2OutlineEarly Ideas of HeredityMendelGene DisordersMultiple AllelesPedigreesGene Disorders Due to Protein AlterationChromosome and InheritanceGenetic RecombinationHuman Chromosomes
3Early Ideas of Heredity Classical assumptionsFirst, heredity occurs within species.species maintained without significant change since time of creationSecond, traits are transmitted directly and independently.paradox - all members of same species should eventually have the same appearancehybrids differ in appearance
4Early Ideas of Heredity Early geneticists demonstrated some forms of an inherited character can:disappear in one generation and reappear, unchanged, in future generations.segregate among offspring of a cross.be more likely to be represented than alternative forms.
5Mendel and the Garden Pea Advantages of garden pea :many hybrids previously producedexpect segregation of traitslarge number of true-breeding varietiessmall and easy to growshort generation timesexual organs enclosed in flowerself-fertilizationcross fertilization
6Mendel and the Garden Pea Mendel’s experimental designallowed pea plants to self-fertilize for several generationsassured pure-breeding traitsperformed crosses between varieties exhibiting alternative character formspermitted hybrid offspring to self-fertilize for several generations
8What Mendel FoundF1 Generation (first filial)Offspring of white flower and purple flower cross had flower color resembling one parent (no intermediate color).All flowers exhibited purple flowers (dominant trait) and none exhibited white flowers (recessive trait).
9Mendel’s ResultsF2 Generation (second filial)Cross between seeds of F1 generation produced some plants exhibiting white flowers (recessive form reappeared).dominant : recessive ratio among F2 plants was always close to 3:1Mendelian Ratiodiscovered ¼ of recessives were always true breedingdisguised 1:2:1 ratio
11Mendel’s Model of Heredity Parents transmit discrete physiological trait information (factors) to offspring.Each individual receives two factors that may code for same, or alternative, character traits.Not all copies of a factor are identical.alleleshomozygous - same allelesheterozygous - different alleles
12Mendel’s Model of Heredity Alleles do not influence each other in any way.Presence of a particular allele does not ensure its encoded trait will be expressed.genotype - totality of an individual’s allelesphenotype - physical appearance
19Mendelian Inheritance Mendel’s Second Law of Heredity(Law of Independent Assortment)Genes that are located on different chromosomes assort independently of one another.
20Mendelian Inheritance Phenotype considerationscontinuous variationThe greater the number of genes influencing a character, the more continuous the expected distribution of character variation will be.pleiotropic effectsIndividual alleles often have more than one effect on the phenotype.
21Phenotypic Considerations Incomplete dominanceHeterozygotes are intermediate in color.Environmental effectsdegree of allele expression may depend on the environmentEpistasisone gene interferes with the expression of another genecoat color in Labrador retrievers
23Gene DisordersGene disorder refers to the harmful effect a detrimental allele produces when it occurs at a significant frequency in a population.Most disorders are rare because affected individuals often die at a relatively young age, or cannot reproduce.Not all defects are recessive.Huntington disease
24Multiple Alleles: ABO Blood Group Codominance - No single allele is dominant, and each allele has its own effect.ABO blood groupshuman gene that encodes enzyme that adds sugar molecules to lipids on the surface of red blood cellsIB adds galactoseIA adds galactosaminei adds no sugar
26PedigreesMutations are accidental changes in genes.rare, random, and usually result in recessive allelespedigrees used to study heredityhemophilia - inherited condition where blood is slow to clot or does not clot at allonly expressed when individual has no copies of the normal alleleRoyal hemophilia - sex-linked
28Gene Disorders Due to Protein Alteration Sickle-cell anemia is a recessive inherited disorder in which afflicted individuals have defective hemoglobin, and thus are unable to properly transport oxygen to tissues.Homozygotes have sickle-cell.Heterozygotes usually appear normal, but are resistant to malaria.
30Curing Defects with Gene Therapy Cystic fibrosisbody cells of affected individuals secrete thick mucus that clogs airways of lungdefect in cf geneResearchers are currently working on transmitting a working copy of cf gene using viruses.Early attempts using adenovirus vectors produced mixed results.
31Chromosomes and Mendelian Inheritance In early 20th century, it was not obvious chromosomes were vehicles of heredity informationchromosomal theory of inheritance first formulated in 1902problems quickly arose in trying to track independent assortment
32Chromosomes and Inheritance A trait determined by a gene on the sex chromosome is said to be sex-linked.In Drosophila, sex is determined by the number of copies of the x chromosome.Mendelian traits assort independently because chromosomes assort independently.
34Genetic Recombination Crossing overGenes located relatively far apart on a chromosome are more likely to cross over than genes located closer together.Frequency of crossings can be used to construct a genetic map.measures distance between genes in terms of recombination frequency
35Human ChromosomesHuman somatic cells normally have 23 pairs of chromosomes.divided into seven groups characterized by size and shape22 pairs of autosomes1 pair of sex chromosomesXX = FemaleXY = Male
36Human ChromosomesOne x chromosome in females is inactivated early in embryonic development.Visible as a darkly staining Barr body attached to the nuclear membrane.
37Alterations in Chromosome Number Failure of chromosomes to separate correctly during meiosis I or II is called primary nondisjunction.Down Syndrome caused by trisomy 211 in 1700 for mothers < 20.1 in 1400 for mothers >20<30.1 in 750 for mothers >30<35.1 in 16 for mothers >45.
38Nondisjunction in Sex Chromosomes XXX or XXY yields Klinefelter syndromeXO yields Turner syndromeY ChromosomeXYY - Jacob syndrome
40Genetic CounselingGenetic counseling identifies parents at risk of producing children with genetic defects and assesses the state of early embryos.High-risk pregnanciescouples with recessive allelesmothers older than 35amniocentesischorionic villi sampling
41Genetic CounselingCounselors can look for three things in cell cultures in search of genetic disorders:aneuploidy or gross alterationsproper enzyme functioningassociation with known genetic markers
42SummaryEarly Ideas of HeredityMendelGene DisordersMultiple AllelesPedigreesGene Disorders Due to Protein AlterationChromosome and InheritanceGenetic RecombinationHuman Chromosomes