Biology Chapter 11: Introduction to Genetics

Transcription

1 Biology Chapter 11: Introduction to Genetics Meiosis - The mechanism that halves the number of chromosomes in cells is a form of cell division called meiosis - Meiosis consists of two successive nuclear divisions. - Before the first division, the DNA is copied, just as it is before mitosis. - the first division, called meiosis I, homologous chromosomes separate into two cells - the second division, called meiosis II, the two chromatids of each chromosome separate into two haploid cells - Thus, one diploid cell that undergoes meiosis produces four haploid cells. - In animals, meiosis results in haploid gametes - In plants, meiosis leads to spores, haploid cells that later lead to the production of gametes - During meiosis, two unique events occur: 1. Crossing-over: In the beginning of meiosis I, homologous chromosomes pair up next to each other. While paired, the arms of the chromosomes exchange reciprocal segments of DNA in a process called crossing-over. 2. Skipping replication: there is only one replication of DNA but two divisions, meiosis halves the number of chromosomes. Meiosis - two nuclear divisions of meiosis are divided into eight stages - Although these stages have the same names as those of mitosis, the events differ (diagram) 1

2 Prophase I - in prophase of mitosis, the chromosomes condense, the nuclear envelope breaks down, the centrioles move to opposite poles, and spindle fibers form - Next, a unique event to meiosis occurs: first the homologous chromosomes pair up and then crossing-over occurs Metaphase I - the pairs of homologous chromosomes are moved by spindle fibers to the equator of the cell - The homologues, each made up of two chromatids, remain together Anaphase I - the homologues separate - chromosomes of each pair are pulled by action of the spindle fibers to opposite poles of the cell - The difference between anaphase of mitosis and anaphase of meiosis I is that the chromatids do not separate at their centromeres. Therefore, each chromosome is still composed of two chromatids joined by a centromere. Telophase I - individual chromosomes gather at each of the two poles - In most organisms, the cytoplasm divides, forming two new cells - Note that each of the cells produced now contains half the number of chromosomes of the original cell. For this reason, meiosis I is often called "reduction division." Meiosis II Separates Chromatids - The stages of meiosis I are similar to those of mitosis - Meiosis II is identical to mitosis except that the chromosomes do not replicate before they divide at their centromeres. - In anaphase II, the centromeres divide, and the chromatids, now called chromosomes, move to opposite poles of the cell. - Meiosis II is followed by cytokinesis, in which new membranes are formed around the four products of meiosis to create four haploid cells. The Importance of Crossing-Over - Crossing-over is an efficient way to produce genetic recombination - the formation of new combmations of genes - As a result of crossing-over, the two chromatids of a chromosome no longer contain identical genetic material - Crossing-over thus provides a source of genetic variation - crossing-over has an enormous impact on how rapidly organisms evolve - change is often limited by the amount of genetic variation available 2

3 Heredity is the transmission of traits (color of eyes, texture of hair, height, etc.) from parent to offspring Beginnings of Genetics - Over a century ago (1866) Gregor Mendel an Austrian monk bred garden peas in the garden of a monastery - From these experiments, he developed a simple set of rules that accurately predicted patterns of heredity. - the branch of biology that studies heredity is called genetics - When Mendel's rules became widely known, scientists all over the world set out to discover the physical mechanism behind these patterns. Eventually, their studies taught them that traits are determined by genes, the instructions encoded in the DNA of the chromosomes an individual receives from each parent. - Mendel was not the first person to try to understand heredity by studying pea plants - Over 200 years ago, British farmers performed similar experiments - the British farmer T. A. Knight bred a variety of the garden pea that had purple flowers with a variety that had white flowers. - All the offspring had purple flowers. - When two of the offspring were bred, however, some of their offspring had purple flowers and some had white. - Knight's explanation of this phenomenon noted only that purple flowers had a "stronger tendency" to appear than white flowers. Why Mendel Chose Peas For his experiments, Mendel chose to study the garden pea. The garden pea is a good subject for genetic study for several reasons. 1. Many varieties of the garden pea (Pisum sativum) exist. He selected seven pairs of varieties that differed in easily distinguishable forms of various traits, such as flower color, seed color, and seed shape. 2. Mendel knew from earlier experiments that he could expect one of the two forms of each trait to disappear in one generation and then reappear in the next. This gave him something to count. 3. the garden pea (P. sativum) is a small, easy-to-grow plant that matures quickly and produces a large number of offspring. Mendel would be able to conduct many experiments and obtain results quickly. 4. The male and female reproductive parts of P. sativum are enclosed within the same flower. When left undisturbed, the flower does not open fully; it simply fertilizes itself through a process called self-pollination. As a result, one individual plant can produce offspring. To cross two pea plants, Mendel first had to remove the anthers (the pollen-producing organs) from a flower of one plant. He could then dust the pistil (the eggproducing organ) with pollen from a flower of a different pea plant. 3

4 Transferring the pollen from the flower of one plant to the flower of a different plant is called crosspollination. Scientists use the term cross to refer to the breeding between two flowers from separate plants. Mendel's Experimental Design Mendel carried out his experiments with garden peas in three steps. Step 1- Mendel began his experiments by allowing each variety of garden pea to self-pollinate for several generations. This method ensured that each variety was true-breeding for a particular trait, which means that all the offspring would display only one form of a particular trait. For example, a true-breeding, purpleflowering plant produced only plants with purple flowers in subsequent generations. Mendel called these plants the parental generation, or p generation. Step 2- Mendel then cross-pollinated two varieties from the p generation that exhibited contrasting traits, such as purpie flowers and white flowers. He called the offspring of these plants the first filial generation, or F1 generation. Step 3 - Mendel allowed the F 1 generation to self- pollinate. He called the offspring of these plants the second filial generation, or F2 generation. These were the plants that he counted. 4

5 Mendel Observed Two Ratios For each cross, Mendel obtained F1 generation plants that had only one form of the crossed traits. The contrasting trait had disappeared. Mendel described the remaining, or expressed trait, as dominant. The trait that was not expressed in the F1 generation was described as recessive. 5

6 - When the F1 generation was allowed to self-pollinate, the recessive trait reappeared in some of the plants in the F2 generation. - Mendel counted each type of plant in the F2 generation and calculated a ratio of approximately 3 purple-flowering plants to every 1 white-flowering plant (3:1) - Mendel obtained the same 3:1 ratio of plants expressing the dominant trait to plants expressing the recessive trait. - Mendel's next question was, Will the 3:1 ratios continue in subsequent generations? - He found that plants showing the recess'ive traits were true-breeding when they were allowed to selfpollinate. - When plants with the dominant trait self-pollinated, Mendel found that only one-third of them were truebreeding, whereas two-thirds were not. - For these plants, Mendel observed a 3:1 ratio of dominant to recessive traits. - These results suggested that the 3:1 ratio in the F2 generation was really a disguised 1:2:1 ratio: 1 truebreeding dominant plant to 2 not-true-breeding dominant plants to 1 true-breeding recessive plant. Mendel Proposed a Theory of Heredity - To explain his results, Mendel proposed a theory that has become the foundation of the science of genetics. His theory has five elements: I. Parents do not trasmit traits directly to their offspring. Rather, they pass on units of information that operate in the offspring to produce the trait. Mendel called these units of information "factors." In 6

7 modern terminology, Mendel's factors are called genes (segment of a DNA that transmits hereditary information) II. For each trait, an individual has two factors: one from its mother and one from its father. The two factors may or may not have the same information. If the factors have the same information (for example, if both factors have information for purple flowers), the individual is said to be homozygous If the factors are different (for example, one factor has information for purple flowers and the other has information for white flowers), the individual is said to be heterozygous. Each copy of a factor, or gene, is called an allele. III. IV. In modern terms, the physical appearance, or phenotype of an individual is determined by the alleles that code for traits. The set of alleles that an individual has is called its genotype. An individual receives one allele from one parent and the other allele from the other parent. Each allele can be passed on when the individual matures and reproduces. V. The presence of an allele does not guarantee that a trait will be expressed in the individual that carries it. In heterozygous individuals, only the dominant allele is expressed; the recessive allele is present but unexpressed. Mendel's Theory Became Laws of Heredity - Mendel's theory predicts the results of his crosses and also accounts for the ratios he observed. - Mendel's theory is often referred to as the law of segregation. In modern terms, the law of segregation states that the members of each pair of alleles separate when gametes are formed. - He found that the inheritance of one trait did not influence the inheritance of any other trait. - This observation eventually became known as the law of independent assortment. - The law of independent assortment states that pairs of alleles separate independently of one another during gamete formation. - We now know that this principle applies only to genes located on different chromosomes or far apart on the same chromosome. Interpreting Mendel's Model - Geneticists still rely on Menders model to predict the likely outcome of genetic crosses. - letters represent the alleles of an organism. - Capital letters refer to dominant alleles, and lowercase letters refer to recessive alleles. - Note that capital and lowercase forms of the same letter must be used to designate the two forms of one gene. - For example, the allele for the dominant trait of tallness in pea plants is represented by T, and the allele for the recessive trait of shortness by t. - Since there are two alleles for each trait, the genotype of a pea plant that is homozygous dominant for tallness is TT. A pea plant that is homozygous recessive for shortness has the genotype tt. If these two plants are crossed with each other, the offspring will be heterozygous for the trait and will be designated Tt. Probability - Mendel's crosses can be interpreted according to rules of probability because these rules can predict how genes will be distributed among the offspring of two parents. - Probability is the likelihood that a specific event will occur 7

8 - For example, when you toss a coin, there's a chance that it will land "heads" - There's also a chance that it will land "tails" - The probability of either event happening can be determined by the following formula: Number of one kind of possible outcome Probability = Total number of all possible outcomes - Thus, when you toss the coin, the chance of its landing heads up is l out of 2 possibilities, or ½ - The same formula can be used to predict the outcome of a genetic cross. - For example, consider Menders crosses that studied seed color. Out of 8,023 F2 pea plants, 6,022 had the dominant yellow seed color and 2,001 had the recessive green seed color. - Using the formula, the probability that the yellow seed color will appear in such a cross is 6,022/8,023, or 0.75 (75 percent), Expressed as 3/4. - The probability that the green seed color will appear in the F2 generation is 2,001/8,023, or 0.25 (25 percent). Expressed as 1/4. - In other words, probability tells us that there are three chances in four that an offspring of two heterozygous individuals will have the dominant trait and one chance in four that it will have the recessive trait. Monohybrid Crosses - A cross that provides data about one pair of contrasting traits is called a monohybrid cross. - A cross between a pea plant that is true-breeding for tallness and one that is true-breeding for shortness is an example of a monohybrid cross. - Biologists can also predict the probable outcome of a cross by using a diagram called a Punnett square - In the Punnett square the genotype of the tall plant and the alleles (TT) can contribute to its offspring are written on the top left side of the square - The genotype of the short plant and the alleles (tt) it can contribute to its offspring are written on the bottom left of the square - The interior of the square is a grid of boxes. - Each box is filled with two letters--0ne letter from the left side of the square and one letter from the top of the square. - These letters indicate the possible genotypes of the offspring. - In the case of the monohybrid cross in 100 percent of the offspring are expected to be heterozygous (Tt), expressing the dominant trait of tallness. - Note the dominant form of the trait is written first, followed by the lowercase letter for the recessive form of the trait. 8

9 - Punnett squares can also be used to predict the outcome of a heterozygous cross. - For example, in rabbits the allele for a black coat (B) is dominant over the allele for a brown coat (b). - Figure 7-7 shows a Punnett square that predicts the results of a monohybrid cross between two rabbits that are both heterozygous (B b) for coat color. - ¼ of the offspring would be expected to have the genotype BB, ½ would be expected to have the genotype Bb, and ¼ would be expected to have the genotype bb. - Since B is dominant over b, ¾ of the offspring would have a black coat, and ¼ would have a brown coat. - Here you can see the two ratios that Mendel observed in his experiments-lbb : 2Bb : lbb (genotype) and 3 black: 1 brown (phenotype). Dihybrid Crosses - A dihybrid cross is a cross that involves two pairs of contrasting traits. 9

10 - Predicting the results of a dihybrid cross is more complicated than a monohybrid cross because you have to consider how the two alleles of each of the two traits from each parent can combine. - For example, crossing a pea plant that is homozygous for round, yellow seeds (RRYY) with a homozygous for wrinkled, green seeds (rryy) would consists of 16 boxes. - When the alleles from each parent are independently sorted and listed the genotype of all the offspring should be RrYy. - Therefore, all offspring should have round, yellow seeds. - In guinea pigs the allele for short hair (S) is dominant over the allele for long hair (s), and the allele for black hair (B) is dominant over the allele for brown hair (b). - The Punnett square predicts the probable offspring of a cross between two individuals heterozygous for both characteristics (SsBb). - The offspring are likely to have nine different genotypes that will result in the following four phenotypes: - 9/16 of the guinea pigs will have short, black hair with the genotypes: SSBB, SsBB, SSBb, and SsBb - 3/16 will have short, brown hair. These include individuals, with genotypes SSbb and Ssbb - Three-sixteenths (3/16) will have long, black hair with the genotypes ssbb and ssbb - One-sixteenth (1/16) will have long, brown hair with the genotype ssbb 10

11 Patterns of Heredity Can Be Complex - The relationships between genes and traits are not always as simple as dominant and recessive alleles discussed so far. - Most of the time, genes display more complex patterns of heredity - Incomplete Dominance is intermediate trait between the two parents - For example, a cross between a red flowers and white flowers produces a pink flowers. - The flowers appear pink because they have less red pigment than the red flowers. - Codominance is when two dominant alleles are expressed at the same time - Codominance is different from incomplete dominance because both traits are displayed. - An example of codominance is the roan coat in horses which consists of red hairs and white hairs. - A cross between a homozygous red horse and a homozygous white horse results in heterozygous offspring with a roan coat, Multiple Alleles - Some traits have genes with more than two alleles called multiple alleles. - For example, there are three alleles that can determine human blood type-a, B, and 0. - The A and B alleles are both dominant over 0, which is recessive, but neither is dominant over the other. - When A and B are both present in the genotype, they are codominant. - The existence of these multiple alleles explains why there are four different blood types-a, B, AB, and 0. Continuous Variation - When several genes influence a trait, such as height or weight, determining the effect of one of these genes is difficult - slight differences in phenotypes are expressed when many individuals are compared. - These traits are said to be exhibiting continuous variation because you see a variety of phenotypes on a continuum from one extreme to the other. Environmental Influences - An individual's phenotype often depends on conditions in the environment. - For example, during the winter, the pigment-producing genes of the arctic fox do not function due to the cold temperature resulting in the coat of the fox as white. In summer, the genes function to produce pigments to darken the coat to a reddish brown