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I.InheritLab - Inheritance & Genetics II. Human...

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Unformatted text preview: Inheritance & Genetics II. Human Genetics Heredity Genes z Passing on a physical characteristic to future generations z Discrete hereditary units that determine physical traits – Segments of DNA coding for protein z Genetics – Eye color, ear shape, genetic diseases, feather color – The study of the transmission of inheritable traits Genes are located on chromosomes in the cell nucleus z Chromosomes are matched in homologous pairs – One member of each pair inherited from Mom, the other from Dad Terminology z Genetic locus: site on a chromosome where a gene is located z Allele: variation of same gene carried on homologous pairs of chromosomes z Human somatic (body) cells – 23 pairs = 46 chromosomes – Homologous = same size, shape & genes z Pairs #1 - 22 = Autosomes – In both males and females z Pair #23 = Sex Chromosomes A a B b – Determine gender – XX = female, XY = male Human karyotype Sexual Reproduction Produces Genetic Variation z Variation arises from – Independent chromosome assortment in meiosis – Random process of fertilization Heyer / Biology Homologous Chromosomes Can Carry Different Versions of Genes z Homologous chromosomes come from different parents z Different versions of same gene = Alleles – Ex. Brown coat, white coat Fig 8.17 Different info on homologous chromosomes 1 Inheritance & Genetics Paired alleles on homologous chromosomes z Homozygous: paired alleles on homologous chromosomes are the same z Heterozygous: paired alleles are different II. Human Genetics Relationship between alleles of the same gene z Dominant: the allele that is expressed in a heterozygote (produces physical appearance) z Recessive: the allele that is masked in a heterozygote Genotype: genes carried on chromosomes z Genotype: genetic make-up – Designated by letters – Dominant gene in upper case letters – Recessive gene in lower case letters AA = homozygous dominant Aa = heterozygous aa = homozygous recessive Mendelian Genetics z Phenotype: physical manifestation of genotype – Brown eyes or blue eyes Phenotype: flower color Parental generation Gregor Mendel was the first to study genetics z And one of the first true experimentalist employing the scientific method z Used pea plants as a model system to study patterns of heritance Heyer / Biology F1: first generation offspring (first filial) F2: second generation Offspring (second filial) 2 Inheritance & Genetics II. Human Genetics Genotype: PP and pp Parental cross: PP X pp What happens to paired alleles during meiosis? z AA genotype – Possible gametes made: only A F1: all heterozygous z aa genotype – Possible gametes made: only a z Aa genotype – Possible gametes made: A or a F2: genotypic ratio 1 PP : 2 Pp : 1 pp Fertilization is random z So if both flowers are Aa genotype – 50% of sperm in pollen are A; 50% are a – 50% of ova in carpal are A; 50% are a Punnett Square: Aa X Aa z Either A or a from one parent can be fertilized by A or a from other parent z Can use a Punnett square to predict possible genotypes of offspring Each allele has an equal chance of combining with either allele of other parent Tracking two characteristics at once: Dihybrid cross z Parental organisms differ in two characteristics z For example, – Seed shape: round vs. wrinkled and – Seed color: yellow vs. green Heyer / Biology Hypothesis: Dependent assortment z Are traits of one parent always inherited together? z NO 3 Inheritance & Genetics II. Human Genetics Mendel’s Principle of Independent Assortment Actual results: z Each different trait segregates independently during gamete formation 9:3:3:1 phenotypic ratio – Seed shape and seed color are inherited separately from each other Independent Assortment How can you determine the genotype of a dominant individual? z Cross a dominant individual with a recessive individual to determine genotype of dominant parent z Recessive allele will unmask a recessive allele carried by a heterozygous parent Test Cross Human Genetics AA X aa Aa X aa All Aa (All dominant) Half Aa (dominant) Half aa (recessive) Heyer / Biology 4 Inheritance & Genetics II. Human Genetics Punnett square to predict inheritance of an autosomal dominant trait Incomplete Dominance AA aa Simple dominance z Heterozygote (Aa) has the same phenotype as homozygous dominant (AA) Incomplete dominance Aa z Alleles blend to produce intermediate phenotype Codominance one pair of the 23 are sex chromosomes z Random fertilization determines sex of the zygote z XX = female z XY = male Two alleles (A or B) are both dominant over a third allele (O), but not over each other Heyer / Biology – XY is NOT a homologous pair 5 Inheritance & Genetics II. Human Genetics Sex-linked genes z Genes carried on sex chromosomes, but not related to sex determination z X-linked traits are expressed in males with only 1 gene (XrY) z Females must be homozygous recessive for phenotype (XrXr) X-linked recessive z Colorblindness: lacking receptors in retina that detect a particular color z Hemophilia: lacking blood clotting factors z Duchenne Muscular Dystrophy (DMD): progressive muscle degeneration Hemophilia Human genetics use family pedigrees z Relationship of family members and those carrying or expressing a genetic trait z Looks like a family tree, but tracks a particular genetic trait Heyer / Biology 6 Inheritance & Genetics II. Human Genetics Reading a pedigree What can a pedigree tell you? z Is a trait dominant or recessive? z Is a trait autosomal or sex-linked? Autosomal recessive traits Example: genetic deafness z Cystic fibrosis: defective ion pump – Excessive secretion of thick mucus z Phenylketonuria (PKU): lacking enzyme to metabolize phenylalanine autosomal recessive! Remember Inborn Errors of Metabolism? Example: phenylalanine metabolism – Results in mental retardation z Sickle cell anemia z Tay-Sachs: lipid accumulation in brain cells; death in childhood PKU pedigree: autosomal recessive Most Inborn Errors are autosomal recessive: z long as e least 1 È allele codes correctly • Instructionsat nzymeone phenylyketonuria [PKU] As Defective for constructing each enzyme in the pathway is coded lcaptonuriamade. . enough functional enzyme will ifferent gene z Defective enzyme5 È a by a d be • As Defective alleles cgÈ albino have two alleles. Only if bothfornzyme ode for nonfunctional z ususal, e each 6 ene you enzyme will the pathway be interrupted. Heyer / Biology 7 Inheritance & Genetics Autosomal dominant II. Human Genetics Huntington’s disease pedigree z Dominant traits can occur in successive generations z Huntington's disease: progressive neurological degeneration – Not expressed until middle-age z Achondroplasia: dwarfism So far, we have discussed only single gene traits (one gene, one protein) z Phenylketonuria – Enzyme z Albinism – Structural protein z Sickle cell anemia Multigene traits z Some traits require many proteins z Produce continuous variation – Range of phenotypes in a population – Transport protein Skin color: 3 loci, 6 alleles Height Number of individuals Height in inches Heyer / Biology 8 Inheritance & Genetics II. Human Genetics Environmental influences on continuous variation z Nutrition during development (height, weight) z Exposure to sun influences skin color (tanning) z Exposure to drugs, toxins, or environmental pollutant z Temperature Alterations of Chromosome Number and Structure z Errors in meiosis can lead to gametes with – Abnormal chromosome # – Alteration of chromosome structure Temperature sensitive melanin production z Pigment production occurs only in parts of body when temperature is lower Down Syndrome z An extra copy of chrom. 21 Trisomy 21 z Most common birth defect z Chance increases with age of mother Down syndrome Aberrations of Chromosome Structure Errors in Crossing-over z Deletions and duplications – “Cat-cry” syndrome, deletion in chrom 5 Deletion and duplication z Translocation – CML, most common of leukemia cancers Translocation of CML Heyer / Biology 9 ...
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