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Patterns of Inheritance Understanding the Chromosome Gene: a unit of hereditary information (nucleotide sequence). Locus: physical location of a gene. Allele: an alternate form of a gene (green vs. brown eyes) Chapter 10 A History of Genetics • Gregor Mendel, a monk who studied at the University of Vienna in the mid-1800s. – Plant breeding – Mathematics • His experiments became the foundation of the modern science of genetics. Pair of Chromosomes During meiosis, each gamete produced receives one copy of each chromosome. Mendel’s Experimental System • Mendel experimented on ordinary peas. • The pea flower is a perfect flower: – Petal, sepal, stamen, carpel are easily accessible and distinguishable. – Can self-fertilize. – Can be crossfertilized by hand. • Flower color is found in one gene. Mendel’s Experiments Mendel’s Experiments • Mendel chose two “truebreeding” pea plants, plants that would always produce one color of flower, purple and white. • He found that all of the first generation offspring were purple. • He cross pollinated them to produce the first generation (F1). Where did the white color go? To try to determine what happened, he created a second generation, F2, by self-pollinating the fist generation. Mendel’s Experiments Cross Pollination • In the F2 generation of plants, Mendel found F that 1/4 of the flowers First Generation Offspring were white and 3/4 were purple. Self-Pollination • The white trait had been “hidden” in the F1 F generation. Second Generation Offspring F1 First Generation Offspring P Parental Generation 1 Purple, 1 White F1 First Generation Offspring All Purple 1 2 Cross Pollination Self-Pollination F2 Second Generation Offspring 3/ Purple, 1/ White 4 4 Mendel’s Conclusions • Mendel was the first to perform controlled breeding experiments with the same plant for a period of time while taking accurate notes. • He came up with 5 theories to explain his inheritance results: 1. Each trait is determined discrete physical units. 2. Certain traits have dominance over others. 3. Traits are segregated from each other during meiosis. 4. Chance determines which trait will go to each gamete. 5. True-breeding organisms have two copies of the same trait. Mendel’s Conclusions 2. Certain traits have dominance over others. - The purple flower color seemed to “hide” the white flower color during the first generation of offspring. - The purple color was the dominant allele. - It blocked expression of the white color gene. - The white color was the recessive allele. - It could not be seen if the purple allele was present in a chromosome copy. Mendel’s Conclusions 1. Each trait is determined discrete physical units. – We now know these “physical units” are genes. – Mendel was lucky since the pea plant has only one gene for flower color. – Each plant of the parental population had a different allele (purple or white) for the flowercolor gene. Mendel’s Conclusions 3. Traits are segregated from each other during meiosis. - Mendel’s Law of Segregation: two alleles of a gene segregate or separate from one another during meiosis. - Each gamete produced receives only one allele. - When the sperm fertilizes the egg, the offspring receives one allele from its mother and one from its father. Mendel’s Conclusions 4. Chance determines which trait will go to each gamete. - Homologous chromosomes separate at random during meiosis. The distribution of alleles in gametes is also random. Purple Flower Allele Mendel’s Conclusions 5. True-breeding organisms have two copies of the same trait. - True-breeding plants are homozygous for flower color : - - They have two copies of the the same allele for the flower color gene. The first generation of offspring were heterozygous for flower color: - They had one copy of each allele, one purple and one white. White Flower Allele Parental “True-Breeding” Generation Purple color is the dominant allele. We will call the allele “P”. Since the purple parent is homozygous for purple color, it has two copies of the allele, or “PP” . White color is the recessive allele. We will call the allele “p”. Since the white parent is homozygous for white color, it is “pp” . Parental “True-Breeding” Generation The First Generation Offspring PP Cross Pollination pp The First Generation Offspring The Second Generation Offspring Since the F1 generation offspring receive one allele from each parent, they get one purple allele (P) and one white allele (p). They are heterozygous for flower color, “Pp”. The Second Generation Offspring PP Homozygous for purple. Pp Heterozygous for purple. Pp Heterozygous for purple. pp Homozygous for white. Pp The F1 generation flowers were self-pollinated to obtain the F2 generation. Each F2 offspring received any combination of the two alleles P (purple) and p (white). The Punnett Square Pp A system for predicting genotypes and phenotypes of offspring. P p The Second Generation Offspring PP Pp Three types of offspring are produced: PP, Pp, and pp. Pp Pp These are the three possible genotypes, the allele combinations. There are only two phenotypes, physical or observable properties, white and purple. The Punnett Square Pp A system for predicting genotypes and phenotypes of offspring. P P Pp Pp p 1. Determine genotypes of each parent and label the square with the possible gametes. p P PP Pp pp p 2. Fill in the genotypes of each offspring by adding gametes from each column and row. Pp pp The Punnett Square Pp A system for predicting genotypes and phenotypes of offspring. P The Punnett Square A system for predicting genotypes and phenotypes of offspring. p P 3. Determine phenotypes by assessing dominant alleles. Pp pp The Punnett Square Pp A system for predicting genotypes and phenotypes of offspring. P p PP 1/4 Pp 1/4 Pp 1/4 1 pp /4 P Pp p 1/ 4 Purple Pp 1/ + 1/ = 1/ 4 4 2 Purple pp 1/ 4 White Pp 1/ 4 Pp p PP p PP 1/4 Pp Pp Phenotype P P PP Genotype Ratio Pp Phenotype Ratio 1/ 4 + 1/2= 3/4 1/ 4 Genotype Ratio = 1:2:1 Phenotype Ratio = 3:1 4. Predict probabilities of each phenotype and genotype by determining the fraction of the offspring that possesses each unique trait. p Pp 1/ 4 1 pp /4 Example Problems Example 1 SS Homozygous Smooth (SS) seed shape vs. Homozygous Wrinkled (ss) shape Yy 1. Determine possible gametes. Heterozygous Tall (Tt) plant vs. Homozygous Dwarf (tt) plant 1. Determine possible gametes. Tt Example 4 Heterozygous Green (Gg) seed pod color vs. Heterozygous Green (Gg) pod color tt Gg 1. Determine possible gametes. YY Homozygous Yellow (YY) seed color vs. Heterozygous Yellow (Yy) color ss Example 3 Example 2 1. Determine possible gametes. Gg Mendel’s Second Experiments Mendel’s Second Experiments • After determining the hereditary statistics of a single gene, Mendel investigated the inheritance of two independent genes. SSYY All F1 generation seeds were heterozygous yellow and smooth (SsYy). Cross Pollination • He cross-pollinated a true-breeding yellow (YY), smooth seeded (SS) pea plant with a true-breeding green (yy), wrinkled seeded (ss) plant. Second Generation Seeds To predict the F2 generation genotypes and phenotypes we must work out the Punnett Square. SsYy Ss Yy First, determine possible gametes. SsYy ssyy For the F2 generation, Mendel self-pollinated the heterozygous F1 generation pea plants. Second Generation Seeds To predict the F2 generation genotypes and phenotypes we must work out the Punnett Square. SsYy SY SY Sy SsYy sY Possible Gametes: SY, Sy, sY, and sy. sy Sy sY sy Second Generation Seeds To predict the F2 generation genotypes and phenotypes we must work out the Punnett Square. SsYy Next, determine genotypes. SsYy SY SsYy SSYY sY sy SSYy SsYY SsYy SsYy Ssyy Sy SSYy SSyy To predict the F2 generation genotypes and phenotypes we must work out the Punnett Square. SsYY SsYy ssYY ssYy SsYy Ssyy ssYy ssyy sY sy SSYy SsYY SsYy SSYy SSyy SsYy Ssyy SsYY SsYy ssYY ssYy SsYy Ssyy ssYy ssyy sY sy SY Sy sY 1/ 16 1/ 16 1/ 16 SSYY SSYy SsYY 1/ 16 1/ 16 1/ 16 SSYy sY Sy Sy Third, determine phenotypes. Second Generation Seeds SsYy Sy SY SSYY sy SY SsYy SY SsYy sY sy Fourth, predict probabilities. Sy SY Second Generation Seeds To predict the F2 generation genotypes and phenotypes we must work out the Punnett Square. Second Generation Seeds 1/ 16 SSyy 1/ 16 SsYy 1/ 16 SsYY SsYy ssYY 1/ 16 1/ 16 1/ 16 Ssyy ssYy SsYy sy Genotype Ratio Phenotype SSYY 1/ 16 Smooth, Yellow SSYy 2/ 16 Smooth, Yellow SsYY 2/ 16 Smooth, Yellow SsYy 4/ 16 Smooth, Yellow Ssyy 2/ 16 Smooth, Green 1/ 16 SsYy 1/ 16 Ssyy 1/ 16 ssYy 1/ 16 ssyy SSyy 1/ 16 Smooth, Green ssYy 2/ 16 Wrinkled, Yellow ssYY 1/ 16 Wrinkled, Yellow ssyy 1/ 16 Wrinkled, Green SsYy SY SY Phenotype Ratio 9/ 16 Sy sY sy 3/ 16 3/ 16 1/ 16 Sy sY sy 1/ 16 1/ 16 1/ 16 SSYY SSYy SsYY 1/ 16 1/ 16 1/ 16 1/ 16 SSYy SSyy SsYy Ssyy 1/ 16 1/ 16 1/ 16 1/ 16 ssYy SsYY SsYy ssYY 1/ 16 1/ 16 1/ 16 SsYy Ssyy ssYy 1/ 16 SsYy 1/ 16 ssyy Phenotype Ratio = 9:3:3:1 Mendel’s Second Conclusions • Different traits can be inherited independently. – Mendel’s Law of Independent Assortment. – During meiosis, chromosomes are randomly pulled to either pole during anaphase I. – Genes on separate chromosomes (like pea seed shape and seed color) will be randomly sorted into gametes. Example Problems Independent Assortment of Alleles on Different Chromosomes Example 5 Homozygous Green and Constricted (GGii) seedpod vs. Heterozygous Green, Homozygous Inflated (GgII) seedpod GgII 1. Determine possible gametes. GGii Inheritance Strategies Example 6 ppll Homozygous White flowers at tips of branches (ppll) vs. Heterozygous Purple flowers at leaf junctions (PpLl) Linked Genes • Genes that are inherited together. – If they are on the same chromosome, they will be inherited together. – If genes are on different chromosomes, they may or may not be inherited together. Flower Color Purple allele (C) PpLl Red allele (c) 1. Determine possible gametes. Pollen Shape Purple allele (C) Long allele (S) Red allele (c) Round allele (s) color shape Long allele (S) Round allele (s) Inheritance Strategies Linked Genes Flower Color Pollen Shape If NOT linked: Inheritance of alleles is linked if genes are found on the same chromosome. color shape OR Chromosome sorting into gametes decides inheritance patterns. Crossing-over can alter inheritance patterns. Inheritance Strategies Inheritance Strategies Crossing-over can alter inheritance patterns. Crossing-over can alter inheritance patterns. Genes have been rearranged. Genetic recombination has occurred. Chromosomes sorted into gametes have a new set of linked genes. Inheritance of Traits • Traits in the pea plant observed by Mendel exhibited simple dominance. – One allele is dominant over the other, completely blocking its expression. • Traits can exhibit more complex dominance strategies: – – – – – – Incomplete dominance. Codominance. Multiple Alleles. Polygenic Inheritance. Pleiotrophy. Sex-linked. Dominance Strategies CRCR Incomplete Dominance • Heterozygous produces an in-between phenotype. • The sweet-pea exhibits this trait: Cross between a red flower (dominant) and a white flower (recessive) produces a heterozygous pink flower (intermediate). CwCw CR Cw CRCw F1 Generation: All CRCw, pink. Dominance Strategies CRCR Incomplete Dominance • Heterozygous produces an in-between phenotype. • The sweet-pea exhibits this trait. CR CR Cw Dominance Strategies Cw F1 CRCw F1 x F1 CRCw CwCw CRCw Multiple Alleles • There may be more than two alleles for a given gene. • In fruit fly, there can be many eye colors (yellow, orange, pink, brown, or red). CR taputea.lbl.gov Cw Dominance Strategies Dominance Strategies Codominance Sickle-Cell Anemia • Both phenotypes are expressed. • Heterozygous individuals for the sickle-cell mutation have immunity to the parasite that causes malaria. Ex: Sickle Cell Anemia • Single amino acid is defective in hemoglobin. – O2 carrying protein in red blood cell. • Blood cells change shape & clump. • Heterozygous (HRHr) would have half normal and half abnormal hemoglobin. • Homozygous recessive person would have sickle-cell anemia. HRHr x HRHr Pg. 239 Distribution of Sickle Cell Anemia 2.5-5.0% 5.0-7.5% 7.5-10.0% 10.0-12.5% >12.5% • Homozygous recessive have sickle-cell disease. A large percentage of the population in regions where malaria is common carry the sickle-cell mutation. Distribution of Malaria HR Hr HR HRHR HRHr Hr HRHr HrHr Dominance Strategies Dominance Strategies Blood Types Blood Types Multiple alleles & co-dominance - More than two alleles present: IA, IB, i - Heterozygous gives you both phenotypes: type AB blood Multiple alleles & co-dominance - More than two alleles present: IA, IB, i - Heterozygous gives you both phenotypes: type AB blood Blood Type Genotype Proteins O A ii No proteins on the surface of the cell. IAIA or IAi B IBIB or IBi A proteins on the exterior of the cell. B proteins on the exterior of the cell. AB IAIB Antibody Blood Type Genotype Antibody Frequency ii Anti-A; Anti-B 4% Anti-B O A IAIA or IAi Anti-B 40% Anti-A B IBIB or IBi Anti-A 10% none AB IAIB none 46% Anti-A; Anti-B Both A and B proteins on the exterior of the cell. Dominance Strategies Polygenic inheritance • Several genes contribute to a single phenotype. Ex: Skin Pigmentation – More than 3 genes as well as environmental factors produce a wide range of skin colors. Dominance Strategies Polygenic Inheritance Ex: Skin Pigmentation – Very pale and very dark are rare alleles for skin color. Universal Donor Universal Recipient Human Disorders m Human Disorders M Albinism • • • • M Follows simple dominance. m Recessive disorder. A mutation causes enzyme (tyrosinase) not to function properly. No melanin is produced. Human Disorders Huntington’s disease • A degenerative disease of the nervous system, which is fatal. • Dominant disorder • On the 4th chromosome Woody Guthrie American folk singer who wrote "This Land Is Your Land" Cystic Fibrosis • Follows simple dominance. • Recessive disorder. • Defective transport protein. • “Salty” forehead. • Mucus in lungs. Dominance Strategies Pleiotropy • One gene with multiple phenotypes Ex: SRY gene on Y chromosome – One gene, activates many other genes to make "male traits" in an embryo. The SRY gene activates all the genes required for male embryonic growth. SRY or “Sex Determining Region of the Y-Chromosome” Dominance Strategies Dominance Strategies w+ Sex-linked traits Sex-linked traits • Trait is carried on the X-chromosome white eyed male Ex: Drosophila (fruit fly) studies. – Thomas Morgan (early 20th century) found a fruit fly w/ white eyes w – Red = Wild type ww all red eyed w+w w w+ w F2 But all the fruit flies with white eyes were male. Males are more likely to inherit a disease on the X chromosome because they only have one. • X chromosome has many traits unrelated to the sex of the individual. w+w+ = w+ Phenotype Ratio = 3:1 – White = Mutant phenotype Sex-linked traits red eyed female F1 • Most common in natural population. Dominance Strategies x F1 x F1 What number do you see? X chromosome XWXw crossed with XWY Healthy Carrier Female x Healthy Male XW XW Y Xw W = normal w = disorder Y chromosome http://www.kcl.ac.uk/teares/gktvc/vc/lt/colourblindness/cblind.htm Dominance Strategies Trace the path between the X’s. Sex-linked traits • Colorblindness Normal, Carrier Female, XCXc XC Xc C Normal X Male, XCY Y The gene for colorblindness is recessive on the X-chromosome. • What % of females will be colorblind? • What % of males will be colorblind? • What % children will be colorblind? http://www.kcl.ac.uk/teares/gktvc/vc/lt/colourblindness/cblind.htm Tracing Genetic Traits • Pedigrees are diagrams that show the genetic relationships between individuals. – A genetic family tree. – Useful for determining disease inheritance. Family Pedigree Human Disorders Human Disorders Hemophilia Hemophilia • Alexis had hemophilia, because his mother was a carrier. • Sex-linked. • Abnormal allele for blood clotting factors. • Bleed excessively when injured. • Pedigree (family tree) uses phenotypes to predict genotypes of members of a family. Queen Victoria was a carrier for hemophilia. Human Disorders Nondisjunction: Error during meiosis where chromosomes do not segregate properly. Human Disorders Nondisjunction – Error during meiosis where chromosomes do not segregate properly. – Results in abnormal number of chromosomes. Human Disorders Nondisjunction • Down Syndrome (Trisomy 21) 1 in 700 children born in the – Caused by 3 copies of chromosome #21 United States (47 chromosomes total, instead of 46). – Results from both copies of the chromosome entering the new gamete during meiosis. Homework Calculate the genotypes, genotype ratios, phenotypes, and phenotype ratios of the following breeding experiments: a. Heterozygous brown eyes crossed with Homozygous Recessive blue eyes. b. Heterozygous brown hair and Homozygous Recessive blue eyes with Homozygous Recessive blonde hair and Heterozygous brown eyes. c. Homozygous Recessive blonde hair and Heterozygous wavy hair with Heterozygous Brown hair and Heterozygous wavy hair. (Note that hair texture exhibits incomplete dominance with curly hair dominant and straight hair recessive) Human Disorders Nondisjunction • Down Syndrome (Trisomy 21) – Frequency increases with maternal age.