notes 14

Molecular Biology----------------------------------- Chapter 14---------------Mendel and the Gene Genetic Variation in gametes due to meiosis Homologous genes Genes- region of DNA that contains information for a specific trait Genes can have variant forms Alleles- variants of genes Homozygote- individual with 2 identical alleles for a gene Heterozygote- individual with 2 different alleles for a gene Chromosomes assort independently, all chromosome combinations are possible So all allele combinations are possible, too Gregor Mendel Chromosomal Theory of Inheritance How is information passed form parent to offspring? Hypothesis 1- traits are blended in the offspring Hypothesis 2- traits are passed by particles, each parent giving 1 These hypotheses make different predictions: If blended inheritance, offspring will resemble? If particles, offspring will resemble? Why peas are great Many easily identifiable variable traits Pure (or true-breeding) line Line with no genetic variation for the trait If lines are true-breeding, a cross between 2 plants that produce purple flowers will only produce purple flowered offspring Monohybrid Cross Monohybrid cross: following 1 trait P generation The parents, true- breeding F1 generation Offspring of cross between 2 P generation individuals F2 generation Offspring of self-fertilized F1s Crossing a true-breeding wrinkled seed parent with a true-breeding round seed parent Mendel crossed true-breeding smooth with true-breeding wrinkled, all offspring were smooth NO BLENDED INHERITANCE Why are they all smooth? What happened to the wrinkled? Mendel let the F1 generation self-fertilize, got 5474 smooth and 1850 wrinkled (2.96: 1 ratio) Combination of particles Genotype- description of alleles in an individual Homozygous- 2 round alleles or 2 wrinkled alleles Heterozygous- 1 round and 1 wrinkled Phenotype- physical appearance related to genotype Round seeds or wrinkled seeds Mendel’s Conclusions Traits are determined by particles In smooth/wrinkled cross there is a particle for smooth and a particle for wrinkled Individuals have 2 particles Both smooth Both wrinkled 1 smooth and 1 wrinkled Dominant/ recessive When you have 2 different particles, only 1 determines what you look like (dominant), and the other is not expressed (recessive) 1 smooth particle and 1 wrinkled particle = smooth peas Segregation When gametes are formed they each get a single particle Working through a cross We can use Mendel’s principles to predict genotype and phenotype frequencies Trait- seed shape Alleles R = round allele r = wrinkled allele By convection, the capitalized allele is dominant Genotypes RR = round Rr = round rr = wrinkled 3 genotypes, but 2 phenotypes Start with the parent’s gneotypes RR for male rr for female What are the possible gametes they can produce? Remember, due to meiosis, wach gamete gets 1 allele Can only put alleles in gametes that you actually have!! If all pollen had R, and all eggs have r, what will the offspring (F1) be? What will they look like? What happens if you cross 2 F1S? More options are possible- let’s use a system Punnett Squares!!! Punnett Squares Steps….. Identify the genotypes of the parents Identify all the possible gametes for each parent Set up a grid Each column is one of mom’s possible gametes Each row is one of dad’s possible gametes Fill the inner squares with their corresponding gametes from their column and row These are the possible genotypes of the offspring Calculate the genotype and phenotype ratios These are also telling us the possibilities What is the probability of… A round seeded offspring? A heterozygous offspring? A wrinkled offspring? Monohybrid crosses 3:1 ratio is found when you cross 2 heterozygotes Mendel found lots of these Practice Cross Cystic fibrosis is caused by having 2 recessive alleles for the CFTR (cystic fibrosis transmembrane conductance regulator) Possible genotypes and phenotypes CC: homogypous dominant, not cystic fibrosis Cc: heterozygous, no cystic fibrosis, but a “carrier” cc: homozygous recessive, has cystic fibrosis 2 people, 1 homozygous dominant and the other heterozygous for the cystic fibrosis trait have a child What is the probability that the child will have cystic fibrosis? What is the probability that the child will be a carrier? Monohybrid problems In humans, albinism is a recessive trait. If an albino woman has a child with a homozygous non-albino man, What is the probability they’ll have an albino child? What is the probability that they’ll have a homozygous, non-albino child? What is the probability they’ll have a heterozygous child? If an albino woman has a child with a man heterozygous for the albino trait, What is the probability they’ll have an albino child? What is the probability that they’ll have a homozygous, non-albino child? What is the probability they’ll have a heterozygous child? Having a big toe shorter than your second tow is caused by a dominant allele. Suppose a man with a short big toe has 2 children with a woman with big big toes. The first child has big big toes. What is the probability that their second child will have short big toes? Huntington’s disease is caused by a dominant allele. A person heterozygous for Huntington’s disease has children with a person homozygous recessive for Huntington’s. What proportion of their children would you predict to have Huntington’s? Dihybrid cross Following more than 1 gene For Mendel: Seed shape: R or r Seed color: Y or y YY= yellow peas Yy= yellow peas yy= green peas 2 hypotheses Alleles for different genes are independent Alleles for different genes are linked What is the genotype for a true breeding smooth yellow pea? What is the genotype for a true breeding wrinkled green pea? In the F1, both hypotheses have the same prediction If alleles for different genes are linked, what gametes ae produced? If alleles for different genes are independent, what gametes are produced? If alleles for different genes are independent Differs from “linked” prediction…. Different ratio All phenotype combinations are possible Independent assortment Principle of independent assortment Each gamete will get 1 allele for shape and 1 allele for color Getting a “R” has no effect on the gamete getting an “Y” or a “y” % chance of getting R or r % chance of getting Y or y Why? Dihybrid example 2 recessive genetic disorders Cystic fibrosis CC, Cc= no disorder, cc= disorder Tay-Sachs TT, Tt= no disorder, tt= disorder If a person is a carrier for cystic fibrosis and Tay-Sachs has a child with someone who is only a carrier for cystic fibrosis, what is the probability their child will have…. Cystic Fibrosis? Tay-Sachs? Both? Parents genotypes CcTt CcTT Possible gametes CT, cT, Ct, ct CT, CT, cT, cT Test crosses If a pea is round, what is its genotype? (dominant traits) With dominant phenotypes you can’t identify the genotype because there is more than one option How can you figure it out? Cross with a homozygous recessive! rr X ?? (round pea) rr X RR rr X Rr with a homozygous recessive parent, the offspring are either heterozygous or homozygous recessive phenotypically you can tell these apart the difference in phenotype is due to the unknown parent’s allele if dominant, offspring is heterozygous if recessive, offspring is homozygous recessive offspring’s phenotype is determined by the parent with the unknown genotype!! If a pea is round and yellow, what is its genotype? (both dominant traits) Genetics problems In a cross between 2 pea plants, both heterozygous for flower color and for pod color, what proportion of their offspring will have purple flowers and yellow pods? In a cross between 2 pea plants, 1 heterozygous for flower color and homozygous recessive for seed shape and 1 homozygous recessive for flower color and heterozygous for seed shape, what percentage of their offspring will have the dominant phenotype for both traits? Incomplete dominance What does it mean to be a “dominant” allele? Does there have to be a dominant allele? Genes produce the enzymes that produce pigments A recessive allele is producing nothing Codominance Both alleles make functional products ABO blood types in humans (and Neanderthals?) 2 dominant alleles, 1 recessive Dominant alleles modify the standard glycoprotein, recessive does nothing Variation in chromosomal inheritance Thomas Hunt Morgan If you mate a white eyed male with a red eyed female, all of the F1 have red eyes If you made a red eyed male with a white eyed female, all of the F1 females have red eyes, a;; the F1 males have white eyes Human karyotype 1 pair of chromosomes isn’t as homologous as the others…… Y chromosome is much smaller than the X X chromosome has genes tht the Y does not But they have regions similar enough for them to recognize each other during meiosis Females have 2x chromosomes Can be homozygous of heterozygous for genes on the X chromosome Males have 1x chromosome Hemizygous Following alleles on the X chromosome In Drosophila, genes are usually named after the mutant W for red eye (dominant) w for white eye (recessive) Alleles in Drosophila X^w, X^w, Y True breeding red-eyed female with white eyed male All the female’s gametes have X^w ½ the male’s gametes have X^w and ½ have Y Offspring are: 50% heterozygous females 50% red eyed males Punnett squares For the F1 cross….. Mom’s gametes X^w and X^w Dad’s gametes X^w and Y What are the phenotype ratios? All females are red-eyed Males 50% red-eyed 50% white-eyed Linkage between sex and eye color Cross white-eyed female with red-eyed male All the female’s gametes have X^w ½ the male’s gametes have X^w and ½ have Y Offspring are: 50% heterozygous females 50% white eyed males Sex chromosomes Sex-linked inheritance X-linked inheritance Females have 2 copies of genes, inheritance will be like it was on an autosome Males have 1 copy, no chance to be heterozygous What does this mean for recessive phenotypes? Y-linked inheritance Only makes have these genes X linked traits in humans Hemophilia A Recessive allele on the X chromosome Alleles: X^H, X^h-a, Y X linked more often seen in boys X-linked color-blindness: 8% men and 0.5% woman Hemophilia: 1:5,000 male births, very rare in female births Incontinentia pigmenti X-linked gene =, IKBKG Codes for nuclear factor-kappa-B (protein makes cells less likely to undergo apoptosis) Characterized by skin abnormalities Blistering rash at birth, grey/brown patches during childhood (fade with age) Very rare, about 900- 1,200 people Only seen in women Why? Genetics problems! What are the predicted genotype and phenotype ratios for the children of a woman with type O blood and a man with type AB blood? Red-green color blindness in an X-linked recessive trait in humans. If a woman who has normal color vision, but whose father was color blind, marries a man with normal color vision, what is the probability that they will have a color-blind son? What is the probability that they will have a color- blind daughter? Stalk-eyed fly, Teleopsis dalmanni If you are a gene on the X chromosome, what’s better from your perspective, sons or daughters? MALES X^D= driver In males with X^D, most sperm will have X^D and not Y Sex ratio is biased towards females 35 males and 65 females in T.dalmanni Where do males get their X chromosome? Alleles are not always independent Chromosomes have more than 1 gene Are they permanently stuck together? NO!!!! Rossing over If crossing over occurs between the 2 genes, linkage is broken Rate of recombination is determined by how close they are physically How many alleles can there be? No rule that limits the number of alleles to 2 or 3 B(beta)- globin has about 500 alleles Pleiotropic genes A gene can affect more than one aspect of a phenotype Marfan syndrome Dominant allele for the FBN1 gene Codes for fibrillin- a protein in the connective tissue This single allele can cause changes to…. Lens of the eye Length of limbs and fingers Foot structure Lungs Lining of the aorta Quantitative traits Many traits have a normal distribution Traits can be influenced nu many genes How can multiple genes with a dominant/recessive system give you a normal distribution? Additive alleles- multiple gene hypothesis Phenotypic traits with continuous variation can be quantifies by measure, weighing, counting Each gene affects the trait in an additive way Each locus has either an additive or nonadditive allele Each additive allele contributes a small, equal amount (incomplete dominance) Additive alleles contribute to a single quantitative traits Gene are not destiny (necessarily) Phenotypes are often the interactions between genotypes and the environment Identifying human alleles Identifying how genes are transmitted (linked/not linked) or how different alleles work (dominant/recessive) can’t be done the same way in humans as it is in other organisms Use historical approaches- pedigrees Follow the trait in a family over time Pedigres Square= males Circle= females Half-filled symbol= does not have the phenotype, but a carrier Filled symbol= has the phenotype