13 Genetics parents children Who are the parents of these children? Heredity= Continuity of biological traits from one generation to the next Variation= Inherited differences among individuals of the same species Genetics= The scientific study of heredity and hereditary variation
gene locus DNA double helix of nucleotides phosphatedesoxyriboseN-base basepair nucleotide 13 Chromosome • Offspring acquire genes fromparents by inheriting chromosomes DNA = Type of nucleic acid that is a polymer of four different kinds of nucleotides. chromosome chromatine wound on histon peptide Chromosomes = Organizational unit of heredity material in the nucleus of eukaryoticorganisms Gene = Unit of hereditary information that is made of DNA and is located onchromosomes Locus = Specific location on a chromosome that contains a gene
13 Asexual life cycle • Asexual reproduction • A type of reproduction involving only one parent that produces genetically identical offspring by budding or by the division of a single cell or the entire organism into two or more parts
12 Mitosis l • Mitotic cell cycle: In a dividing cell, the mitotic (M) phase alternates with interphase, a growth period. The first part of interphase, called G1, is followed by the S phase, when the chromosomes replicate; the last part of interphase is called G2. In the M phase, mitosis divides the nucleus and distributes its chromosomes to the daughter nuclei, and cytokinesis divides the cytoplasm, producing two daughter cells.
13 Sexual life cycle • Meiosisand fertilization result in alternation between the haploid and diploid condition Diploid = Condition in which cells contain two sets (2n) of chromosomes Haploid = Condition in which cells contain one set (1n) of chromosomes Gamete = A haploid reproductive cell (sperm cells and ova) The diploid number is restored when two haploid gametes unite Fertilization = The union of two gametes to form a zygote Zygote = A diploid cell that results from the union of two gametes
13 Variety of life cycles • Three basic patterns of sexual life cycles a. Animal In animalsgametes are the only haploid cells b. Fungi and some protists In many fungi and some protists, theonly diploid stage is the zygote c. Plants and some algae Plants and some species of algaealternate between multicellularhaploid and diploid generations
13 (A)sexual reproduction • Like begets like, more or less: a comparison of asexual versus sexual reproduction
13 Meiosis – overview What happens during meiosis?
13 Origins of genetic variation l 1. Independent assortment of chromosomes Independent assortment = the random distribution of maternal and paternalhomologues to the gametes Theprocess produces 2npossible combinations of maternal and paternalchromosomes in gametes
13 Origins of genetic variation ll • 2. Crossing over • Crossing over = The exchangeof genetic material between homologues; occursduring prophase of meiosis I. • In humans, there is an average of two or three crossovers per chromosomepair
13 Origins of genetic variation lll • 3. Random fertilization • Random fertilization is another source of genetic variation in offspring • Theprocess produces 2nx 2n possible combinations of maternal and paternalchromosomes in the zygote • 4. Mutation • The ultimate source of variation: random and relatively rare structural changes made during DNA replication in a gene as a result of mistakes
13 Summary • Offspring acquire genes from parents by inheriting chromosomes • Like begets like, more or less • Fertilization and meiosis alternate in sexual life cycles • Meiosis reduces chromosome number from diploid to haploid • Sexual life cycles produce genetic variation among offspring • Evolutionary adaptation depends on a population’s genetic variation
13 Key terms • heredity karyotype • zygote meiosis I and II • variation homologous • diploid cells synapsis • genetics chromosomes • meiosis tetrad • gene sex chromosomes • alternation of generations chiasma (chiasmata) • asexual reproduction autosome • clone gamete • sporophyte crossing over • sexual reproduction haploid cell • spores life cycle • fertilization gametophyte • somatic cell syngamy
14 Heredity Theories of heredity Blending theory of heredity = Pre-Mendelian theory of heredity proposing thathereditary material from each parent mixes in the offspring; once blended the hereditary material is inseparable and the offspring's traits aresome intermediate between the parental types Particulate theory of heredity = Gregor Mendel's theory that parents transmit to theiroffspring discrete inheritable factors (now called genes) that remain as separate factorsfrom one generation to the next
14 Mendel’s experiments • Terms • Character = Detectable inheritable feature of an organism gene • Trait = Variant of an inheritable character allele • True breeding = Always producing offspring with the same traits as the parents whenthe parents are self-fertilized • P = true-breeding parental plants of a cross • F1= hybrid offspring of the P-generation • F2 =generation of self-pollinated F1-plants
14 Monohybrid cross • Monohybrid cross • Hypothesis: If the inheritable factor for white flowers hadbeen lost, then a cross between F1 plants should produce only purple-flowered plants. • Experiment: Mendel allowed the F1 plants to self-pollinate. • Results: There were 705 purple-flowered and 224 white-flowered plants in the F2generation—a ratio of 3:1. • Conclusion: The inheritable factor for white flowers was not lost, so thehypothesis was rejected
14 Testing hypotheses using the 2-statistic • There were 705 purple-flowered and 224 white-flowered plants in the F2generation. Hypothesis: Is this a ratio of 3:1? • Calculate expected values based on the result total (929):3 : 1 = 696.75 : 232.25 • Calculate the differences between observed (O) and expected (E) • Standardise the differences: |O – E |2 2 = E • Calculate degrees of freedom (DF) = number of differences – 1 • Lookup in 2 table at DF • If p<0.05 then reject hypothesis O E |O–E| 705 696.75 8.25 224 232.25 8.25 929 929 (8.25-0.5)2 (8.25-0.5)2 2 = + 696.75 232.25 The correction for continuity of 0.5 is only applied when DF=1 2 = 0.086 + 0.259 = 0.345 At DF=1 0.345 lies between 0.016 and 0.455 with 0.900>p>0.500, so accept hypothesis
14 Law of segregation • By the law of segregation, the two alleles for a character are packaged intoseparate gametes • 1. Alternative forms of genes are responsible for variations in inheritedcharacters. • 2. For each character, an organism inherits two alleles, one from each parent. • 3. If the two alleles differ, one is fully expressed (dominant alleleP); the other iscompletely masked (recessive allelep). • 4. The two alleles for each character segregate during gamete production.
14 Genotype and phenotype • Homozygous = Having two identical alleles for a given trait (e.g., PP or pp). • Heterozygous = Having two different alleles for a trait (e.g., Pp). • Phenotype = An organism's expressed traits (e.g., purple or white flowers). • Genotype = An organism's genetic makeup (e.g., PP, Pp, or pp).
14 Testcross • Testcross = The breeding of an organism of unknown genotype with a homozygousrecessive
14 Dihybrid cross • Mendel's law of independent assortment = Each allele pair segregates independently ofother gene pairs during gamete formation
14 Laws of Probability • 1. Rule of multiplication • Rule of multiplication = The probability that independent events will occursimultaneously is the product of their individual probabilities Question: In a Mendelian cross between pea plants that are heterozygous forflower color (Pp), what is the probability that the offspring will be homozygousrecessive? • 2. Rule of addition • Rule of addition = The probability of an event that can occur in two or moreindependent ways is the sum of the separate probabilities of the different ways Question: In a Mendelian cross between pea plants that are heterozygous forflower color (Pp), what is the probability of the offspring being a heterozygote?
14 Extended Mendelian genetics l • Incomplete dominance = dominant phenotypeis not fully expressed in the heterozygote, resulting in a intermediatephenotype Complete dominance = an allele is fully expressedin the phenotype of a heterozygote and masks the phenotypic expression ofthe recessive allele; the phenotypes of the heterozygote anddominant homozygote are indistinguishable Codominance = full expression of both alleles in theheterozygote
14 Extended Mendelian genetics ll • Important points about dominance/recessiveness relationships: • 1. They range from complete dominance, through various degrees of incomplete dominance, to codominance • 2. They reflect the mechanisms by which specific alleles are expressed in phenotype and do not involve the ability of one allele to subdue another at the level of the DNA • 3. They do not determine or correlate with the relative abundance of alleles in a population
14 Extended Mendelian genetics lll • Multiple alleles = Some genes may have more than just two alternative formsof a gene
14 Extended Mendelian genetics lV • Pleiotropy = The ability of a single gene to have multiple phenotypic effects Epistasis = Interaction between two nonallelic genes in which one modifies the phenotypic expression of the other (9:3:4)
14 Extended Mendelian genetics V • Polygenic inheritance = Mode of inheritance in which the additive effect of two ormore genes determines a single phenotypic character
14 Nature versus nurture • Norm of reaction= Range of phenotypic variability produced by a single genotypeunder various environmental conditions • The expression of most polygenic traits, such as skin color, is multifactorial; thatis, it depends upon many factors - a variety of possible genotypes, as well as avariety of environmental influences
14 Human genetics • Humans are difficult to investigate • long generation time • few offspring • experiments unacceptable • Pedigree = A family tree that diagrams the relationships among parents and childrenacross generations and that shows the inheritance pattern of a particular phenotypiccharacter
14 Human genetic disorders l • Recessive disorders: • show up in homozygous individuals • heterozygote are called carriers • frequencies differ worldwide due to local selective forces • Cystic fibrosisTay-Sachs diseaseSickle cell anemia
14 Human genetic disorders ll • Phenylketonuria • Dominant disorders • Achondroplasia (dwarfism) 1:10,000 people • Lethal dominant alleles are not passed to next generation • Late-acting alleles escape elimination: Huntington’s disease • Multifactoral disorders (heart disease, diabetes, cancer, • schizophrenia)
14 Genetic testing • Carrier recognition • Fetal testing • Newborn screening (PKU)
14 Summary • Mendel brought an experimental and quantitative approach to genetics • By the law of segregation, the two alleles for a character are packaged into separate gametes • By the law of independent assortment, each pair of alleles segregates into gametes independently • Mendelian inheritance reflects rules of probability • Mendel discovered the particulate behavior of genes • The relationship between genotype and phenotype is rarely simple • Pedigree analysis reveals Mendelian patterns in human inheritance • Many human disorders follow Mendelian patterns of inheritance • Technology is providing news tools for genetic testing and counseling
14 Key terms • character dominant allele • polygenic inheritancetrait • law of segregation true-breeding • incomplete dominance multifactorial • complete dominance homozygous • monohybrid cross heterozygous • cystic fibrosisP generation • multiple alleles F1 generation • genotype pleiotropy • F2 generation testcross • alleles dihybrid cross • recessive allelehybridization • norm of reactioncarriers • codominanceepistasis • phenotypesickle-cell disease • quantitative characterlaw of independent assortment
15 Chromosomal basis of inheritance • Chromosome theory of inheritance: • Mendelian genes have specific loci on chromosomes, which undergo segregation and independant assortment • Recombinants have new combinations of traits • RY and ry are parental phenotypes • rY and Ry are recombinant phenotypes
15 Morgan’s experiments with fruit flies l • Morgan used Drosophila melanogaster, a fruit fly species • Short generation time • Fruit flies have three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males) • The normal character phenotype is the wild type • Alternative traits are mutant phenotypes
15 Morgan’s experiments with fruit flies ll • Cross white-eyed male with a red-eyed female: F1 had red eyes • Crosses between the F1 offspring produced the classic 3:1 phenotypic ratio in the F2 offspring • The white-eyed trait appeared only in males • All the females and half the males had red eyes • Morgan concluded that a fly’s eye color is a sex-linked gene
15 Linked genes • Dihybrid test cross did not produce 1:1:1:1 ratio • genes must be on the same chromosome: linked genes • Complete linkage gives ratio 1:1:0:0 • 17% recombinants must be the resultof crossing over
15 Crossing over Recombination is result of crossing over and independant assortment
15 Genetic maps • The recombination frequency between cn and b is 9% • The recombination frequency between cn and vg is 9.5% • The recombination frequency between b and vg is 17% • How are the loci arranged?b – vg – cn orvg – b - cn • Geneticists can use recombination data to map a chromosome’s genetic loci:linkage map The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore a higher recombination frequency Map distance: 1 unit is 1% recombination
15 Genetic maps • Why is 9,0 (b-cn)+ 9,5 (cn-vg) > 17% (b-vg)? This results from multiple crossing over:the further loci are apart, the greater the change for multiple crossing over events Some genes on a chromosome are so far apart that a crossover between them is virtually certain: independent inheritance, no linkage