GeneticsHonors BiologyMs. Pagodin

Gregor Mendel (1822-1884)Austrian Monk, Father of Genetics

Bred Garden Peas (Pisum sativum)

Developed a simple set of rules to accurately predict patterns of heredity which form the basics of genetics

Years later we found that traits are determined by genes encoded in DNA

Heredity HistoryHeredity transmission of traits from parents to offspring before DNA was discovered it was one of the great mysteries of science!

Modeled experiments after British farmer T.A. Knight who bred garden peas and concluded purple flowers show a stronger tendency to appear than white flowers

Mendel used a mathematical approach and counted the number of each kind of offspring

Why did Mendel choose peas? Many easily distinguishable characteristics2 possible traits (forms) of each characteristic

Quantitative he could count plants with or with out trait

P. sativum were small, easy to grow, mature quickly, and produce lots of offspring

Pea plants can self-pollinateMale (pollen) and Female (pistil) parts are enclosed in the same flower and it can fertilize itself

Pea plants can cross-pollinateTransfer pollen from one plant to the pistil of another plant

Anatomy of a flowering plant

Self pollination vs. Cross Pollination

Mendels Experimental DesignParental Generation (P generation): ensure that ea/plant was true breeding all offspring display only one form of the characteristics for subsequent generations

First Filial Generation (F1 generation): Mendel cross pollinated 2 plants from P generation w/ contrasting traits, offspring called F1 generation

Second Filial Generation (F2 generation): Mendel allowed the F1 generation to self-pollinate, offspring called the F2 generation

Mendel then counted his results

Mendels ResultsF1The recessive traits disappears The expressed trait is said to be dominantF2The recessive trait reappears!!Mendel obtained a 3:1 ratio of dominant to recessive for each trait of the F2 generation!

Mendel proposed a Theory of HeredityParents pass on units of information that operate in the offspring to produce a trait (today we know these to be genes!)For each characteristic there are 2 factors or alleles(1 from mom and 1 from dad) at ea/locusHomozygous - if 2 of the same alleles are inherited (true-breeding)Heterozygous if 2 different alleles are inherited (hybrid)Genotype combination of alleles an individual hasPhenotype physical appearance as a result of the alleles inherited

Mendels Theory Became Laws of HeredityLaw of SegregationThe members of each pair of alleles separate when gametes are formedLaw of Independent AssortmentPairs of alleles separate independently of one another during gamete formation (only applies to genes far apart on the same chromosome or separate chromosomes)Mendel published paper in 1866 no interest, rediscovered in early 1900s

Analyzing HeredityUse letters to represent alleles Capital letters represent dominant allelesLowercase letters represent recessive allelesSame letter designates 2 forms of the same trait (letter of dominant trait)Ex. Tallness in pea plantsT = tall dominant allelet = short recessive allele

Genotype vs. Phenotype2 alleles for each trait make up genotype

GenotypePhenotypeHomozygous dominantTTTallHeterozygousTtTallHomozygous recessivett Short

ProbabilityProbability likelihood that a specific event will occurProbability = # of specific outcometotal # of all possible outcomesUse this formula to predict the outcome of a genetic cross

Monohybrid CrossMonohybrid Cross - provides data about 1 pair of contrasting traitsEx. Homozygous tall x homozygous shortPunnett Square diagram used to predict the probable outcome of a crossWrite parental cross (genotypes)Draw box, genotype of 1 parent goes on one side, other parents genotype on the other sideFill in the boxes with 1 allele from each parent to indicate possible offspring genotypesDetermine probability of traitsGenotypic Ratio: homozygous dominant : heterozygous : homozygous recessivePhenotypic Ratio: dominant: recessive

Test CrossTest cross is used to determine unknown genotypesCross unknown with a homozygous recessive individual for that traitIf ALL offspring show dominant trait, then the unknown is homozygous dominantIf any (about 1/2 ) offspring show recessive trait, then the unknown is heterozygous

Do Now: Leslie has a long palmar muscle. Leslie has a brother, who does not have a long palmar muscle. Leslies parents also lack the muscle. Leslie is married to Lamont, who does have the long palmar muscle. Their first two children are identical twin boys (Larry and Lance), who both have a long palmar muscle. Use the letters M and m to represent the alleles for this trait. What are the genotypes of everyone in this problem?Leslie, Louis, Lamont, Larry, Lance, Leslies ParentsWhat is the most probable method of inheritance (dominant or recessive) for this trait? Explain.

Dihybrid CrossDihybrid Cross involves 2 pairs of contrasting traitsEx. Homozygous round yellow seeds (RRYY) x homozygous green wrinkled seeds (rryy)Punnett Square has 16 boxesDetermine possible allele combinations for each parent and put on sides of Punnett squareFill in boxes with possible allele combinations for offspring

Dihybrid Cross (RrYy x RrYy)

Extra Credit Trihybrid CrossRound is dominant to wrinkled seedsYellow seeds are dominant to green seedsPurple flower color is dominant to white flower colorShow a trihybrid cross, and use a Punnett square to determine the phenotypic ratio for possible offspring from parents that are each heterozygous for all traits

Complex Patterns of HeredityDo not follow Mendelian GeneticsIncomplete DominanceCodominanceMultiple AllelesAutosomal linked traitsSex linked traitsGene InteractionPolygenic traitsEpistasis

Incomplete DominanceIncomplete dominance occurs when an intermediate form of the trait is displayed in heterozygous individuals Ex. Snapdragons Red x White = 100% Pink!

CodominanceCodominance 2 dominant alleles are both expressed at the same time

Ex. Roan horsesRed x White horse= 100% Roan horse(has both red and white hair)

Do Now:Thomas has sickle cell but his wife, Susie, does not have sickle cell. Their daughter, Kelly has both regular cells and sickle cells. What pattern of inheritance does sickle cell follow? How do you know? What is the probability that Kelly and her husband Regis (who does not have sickle cell) will have a child with all normal red blood cells?

Multiple AllelesTraits with more than 2 possible allelesEx. Blood Type (A,B, and O)3 possible alleles IA,IB (dominant), i (recessive)

Linked GenesDiscovered by Thomas Hunt Morgan (1910)Studied Drosophila melanogastar4 pairs of chromosomesBreed every 2 weeks100s of offspringId 50+ Drosophila genesWildtype normal phenotypeEx. Red eyes (w+) Mutant mutant phenotypeEx. White eyes (w)

Autosomal Linked GenesLinked Genes on same chromosome tend to be inherited together Deviates from Mendels law of independent assortment The further apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher recombination frequencyRecombination frequency - % of offspring with new gene combinations (different from parents)

b+vg+ =gray body and normal wingsbvg = black body and vestigial wingsTest Cross: b+bvg+vg x bbvgvgResult: 965 gray-normal944 black-vestigial206 gray-vestigial185 black noramlmost offspring demonstrated parental phenotypessome some non-parental phenotypes also produced (called recombinants)

Genetic Recombination and Linkage MapsUnlinked Genes - typically see 50% freq of recombination for any 2 genes located on different chromosomes due to independent assortment of metaphase ILinked Genes freq of recombination varies depending on distance between linked genes due to crossing over during prophase IUsing the freq of recombination can construct a genetic map (ordered list of loci along chromosome)One map unit (centimorgans) = 1% recombinationEx. 3 drosophila gene pairs b-cn 9.5%, cn-vg 9.5%, b-vg 17%Linear order: b---9.5----cn-----9.5----vg

Sex-linked TraitsCrossed wildtype red-eyed female x mutant white-eyed maleConcluded white-eye mutation linked to sex chromosome (X)Sex-linked traits genes are found on the X chromosome but not on the Y chromosomeFemales have 2 X chromosomes, therefore 2 alleles for each trait and a heterozygous female would exhibit the dominant traitMales have only 1 X chromosome, therefore only 1 allele to determine traits found on the x chromosome and will always exhibit that trait even if it is recessiveEx. Sex-linked traits: Hemophilia, Red-Green color blindness, Male-Pattern baldness, Duchenne Muscular Dystrophy

Punnett Squares for Sex-linked Traits

Gene InteractionFunction of gene product is related to development of a common phenotypeDiscontinuous variation qualitativeEpistasis expression of one gene masks the expression of another gene

Continuous variation quantitativeMultiple Genes (Polygenic) contribute to the phenotype in a cumulative way

Polygenic (Multi-gene Inheritance)Polygenic Inheritance several genes influence 1 trait, therefore we see a variety of phenotypes and a continuum from one extreme to another

Sample ProblemThe size of the eggs laid by one variety of hens is determined by 3 pairs of alleles. Hens with the genotype AABBCC lay eggs weighing 90 grams, and hens with the genotype lay eggs weighing 30 grams. When a hen from the 90g strain is mated with a rooster from the 30g strain, the hens of the F1 generation lay eggs weighing 60g. How much does each allele contribute? What pattern of inheritance does this exemplify? If a hen and a rooster from this F1 generation are mated, what will be the weight of the eggs laid by hens of the F2?

X Inactivation in Female MammalsAlthough female mammals inherit 2 copies of the X chromosome, one X chromosome becomes inactivated during embryonic development and is called a Barr BodyThe inactivation of an X chromosome occurs randomly in each embryonic cell, therefore females consist of a mosaic of 2 types of cells (active x from mom or active x from dad)Ex. Tortoise shell catsSome cells express black fur and others express orange fur

Pedigree AnalysisPedigree - diagram of family history of a trait or disease used to study heredityBy studying a pedigree, it is possible to infer the pattern of heredity

Analyzing a PedigreeDetermine if trait is sex-linked or autosomalSex-linked usually seen in malesAutosomal appears in both sexes equallyDetermine if trait is dominant or recessiveIf every individual w/trait has a parent w/trait then it is dominantIf individual has parents w/o trait then it is recessiveDetermine if the trait is determined by a single gene or severalIf determined by a single recessive gene, than normal parents should produce affected children with a 3:1 ratioIf determined by several genes the proportion would be much lower

Example PedigreeEx. Pedigree 1

Pedigree 2