3: Mendelian Genetics Flashcards
Inheritance is governed by information stored in discrete unit factors called ______.
genes
Genes are transmitted from generation to generation on vehicles called ______.
chromosomes
Chromosomes, which exist in pairs in ______ organisms, provide the basis of ______.
diploid, biparental inheritance
During gamete formation, chromosomes are distributed according to postulates first described by ______, based on his nineteenth-century research with the ______.
Gregor Mendel, garden pea
Mendelian postulates prescribe that ______ chromosomes ______ from one another and ______ with other segregating homologs during gamete formation.
homologous, segregate, assort independently
Genetic ratios, expressed as ______, are subject to ______ and may be evaluated statistically.
probabilities, chance deviation
The analysis of ______ allows predictions concerning the genetic nature of human traits.
pedigrees
Although inheritance of biological traits has been recognized for thousands of years, the first significant insights into how it takes place only occurred about ______ years ago. In 1866, ______ published the results of a series of experiments that would lay the foundation for the formal discipline of genetics.
150, Gregor Johann Mendel
Mendel’s work went largely unnoticed until the turn of the twentieth century, but eventually, the concept of the ______ as a distinct hereditary unit was established. Since then, the ways in which genes, as segments of ______, are transmitted to offspring and control traits have been clarified. Research continued unabated throughout the twentieth century and into the present—indeed, studies in genetics, most recently at the molecular level, have remained at the forefront of biological research since the early 1900s.
gene, chromosomes
When Mendel began his studies of inheritance using ______, the garden pea, chromosomes and the role and mechanism of meiosis were totally unknown. Nevertheless, he determined that discrete units of inheritance exist and predicted their behavior in the formation of ______. Subsequent investigators, with access to cytological data, were able to relate their own observations of chromosome behavior during meiosis and Mendel’s principles of inheritance. Once this correlation was recognized, Mendel’s postulates were accepted as the basis for the study of what is known as ______— how genes are transmitted from parents to offspring. These principles were derived directly from Mendel’s experimentation.
Pisum sativum, gametes, transmission genetics
Mendel Used a Model Experimental Approach to Study Patterns of Inheritance
Johann Mendel was born in ______ to a peasant family in the Central European village of ______. An excellent student in high school, he studied philosophy for several years afterward and in 1843, taking the name Gregor, was admitted to the ______ in ______, now part of the Czech Republic. In 1849, he was relieved of pastoral duties, and from 1851 to 1853, he attended the ______, where he studied physics and botany. He returned to Brno in 1854, where he taught physics and natural science for the next 16 years. Mendel received support from the monastery for his studies and research throughout his life.
1822, Heinzendorf, Augustinian Monastery of St. Thomas, Brno, University of Vienna
Mendel Used a Model Experimental Approach to Study Patterns of Inheritance
In 1856, Mendel performed his first set of hybridization experiments with the ______, launching the research phase of his career. His experiments continued until 1868, when he was elected abbot of the monastery. Although he retained his interest in genetics, his new responsibilities demanded most of his time. In 1884, Mendel died of a ______ disorder. The local newspaper paid him the following tribute:
His death deprives the poor of a benefactor, and man- kind at large of a man of the noblest character, one who was a warm friend, a promoter of the natural sciences, and an exemplary priest.
garden pea, kidney
Mendel Used a Model Experimental Approach to Study Patterns of Inheritance
Mendel first reported the results of some simple genetic crosses between certain strains of the garden pea in ______. Although his was not the first attempt to provide experimental evidence pertaining to inheritance, Mendel’s success where others had failed can be attributed, at least in part, to his elegant experimental design and analysis.
1865
Mendel Used a Model Experimental Approach to Study Patterns of Inheritance
Mendel showed remarkable insight into the methodology necessary for good experimental biology. First, he chose an organism that was easy to ______ and to ______ artificially. The pea plant is self-fertilizing in nature, but it is easy to cross-breed experimentally. It reproduces well and grows to maturity in a single season. Mendel followed ______ visible features (we refer to them as ______, or ______), each represented by two contrasting forms, or ______. For the character stem height, for example, he experimented with the traits ______ and ______. He selected six other contrasting pairs of traits involving seed shape and color, pod shape and color, and flower color and position. From local seed merchants, Mendel obtained true-breeding strains, those in which each trait appeared unchanged generation after generation in self-fertilizing plants.
grow, hybridize, seven, characters, characteristics, traits, tall, dwarf
Mendel Used a Model Experimental Approach to Study Patterns of Inheritance
There were several other reasons for Mendel’s success. In addition to his choice of a suitable organism, he restricted his examination to one or very few pairs of contrasting traits in each experiment. He also kept accurate ______ records, a necessity in genetic experiments. From the analysis of his data, Mendel derived certain postulates that have become the principles of ______.
quantitative, transmission genetics
Mendel Used a Model Experimental Approach to Study Patterns of Inheritance
The results of Mendel’s experiments went unappreciated until the turn of the century, well after his death. However, once Mendel’s publications were rediscovered by geneticists investigating the function and behavior of chromosomes, the implications of his postulates were immediately apparent. He had discovered the basis for the transmission of ______ traits!
hereditary
The Monohybrid Cross Reveals How One Trait Is Transmitted from Generation to Generation
Mendel’s simplest crosses involved only one pair of contrasting traits. Each such experiment is called a ______.
monohybrid cross
The Monohybrid Cross Reveals How One Trait Is Transmitted from Generation to Generation
A monohybrid cross is made by mating true-breeding individuals from ______ parent strains, each exhibiting one of the two contrasting forms of the character under study. Initially, we examine the first generation of offspring of such a cross, and then we consider the offspring of selfing, that is, of self-fertilization of individuals from this first generation. The original parents constitute the ______, or ______; their offspring are the ______, or ______; the individuals resulting from the selfed F1 generation are the ______, or ______; and so on.
two, P1, parental generation, F1, first filial generation, F2, second filial generation
The Monohybrid Cross Reveals How One Trait Is Transmitted from Generation to Generation
The cross between true-breeding pea plants with tall stems and dwarf stems is representative of Mendel’s ______ crosses. Tall and dwarf are contrasting traits of the character of stem height. Unless tall or dwarf plants are crossed together or with another strain, they will undergo self-fertilization and breed true, producing their respective traits generation after generation. However, when Mendel crossed tall plants with dwarf plants, the resulting F1 generation consisted of only ______ plants. When members of the F1 generation were selfed, Mendel observed that 787 of 1064 F2 plants were tall, while 277 of 1064 were dwarf. Note that in this cross, the ______ trait disappeared in the F1 generation, only to reappear in the F2 generation.
monohybrid, tall, dwarf
The Monohybrid Cross Reveals How One Trait Is Transmitted from Generation to Generation
Genetic data are usually expressed and analyzed as ______. In this particular example, many identical P1 crosses were made and many F1 plants—all tall—were produced. As noted, of the 1064 F2 offspring, 787 were tall and 277 were dwarf—a ratio of approximately 2.8:1.0, or about 3:1.
ratios
The Monohybrid Cross Reveals How One Trait Is Transmitted from Generation to Generation
Mendel made similar crosses between pea plants exhibiting each of the other pairs of contrasting traits; the results of these crosses are shown in Figure 3.1. In every case, the outcome was similar to the tall/dwarf cross just described. For the character of interest, all F1 offspring expressed the same trait exhibited by one of the parents, but in the F2 offspring, an approximate ratio of ______ was obtained. That is, three-fourths looked like the F1 plants, while one-fourth exhibited the contrasting trait, which had disappeared in the F1 generation.
3:1
The Monohybrid Cross Reveals How One Trait Is Transmitted from Generation to Generation
We note one further aspect of Mendel’s monohybrid crosses. In each cross, the F1 and F2 patterns of inheritance were similar regardless of which P1 plant served as the source of ______ and which served as the source of the ______. The crosses could be made either way— pollination of dwarf plants by tall plants, or vice versa. Crosses made in both these ways are called reciprocal crosses. Therefore, the results of Mendel’s monohybrid crosses were not ______ dependent.
pollen (sperm), ovum (egg), sex
The Monohybrid Cross Reveals How One Trait Is Transmitted from Generation to Generation
To explain these results, Mendel proposed the existence of particulate ______ for each trait. He suggested that these factors serve as the basic units of ______ and are passed unchanged from generation to generation, determining various traits expressed by each individual plant. Using these general ideas, Mendel proceeded to hypothesize precisely how such factors could account for the results of the monohybrid crosses.
unit factors, heredity
Mendel’s First Three Postulates:
- UNIT FACTORS IN PAIRS
- DOMINANCE/RECESSIVENESS
- SEGREGATION
Mendel’s First Three Postulates
______
Genetic characters are controlled by unit factors existing in pairs in individual organisms.
UNIT FACTORS IN PAIRS
Mendel’s First Three Postulates
______
When two unlike unit factors responsible for a single character are present in a single individual, one unit factor is dominant to the other, which is said to be recessive.
DOMINANCE/RECESSIVENESS
Mendel’s First Three Postulates
______
During the formation of gametes, the paired unit factors separate, or segregate, randomly so that each gamete receives one or the other with equal likelihood.
SEGREGATION
Mendel’s First Three Postulates
UNIT FACTORS IN PAIRS
In the monohybrid cross involving tall and dwarf stems, a specific ______ exists for each trait. Each diploid individual receives one ______ from each parent. Because the factors occur in pairs, ______ combinations are possible: two factors for tall stems, two factors for dwarf stems, or one of each factor. Every individual possesses one of these three combinations, which determines stem height.
unit factor, factor, three
Mendel’s First Three Postulates
DOMINANCE/RECESSIVENESS
In each monohybrid cross, the trait expressed in the F1 generation is controlled by the ______ unit factor. The trait not expressed is controlled by the ______ unit factor. The terms dominant and recessive are also used to designate ______. In this case, ______ stems are said to be dominant over recessive ______ stems.
dominant, recessive, traits, tall, dwarf
Mendel’s First Three Postulates
SEGREGATION
If an individual contains a pair of like unit factors (e.g., both specific for tall), then all its gametes receive ______ of that same kind of unit factor (in this case, tall). If an individual contains unlike unit factors (e.g., one for tall and one for dwarf), then each gamete has a ______ percent probability of receiving either the tall or the dwarf unit factor.
one, 50
Mendel’s First Three Postulates
These postulates provide a suitable explanation for the results of the monohybrid crosses. Let’s use the tall/dwarf cross to illustrate. Mendel reasoned that P1 tall plants contained ______ paired unit factors, as did the P1 dwarf plants. The gametes of tall plants all receive one tall unit factor as a result of ______. Similarly, the gametes of dwarf plants all receive ______ dwarf unit factor. Following fertilization, all F1 plants receive one unit factor from each parent—a tall factor from one and a dwarf factor from the other—reestablishing the paired relationship, but because tall is dominant to dwarf, all F1 plants are ______.
identical, segregation, one, tall
Mendel’s First Three Postulates
When F1 plants form gametes, the postulate of ______ demands that each gamete randomly receives either the tall or dwarf unit factor. Following random fertilization events during F1 selfing, four F2 combinations will result with equal frequency:
1. tall/tall
2. tall/dwarf
3. dwarf/tall
4. dwarf/dwarf
segregation
Modern Genetic Terminology
To analyze the monohybrid cross and Mendel’s first three postulates, we must first introduce several new terms as well as a symbol convention for the unit factors. Traits such as tall or dwarf are physical expressions of the information contained in ______. The physical expression of a trait is the ______ of the individual.
unit factors, phenotype
Modern Genetic Terminology
Mendel’s unit factors represent units of inheritance called ______ by modern geneticists. For any given character, such as plant height, the phenotype is determined by alternative forms of a single gene, called ______. For example, the unit factors representing tall and dwarf are alleles determining the height of the pea plant.
genes, alleles
Modern Genetic Terminology
Geneticists have several different systems for using symbols to represent genes. Later in the text, we will review a number of these conventions, but for now, we will adopt one to use consistently throughout this chapter. According to this convention, the first letter of the recessive trait symbolizes the character in question; in lowercase italic, it designates the allele for the ______ trait, and in uppercase italic, it designates the allele for the ______ trait. Thus for Mendel’s pea plants, we use d for the dwarf allele and D for the tall allele. When alleles are written in pairs to represent the two unit factors present in any individual (DD, Dd, or dd), the resulting symbol is called the ______.
recessive, dominant, genotype
Modern Genetic Terminology
The genotype designates the genetic makeup of an individual for the trait or traits it describes, whether the individual is ______ or ______. By reading the genotype, we know the phenotype of the individual: DD and Dd are ______, and dd is ______. When both alleles are the same (DD or dd), the individual is ______ for the trait, or a ______; when the alleles are different (Dd), we use the terms ______ and ______.
haploid, diploid, tall, dwarf, homozygous, homozygote, heterozygous, heterozygote
Punnett Squares
The genotypes and phenotypes resulting from combining gametes during fertilization can be easily visualized by constructing a diagram called a ______, named after the person who first devised this approach, ______. Figure 3.3 illustrates this method of analysis for our F1 * F1 monohybrid cross. Each of the possible gametes is assigned a column or a row; the vertical columns represent those of the female parent, and the horizontal rows represent those of the male parent. After assign- ing the gametes to the rows and columns, we predict the new generation by entering the male and female gametic information into each box and thus producing every possible resulting genotype. By filling out the Punnett square, we are listing all possible random fertilization events. The genotypes and phenotypes of all potential offspring are ascertained by reading the combinations in the boxes.
Punnett square, Reginald C. Punnett
Punnett Squares
The ______ is particularly useful when you are first learning about genetics and how to solve genetics problems. Note the ease with which the 3:1 phenotypic ratio and the 1:2:1 genotypic ratio may be derived for the F2 generation in Figure 3.3.
Punnett square method
The Testcross: One Character
Tall plants produced in the F2 generation are predicted to have either the DD or the Dd genotype. You might ask if there is a way to distinguish the genotype. Mendel devised a rather simple method that is still used today to discover the genotype of plants and animals: the ______.
testcross
The Testcross: One Character
The organism expressing the dominant phenotype but having an unknown genotype is crossed with a known ______ individual. For example, as shown in Figure 3.4(a), if a tall plant of genotype DD is testcrossed with a dwarf plant, which must have the dd genotype, all offspring will be tall phenotypically and Dd genotypi- cally. However, as shown in Figure 3.4(b), if a tall plant is Dd and is crossed with a dwarf plant (dd), then one-half of the offspring will be tall (Dd) and the other half will be dwarf (dd). Therefore, a 1:1 tall/dwarf ratio demonstrates the heterozygous nature of the tall plant of unknown genotype. The results of the testcross reinforced Mendel’s conclusion that separate unit factors control traits.
homozygous recessive
Mendel’s Dihybrid Cross Generated a Unique F Ratio
As a natural extension of the monohybrid cross, Mendel also designed experiments in which he examined two characters simultaneously. Such a cross, involving two pairs of contrasting traits, is a ______, or a two-factor cross. For example, if pea plants having yellow seeds that are round were bred with those having green seeds that are wrinkled, the results shown in Figure 3.5 would occur: the F1 offspring would all be yellow and round. It is therefore apparent that yellow is dominant to green and that round is dominant to wrinkled. When the F1 individuals are selfed, approximately 9/16 of the F2 plants express the yellow and round traits, 3/16 express yellow and wrinkled, 3/16 express green and round, and 1/16 express green and wrinkled.
dihybrid cross
Mendel’s Fourth Postulate: ______
Independent Assortment
Mendel’s Fourth Postulate: Independent Assortment
We can most easily understand the results of a dihybrid cross if we consider it theoretically as consisting of two ______ crosses conducted separately. Think of the two sets of traits as being inherited ______ of each other; that is, the chance of any plant having yellow or green seeds is not at all influenced by the chance that this plant will have round or wrinkled seeds. Thus, because yellow is dominant to green, all F1 plants in the first theoretical cross would have yellow seeds. In the second theoretical cross, all F1 plants would have round seeds because round is dominant to wrinkled. When Mendel examined the F1 plants of the dihybrid cross, all were yellow and round, as our theoretical crosses predict.
The predicted F2 results of the first cross are 3/4 yellow and 1/4 green. Similarly, the second cross would yield 3/4 round and 1/4 wrinkled. Figure 3.5 shows that in the dihy- brid cross, 12/16 F2 plants are yellow, while 4/16 are green, exhibiting the expected 3:1 (3/4:1/4) ratio. Similarly, 12/16 of all F2 plants have round seeds, while 4/16 have wrinkled seeds, again revealing the 3:1 ratio.
monohybrid, independently
Mendel’s Fourth Postulate: Independent Assortment
These numbers demonstrate that the two pairs of contrasting traits are inherited independently, so we can predict the frequencies of all possible F2 phenotypes by applying the ______ of probabilities: the probability of two or more independent events occurring simultaneously is equal to the product of their individual probabilities. For example, the probability of an F2 plant having yellow and round seeds is (3/4)(3/4), or 9/16, because 3/4 of all F2 plants should be yellow and 3/4 of all F2 plants should be round.
In a like manner, the probabilities of the other three F2 phenotypes can be calculated: yellow (3/4) and wrinkled (1/4) are predicted to be present together 3/16 of the time; green (1/4) and round (3/4) are predicted 3/16 of the time; and green (1/4) and wrinkled (1/4) are predicted 1/16 of the time. These calculations are shown in Figure 3.6.
It is now apparent why the F1 and F2 results are iden- tical whether the initial cross is yellow, round plants bred with green, wrinkled plants, or whether yellow, wrinkled plants are bred with green, round plants. In both crosses, the F1 genotype of all offspring is identical. As a result, the F2 generation is also identical in both crosses.
product law
______
During gamete formation, segregating pairs of unit factors assort independently of each other.
INDEPENDENT ASSORTMENT
INDEPENDENT ASSORTMENT
This postulate stipulates that segregation of any pair of unit factors occurs ______ of all others. As a result of random segregation, each gamete receives one member of every pair of unit factors. For one pair, whichever unit factor is received does not influence the outcome of segre- gation of any other pair. Thus, according to the postulate of independent assortment, all possible combinations of gametes should be formed in ______.
independently, equal frequency
Mendel’s Fourth Postulate: Independent Assortment
In every F1 * F1 fertilization event, each zygote has an ______ probability of receiving one of the four combinations from each parent. If many offspring are produced, 9/16 have yellow, round seeds, 3/16 have yellow, wrinkled seeds, 3/16 have green, round seeds, and 1/16 have green, wrinkled seeds, yielding what is designated as Mendel’s 9:3:3:1 ______. This is an ideal ratio based on probability events involving ______, ______, and ______. Because of deviation due strictly to chance, particularly if small numbers of offspring are produced, actual results are highly unlikely to match the ideal ratio.
equal, dihybrid ratio, segregation, independent assortment, random fertilization
The Testcross: Two Characters
The testcross may also be applied to individuals that express ______ dominant traits but whose genotypes are unknown. For example, the expression of the yellow, round seed pheno- type in the F2 generation just described may result from the GGWW, GGWw, GgWW, or GgWw genotypes. If an F2 yellow, round plant is crossed with the homozygous recessive green, wrinkled plant (ggww), analysis of the offspring will indicate the exact genotype of that yellow, round plant. Each of the above genotypes results in a different set of gametes and, in a testcross, a different set of phenotypes in the resulting off- spring. You should work out the results of each of these four crosses to be sure that you understand this concept.
two
The Trihybrid Cross Demonstrates That Mendel’s Principles Apply to Inheritance of Multiple Traits
Thus far, we have considered inheritance of up to two pairs of contrasting traits. Mendel demonstrated that the processes of segregation and independent assortment also apply to three pairs of contrasting traits, in what is called a ______, or ______.
trihybrid cross, three-factor cross
The Trihybrid Cross Demonstrates That Mendel’s Principles Apply to Inheritance of Multiple Traits
Although a trihybrid cross is somewhat more complex than a dihybrid cross, its results are easily calculated if the principles of segregation and independent assortment are followed. For example, consider the cross shown in Figure 3.8 where the allele pairs of theoretical contrast- ing traits are represented by the symbols A, a, B, b, C, and c. In the cross between AABBCC and aabbcc individuals, all F1 individuals are heterozygous for all three gene pairs. Their genotype, AaBbCc, results in the phenotypic expres- sion of the dominant A, B, and C traits. When F1 individu- als serve as parents, each produces eight different gametes in equal frequencies. At this point, we could construct a Punnett square with ______ separate boxes and read out the phenotypes—but such a method is cumbersome in a cross involving so many factors. Therefore, another method has been devised to calculate the predicted ratio.
64
The Forked-Line Method, or Branch Diagram
It is much less difficult to consider each contrasting pair of traits separately and then to combine these results by using the ______, first shown in Figure 3.6. This method, also called a ______, relies on the simple application of the laws of probability established for the dihybrid cross. Each gene pair is assumed to behave inde- pendently during gamete formation.
forked-line method, branch diagram
Mendel’s Work Was Rediscovered in the Early Twentieth Century
Mendel published his work in ______. While his findings were often cited and discussed, their significance went unappreciated for about 35 years. Then, in the latter part of the nineteenth century, a remarkable observation set the scene for the recognition of Mendel’s work: ______’s discovery of chromosomes in the nuclei of salamander cells. In 1879, Flemming described the behavior of these threadlike structures during cell division. As a result of his findings and the work of many other cytologists, the presence of discrete units within the nucleus soon became an integral part of scientists’ ideas about inheritance.
1866, Walter Flemming
Mendel’s Work Was Rediscovered in the Early Twentieth Century
In the early twentieth century, hybridization experiments similar to Mendel’s were performed independently by three botanists, ______, ______, and ______. De Vries’s work demonstrated the principle of ______ in several plant species. Apparently, he searched the existing literature and found that Mendel’s work had anticipated his own conclusions! Correns and Tschermak also reached conclusions similar to those of Mendel.
Hugo de Vries, Carl Correns, Erich Tschermak, segregation
Mendel’s Work Was Rediscovered in the Early Twentieth Century
About the same time, two cytologists, ______ and ______, independently published papers linking their discoveries of the behavior of chromosomes during meiosis to the Mendelian principles of ______ and ______. They pointed out that the separation of chromosomes during meiosis could serve as the cytological basis of these two postulates. Although they thought that Mendel’s unit factors were probably chromosomes rather than genes on chromosomes, their findings reestablished the importance of Mendel’s work and led to many ensuing genetic investigations. Sutton and Boveri are credited with initiating the ______, the idea that the genetic material in living organisms is contained in chromosomes, which was developed during the next two decades.
Walter Sutton, Theodor Boveri, segregation, independent assortment, chromosomal theory of inheritance
Unit Factors, Genes, and Homologous Chromosomes
As we know, each species possesses a specific number of chromosomes in each somatic cell nucleus. For diploid organisms, this number is called the ______ (2n) and is characteristic of that species. During the formation of gametes (______), the number is precisely halved (n), and when two gametes combine during fertilization, the ______ is reestablished. During meiosis, however, the chromosome number is not reduced in a random manner. It was apparent to early cytologists that the diploid number of chromosomes is composed of homologous pairs identifiable by their morphological appearance and behavior. The gametes contain one member of each pair—thus the chromosome complement of a gamete is quite specific, and the number of chromosomes in each gamete is equal to the haploid number.
With this basic information, we can see the correlation between the behavior of unit factors and chromosomes and genes. Figure 3.10 shows three of Mendel’s postulates and the chromosomal explanation of each. Unit factors are really genes located on homologous pairs of chromosomes. Members of each pair of homologs separate, or segregate, during gamete formation. In the figure, two different alignments are possible, both of which are shown.
diploid number, meiosis, diploid number
Unit Factors, Genes, and Homologous Chromosomes
To illustrate the principle of independent assortment, it is important to distinguish between members of any given homologous pair of chromosomes. One member of each pair is derived from the ______, whereas the other comes from the ______. (We represent the different parental origins with different colors.) As shown in Figure 3.10(c), following independent segregation of each pair of homologs, each gamete receives one member from each pair of chromosomes. All possible combinations are formed with equal probability. If we add the symbols used in Mendel’s dihybrid cross (G, g and W, w) to the diagram, we can see why equal numbers of the four types of gametes are formed. The independent behavior of Mendel’s pairs of unit factors (G and W in this example) is due to their presence on separate pairs of homologous chromosomes.
maternal parent, paternal parent
Unit Factors, Genes, and Homologous Chromosomes
Observations of the phenotypic diversity of living organisms make it logical to assume that there are many more genes than ______. Therefore, each homolog must carry genetic information for more than one trait. The currently accepted concept is that a chromosome is composed of a large number of linearly ordered, information-containing genes. Mendel’s paired unit factors (which determine tall or dwarf stems, for example) actually constitute a pair of genes located on one pair of homologous chromosomes. The location on a given chromosome where any particular gene occurs is called its ______. The different alleles of a given gene (for example, G and g) contain slightly different genetic information (green or yellow) that determines the same character (seed color in this case). Although we have examined only genes with two alternative alleles, most genes have more than two allelic forms.
chromosomes, locus (pl. loci)
Independent Assortment Leads to Extensive ______
Genetic Variation
Laws of Probability Help to Explain Genetic Events
When two or more events with known probabilities occur independently but at the same time, we can calculate the probability of their possible outcomes occurring together. This is accomplished by applying the ______, which states that the probability of two or more independent events occurring simultaneously is equal to the product of their individual probabilities. Two or more events are independent of one another if the out- come of each one does not affect the outcome of any of the oth- ers under consideration.
product law
Laws of Probability Help to Explain Genetic Events
If we want to calculate the probability when the possible outcomes of two events are independent of one another but can be accomplished in more than one way, we can apply the ______. For example, what is the probability of tossing our penny and nickel and obtaining one head and one tail? In such a case, we do not care whether it is the penny or the nickel that comes up heads, provided that the other coin has the alternative outcome. As we saw above, there are two ways in which the desired outcome can be accomplished, each with a probability of 1/4.
sum law
Laws of Probability Help to Explain Genetic Events
The ______ states that the probability of obtaining any single outcome, where that outcome can be achieved by two or more events, is equal to the sum of the individual probabilities of all such events.
sum law
Chi-Square Analysis Evaluates the Influence of Chance on Genetic Data
Mendel’s 3:1 monohybrid and 9:3:3:1 dihybrid ratios are hypothetical predictions based on the following assumptions: (1) each allele is dominant or recessive, (2) segregation is unimpeded, (3) independent assortment occurs, and (4) fertilization is random. The final two assumptions are influenced by chance events and therefore are subject to random fluctuation. This concept of ______ is most easily illustrated by tossing a single coin numerous times and recording the number of heads and tails observed.
chance deviation
Chi-Square Calculations and the Null
Hypothesis
In genetics, being able to evaluate observed deviation is a crucial skill. When we assume that data will fit a given ratio such as 1:1, 3:1, or 9:3:3:1, we establish what is called the ______. It is so named because the hypothesis assumes that there is no real difference between the measured values (or ratio) and the predicted values (or ratio). Any apparent difference can be attributed purely to chance.
null hypothesis (H0)
Chi-Square Calculations and the Null
Hypothesis
The validity of the null hypothesis for a given set of data is measured using ______. Depending on the results of this analysis, the null hypothesis may either (1) be rejected or (2) fail to be rejected. If it is rejected, the observed deviation from the expected result is judged not to be attributable to chance alone. In this case, the null hypothesis and the underlying assumptions leading to it must be reexamined. If the null hypothesis fails to be rejected, any observed deviations are attributed to chance.
statistical analysis
Chi-Square Calculations and the Null
Hypothesis
One of the simplest statistical tests for assessing the goodness of fit of the null hypothesis is ______. This test takes into account the observed devia- tion in each component of a ratio (from what was expected) as well as the sample size and reduces them to a single numerical value.
chi-square (x2) analysis
Chi-Square Calculations and the Null
Hypothesis
The final step in chi-square analysis is to interpret the x2 value. To do so, you must initially determine a value called the ______, which is equal to n - 1, where n is the number of different categories into which the data are divided, in other words, the number of possible outcomes. For the 3:1 ratio, n = 2, so df = 1. For the9:3:3:1ratio,n = 4anddf = 3.Degreesoffreedommust be taken into account because the greater the number of categories, the more deviation is expected as a result of chance.
degrees of freedom (df)
Chi-Square Calculations and the Null
Hypothesis
Once you have determined the degrees of freedom, you can interpret the x2 value in terms of a corresponding ______. Since this calculation is complex, we usually take the p value from a standard table or graph. Figure 3.11 shows a wide range of x2 values and the corresponding p values for various degrees of freedom in both a graph and a table. Let’s use the graph to explain how to determine the p value.
probability value (p)
