Ms. Petrauskas' Class



Mendelian GeneticsThe idea that biological traits can be inherited has existed for over 6000 years (the time of the Babylonians), where pedigrees were depicted in cave paintings.Traits were initially thought to be a blending of parental characteristicsA trait is a particular version of a characteristic that is inherited , such as hair colour or blood type (Characteristic = eye colour; trait =blue eyes)Gregor Mendel (1822-1884):Pioneer of geneticsAustrian monk who worked with garden peasPea plants reproduce quickly and can be either self-pollinated or cross-pollinatedA true-breeding plant can be crossed over and over with itself or similarly looking pea plants to produce offspring with the same traitsA hybrid is the offspring of two true-breeding plantsHe cut the anthers off parent plant that he was pollinating, to eliminate self -pollinationHis experiments explained the mechanism of inheritance in plants and heredity in generalMendel examined 7 characteristics in pea plants and only crossed plants that only differed at a single trait at one time (i.e. plants with purple or white flowers, while all other traits were the same)CharacteristicTraitFlower colourPurple/whiteFlower positionAxial (along stems)/ terminal (at tips)Stem lengthTall/dwarfSeed shapeWrinkled/ roundSeed colourYellow/ greenPod shapeInflated/constrictedPod colourYellow/greenAccording to prior belief if Mendel crossed a plant with wrinkled peas with one with smooth seeds then it would have been expected to get a mixture of the two traitsMendel proved this to be incorrect!!!!Wrinkled + smooth = smoothWith the above traits one always dominates the otherMendels Experiments:Definitions:Cross: The breeding of two organisms with different traits is called a cross.P- generation: the parental generation. True-breeding.F1 generation: the filial generation. The hybrid offspring are produced from crossing the P-generation. They are referred to as monohybrids or offspring of two true-breeding plants that differ in only one characteristic.F2 generation: the offspring of an F1 generation cross. Resulted when Mendel allowed the F1 generation to self-pollinate.Monohybrid cross: a cross designed to study the inheritance of only one trait.What he did:1. Crossed a true-breeding pea plant with purple flowers, with one with white flowers. Wondered if he would get individuals with pink (“blended”) flowers2. Mendel observed that the F1 generation all had purple flowers3. He initially believed that the white trait had disappeared, but when he crossed the F1 generation, he observed both purple and white flowers again4. With repeated experimentation he was able to observe that not only did the white trait not disappear, but that these traits in his F2-generation were always expressed in a 3:1 ratio (purple to white)CharacterisiticPF1F2F2 RatioFlower colourPurple x whiteAll purplePurple (705) and white (224)3:1Seed shapeRound x wrinkledAll roundRound (5474) and wrinkled (1850)3:1Seed colourYellow x greenAll yellowYellow (6022) and green (2001)3:1Pod shapeInflated x constrictedAll inflatedInflated(882) and constricted (299)3:1Pod colourGreen x yellowAll greenGreen (428) and yellow (152)3:1Flower positionAxial x terminalAll axialAxial (651) and terminal (207)3:1Stem lengthTall x dwarfAll tallTall (787) and dwarf (277)3:1Mendel’s Conclusions:Mendel concluded that traits must be passed in distinct heredity units, factors, which we now call genesEven if not expressed in the F1 generation they can still be passed onThose that were expressed in all F1 generations were called “dominant factors” and those that were hidden but expressed in the F2 generation “recessive factors”From his conclusions he came up with the first law of inheritance; the law of segregationLaw of segregation:A scientific law stating that;organisms inherit two copies of genes, one from each parentorganisms only donate one copy of each gene to their gametes because the genes separate during gamete formation (meiosis)Alleles: Dominant or RecessiveMendel also realized that each characteristic had multiple traits. Today the different forms of a gene are known as alleles. One allele from your father and one from your motherWhich allele is passed is purely random. The combination of alleles can be the same as one another or different (i.e. both code for a purple flower or one codes purple and one codes white).If two alleles are the same this condition is called homozygous.If the two alleles are different this condition is called heterozygous.An individual’s genotype includes all forms of an individual’s genes, even if “hidden”.An individual phenotype includes the allele that has been expressed or the appearance of that individual.A dominant allele is one that does not stay hidden, and is always expressed when the recessive gene is presentFor Mendel’s pea plant we assign letters to the different alleles. Upper case for dominant and lower case for recessive.If the dominant allele is present then the plant will look like the dominant gene. Re-cap:Mendel concluded that sex cells contain 1 allele and today we know this because of _________Factors=______________ and different forms of a gene =__________________TraitHomozygous Dominant Heterozygous DominantHomozygous RecessiveFlower colour (P, p)PP (purple phenotype)Pp (Purple phenotype but purple and white genotype)pp (white phenotype)Seed colour (Y, y)YYYyyySeed shape ( R, r)RRRrrrPod shape (G, g)Flower Position (A,a)Stem Length (T, t)Law of Independent Assortment:A scientific law stating that;Segregation for different pairs of alleles occurs independently (a plant that is TtRr forms gametes that are TR, Tr, tR, tr) Punnet Squares and Predicting InheritancePPA Punnett square is a diagram used to predict the proportions of genotypes in the offspring resulting from a cross between two individualsCross between a true-breeding white plant pp and true-breeding purple plant, PPpPossible GenotypespIt can help us determine the probability, or likelihood that an outcome will occur. Ex. If we flip a coin 10 times there is a 50% it will be heads or tail; a 50:50 ratio. This does not mean you will get head 5 times and tails 5 times. Each flip is an independent event. The same is true for the inheritance of alleles.Human TraitsDuring meiosis we observed that during gamete formation only 1 set of chromosomes (n=23) from each parent is used in the process of fertilizationAfter fertilization, our genes are made up of a set of homologous chromosomes (2n=46)Therefore, half of our genes come from our mother and half come from our fatherRecall that each homologous chromosome codes for similar traits ( ex. Eye colour)Every individual is made up of many different traits, some of which you are already familiar with like baldness, hair colour, colour blindness and blood types.Others include position of eyes, shape of fingers, body size and proportion (these traits can be influenced by our environment as well)The following traits are known as single-gene traits, expressions of two alleles at a single gene locus.Trait (alleles)ExpressionBent pinky (B, b)Dominant allele causes the distal segment of the fifth finger to bend distinctly toward the fourth fingerPTC taster (T, t)Phenylthiocarbamide (PTC) tastes bitter to heterozygous or homozygous dominant, but tasteless to homozygous recessive. Several related compounds occur naturally in foods and if you are a PTC taster there is a good chance you are a non-smoker, do not like Brussels sprouts, grapefruit juice or green tea.Blue Eyes (E, e)Blue-eyed individuals are homozygous recessive and lack pigment in their iris. Homozygous and heterozygous dominant individuals have pigment and the colour is determined by another geneMid-digital hair (M, m)People lacking hair in the middle segments of their fingers are homozygous recessive. The presence of hair on one or more of the middle segments of the fingers is governed by a series of alleles all dominant to the recessive.Tongue rolling (R, r)Persons with a dominant allele in heterozygous or homozygous condition can roll their tongue into a tube-like shape. Homozygous recessives are non-rollers and can never learn to roll their tongue.Widow’s Peak (W, w)Dominant allele in heterozygous or homozygous individuals results in a V-shaped front hairline; homozygous have a straight hairline.Thumb crossing (C, c)In relaxed interlocking fingers, left over right indicates the dominate allele in either homozygous of heterozygous individuals and right over left indicated the homozygous recessive conditionEar lobes (A, a)Homozygous recessives have attached earlobes and the heterozygous or homozygous dominant individuals have detached earlobesHitchhiker’s thumb (H, h)Homozygous recessives can bend the distal joint o the thumb backward to nearly a 90 degree angle; heterozygous or homozygous dominant individual cannot bend further than approximately 30 degrees ................
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