Introduction to Genetics PDF

20 July 1822 – 6 January 1884 Chapter 10 Mendel and the Gene Idea Note: Darwin: 12 February 1809 – 19 April 1882  How do traits pass from parents to offspring? (of a characteristic)...

20 July 1822 – 6 January 1884 Chapter 10 Mendel and the Gene Idea Note: Darwin: 12 February 1809 – 19 April 1882  How do traits pass from parents to offspring? (of a characteristic) transmissible from parent to offspring: blending The “_________” hypothesis is the The “__________” particulate hypothesis is idea that genetic material from the the idea that parents pass on two parents blend together (like discrete heritable units (genes). blue and yellow paint to make Mendel’s work supported this. green). Mendel’s work disproved individually separate and distinct This hypothesis helps to explain the this. reappearance of traits after several generations Mendel experimented with garden peas to document the particulate mechanism.  Genetics terms Gamete: ____ sex cells Gene: a segment of DNA that codes for a _______ protein Allele: different ______ versions of a gene Phenotype: the _____________ apperence of a trait (physical expression) – Tall, yellow, round, purple, etc. Genotype: the allele combination which causes the phenotype: alleles (genes) are designated by letters (A, b, K, m, Z, etc.) – PP, Tt, tt, etc. – Types of genotypes ______zygous homo (pure): PP, tt, KK, MM, qq, etc. – Also known as “true breed” _______zygous hetero (hybrid): Pp, Tt, Kk, Mm, Qq, etc.  Mendel and his Laws Austrian monk – Left from teaching high school science to study heredity – Chose to use __________ garden peas as his subject They reproduce _______ sexually Male and female produce sex cells -- _______ gamete Male and female parts are in the same flower in pea plants – Chose 7 traits (with distinct opposite expressions) to study Tall vs. short; purple flowers vs. white; green seeds vs. yellow, etc.  Gregor Mendel’s Peas Mendel hand-pollinated (transferred pollen to carpel) each flower fertilization _________ occurs shortly after pollination uniting of male and female gametes Mendel was able to produce seeds that carpel had ____ different two anther (female, (male) eggs) parents. This process is called cross-pollination. Experiment: Cross white flower by purple flower plants single This is called a monohybrid cross: involving a ______ trait flower color Parent generation First filial F1: generation He noted that all were purple. What happened to the white petal trait? Mendel then crossed two of the hybrid offspring together. Suddenly the white trait shows up again. This means the trait never left, it is just not being expressed in the hybrid condition. Conclusion: The alleles are passed on separately! Some are dominant to others!  P=parent / F1=first filial / F2=second filial Writing out Mendel’s Flower Color Experiment. Law of Segregation of Unit pp PP Factors (Characters): Mendel called genes factors and showed that each parent contributed one factor each to produce the offspring. P: PP X pp F1: Pp (purple hybrids) Law of Dominance: Pp X Pp The dominant allele (gene) will be expressed (show) when F2: PP 2Pp pp paired with a recessive allele (pure) (hybrid) (pure)  Figure 14.4 Allele for purple flowers Pair of Locus for flower-color gene homologous chromosomes Allele for white flowers Law of Dominance: for each character, an organism inherits two alleles, one from each parent, and some alleles are dominant and others are recessive. Mendel made this deduction without knowing about the role of chromosomes The two alleles at a particular locus may be identical, as in the true- breeding plants of Mendel’s P generation Alternatively, the two alleles at a locus may differ, as in the F1 hybrids  What ratio of phenotypes will appear in the F2 generation if you cross these two traits below? Which trait is shown in this picture? Solution: P: Round seed x Wrinkled Seed RR rr [move one allele (gene/factor) from each parent into F1 generation line] F1: all round seeds Rr Law of Segregation of Alleles F2: Rr x Rr (Chromosomes). Parent allele pairs separate into different gametes. Use a Punnett Square to solve the predicted F2 generation ratios R r Gamete genotypes from one parent Gamete R genotypes Phenotype ratio = from 3 Round:1 Wrinkled one parent r  Predict next generation phenotype ratios if: P: Yellow x Green (Gg) (gg) F1: Yellow : Green Answer = ?:? DO THE PUNNETT SQUARE, ALWAYS! I cannot tell you how many times a student makes a simple mistake and gets a test question like this wrong when they just needed to write it out and see the answer Probabilities Predict Averages Probabilities predict the ________ average outcome of a large number of events. Probability cannot predict the precise outcome of an individual event. In genetics, the larger the number of offspring, the closer the resulting numbers will get to expected values. Review Questions follow The Testcross How can we tell the genotype of an individual with the dominant phenotype? Such an individual could be either homozygous dominant or heterozygous Test Cross (a variation of a monohybrid cross) is performed when you don’t know the genotype of a dominant-looking phenotype Drawing Inferences From Data: Data for a monohybrid cross of Rr x Rr plants 𝑅𝑜𝑢𝑛𝑑𝑆𝑒𝑒𝑑𝑠 882 2.95 Mendel had the following raw data: = = 𝑊𝑟𝑖𝑛𝑘𝑙𝑒𝑑𝑠𝑒𝑒𝑑𝑠 299 1 Raw data can sometimes be clunky and obscure to analyze. Organizing data into something meaningful is key. One way to accomplish this: simplify raw data by determining the ratio of one value to another. The ratio of one value to another may be determined by dividing the larger number by the smaller one: The ratio of round seeds plants to wrinkled seed plants is said to be 2.95 to 1. In an experiment of this kind, it is convenient if the value is an integer, or whole number. If it is not, rounding off may be appropriate. Drawing Inferences From Data: Your turn! The calculated value of 2.95 to 1 is nearly a 3 to 1 ratio if it is rounded off. That is, one could reasonably infer that the relationship is actually a 3 to 1 ratio between these two types of plants. Try determining the ratio in each of the following sets of numbers, and then try to infer their relationship. 23 grey mice and 7 albino (white) mice in a litter: – ratio: ____________________ – inferred ratio: ____________________ 75 female fruit flies and 35 male fruit flies in the first generation of a mating: – ratio: ____________________ – inferred ratio: ____________________ Ratios between several values may also be determined with this technique. As before, the smallest value is divided into each of the other values. Again, the experimenter may round off the actual ratio to an integer if it seems reasonable. The following examples illustrate the point: 27 red-flowered snapdragons, 52 pink snapdragons, and 30 white snap dragons: – ratio: ____________________ (need 3 numbers) – inferred ratio: ____________________ 250 round, yellow-seeded pea plants; 85 round, green- seeded plants; 76 wrinkled, yellow-seeded plants; and 27 wrinkled, green-seeded plants: – ratio: ____________________(need 4 numbers) – inferred ratio: ____________________  Dihybrid crosses Are two different traits inherited together, or independently of each other? Ex. Plant height and seed shape Let’s find out: Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters P: TTRR x ttrr tall, round short, wrinkled F1: all TtRr tall, round A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package p or independently. TtRr x TtRr  A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently If dependent assortment TR sperm tr is the case: If the two traits are dependent on one TR another, then a “T” TTRR might only ever be attached to a “R”, eggs and a “t” with a “r”. What is the predicted tr phenotypic ratio of this crossing? A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently So how do we figure out the phenotypic ratio if traits are sorted independently for a dihybrid cross between two heterozygous offspring? – We need to account for each possible combination of alleles TtRr x TtRr What allele combinations are possible for each parent? Remember, you do not pass on both “related” alleles (homologs).  Law of Independent Assortment Dihybrid cross T T T R r X t R r Tall t t What are the possible combinations of alleles in the gametes of a parent? Short R Round r Wrinkled  Law of Independent Assortment Dihybrid cross T T T R r X t R r Tall t t What are the possible combinations of alleles in the gametes of a parent? Short R TR Round r Tr Wrinkled  Law of Independent Assortment Dihybrid cross T T T R r X t R r Tall t t What are the possible combinations of alleles in the gametes of a parent? Short R TR tR Round tr r Tr Wrinkled  Law of Independent Assortment Dihybrid cross T T T R r X t R r Tall t t What are the possible combinations of alleles in the gametes of a parent? Short R TR tR Round tr r Tr Wrinkled  Law of Independent Assortment Dihybrid cross T T T R r X t R r Tall t t TR Tr tR tr TR Short R Tr Round tR r tr Wrinkled TtRr  EXPERIMENT You can see that using these P Generation YYRR yyrr ratios to predict the outcome Gametes YR yr can indicate which hypothesis F1 Generation is supported and which is YyRr refuted. Predictions Hypothesis of Hypothesis of dependent assortment independent assortment We expect a 3:1 ratio if they are Sperm Predicted or assort dependently. offspring of F2 generation Sperm 1/ 4 YR 1/ 4 Yr 1 /4 yR 1/ 4 yr 1 /2 YR 1/ yr We expect a 9:3:3:1 ratio if they 2 1 /4 YR YYRR YYRr YyRR YyRr assort independently. 1 /2 YR YYRR YyRr 1/ 4 Yr Eggs YYRr YYrr YyRr Yyrr 1 /2 yr Eggs Based on the results, which YyRr yyrr 1 /4 yR YyRR YyRr yyRR yyRr hypothesis is supported? 3 /4 1 /4 1/ yr 4 Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9/ 3/ 3/ 1/ 16 16 16 16 Phenotypic ratio 9:3:3:1 RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1  Principal of Independent Assortment Mendel's experimental results were very close to the 9 : 3 : 3 : 1 ratio predicted by the Punnett square. Mendel had discovered a 3rd law of inheritance. The alleles for seed shape segregated independently of those for seed color. This principle is known as independent assortment. dont Genes that segregate independently _______ influence each other's inheritance. – Helps account for the many genetic variations observed in sexually reproducing organisms. Strictly speaking, this law applies only to genes on different, non- homologous chromosomes or those far apart on the same chromosome – Genes located near each other on the same chromosome tend to be inherited together Beyond Dominant and Recessive Alleles What inheritance patterns exist aside from simple dominance? –Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.  Beyond Dominant and Recessive Alleles –Incomplete Dominance RR When one allele is not completely dominant over another it is called incomplete dominance. In incomplete dominance, the heterozygous phenotype is WW between the two homozygous phenotypes. A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW). Incomplete dominance: In heterozygous condition, dominant allele is not completely expressed. Example: Sweet pea flower colors P: RR x R’R’ (red) (white) F1: RR’ (pink) RR’ x RR’ F2: RR : 2 RR’ : R’R’ Beyond Dominant and Recessive Alleles: Codominance In codominance, both alleles contribute to the phenotype. In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens are speckled with both black and white feathers, each appearing separately rather than blending. Beyond Dominant and Recessive Alleles: Multiple Alleles more than two alleles Genes that are controlled by _______________ are said to have multiple alleles. An individual can’t have more than two alleles. However, within a population, more than two possible alleles can exist. A rabbit population has at least four different alleles that exist. – C, cch , ch, c – An individual rabbit’s coat color is determined by the pair of alleles that they receive.  Beyond Dominant and Recessive Alleles Different combinations of alleles result in the colors shown here. KEY C = full color; dominant to all other alleles cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles ch = Himalayan; color in certain parts of the body; dominant to c allele AIbino: Chinchilla: Himalayan: cc c Full color: cchhcc,hCc CC, ch,cCc cch , or h chc,hhor cch , or c Cc c = albino; no color; recessive to all other alleles  Beyond Dominant and Recessive Alleles What is the KEY phenotypic ratio of C = full color; dominant to all other alleles the possible offspring obtained cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles from a cross of these two parental ch = Himalayan; color in certain parts of the genotypes: body; dominant to c allele Ccch x chc c = albino; no color; recessive to all other alleles Beyond Dominant and Recessive Alleles: Polygenic Traits Traits controlled by two or more genesare said to be _____________ polygenic traits. Show a wide range of phenotypes (bell curve distribution) – Hair color – Eye color – Skin color – Height  AaBbCc AaBbCc Sperm 1/ 1/ 1/ 1/ 1/ 1/ 1/ 1/ 8 8 8 8 8 8 8 8 1/ 8 1/ 8 1/ 8 1/ 8 Eggs 1/ 8 If you were to graph the probabilities 1/ as percentages, what would the graph 8 of this data look like? 1/ 8 1/ 8 Phenotypes: Number of 1/ 64 6/ 64 15/ 64 20/ 64 15/ 64 6/ 64 1/ 64 dark-skin 0 1 alleles: 2 3 4 5 6  A Summary of Mendel’s Principles A Summary of Mendel's Principles Genes are passed from _________parents offspring to their ________. If two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be ________, dominant and others may be recessive. In most sexually reproducing organisms, each adult has _____ 2 copies of each gene. These genes are segregated from each other when gametes are formed. The alleles for different genes usually segregate ____________ independantly of one another.  Applying Mendel's Principles Applying Mendel's Principles Thomas Hunt Morgan used fruit flies to advance the study of genetics. Morgan and others tested Mendel’s principles and learned that they applied to other organisms as well as plants. Mendel’s principles can be used to study inheritance of human traits and to calculate the probability of certain traits appearing in the next generation. Genetics and the Environment Characteristics of any organism are determined by the interaction between genes and the environment. Review Questions follow CHROMOSOMES AND MEIOSIS Sex Cell Formation  Chromosomes and Meiosis n n Chromosomes contain genes lined up and connected to each other linked (________) Chromosomes occur in sets designated by the letter n. One set = n – Garden pea plant cells contain a total of 14 chromosomes. There are 7 chromosomes in each set Organisms produce gametes which contain only one set of haploid or n. chromosomes and are said to be _______ Cells which contain 2 sets of chromosomes are said to be _______ diploid and the symbol 2n is used. Only gametes are haploid, all other (somatic) cells are diploid.  Let’s test this concept: – If the diploid number for a frog is 26, how many chromosomes will be in its gametes? – If dogs produce gametes with 39 chromosomes, how many chromosomes will occur in a skin cell? Every species has its own unique number and sizes of chromosomes. – Humans have how many chromosomes in their somatic cells? – How many in their gametes? Homologous Chromosomes karyotype is an image of a person's A __________ chromosomes isolated from an individual cell and arranged in numerical order. Chromosomes occur in pairs, one from each parent (a maternal set and a paternal set). Humans have ___ 22 pairs of autosomes, and 1 pair of ____ sex chromosomes. Each pair of homologous chromosomes contains the same type of allele (gene) in the same position (locus). See fig. 11-16 on p. 277)  What is the number of chromosomes in each body cell of these fruit flies? How many chromosomes must each body cell of normal offspring have?  Mitosis produces daughter cells that are genetically identical to the parent cell. ________ 2n parent → 2 (2n daughters) Why meiosis 2n cells cannot be used in sexual reproduction. (It would double the chromosome number for each is necessary generation). Thus, there is a need for a special kind of cell division which will reduce the chromosome number by ½ Sperm (n) + Egg (n) → Zygote (2n) How many chromosomes does a normal human have?  Basics of meiosis _____ germ cells (the starting cells in sex organs) go through a cell cycle which enters meiosis instead of mitosis. – G1 → S → G2 → Meiosis I and II – Meiosis I Prophase I, Metaphase I, Anaphase I, Telophase I – [ No G1, S, or G2 → cell goes directly into II ] – Meiosis II Prophase II, Metaphase II, Anaphase II, Telophase II – Cytokinesis produces __ haploid gametes  2n 2n When do haploid Meiosis I cells form? n Meiosis II n Phases of Meiosis When homologous chromosomes form ______ tetrads in prophase I of meiosis I, they exchange portions of crossing over their chromatids in a process called ____________. Crossing-over produces new combinations of alleles. Independent Assortment When both crossing over and independent assortment are combined…  Phases of Meiosis – Meiosis I Meiosis I Metaphase I Anaphase I Prophase I Interphase I Telophase I and Cytokinesis Phases of Meiosis Meiosis II Telophase I and Cytokinesis I Meiosis II Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis  Overview Meiosis II results in four haploid (N) daughter cells. MEIOSIS II Telophase II and Cytokinesis Gamete Formation In many female animals, only one egg results from In male animals, meiosis. The other three meiosis results in four cells, called ________, equal-sized gametes are usually not involved in called ______. reproduction.  Things to notice in meiosis- - – crossing over during tetrad formation in ____________ – independent assortment of homologous chromosomes during ___________ – that Meiosis I ends with haploid cells and sister chromatids still joined. – Sister chromatids line up singly in ___________ (just like in mitosis) – Gametes contain one each of the beginning sister chromatids  Methods of Genetic Recombination Genetic recombination = making new combinations of genes in offspring What forms of genetic recombination have we seen thus far? – 1. Crossing over during Prophase I – 2. Independent assortment (mixing) of paternal/maternal homologues during Metaphase I (remember: homologous pairs can come to lie one of 2 ways across the equator) See lab 11-2 Genetic mistakes: Nondisjunction: ___________ fail to n -1 separate during anaphase –– the centromere doesn’t divide n +1 Nondisjunction in Plants Researchers can “force” this to occur by treating plant cells with the drug colchicine, but it also happens naturally too. – usually results on whole additional sets of chromosomes remaining in a gamete. leads to _____ploidy of some kind: triploid, tetraploid, hexaploid, octoploid May be a benefit to plants (bananas = 3n, daylilies = 4n, pansies = 6n, apples = 3n, wheat = 4n) Increases resistance to disease and pests, larger fruit, etc.  Nondisjunction in humans Creates a situation called ____ploidy (an unusual number of chromosomes) Usually involves only one chromosome – Final gamete will contain either 2 chromosomes of a kind or no chromosome for that kind. Total count of person’s chromosomes will either be 45 (2n-1) or 47 (2n+1)chromosomes. 45 chromosomes means that somewhere among the chromosomes there is a single (not paired) chromosome = ____________ 47 chromosomes means that somewhere among the chromosomes there is a triplet of chromosomes = ________ Examples Turner Female = Monosomy X (infertile female; not lethal) Klinefelter male = Trisomy (XXY)(not lethal) Down syndrome is a trisomy of chromosome ___. Patau Syndrome = Trisomy 13 (many physical defects; lethal by 3 days avg.) Edwards Syndrome = Trisomy 18 (severe mental retardation; only 10% live beyond 1 year)  Down Syndrome A karyotype Although both graphics show Down Syndrome, how do we know that this particular karyotype doesn't belong to this particular individual?  Quick check How do the following word combinations relate? – Pollination –– fertilization – Allele –– [dominant –– recessive] – Genotype –– phenotype – Homozygous –– heterozygous – Monohybrid –– dihybrid – Diploid –– haploid – Homologous chromosomes –– allele – Sperm –– egg –– zygote – Meiosis –– gamete – Crossing over –– genetic recombination  11-5 Linkage 11-5 Linkage and and Gene Maps Gene Maps  Gene Linkage Independent assortment revisited and revised: Gene Linkage Morgan discovered that many of the genes was studying seemed to violate the principle of independent assortment. They appeared to be “linked” together. So, Morgan and his associates grouped the “linked genes” into four “linkage groups”. Each linkage group assorted independently but all the genes in one group were inherited together. Thus, each chromosome is actually a group of linked genes. Morgan concluded that Mendel’s principle of independent assortment still holds true, but it is the chromosomes that assort independently, not individual genes. Gene Maps Gene Maps We know that crossing-over during meiosis sometimes separates genes that had been on the same chromosomes onto the opposite homologous chromosomes. Crossover events occasionally separate and exchange linked genes and produce new combinations of alleles. Alfred Sturtevant, a student of Morgan, reasoned that the farther apart two genes were, the more likely they were to be separated by a crossover in meiosis. So, looking at how frequently genes are recombined should be indicate how far apart they are on the chromosome.  Gene Maps If two genes are close together, the recombination frequency between them should be ____, since crossovers are rare. If they are far apart, recombination rates between them should be _____. With this information Sturtevant created a gene map showing the relative locations of each known gene on one of the Drosophila chromosomes.  Gene Maps Exact location on chromosome Chromosome 2 0.0 Aristaless (no bristles on antenna) 13.0 Dumpy wing 48.5 Black body 54.5 Purple eye 67.0 Vestigial (small) wing 99.2 Arc (bent wings) 107.0 Speck wing  Gene Maps Exact location on chromosome Chromosome 2 1.3 Star eye 31.0 Dachs (short legs) 51.0 Reduced bristles 55.0 Light eye 75.5 Curved wing 104.5 Brown eye Review Questions follow That’s all... Do well on your test :)