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Genetics What accounts for the passing of genetic traits from parents to offspring? Are traits blended in the offspring? Or: are traits inherited as single, discrete and separate units (genes)? Today, much of what we know about genetics and heredity started with the work of an Austrian monk in the 1800s—Gregor Mendel Mendel discovered the basic principles of heredity with his experiments with garden peas. Fig. 14-1 Varieties include those with distinct heritable characteristics (traits) such as flower color or plant height. Pea plants are normally self-pollinated, but can be easily cross-pollinated by the plant breeder. Therefore, the breeder can control which traits are crossed. Pea Flowers have Petals that are closed over the Stamens and Carpels. They are not open to wind and other pollinators such as insects. Therefore, they are selfpollinating. What does this mean regarding their genetic diversity? Mendel chose traits that were either..or He also chose plants that were true-breeding for a particular trait. (Purebred). And, since peas plants are normally selfpollinated, he knew what genes they carried. P In his experiments, Mendel crossed two plants with contrasting traits, producing offspring called hybrids. The true-breeding parents are the P (parental) generation The offspring are called the F1 generation. When the F1 generation plants self-pollinate, their offspring are called the F2 generation. • • • When Mendel crossed contrasting, truebreeding white and purple flowered pea plants, all of the F1 hybrids were purple When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation Fig. 14-3-3 EXPERIMENT P Generation (true-breeding parents) F1 Generation (hybrids) Purple flowers White flowers All plants had purple flowers F2 Generation 705 purple-flowered plants 224 white-flowered plants In the first experiment, when only purple flowers were produced, Mendel thought that the white trait had possibly been absorbed and had disappeared. However, when it reappeared in the F2, he knew it had just been hidden. So, he called the purple color the dominant trait and the white color the recessive trait. Table 14-1 • • Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring Four related concepts that make up this model can be related to what we now know about genes and chromosomes Mendel’s garden in the abbey in Austria where he conducted his experiments. Alternative versions of genes account for variations in traits. For example: there are two versions of the gene for flower color in peas: purple and white. These alternative versions are called alleles—each of which resides in a particular place on a chromosome (the locus). For each characteristic, an organism inherits two alleles: one from the male parent and one from the female parent. The alleles may be the same (like the truebreeding plants), or they may be different (like the F1 hybrids). Mendel figured all of this out without ever knowing anything about chromosomes! If the two alleles at a locus are different, then one of them (the dominant allele) determines what the organism will look like. The recessive allele has no noticeable affect on the appearance. Why are all of the F1 offspring purple? The law of segregation states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes An egg or sperm gets only one of the two alleles that are present in the somatic cell. Mendel figured this out without ever knowing anything about meiosis! How do we explain the 3:1 results that Mendel got in the F2 generation? If we know the genetics of the parents, a Punnett Square can show the possible combinations of genes the offspring can inherit. Use CAPITAL letters for dominant genes, lower case letter for recessive Homozygous—an organism with two alleles for a given trait that are identical (also called purebred) Heterozygous—an organism with two alleles for a given trait that are different. (these are also known as hybrids) An organisms’ traits may not always reveal its true genetics. (think about those hybrid purple plants) Phenotype: what the organisms’ physical traits are—what it looks like Genotype: what genes the organism carries— what are its actual genes? Notice that the Genotype Ratio is 1:2:1 and the Phenotype Ratio is 3:1. Why are they different? How can we tell the genotype of an individual with the dominant phenotype? Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual If any offspring display the recessive phenotype, the mystery parent must be heterozygous Test Cross Technique TECHNIQUE Dominant phenotype, Recessive phenotype, unknown genotype: known genotype: PP or Pp? pp Predictions If PP Sperm p p P Pp Eggs If Pp Sperm p p or P Pp Eggs P Pp Pp pp pp p Pp Pp RESULTS or All offspring purple 1 /2 offspring purple and offspring white 1 /2 Mendel figured out the Law of Segregation by studying the results of crosses involving only one trait (for example, flower color). These are called monohybrid crosses. Mendel derived his second law by studying crosses involving pea plants who differed in two traits. These are called dihybrid crosses. An organism who is hybrid for two traits is called a dihybrid. A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently 1. Figure out the genotypes of the parents. 2. Figure out the possible combinations of genes that could be in the gametes of these parents. Example: TtYy could produce 4 kinds of gametes: TY, Ty, tY, ty 3. Put the gametes into a Punnett Square & Solve 4. Figure out genotypic and phenotypic ratios. Law of Independent Assortment • • Using a dihybrid cross, Mendel developed the law of independent assortment The Law of Independent Assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation Remember that in Meiosis, it is random which direction the chromosomes go during Anaphase I and II. There are all kinds of possibilities! • • The Law of Independent Assortment in general applies only to genes on different, nonhomologous chromosomes Genes located near each other on the same chromosome tend to be inherited together Remember that in Prophase I of Meiosis, synapsis and crossing over occurs. When crossing over occurs, it could happen anywhere on the chromosome, but it is more likely to happen on the ends. Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: When alleles are not completely dominant or recessive When a gene has more than two alleles When a gene produces multiple phenotypes Why does everything have to be so complicated???? Complete dominance—when the dominant gene totally dominates over the recessive. Most of the traits Mendel studied showed complete dominance. In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties Incomplete Dominance in Snapdragons P Generation Red CRCR White CWCW CR Gametes CW Pink CRCW F1 Generation 1 Gametes /2 CR 1/ CW 2 Sperm 1/ 2 CR 1/ 2 CW F2 Generation 1/ 2 CR Eggs 1/ 2 CRCR CRCW CRCW CWCW CW In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways There isn't a blending of the traits, but rather both alleles are present in the phenotype. In this flower, both the dark pink allele and the white allele are co-dominant. Neither one dominates over the over, so the phenotype shows both alleles. Polygenic Traits are characteristics that are affected by more than one gene. Examples: Eye Color, Skin Color Eye color comes from different genes which affect tone, amount and position of the pigments. Skin color is determined by at least 3 different genes working together to produce a wide variety of tones. Most genes exist in more that two allelic forms. A classic example of this is human blood types. Four major blood types exist: O, A, B, and AB They are named for the presence or absence of certain carbohydrates on the surface of the red blood cells. The gene for blood type has 3 possible alleles: IA, IB, and i Both IA and IB are dominant over i IA and IB are codominant to each other Type A Blood Genotypes: IAIA or IAi Type B Blood Genotypes: IBIB or IBi Type AB Blood Genotype: IAIB Type O Blood Genotype: ii In epistasis the expression of one gene affects the expression of another gene. Example: Coat color in mice. Black (B) is dominant over brown (bb). However, there is also another gene locus independent of the first. This gene (Cc or CC) controls whether there will be ANY pigment deposited on the fur (cc will always have white fur) In pleiotropy (“pleion”=more), a single gene has multiple effects on an organism. A good example is the gene that causes Sickle Cell Anemia. This single gene causes blood cells to be sickle-shaped. This leads to many severe problems such as heart, lung, and kidney damage; muscle pain; weakness and fatigue It can eventually lead to kidney failure and death—all because of one gene. Probability is a concept important to a full understanding of heredity. What is the probability that a flipped coin will be heads? ½ What is the probability that 2 coins flipped will BOTH be heads? A di-hybrid or other multi-character cross is equivalent to two or more independent monohybrid crosses occurring simultaneously. The multiplication rule states that to determine the probability of two events occurring together, we multiply the probability of one event X the probability of the second event. What is the probability of flipping a coin twice and getting “heads” both times? ½ X ½= ¼ What is the probability of getting heads 3 times in a row? What is the chance of getting a recessive trait if two heterozygous individuals mate? ½ X ½ = ¼ (25%) Each egg has a ½ chance of carrying the “G” gene and ½ chance of carrying the “g” gene. The same odds apply to the sperm cell. The probability that two “g” genes will appear is 25%. Sometimes, the phenotype that is observed depends on environmental factors as well as the genotype. For example: in humans, nutrition affects height Even identical twins accumulate phenotypic differences as they experience different environments. Twins holding twins Pedigrees are family trees used to describe the genetic relationships within a family. Pedigrees can be used to determine the risk of parents passing certain conditions to their offspring