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Genetics Biology 12 A. Allen 1 Mendelian Genetics Augustinian While assigned to Monk at Brno teach, he was also Monastery in assigned to tend Austria (now the gardens and Czech Republic) grow vegetables for the monks to Not a great teacher eat. but well trained in math, statistics, probability, physics, and interested in plants and heredity. Mountains with Gregor Mendel short, cool growing season meant pea “Father of Genetics” (Pisum sativum)2was an ideal crop plant. Contributions in 1860s (US Civil War Era) • Discovered Genes as Particles of Inheritance • Discovered Patterns of Inheritance • Discovered Genes Come from Both Parents Egg + Sperm = Zygote Nature vs Nurture http://academic.evergreen.edu/v/vivianoc/homunculus.gif Sperm means Seed (Homunculus) • Discovered One Form of Gene (Allele) Dominant to Another • Discovered Recessive Allele Expressed in Absence of Dominant Allele 3 Mendel worked with peas (Pisum sativum) • Good choice for environment of monastery • Network provided unusual varieties for testing • Obligate self-pollination reproductive system Permits side-by-side genetic barriers Cross-pollinations require intentional process • Crosses meticulously documented • Crosses numerically/statistically analyzed • Scientists of 1860s could not understand math • Work lost in journals for 50 years! • Rediscovered in 1900s independently by 3 scientists • Recognized as landmark work! 4 • Peas naturally self-pollinate because the male and female flower parts are enclosed by the petal arrangement 5 6 7 P ‘True breeding’ or ‘pure stock’ tall and short plants Mendel noticed that F1 tall peas crossed with short peas yielded all tall peas in the F1 generation (first group of offspring) 8 One Example of Mendel’s Work Tall x Dwarf Phenotype P TT tt Genotype Homozygous Homozygous Dominant Recessive All Tall Clearly Tall is Inherited… F1 What happened to Dwarf? Tt Heterozygous Tall is dominant to Dwarf F1 x F1 = F2 possible gametes Punnett Square: T t 3/ Tall 4 F2 T Tall Tall 1/ 4 Dwarf possible TT Tt gametes Dwarf is not missing…just masked as Tall Dwarf “recessive” in a diploid state… there t Tt tt IS a contribution from the short plant 9 which reappears in F2 generation. Mendel then took the F1 peas and F1 crossed them with themselves to produce an F2 generation (2nd group of offspring) F2 10 Mendel as a Scientist F2 F1 x F1 = F2 possible gametes Test Cross: Punnett Square: T t Unknown Tall x Dwarf Tall Tall tt T possible TT Tt gametes Tall Dwarf possible gametes t If Unknown is TT: Tt tt t t Tall Tall T possible Tt Tt gametes Test Progeny All Tall Tall Tall T Tt Tt 1/ of F2 Tall are TT possible gametes 3 If Unknown is Tt: t t 2/ of F2 Tall are Tt 3 Tall Tall T possible Tt Tt gametes Test Progeny Half Tall Dwarf Dwarf t Half Dwarf tt tt 11 Another Example of Mendel’s Work Green x Yellow Phenotype P yy YY Genotype Homozygous Homozygous Recessive Dominant All Yellow Clearly Yellow is Inherited… F1 What happened to Green? Yy Heterozygous Yellow is dominant to Green F1 x F1 = F2 possible gametes Punnett Square: Y y 3/ Yellow F2 Y Yellow Yellow 1/ 4 possible YY Yy 4 Green gametes Yellow Green Green is not missing…just masked y as “recessive” in diploid state Yy yy 12 Mendel as a Scientist F2 F1 x F1 = F2 possible gametes Punnett Square: Test Cross: Y y Unknown Yellow x Green Y Yellow Yellow yy possible YY Yy gametes Yellow Green possible gametes y If Unknown is YY: yy yy y y Yellow Yellow Y possible Yy Yy gametes Test Progeny All Yellow Yellow Yellow Y Yy Yy 1/ of F2 Yellow are YY 3 possible gametes If Unknown is Yy: 2/ y y 3 of F2 Yellow are Yy Yellow Yellow Y possible Yy Yy gametes Test Progeny Half Yellow Green Green y Half Green yy yy 13 Mendel as a Scientist Actual Results Decision Test Cross: 3 Yellow 2 Green Yy Unknown Yellow x Green Y? yy 2 Yellow 3 Green Yy possible gametes 1 Yellow 4 Green Yy If Unknown is YY: y y Small families do not follow Yellow Yellow expected ratios perfectly! Y possible Yy Yy 0 Yellow 5 Green Yy gametes Yellow Yellow Rare, but it can happen! Y Yy Yy 4 Yellow 1 Green Yy It only takes 1 green to be sure possible gametes the unknown is Yy! If Unknown is Yy: y y 5 Yellow 0 Green YY Yellow Yellow <5% chance unknown is Yy Y possible Yy Yy 1/ • 1/2 • 1/2 • 1/2 • 1/2 = 1/32 2 gametes Green Green y You could be wrong (rarely)! yy yy 14 Yet Another Example of Mendel’s Work Wrinkled x Round Phenotype P rr RR Genotype Homozygous Homozygous Recessive Dominant All Round 1. Round is dominant to Wrinkled F1 Rr Heterozygous F1 x F1 = F2 NEVER use W/R or w/r possible gametes Punnett Square: R r F2 R Round Round 3/ possible RR Rr 4 Round gametes 1/ r Round Wrinkled 4 Wrinkled Rr rr 15 Vocabulary Challenge Gene monohybrid cross Homozygous genotype Allele Dominant Heterozygous dihybrid cross Recessive Phenotype 16 Mendel’s Laws of Heredity 1. The Law of Segregation: The two members of a gene pair segregate (separate) from each other into the gametes, so that one half of the gametes carry one member of the pair and the other half of the gametes carry the other member of the gene pair. Each pair of alleles (which are located on homologous chromosomes) segregates during the formation of sex cells. Think in terms of meiosis: a diploid cell undergoes meiosis to produce haploid cells (only one allele in a gamete for a given gene) 17 …Mendel’s Laws of Heredity 2. Law of Unit Characters: Each parent contributes one allele during cross-fertilization. • Mendel emphasized this to explain the continued presence of shortness in the F1 tall peas and to dispel the idea of “Blending Inheritance” common in his time. • Mendel discovered that the recessive allele reappeared in the F2 generation after the F1 plants were crossed. 18 …Mendel’s Laws of Heredity 3. Law of Dominance: The dominant allele is always expressed when the recessive allele is present. Ex: Genotypes TT and Tt both result in tall plants. The phenotype of a Tt plant is tall even though it has a ‘short’ (t) allele. 19 …Mendel’s Laws of Heredity 4. Law of independent assortment (which says that an organism's individual traits are passed on independently of one another). 20 Genetics After Mendel Incomplete Dominance • A Heterozygous genotype that creates an intermediate phenotype. In other words, when two different alleles are present, the phenotypes are blended. 21 Incomplete Dominance Red Yellow After 1900 several scientists tried to P x replicate Mendel’s crosses using PRPR PYPY other species including snapdragon. When these alleles go walking, they both do some talking ! All Orange F1 OK, so we cannot use R/r nor Y/y so we pick PRPY a third letter…P for the petal color gene. F1 x F1 = F2 possible gametes Punnett Square: PR PY F2 PR Red Orange This F2 will NOT have a 3:1 ratio of phenotypes. possible PRPR PRPY gametes Orange Yellow Instead it shows a 1:2:1 ratio! PY PRPY P YP Y The exception here proves the rule. 22 Make a Prediction • Draw a Punnett Square to predict the phenotypes of the offspring from a Red x Orange cross 23 In addition to this, there are multiple alleles possible: PR = red PY = yellow p = no pigment The combination of alleles in a diploid determine the flower color: PRPR = red PRp = pink PRPY = orange PYp = cream PYPY = yellow pp = white Human hair color follows a similar pattern: Alleles: HBn = brown HBd = blonde hR = red hbk = black The combinations of these alleles determine the base hair color: HBnHBn = dark brown HBdHBd = blonde hRhR = red H BnHBd = sandy brown HBdhR = strawberry hRhbk = red HBnhR = auburn blonde HBnhbk = dark brown HBdhbk = blonde hbkhbk = black Recessive can Dominant does NOT mean frequent! 25 be common! Genetics After Mendel Codominance • The situation in which two different alleles for a trait are expressed unblended in the phenotype of heterozygous individuals. Neither allele is dominant or recessive, so that both influence the phenotype. Type AB Blood is an example. Such traits are said to be codominant. 26 Codominance: ABO Blood Type • There are 4 ABO blood types; A, B, AB, & O Blood Allelic • Your blood type is determined by the Type combinations presence of antigens on the surface of your erythrocytes (red blood cells). A IA IA or IA i • The three alleles are: – IA codes for production of A antigen B IB IB or IB i – IB codes for production of B antigen – i no antigens produced on AB IA IB erythrocytes • The IA and IB alleles are dominant over the i allele, which is always recessive. O ii • Presence of both IA and IB alleles exhibit codominance 27 Try this Blood Type Problem • Lauren is blood type A. Her dad is type O. • Lauren marries her high school sweetheart who is type B. Lauren’s father-in-law is type O. • What are the possible blood types of their baby? 28 Another Example of Recessive Being Common: Pisum sativum Garden Peas: green seed, wrinkled seed, dwarf stature, white flower yy rr tt aa In other words: a quadruple double-recessive is the most common garden pea on Earth! Quantitative Inheritance: multiple genes control trait Highest Crop Yield: AABBCCDDEE Intermediate Crop Yield: AabbCCDdEe Lowest Crop Yield: aabbccddee Darkest Skin Color: AABBCCDDEE Intermediate Skin Color: AaBbCcDdEe Lightest Skin Color: aabbccddee AaBbCcDdEe x AaBbCcDdEe can produce a huge range of colors! Yet TV talk show guests argue this point for Maury, etc.30 Sex-Linked Traits 31 What is a Sex-Linked trait? • Sex-linked traits are due to genes located on sex chromosomes. • Males have XY sex chromosomes • Females have XX sex chromosomes. – The X chromosome contains over 1000 genes while the Y chromosome contains as few as 26. Therefore, many sex-linked traits are discussed in terms of the X-chromosomes. 32 Carriers • Because females have two copies of the X chromosome, it is possible to have certain traits “hidden” by a dominant copy. – However, because males only have one X chromosome, the observable phenotype is obvious and identifies the genotype. 33 Carriers • When a female contains a recessive allele that is hidden by the dominant allele, we call them carriers. – A carrier maintains the ability to pass on a trait even if they do not express/show it. 34 Examples of X-linked genes • Other than determining sex, genes on the X chromosome are responsible for traits. Some examples are: – Hemophilia – Red-green color blindness – Muscular dystrophy 35 …Examples of X-linked genes Red-green color blindness • Three types of cones, “red’, green’ and ‘blue’ are receptive different wavelengths of light. • an inability to distinguish between red and green, is caused by a defect in one of the three color- sensitive cells in the retina. 36 …Examples of X-linked genes Hemophilia • Human blood contains special proteins, known as clotting factors. • Clotting factors help stop bleeding and allow a blood vessel to heal after an injury. • The last step in the clotting process (also called coagulation) is the creation of a "net" that closes the torn blood vessel and stops the bleeding. This part of the process involves clotting factors VIII and IX. • People with hemophilia are deficient in one of those factors due to their abnormal genes and, as a result, their blood can't clot properly. 37 …Examples of X-linked genes Muscular dystrophy • incorrect or missing genetic information that prevents the body from making the proteins needed to build and maintain healthy muscles. • Symptoms may but may include: • Muscle weakness that slowly gets worse – Frequent falls – Delayed development of muscle motor skills (children) – Problems walking – Difficulty using one or more muscle groups – Eyelid drooping (ptosis) – Drooling • Hypotonia (low muscle tone) • Joint contractures (loss of joint movement) clubfoot, clawhand, or others 38 • Scoliosis (curved spine) Punnett Practice • ‘Elephant ears’ (e) is a fictional recessive sex-linked trait: • The woman is a carrier and her partner does not have the trait. – What % chance do the boys & girls have of receiving the trait? – What % chance do the boys & girls have of having elephant ears? • If the man has an X-recessive trait and his partner does not have it (nor is a carrier). – What % chance do the boys & girls have of receiving the trait? – What % chance do the boys & girls have of having elephant ears? 39 Recessive Lethals • In some rare instances, recessive traits are lethal-- meaning the organism is born very weak and sickly or dies not long after birth. – When looking at genes on non-sex chromosomes (AKA autosomal), we can apply standard Punnett square probability. – However, when looking at genes on sex chromosomes we see that males have a much higher tendency for recessive lethals. Why? 41 Human Pedigree Symbols 42
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