel cross-fertilized the 2 true-breeding strains for each trait8Monohybrid CrossesF1 generation (1st filial generation): offspring produced by crossing 2 true-breeding strainsFor every trait Mendel studied, all F1 plants resembled only 1 parent-no plants with characteristics intermediate between the 2 parents were produced910Monohybrid CrossesF1 generation: offspring resulting from a cross of true-breeding parentsF2 generation: offspring resulting from the self-fertilization of F1 plantsdominant: the form of each trait expressed in the F1 plantsrecessive: the form of the trait not seen in the F1 plants11Monohybrid CrossesF2 plants exhibited both forms of the trait in a very specific pattern: ¾ plants with the dominant form ¼ plant with the recessive formThe dominant to recessive ratio was 3 : 1.Mendel discovered the ratio is actually: 1 true-breeding dominant plant 2 not-true-breeding dominant plants 1 true-breeding recessive plant1213Monohybrid Crossesgene: information for a trait passed from parent to offspringalleles: alternate forms of a genehomozygous: having 2 of the same alleleheterozygous: having 2 different alleles14Monohybrid Crossesgenotype: total set of alleles of an individual PP = homozygous dominant Pp = heterozygous pp = homozygous recessivephenotype: outward appearance of an individual15Monohybrid CrossesPrinciple of SegregationTwo alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization.161718Monohybrid CrossesSome human traits are controlled by a single gene. -some of these exhibit dominant inheritance -some of these exhibit recessive inheritancePedigree analysis is used to track inheritance patterns in families.192021Dihybrid CrossesDihybrid cross: examination of 2 separate traits in a single cross -for example: RR YY x rryyThe F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait.22Dihybrid CrossesThe F2 generation is produced by crossing members of the F1 generation with each other or allowing self-fertilization of the F1. -for example RrYy x RrYyThe F2 generation shows all four possible phenotypes in a set ratio: 9 : 3 : 3 : 12324Dihybrid CrossesPrinciple of Independent AssortmentIn a dihybrid cross, the alleles of each gene assort independently.25Probability – Predicting ResultsRule of addition: the probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities.When crossing Pp x Pp, the probability of producing Pp offspring is probability of obtaining Pp (1/4), PLUS probability of obtaining pP (1/4) ¼ + ¼ = ½ 26Probability – Predicting ResultsRule of multiplication: the probability of 2 independent events occurring simultaneously is the PRODUCT of their individual probabilities.When crossing Rr Yy x RrYy, the probability of obtaining rr yy offspring is: probability of obtaiing rr = ¼ probability of obtaining yy = ¼ probability of rr yy = ¼ x ¼ = 1/1627TestcrossTestcross: a cross used to determine the genotype of an individual with dominant phenotype-cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp)-the phenotypic ratios among offspring are different, depending on the genotype of the unknown parent2829Extensions to MendelMendel’s model of inheritance assumes that:-each trait is controlled by a single gene-each gene has only 2 alleles-there is a clear dominant-recessive relationship between the allelesMost genes do not meet these criteria.30Extensions to MendelPolygenic inheritance occurs when multiple genes are involved in controlling the phenotype of a trait.The phenotype is an accumulation of contributions by multiple genes.These traits show continuous variation and are referred to as quantitative traits.For example – human height3132Extensions to MendelPleiotropy refers to an allele which has more than one effect on the phenotype.This can be seen in human diseases such as cystic fibrosis or sickle cell anemia.In these diseases, multiple symptoms can be traced back to one defective allele.33Extensions to MendelIncomplete dominance: the heterozygote is intermediate in phenotype between the 2 homozygotes.Codominance: the heterozygote shows some aspect of the phenotypes of both homozygotes.3435Extensions to MendelThe human ABO blood group system demonstrates:-multiple alleles: there are 3 alleles of the I gene (IA, IB, and i)-codominance: IA and IB are dominant to i but codominant to each other3637Extensions to MendelThe expression of some genes can be influenced by the environment.for example: coat color in Himalayan rabbits and Siamese cats-an allele produces an enzyme that allows pigment production only at temperatures below 30oC38Extensions to Mendel39Extensions to MendelThe products of some genes interact with each other and influence the phenotype of the individual.Epistasis: one gene can interfere with the expression of another gene4041