3 Early Ideas of Heredity Before the 20th century, 2 concepts were the basis for ideas about heredity:heredity occurs within speciestraits are transmitted directly from parent to offspringLed to the belief that inheritance is a matter of blending traits from the parents.
4 Early Ideas of Heredity Botanists 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.
6 Early Ideas of Heredity Gregor MendelStudied the pea plant:other research showed that pea hybrids could be producedmany pea varieties were availablepeas are small plants and easy to growpeas can self-fertilize or be cross-fertilized
7 Early Ideas of Heredity Gregor Mendel’s experimental method:Produce true-breeding strains for each trait he was studyingCross-fertilize true-breeding strains having alternate forms of a traitperform reciprocal crosses as wellAllow the hybrid offspring to self-fertilize and count the number of offspring showing each form of the trait
11 Monohybrid CrossesMonohybrid cross: a cross to study only 2 variations of a single traitMendel produced true-breeding pea strains for 7 different traitseach trait had 2 alternate forms (variations)flower color, pea color, pea shape...Mendel cross-fertilized the 2 true-breeding strains for each trait
12 Monohybrid Crosses F1 generation (1st filial generation): offspring produced by crossing 2 true- breeding strainsFor every trait Mendel studied, all F1 plants resembled only 1 parentNo plants with characteristics intermediate between the 2 parents were produced
17 Monohybrid CrossesF2 generation (2nd filial generation): the offspring resulting from the self-fertilization of F1 plantsDominant traits: the form of each trait most commonly expressed in the F1 plantsRecessive traits: the form of the trait not seen in the F1 plants; hidden traits
18 Monohybrid CrossesF2 plants exhibited both forms of the trait in a very specific pattern:¾ plants with the dominant form¼ plant with the recessive formThe dominant to recessive ratio was 3 : 1.Mendel discovered the ratio is actually:1 true-breeding dominant plant2 not-true-breeding dominant plants1 true-breeding recessive plant
21 Monohybrid CrossesGene: information for a trait passed from parent to offspringAlleles: alternate forms of a geneHomozygous: having 2 of the same alleleHeterozygous: having 2 different alleles
22 Monohybrid Crosses Genotype: total set of alleles of an individual PP = homozygous dominantPp = heterozygouspp = homozygous recessivePhenotype: outward appearance of an individualphysical manifestation of genotype
23 Monohybrid Crosses Principle of Segregation: Two alleles for a gene segregate during gamete formationhomologous chromosome separation during meiosisAlleles are re-paired at random, one from each parent, during fertilization.
26 Monohybrid Crosses Some human traits are controlled by a single gene. some of these exhibit dominant inheritancesome of these exhibit recessive inheritanceCarriers are heterozygousPedigree analysis is used to track inheritance patterns in families.
29 Dihybrid CrossesDihybrid cross: examination of 2 separate traits in a single crossfor example: RR YY x rryyThe F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait.
31 Dihybrid 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 : 1
33 Dihybrid Crosses Principle of Independent Assortment: In a dihybrid cross, the alleles of each gene assort independently.
34 Probability – Predicting Results Rule 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
35 Probability – Predicting Results Rule 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 = ¼probability of obtaining yy = ¼probability of rr yy = ¼ x ¼ = 1/16
36 TestcrossTestcross: a cross used to determine the genotype of an individual with dominant phenotypecross 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 parent
38 Extensions to Mendel Mendel’s model of inheritance assumes: each trait is controlled by a single geneeach gene has only 2 allelesthere is a clear dominant-recessive relationship between the allelesMost genes do not meet these criteria.Other types of inheritance:Polygenic, Pleiotropy, Incomplete dominance, codominance
39 Extensions 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 height
41 Extensions 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.
42 Extensions 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.
46 Extensions to MendelThe expression of some genes can be influenced by the environment.for example: coat color in Himalayan rabbits and Siamese catsan allele produces an enzyme that allows pigment production only at temperatures below 30oC
50 Chromosomes, Mapping, and the Meiosis-Inheritance Connection Chapter 13
51 Chromosome Theory Chromosomal theory of inheritance developed in 1902 by Walter Suttonproposed that genes are present on chromosomesbased on observations that homologous chromosomes pair with each other during meiosissupporting evidence was provided by work with fruit flies
52 Chromosome Theory T.H. Morgan isolated a mutant white-eyed Drosophila Red-eyed female X white-eyed male gave a F1 generation of all red eyeMorgan concluded that red eyes are dominant
54 Chromosome Theory Morgan crossed F1 females X F1 males F2 generation contained red and white- eyed flies but all white-eyed flies were maleTestcross of a F1 female with a white-eyed male showed the viability of white-eyed femalesMorgan concluded that the eye color gene is linked to the X chromosome
57 Sex ChromosomesSex determination in Drosophila is based on the number of X chromosomes2 X chromosomes = female1 X and 1 Y chromosome = maleSex determination in humans is based on the presence of a Y chromosomehaving a Y chromosome (XY) = male
58 Sex ChromosomesSex-linked traits: traits controlled by genes present on the X chromosomeSex-linked traits show inheritance patterns different than those of genes on autosomes.In many organisms, the Y chromosome is greatly reduced or inactive.Genes on the X chromosome are present in only 1 copy in males
60 Sex ChromosomesDosage compensation ensures an equal expression of genes from the sex chromosomes even though females have 2 X chromosomes and males have only 1.In each female cell, 1 X chromosome is inactivated and is highly condensed into a Barr body.Females heterozygous for genes on the X chromosome are genetic mosaics.Phenotype depends on which X chromosome inactivated
62 Chromosome Theory Exceptions Mitochondria and chloroplasts contain genes.Traits controlled by these genes do not follow the chromosomal theory of inheritanceGenes from mitochondria and chloroplasts are often passed to the offspring by only one parent
63 Chromosome Theory Exceptions Maternal inheritance: uniparental (one- parent) inheritance from the motherThe mitochondria in a zygote are from the egg cell; no mitochondria come from the sperm during fertilizationIn plants, the chloroplasts are often inherited from the mother, although this is species dependent
64 Genetic MappingThe science of determining the location of a gene on a chromosomeEarly geneticists realized that they could obtain information about the distance between genes on a chromosome.This is genetic mappingMapping is based on genetic recombination (crossing over) between genes.
67 Genetic Mapping To determine the distance between genes: dihybrid organisms are testcrossedoffspring resembling the dihybrid parent result from homologues that were not involved in the crossoveroffspring resulting from a crossover are called recombinant progeny
68 Genetic MappingThe distance between genes is proportional to the frequency of recombination events.recombination recombinant progenyfrequency total progeny1% recombination = 1 map unit (m.u.)1 map unit = 1 centimorgan (cM)=
72 Genetic MappingDetermining the order of genes can be done with a three-point testcrossThe frequency of double crossovers is the product of the probabilities of each individual crossoverTherefore, the classes of offspring with the lowest numbers represent the double crossovers and allow the gene order to be determined
75 Human Genetic Disorders Some human genetic disorders are caused by altered proteins:the altered protein is encoded by a mutated DNA sequencethe altered protein does not function correctly, causing a change to the phenotypethe protein can be altered at only a single amino acid (e.g. sickle cell anemia)
79 Human Genetic Disorders Some genetic disorders are caused by a change in the number of chromosomes.Nondisjunction during meiosis can create gametes having one too many or one too few chromosomesFertilization of these gametes creates trisomic or monosomic individualsDown syndrome is trisomy of chromosome 21
81 Human Genetic Disorders Nondisjunction of sex chromosomes can result in:XXX triple-X femalesXXY males (Klinefelter syndrome)XO females (Turner syndrome)OY nonviable zygotesXYY males (Jacob syndrome)
82 Sex Determination in Humans The sex chromosomes, X and Y, are a homologous pair:this pair is unique because X and Y carry different sets of genesthe Y chromosome has genes that determine malenessthe X chromosome has a variety of genes on itXX = female; XY = maleX_ = female; _Y = does not surviveXXY = male82
83 Sex Determination in Humans Genotype: X__Sex: FemalePhenotype: Turner’s Syndromeshort stature (less than 5’)‘webbed’ neck and other physical characteristicsinfertility83
84 Sex Determination in Humans Genotype: XXXSex: FemalePhenotype: ‘Super-females’, metafemalestall staturelonger legs and torsomay have learning disabilities or emotionally underdevelopedcommonly labeled as ‘trouble makers’ in school84
85 Sex Determination in Humans Genotype: XYYSex: MalePhenotype: ‘Super-males’produce higher levels of testosteronemay be taller than averageno known significant abnormalities85
86 Sex Determination in Humans Genotype: XXY or XXXYSex: MalePhenotype: Klinefelter Syndromeproduce very little testosteronetaller and more overweight than averagemay have feminine characteristicssterile or nearly sterilemost have normal cognitive abilitiescan be treated with testosterone early in life86
90 Human Genetic Disorders Genomic imprinting occurs when the phenotype exhibited by a particular allele depends on which parent contributed the allele to the offspringA specific partial deletion of chromosome 15 results in:Prader-Willi syndrome if the chromosome is from the fatherAngelman syndrome if it’s from the mother
91 Human Genetic Disorders Genetic counseling can use pedigree analysis to determine the probability of genetic disorders in the offspring.Some genetic disorders can be diagnosed during pregnancy:amniocentesis collects fetal cells from the amniotic fluid for examinationchorionic villi sampling collects cells from the placenta for examination