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Chapter 14: Genetics & The Work of Mendel
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Gregor Mendel Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas used experimental method used quantitative analysis collected data & counted them excellent example of scientific method He studied at the University of Vienna from 1851 to 1853 where he was influenced by a physicist who encouraged experimentation and the application of mathematics to science and a botanist who aroused Mendel’s interest in the causes of variation in plants. After the university, Mendel taught at the Brunn Modern School and lived in the local monastery. The monks at this monastery had a long tradition of interest in the breeding of plants, including peas. Around 1857, Mendel began breeding garden peas to study inheritance.
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Mendel’s work Bred pea plants P F1 F2
Pollen transferred from white flower to stigma of purple flower Bred pea plants cross-pollinate true breeding parents (P) P = parental raised seed & then observed traits (F1) F = filial (kids) allowed offspring to self-pollinate & observed next generation (F2- grandkids) P anthers removed all purple flowers result F1 P = parents F = filial generation self-pollinate F2
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Looking closer at Mendel’s work
true-breeding purple-flower peas true-breeding white-flower peas X P Where did the white flowers go? 100% F1 generation (hybrids) purple-flower peas White flowers came back! In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties. The true-breeding parents are the P generation and their hybrid offspring are the F1 generation. Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation. self-pollinate F2 generation 3:1 75% purple-flower peas 25% white-flower peas
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What did Mendel’s findings mean?
Traits come in alternative forms = alleles purple vs. white flower color different alleles vary in the sequence of nucleotides at the specific locus of a gene some difference in sequence of A, T, C, G purple-flower allele & white-flower allele are two DNA variations at flower-color locus different versions of gene at same location on homologous chromosomes
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What did Mendel’s findings mean?
Some traits mask others purple & white flower colors are separate traits that do not blend purple x white ≠ light purple purple masked white dominant allele functional protein masks other alleles recessive allele allele makes a malfunctioning protein I’ll speak for both of us! wild type allele producing functional protein mutant allele producing malfunctioning protein homologous chromosomes
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Genotypes Homozygous = same alleles = PP, pp
Heterozygous = different alleles = Pp homozygous dominant heterozygous homozygous recessive
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Genotype vs. phenotype Difference between how an organism “looks” & its genetics phenotype description of an organism’s trait the “physical” genotype description of an organism’s genetic makeup F1 P X purple white all purple Explain Mendel’s results using …dominant & recessive …phenotype & genotype
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Mendel’s 1st law of heredity
PP P Mendel’s 1st law of heredity Law of segregation during meiosis, alleles segregate homologous chromosomes separate each allele for a trait is packaged into a separate gamete pp p Pp P p
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Whoa! And Mendel didn’t even know DNA or genes existed!
Law of Segregation Which stage of meiosis creates the law of segregation? Whoa! And Mendel didn’t even know DNA or genes existed! Anaphase 1
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Monohybrid cross Some of Mendel’s experiments followed the inheritance of single characters flower color seed color monohybrid crosses
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PP pp Pp x Making crosses Can represent alleles as letters
flower color alleles P or p true-breeding purple-flower peas PP true-breeding white-flower peas pp F1 P X purple white all purple PP x pp Pp
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phenotype & genotype can have different ratios
Aaaaah, phenotype & genotype can have different ratios Punnett squares Pp x Pp F1 generation (hybrids) % genotype % phenotype P p male / sperm PP 25% 75% Pp 50% P p female / eggs PP Pp Pp Pp pp 25% 25% pp 1:2:1 3:1
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Can’t tell by lookin’ at ya!
Phenotype vs. genotype 2 organisms can have the same phenotype but have different genotypes homozygous dominant PP purple Pp heterozygous purple Can’t tell by lookin’ at ya! How do you determine the genotype of an individual with with a dominant phenotype?
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Test cross Breed the dominant phenotype — the unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele x How does that work? is it PP or Pp? pp
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How does a Test cross work?
x x Am I this? Or am I this? PP pp Pp pp p p p p P P Pp Pp Pp Pp P p Pp Pp pp pp 100% purple 50% purple:50% white or 1:1
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Mendel was working out many of the genetic rules!
Dihybrid cross Other of Mendel’s experiments followed the inheritance of 2 different characters seed color and seed shape dihybrid crosses Mendel was working out many of the genetic rules!
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Dihybrid cross P YYRR yyrr 100% F1 YyRr 9:3:3:1 F2 x true-breeding
yellow, round peas true-breeding green, wrinkled peas x YYRR yyrr Y = yellow R = round y = green r = wrinkled 100% F1 generation (hybrids) yellow, round peas YyRr Wrinkled seeds in pea plants with two copies of the recessive allele are due to the accumulation of monosaccharides and excess water in seeds because of the lack of a key enzyme. The seeds wrinkle when they dry. Both homozygous dominants and heterozygotes produce enough enzyme to convert all the monosaccharides into starch and form smooth seeds when they dry. self-pollinate 9:3:3:1 F2 generation 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas
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Which system explains the data?
What’s going on here? If genes are on different chromosomes… how do they assort in the gametes? together or independently? YyRr Is it this? Or this? YyRr YR yr YR Yr yR yr Which system explains the data?
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NO! Is this the way it works? YyRr x YyRr YR yr YR YYRR YyRr yr YyRr
9/16 yellow round YR yr 3/16 green round Well, that’s NOT right! YR YYRR YyRr 3/16 yellow wrinkled yr YyRr yyrr 1/16 green wrinkled
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YES! Dihybrid cross YyRr x YyRr YR Yr yR yr YR Yr yR yr YYRR YYRr YyRR
or Dihybrid cross YyRr x YyRr 9/16 yellow round YR Yr yR yr YR Yr yR yr 3/16 green round YYRR YYRr YyRR YyRr BINGO! YYRr YYrr YyRr Yyrr 3/16 yellow wrinkled YyRR YyRr yyRR yyRr 1/16 green wrinkled YyRr Yyrr yyRr yyrr
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Mendel’s 2nd law of heredity
Can you think of an exception to this? Mendel’s 2nd law of heredity Law of independent assortment different loci (genes) separate into gametes independently non-homologous chromosomes align independently classes of gametes produced in equal amounts YR = Yr = yR = yr only true for genes on separate chromosomes or on same chromosome but so far apart that crossing over happens frequently yellow green round wrinkled YyRr Yr Yr yR yR YR YR yr yr 1 : 1 : 1 : 1
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Law of Independent Assortment
Which stage of meiosis creates the law of independent assortment? Metaphase 1 Remember Mendel didn’t even know DNA —or genes— existed! EXCEPTION! If genes are on same chromosome & close together will usually be inherited together rarely crossover separately “linked”
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The chromosomal basis of Mendel’s laws…
Trace the genetic events through meiosis, gamete formation & fertilization to offspring
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Review: Mendel’s laws of heredity
Law of segregation monohybrid cross single trait each allele segregates into separate gametes established by Anaphase 1 Law of independent assortment dihybrid (or more) cross 2 or more traits genes on separate chromosomes assort into gametes independently established by Metaphase 1 EXCEPTION! linked genes metaphase1
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Probability & Genetics
Laws of Probability Probability & Genetics
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Probability & Genetics
Calculating probability of making a specific gamete is just like calculating the probability in flipping a coin probability of tossing heads? 50% probability making a P gamete… Outcome of 1 toss has no impact on the outcome of the next toss probability of tossing heads each time? 50% probability making a P gamete each time?
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Rule of Addition Chance that an event can occur 2 or more different ways SUM of the separate probabilities Use for heterozygous possibilities Two ways to be heterozygous: Pp or pP Key word is “or”. Ex: Probability of getting 2 or a 6 on the roll of a die. 1/6 + 1/6 = 2/6 = 1/3 Ex: Probability of having offspring with dominant phenotype? PP or Pp or pP ¼+ ¼ + ¼ = ¾
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Rule of multiplication
Chance that 2 or more independent events will occur together probability that 2 coins tossed at the same time will land heads up probability of pp or PP offspring Ex: Probability of getting a head and a tail with two different coins. ½ x ½ = 1/4 Key word is “and”
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Calculating Probability of Pp x Pp ½ x ½ = ¼ = PP ½ x ½ = ¼ = pp What about Pp?
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Calculating Dihybrid Probability
Rule of multiplication application with Dihybrid crosses: heterozygous parents — YyRr probability of producing yyrr? probability of producing y gamete = 1/2 probability of producing r gamete = 1/2 probability of producing yr gamete = 1/2 x 1/2 = 1/4 probability of producing a yyrr offspring = 1/4 x 1/4 = 1/16
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Beyond Mendel’s Laws of Inheritance
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Incomplete dominance Heterozygote shows an intermediate, blended phenotype example: RR = red flowers rr = white flowers Rr = pink flowers make 50% less color RR WW RW RR RW WW
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Incomplete dominance P F1 100% 1:2:1 F2 X true-breeding red flowers
white flowers 100% 100% pink flowers F1 generation (hybrids) self-pollinate 25% white F2 generation 25% red 1:2:1 50% pink
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Codominance There are two or more alleles that are dominant in a phenotype. Both alleles are expressed in heterozygous condition. Ex: Red – RR, white – WW, red and white - RW
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Multiple Alleles 2 alleles affect the phenotype equally & separately
not blended phenotype human ABO blood groups 3 alleles IA, IB, i IA & IB alleles are co-dominant glycoprotein antigens on RBC IAIB = both antigens are produced i allele recessive to both
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Polygenic inheritance
Some phenotypes determined by additive effects of 2 or more genes on a single characteristic phenotypes on a continuum human traits skin color height weight intelligence behaviors
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Sex linked traits 1910 | 1933 Genes are on sex chromosomes
as opposed to autosomal chromosomes first discovered by T.H. Morgan at Columbia U. Drosophila breeding good genetic subject prolific 2 week generations 4 pairs of chromosomes XX=female, XY=male
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Discovery of sex linkage
true-breeding red-eye female true-breeding white-eye male X P Huh! Sex matters?! 100% red eye offspring F1 generation (hybrids) 100% red-eye female 50% red-eye male 50% white eye male F2 generation
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Genetics of Sex X Y X XX XY X XX XY
In humans & other mammals, there are 2 sex chromosomes: X & Y 2 X chromosomes develop as a female: XX gene redundancy, like autosomal chromosomes an X & Y chromosome develop as a male: XY no redundancy X Y X XX XY X XX XY 50% female : 50% male
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Morgan’s flies… x x XRXR XrY XRXr XRY Xr Y XR Y XR XR XRXr XRY XRXR
100% red females 50% red males; 50% white males 100% red eyes
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Genes on sex chromosomes
Y chromosome few genes other than SRY sex-determining region master regulator for maleness turns on genes for production of male hormones X chromosome traits other than sex determination mutations: (all are recessive) hemophilia Duchenne muscular dystrophy color-blindness Duchenne muscular dystrophy affects one in 3,500 males born in the United States. Affected individuals rarely live past their early 20s. This disorder is due to the absence of an X-linked gene for a key muscle protein, called dystrophin. The disease is characterized by a progressive weakening of the muscles and loss of coordination.
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Hemophilia XHXh XHY Hh x HH XH XHXh XH Y Xh XHXH XHXH XHY XHY XH Xh XH
sex-linked recessive Hemophilia XHXh XHY Hh x HH XH XHXh XH Y male / sperm Xh XHXH XHXH XHY XHY XH Xh female / eggs XH XHY XHXh XHXh XhY XhY Y carrier disease
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X-inactivation Female mammals inherit 2 X chromosomes XH XHXh Xh
one X becomes inactivated during embryonic development condenses into compact object = Barr body which X becomes Barr body is random patchwork trait = “mosaic” patches of black XH XHXh Xh tricolor cats can only be female patches of orange
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Environmental effects
Phenotype is controlled by both environment & genes Human skin color is influenced by both genetics & environmental conditions Coat color in arctic fox influenced by heat sensitive alleles The relative importance of genes & the environment in influencing human characteristics is a very old & hotly contested debate a single tree has leaves that vary in size, shape & color, depending on exposure to wind & sun for humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests even identical twins — genetic equals — accumulate phenotypic differences as a result of their unique experiences Color of Hydrangea flowers is influenced by soil pH
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Pedigree Analysis A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees
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(a) Is a widow’s peak a dominant or recessive trait?
Fig b 1st generation (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, and uncles) Ww ww ww Ww Ww ww 3rd generation (two sisters) WW ww or Figure 14.15a Pedigree analysis Ww Widow’s peak No widow’s peak (a) Is a widow’s peak a dominant or recessive trait?
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(b) Is an attached earlobe a dominant or recessive trait?
Fig c 1st generation (grandparents) Ff Ff ff Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff 3rd generation (two sisters) ff FF or Ff Figure 14.15b Pedigree analysis Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait?
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