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38 Mendelian Inheritance

Inheritance follows the rules of probability.

Dice and the law of probability.Predicting the likelihood of inheriting a particular trait is similar to predicting the possible outcomes of rolling dice. It isimportant to understand probability to study genetic inheritance.Adrienne Hart-Davis/Science Source.

Topics Covered in this Module Laws of ProbabilityThe Punnett Square

Major Objectives of this Module Analyze a basic Punnett square problem.Analyze a test cross problem.Determine probabilities in monohybrid and dihybrid crosses.



Laws of ProbabilityProbability examines how likely it is that an event will occur. Probabilitiesrange from 0, where an event has absolutely no chance of occurring, to 1,where there is no chance that the event will not occur. There are two basicrules to calculating probabilities depending on the type of events.

Multiplication rule: Predicts the probability of independent events (aparticular event in one experiment does not affect the probability of event in asecond experiment). For example, when rolling a dice twice, the outcome inthe first roll will not affect the probable outcomes when the dice is rolled asecond time. The results of the two dice rolls are independent of each other.Each individual probability is multiplied to obtain an overall probability.

Addition rule: Predicts the probability of mutually exclusive events. Mutuallyexclusive means that if one event occurs (such as a flipped coin comes upas a heads), the other event cannot simultaneously occur (i.e., it cannot betails). The probability of either of the mutually exclusive events, either A or B,occurring is equal to the probability of event A added to the probability ofevent B (in the simple coin tossing case, the additions rule tells us that theprobability that a flipped coin will be either heads or tails is 100%, because itwill turn up heads approximately half of the time and tails the other half).

Gregor Mendel studied the patterns of inheritance by breeding pea plants. Inone set of experiments, he bred lines of plants that each had a different trait(for example, seed color or shape). He then crossed those lines with differenttraits to create a monohybrid cross; the offspring (F1) did not have all of thetraits that he observed in the parent generation. After crossing individuals inthis F1 generation, the traits that had disappeared in the F1 generationreappeared in the offspring of these crosses (F2). However, the trait did notreappear in all of the offspring — on average, it appeared in 1 of 4 plants. Hecalled this trait recessive because it could be masked by the other, dominanttrait. What if Mendel had bred only four plants instead of hundreds? Wouldhe have seen this pattern?

What does probability have to do with genetics?The products of a genetic cross can be predicted using laws of probability.

Mendel's principle of segregation states that two alleles in a parentsegregate to form gametes with only one of those alleles through meiosis(Figure 1). The discovery of chromosomes in the latter part of the nineteenthcentury by Walther Flemming, along with the identification of the cell divisionprocess that gives rise to gametes (meiosis), reestablished the importance ofMendel's work. In meiosis, the parent reproductive cell, which contains a pairof each chromosome (called homologous chromosomes), undergoes twosteps of division (meiosis I and meiosis II). Each parent cell divides into twocells during meiosis I, and the homologous chromosomes separate, followedby a second division in meiosis II where the two sister chromatids separate,forming four gametes that all contain a single allele (Figure 1). When allelesare described as dominant or recessive, it is important to keep in mind thatdominance and recessiveness are not intrinsic allelic properties. Instead,they are effects that may only be measured in relation to the effects of thealternative alleles of the same gene.

Figure 1: Mendel's principle of segregation.

© 2013 Nature Education All rights reserved. Figure Detail

Homologous chromosomes contain alternative versions of the sameallele. In this case, the blue chromosomes contain the allele "G" or "g," andthe red chromosomes contain either "W" or "w." Note how the homologouschromosomes and their sister chromatids randomly segregate into thefour different gametes based on their positions during meiosis I, leading tothe two alternative pathways. The properties of the allele (dominant orrecessive) do not influence the location of the alleles in gametes.

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Which allele ends up in which gamete is random. However, one could predictthe likelihood of the offspring possessing specific alleles (the genotype), andultimately the phenotype, using the rules of inheritance that are governed byprobability. Probability examines how likely it is that an event will occur.

What Is the Probability of a Coin Toss?Every time one flips a standard (fair) coin, the chance of getting heads is 1 in2, or 50% (Figure 2). Each toss is one independent probability event —the outcome of one toss does not impact the outcome of any other tosses.You likely understand the reasoning behind the concept of independentprobability, but it is not so easy to observe. If one flips a coin twice, there is agood chance that it may be heads both times. If one flips a coin 10 times,there is a chance, but a much smaller chance, that it will be heads all 10times. During World War II, a South African statistician, John Kerrich, passedsome of the time he spent as a prisoner of war flipping a coin 10,000 times.

Figure 2: Calculating the probability of several independent events.


The total probability of multiple independent events is calculated by firstdetermining the probability of one of the independent events. In this case,the probability of a single coin toss resulting in heads is one-half, or 50%.Therefore, the total probability of four coin tosses resulting in heads is ½ ×½ × ½ × ½, or one-half to the fourth power.

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He ended up with heads 50.67% of the time. The more events there are, themore the results reflect the predicted probabilities.

Consider a person flipping a coin four times. What is the chance of gettingheads on all four tries? Each of the four tries has a probability of 1/2 that itwill be heads. Since none of the flips will impact the outcome of any of theother flips, you multiply the probability of heads for each flip together todetermine the probability of four heads in a row.

½ x ½ x ½ x ½ = (½)4 = 1/16 = 0.0625 = 6.25%

There is a bit more than a 6% chance of getting heads on all four coin flips.This example demonstrates the multiplication rule for independentprobability.

How does the multiplication rule apply to genetics?A recessive trait is expressed only when the allele coding for the recessivetrait is inherited from both parents (that is, the organism is homozygous forthe recessive trait). If the recessive allele is only inherited from one parent(i.e., the organism is heterozygous), the dominant trait is expressed. InMendel's pea plants, white flowers were recessive, and purple flowers weredominant. Mendel reasoned that if each parent carried the alleles coding forone dominant and one recessive trait, the offspring had a 50% chance ofinheriting the allele coding for the recessive trait from one parent and a 50%chance of inheriting the allele coding for the recessive trait from the otherparent.

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Test Yourself

Imagine that two heterozygous plants, each with one dominant allele and one recessive allele,are mated with each other. Since a plant has a 50% chance of inheriting a particular allele fromeach parent, what is the probability that the offspring of the two plants will inherit two copiesof the allele coding for the recessive trait?

There is a 25% chance that such a plant will have white flowers and,conversely, a 75% chance that it will have purple flowers. The reason thatwhite flowers, the recessive trait, are less likely is that the recessive trait canonly occur under one scenario: namely, the offspring's inheritance of bothrecessive alleles; conversely, there is a greater chance that the offspring willhave purple flowers because purple flowers can result in two different ways:plants that are homozygous for the dominant trait and those that areheterozygous both express the purple flower trait. As a result, when countingplants with different flower colors, we expect that purple flowers should occurin a 3:1 ratio with white flowers.

Mendel wanted to test this hypothetical ratio, but he knew that patterns inprobability are most reliable when the numbers are large. As a result, hemeasured hundreds of offspring from multiple monohybrid crosses overmany generations of pea plants.

Laws of Probability

The Punnett Square

Summary

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Genetics in the Lab

Let the Computer Do the Work

Roll the Dice

Model Organism

In Mendel's Words

Understanding the Spectrum of Dominance

IN THIS MODULE

PRIMARY LITERATURE

Using sterile mates andengineered toxins to beat bugsSuppressing resistance to Bt cotton withsterile insect releases.

Classic paper: How scientistscloned the first mammal (1997)Viable offspring derived from fetal and adultmammalian cells.

Classic paper: Fruit fly researchreveals how complex organismsform (1980)Mutations affecting segment number andpolarity in Drosophila.

SCIENCE ON THE WEB

Simulate a laboratory to discover the geneticinheritance patterns of fruit flies

Use this website to compare observed andexpected frequencies using the chi-squaretest

Observe the odds of rolling a certain number

Explore this NIH website about modelorganisms used in biomedical research

Read Gregor Mendel's original paperExperiments in Plant Hybridization (1865)

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Alleles are never exclusively "dominant" or"recessive."



The Punnett SquareThe Punnett square, invented by geneticist Reginald Punnett, is used toassess probability in genetics (Figure 3). The father's alleles are listed onone side of the square and the mother's on the other side. The cells of thetable represent offspring, each of which receives one allele from the fatherand one from the mother. Drawing a Punnett square is one way to figure outthe probability of parents with particular genotypes having offspring of acertain genotype or phenotype. Punnett squares are often easier to visualizethan mathematical calculations of probability.

Illustrating probability and the Punnett square with Mendel's peas.Not all pea plants with purple flowers have the same set of alleles. A purple-flowered plant may be homozygous, having inherited the dominant allelefrom each parent, or heterozygous, having inherited one dominant and onerecessive allele. What is the probability that a plant with heterozygousparents will also be heterozygous?

A plant is heterozygous when it receives

an allele coding for the dominant trait from the female and an allelecoding for the recessive trait from the male (event A)

or

an allele coding for the dominant trait from the male and an allelecoding for the recessive trait from the female (event B).

These two possibilities are not independent — in fact, they are mutuallyexclusive. In such cases, probabilities are calculated using the addition rule.What is the probability that one of these events will occur?

In this case, the two events have the same likelihood of occurring. Amonohybrid cross between two heterozygotes gives four offspring. Theprobability of event A is one-quarter, and the probability of event B isone-quarter. Applying the addition rule to this situation, we would addone-quarter to one-quarter to conclude that there is a 50% chance (¼ + ¼ =½ ) that one of the offspring will be heterozygous. Thus, if two heterozygotesbreed, half of their offspring, on average, will also be heterozygotes.

Because purple flowers are dominant, this phenotype does not reveal theplant's genotype. What is the probability that a plant is heterozygous if itsparents are heterozygotes with purple flowers? The probability that a plant isheterozygous, given that it has purple flowers, is two-thirds. Note that this ishigher than the chance of the plant being heterozygous when its phenotypeis unknown, which we calculated to be one-half. An easier way to visualizethese probabilities is by taking the alleles of each parent and combining themby means of a Punnett square (Figure 3).

Figure 3: Probability predicts genotype and phenotype in Mendel'smonohybrid cross.

© 2014 Nature Education All rights reserved.

Probability is used to assess the genetic outcomes (both genotype andphenotype of the offspring). This Punnett square is a straightforward wayto visualize the possible combinations of alleles the offspring of twoparents will have. In this example, two individuals that are heterozygous(Pp) are mated. The alleles in the parental gametes are on the outside ofthe square, and each cell of the Punnett square represents the possibleallelic combinations for the offspring. (Each offspring, of course, receivesone allele from the male's gametes (sperm) and one allele from thefemale's gametes (egg).) The cells show that on average three of everyfour offspring should have purple flowers; on average, two out of thosethree will be heterozygous (Pp), and one out of three will be homozygousfor the dominant trait (PP). Only one of four offspring, on average, will havewhite flowers as a result of being homozygous recessive (pp).

How do I use a Punnett square?The body of a fruit fly may be black or brown. Brown is the dominant color (B)and black is the recessive color (b). You can create a Punnett square tofigure out what the offspring will look like given the parents' genotypes

Figure 4: How to draw a Punnett square.


Punnett squares may predict the traits of the offspring that result from aparticular cross.

(Figure 4).

The below instructions describe how to create a Punnett square for a crossbetween a homozygous brown female fly (BB) and a homozygous black malefly (bb). These instructions could also be used for monohybrid and dihybridcrosses. Larger crosses, however, are more complicated.

Step 1. Write down the genotype of the female and male. In this case, thefemale's genotype is BB, and the male's is bb. Count each parent's alleles andmultiply the numbers. The result determines how big the Punnett squareneeds to be. In this case, because there are only two alleles for each parent,there are four possible outcomes of allelic combinations (2 * 2 = 4), and asquare with 4 cells is needed.

Step 2. Draw a square with the required number of cells.

Step 3. List the female's alleles along one side of the square and the male'salleles along the other. In this case, the alleles from the female are written inthe column on the left (B and B) and the alleles from the male are written inthe row on the top (b and b).

Step 4. Copy each combination of alleles that can be received from each

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© 2013 Nature Education All rights reserved. Transcript

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parent in the respective cells. Each B from the female is combined with eachb from the male in each cell, thereby providing the possible genotypes of theoffspring.

Step 5. Determine the phenotypes of the offspring based on their genotypes.In this case, all of the individuals are heterozygous (Bb), and because theallele for brown (B) is dominant over the allele for black (b), all the fruit flies ofthis pairing would be brown.

Crossing Flies: Practicing the Punnett SquarePractice determining phenotypes and phenotypic ratios expected from theoffspring (F1 generation) and the offspring's offspring (F2 generation) of amonohybrid cross between a brown female with dominant homozygousalleles (BB) and a black male with recessive homozygous alleles (bb) (Figure5).

Figure 5: Monohybrid crosses.Determine the phenotypes and phenotypic ratio expected from theoffspring (F1 generation) and the offspring's offspring (F2 generation) of amonohybrid cross between a brown female with dominant homozygousalleles (BB) and a black male with recessive homozygous alleles (bb).

Test Yourself

What are the possible genotypes that could produce a brown phenotype? What about a blackphenotype?

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Figure 6: Dihybrid Cross.


When both parents are heterozygous for two traits, the offspring may haveany of four possible phenotypes (brown body and red eyes, brown bodyand brown eyes, black body and red eyes, and black body and browneyes). Note that allele combinations for a dihybrid cross are made bydetermining the possible combinations of alleles from the two differentgenes.

Using Punnett squares for dihybrid crosses.The F2 generation in the interactive in Figure 5 represented a monohybridcross, which is a pairing of two parents that are both heterozygous for onetrait (in this case body color). What if the parents differ not just in one trait butin two or more? Fruit flies may have brown or black bodies and brown or redeyes. A dihybrid cross is a cross between parents that are heterozygous fortwo traits. Although a Punnett square for a dihybrid cross is more complexthan Punnett squares for monohybrid crosses, the purpose of the square isthe same: determining the probable genotypes and phenotypes of theoffspring.

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Figure 7: Test cross.


Crossing an organism of unknown genotype with one expressingrecessive traits will determine the genotype of the unknown parent (i.e.,whether it is homozygous or heterozygous).

Test Yourself

What is the phenotypic ratio among the offspring of the dihybrid cross shown in Figure 6?

What is a test cross?In Mendel's peas, the purple trait is dominant, so a purple flower could eitherbe homozygous (PP) or heterozygous (Pp) for the purple allele. How couldyou experimentally determine the genotype if you had a pea plant and allthat you knew about it was that it had a purple flower? In a test cross, thepurple flower, whose genotype is unknown, is bred with a white flowered peaplant that expresses the recessive trait (Figures 7 and 8). If the purple plantis homozygous, all of the offspring will be purple because they will all haveat least one copy of the dominant purple allele. If the parent plant isheterozygous, having both an allele coding for the dominant trait and anallele coding for the recessive trait, half of the offspring will be white and theother half will be purple. Therefore, you could determine whether the plantwith an unknown genotype is heterozygous or homozygous by looking at thephenotypes of the offspring from a test cross.

Figure 8: Test-cross problem.

© 2013 Nature Education All rightsreserved.

What is a brown female parent'sgenotype if half of her offspring arebrown (B), the other half are black (b),the black allele is recessive, and themale parent is black (bb)?

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Test Yourself

In a cross between a brown female fly and a black male fruit fly, if half of the offspring areblack, what is the female's genotype? What if all of the offspring are brown?

A Chi-Square Test Uses Probability to Detect Differences betweenExpected and Observed ValuesThe predictions made in Mendel's monohybrid and dihybrid cross are basedon some assumptions that certain events are random and will occur with acertain probability. For example, it assumes that alleles randomly andindependently assort into gametes and that fertilization between gametes israndom. Therefore, the simple predictions from Mendelian genetics are notalways observed. If the observed data differ substantially from what wouldhave been expected from our Punnett square and probability calculations,we can infer that the simple assumptions, such as independent assortment,are not true. When we assume that the observed data of a genetic cross willfit a given Mendelian ratio, a null hypothesis (H0) is generated, which is thatthere will be no real difference between the measured values (or ratio) andthe expected values (or ratio). The null hypothesis can either be rejected ornot rejected for a given set of data measured using statistical analysis.

One of the simplest statistical tests for assessing whether the observedvalues differ substantially from those expected is the chi-square test, oftenabbreviated χ2. In figure 9, a chi-square test is illustrated using a crossbetween genes for body color and wing type in fruit flies. In these genes,brown bodies (B) and normal wings (W) are dominant traits, and black bodies(b) and vestigial wings (w) are recessive traits. A female fruit fly heterozygousfor brown body and normal wings (BbWw) was crossed with a male fruit flywith a black body and vestigial wings (bbww) (Figure 9).

Figure 9: Crossing a heterozygous dominant female with ahomozygous recessive male.

© 2013 Nature Education All rights reserved.Four equally probable phenotypes are expected in the offspring.

Figure 10: Data for χ2 analysis.A chi-square analysis is used to determine if the observed counts aresignificantly different from the expected counts for each of these four fruitfly phenotypes.

If the assumptions are that these two genes are independent and randomlyassort into gametes, we can predict the proportions of genotypes andphenotypes using a Punnett square as above. In this cross, we expect a1:1:1:1 ratio of the following phenotypes: brown body, normal wings; blackbody, vestigial wings; black body, normal wings; and brown body, vestigialwings. That is, on average, one-quarter (25%) of the offspring from this crossshould be in each of the four phenotype categories. If a scientist breeds2,300 fruit flies, each phenotype should be represented by approximately575 flies (one-quarter expected in each group x 2,300 flies = 575 flies pergroup). Remember that it may take many crosses before a reliable estimateof proportions can be calculated (if the scientist only bred five or 10individuals, the proportions would almost certainly differ from the expectedratios, but this could have easily happened by chance). However, in thiscase, the observed results look rather different from the results that would beexpected from random processes (Figure 10). Are these results significantlydifferent than expected?

© 2011 Nature Education All rights reserved.

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This is where the chi-square test comes in. The formula is

χ2 = the sum of ([observed count - expected count]2/expected count)

If the observed counts were exactly as expected, the χ2 value will be 0. Asthe observed results increasingly differ from the expected results, the χ2

value increases. To interpret the χ2 result, the degrees of freedom (df) mustbe also determined, which are equal to "n-1," where "n" is the number ofdifferent categories into which the data are divided. In the example above,there are 4 possible phenotypes, and thus "n" is 4; the df is 3 (4-1).

A χ2 probability table is used to determine the statistical significance of the χ2

value we calculate; that is, the table tells us what the probability is that ourobserved values differ from the expected value by chance alone. A highprobability (p), such as p > 0.5, tells us that there is more than a 50% chancethat the differences that we found between the observed and the expectedresults could have been simply due to chance. This would not give us muchconfidence that the results were in fact significantly different and we wouldnot reject the null hypothesis that any observed differences were simply dueto chance. To find the significance value, match the df and χ2 values in a χ2

probability distribution table (available in most statistical textbooks or online).One common statistical standard is p < 0.05, which means that there is lessthan a 5% probability that the observed results would have occurred simplyby chance and were due to random sampling; as a corollary, there is agreater than 95% probability that the observed results are due tonon-random factors, and that the null hypothesis can be rejected.

For these fruit fly crosses, χ2 = 1,002.65, and df = 3. For three degrees offreedom, the χ2 value that must be found for there to be only a 5% that thedifferences are due to chance (p < 0.05) is 7.82. That is, any χ2 value greaterthan 7.82 has less than a 5% probability of occurring by chance. Our χ2

value of more than 1,000 is much greater than 7.82! In other words, thisresult is highly unlikely to have occurred by chance. What could account for it?

Observed phenotypic ratios often differ from those expected by Mendelianinheritance. This is because many genes are not inherited according toMendel's three principles of inheritance (dominance, segregation andindependent assortment). For example, some genes do not code for cleardominant or recessive traits, violating the principle of dominance. In addition,genes that are near each other on the same chromosome are sometimeslinked so that they are not sorted independently into gametes, violating theprinciple of independent assortment. Although Mendelian inheritance onlyapplies to a minority of genes, Mendel's work pioneered the quantitativegenetic methods that allowed the discovery of non-Mendelian traits.

What Makes a Suitable Model Organism?A model organism is one that is well suited to scientific research.Experiments using model organisms help scientists understand thefundamentals of biological principles, which can then be extended to otherorganisms, including humans. Gregor Mendel used pea plants as a modelorganism in the mid-1800s, and in the early 1900s Thomas Morgan used thefruit fly Drosophila melanogaster. Other organisms now commonly used in geneticresearch include Caenorhabditis elegans (a nematode worm), Arabidopsis thaliana (asmall plant in the mustard family), mice and yeast. What makes theseorganisms useful as models?

Pea plants are inexpensive and easy to grow from seeds. Theymature quickly, produce many seeds, and show a number of visible


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variations in flower color, seed color and shape. Gregor Mendel alsocut the flowers' male reproductive appendages, or stamens, to onlyallow cross-pollination. This allowed the parents of the offspring to beknown and traced. Today, the most common plant used as a modelorganism is Arabidopsis thaliana, which shares some of the sameadvantages as pea plants, such as high variability in traits, but is alsomuch smaller and can grow more rapidly. It also has a small genomewith relatively few chromosomes, making modern genetic analysesmore streamlined.Drosophila melanogaster and Caenorhabditis elegans offer many of the samebenefits described above. These invertebrates are small and easy toraise in a lab. Both lay many eggs, and their eggs mature quickly,allowing many generations of inheritance to be observed over a shortperiod of time. Drosophila, in particular, have many visible variablecharacteristics, such as wing type, body color and eye color (asmentioned above), which allow their genotypes to be traced. Fruitflies also have relatively small genomes with only four pairs ofchromosomes.Mice are mammals and share many characteristics with humans. Infact, about 75% of rodent genes share largely the same function asthose in humans, making mice genetically closer to humans than anyof the other common model organisms. Mice may be used toexamine questions that researchers are interested in but that wouldbe unethical to test using human subjects. The mouse genome isalso useful in research because genes may be "knocked out," anexperimental technique in which scientists inactivate a target gene orreplace it with an inactivated mutant version to see how its absenceaffects the organism. The other model organisms may also have theirgenes knocked out to address important questions in gene function.The simplicity of baker's yeast (Saccharomyces cerevisiae), a eukaryoticorganism that is unicellular, has made it useful for studying celldivision and gene regulation — how genes are turned "on" and "off."It is the easiest eukaryotic genome to manipulate because it is sosimple compared to other eukaryotes, but its genes still share manyof the basic functions of more complex eukaryotes. This helpsscientists identify how genes work on a cellular level in yeast, andapply that understanding to organisms like humans.

Test Yourself

Why have so many genetic experiments been done on fruit flies? Name two characters thatmake fruit flies a good model organism.

Laws of Probability

The Punnett Square

Summary

Test Your Knowledge

IN THIS MODULE



OBJECTIVE Analyze a basic Punnett square problem.Punnett squares are a way to visualize the probabilities of genotypes andphenotypes in offspring using parental genotypes. For example, using aPunnett square to analyze a monohybrid cross, a phenotypic ratio of 3:1dominant to recessive in offspring may be predicted. In this case, one-third ofthe offspring with the dominant phenotype are homozygous, and two-thirdsare heterozygous for the trait.

OBJECTIVE Analyze a test cross problem.To identify the unknown genotype of an organism showing the dominantphenotype, a test cross of an organism expressing the dominant phenotypeis made with an organism displaying recessive traits. The distribution ofphenotypes in the offspring will reveal whether the unknown parent genotypeis heterozygous or homozygous. A chi-square test is used to evaluatewhether the difference between the observed results and the expectedresults of a test cross is likely to have occurred by chance or whether itrepresents an inheritance pattern different from the one predicted.

OBJECTIVE Determine probabilities in monohybrid and dihybrid crosses.The principle of independent assortment states that when gametes form,each pair of alleles assorts independently. Which offspring inherit whichalleles is random, but we are able to predict the likelihood of phenotypes inthe offspring based on the parents' genotypes and the rules of probability. Ina monohybrid cross, the ratio of the phenotypes in the offspring is 3:1dominant:recessive, or three-quarters dominant and one-quarter recessive.In a dihybrid cross, the ratio of phenotypes in the offspring is 9:3:3:1(dominant, dominant):(dominant, recessive):(recessive, dominant):(recessive, recessive); stated differently, 9/16 of the offspring have bothdominant phenotypes, 3/16 of the offspring have the dominant phenotype forgene 1 and the recessive phenotype for gene 2, 3/16 have the recessivephenotype for gene 1 and the dominant phenotype for gene 2, and 1/16 ofthe offspring have both recessive phenotypes.

addition ruleConsidering mutually exclusive events, the probability of both occurring is the sumof the probabilities of each event.

heterozygousHaving two different alleles for a trait.

homozygousHaving identical alleles for a trait.

independent probabilityProbability of one event occurring has no effect on the probability of the other.

model organismAn organism that is well suited for scientific research for a variety of reasons.

multiplication ruleIf the events are truly independent, the probability of both events happening isfound by multiplying the probabilities of each event.

null hypothesisA hypothesis assuming that there is no difference between the observed values

Summary

Key Terms


and the expected values.

probabilityCalculation of the chance of an event occurring.

Punnett squareTable method used to assess probabilities of genotypic and phenotypic outcomesamong genetic crosses.

test crossA cross between an organism with a dominant phenotype but an unknowngenotype and an individual that is homozygous recessive.

Laws of Probability

The Punnett Square

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Genetics in the Lab


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Model Organism

In Mendel's Words


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1.

40%25%66%10%50%

Huntington's disease, a progressive nervous system disorder, is inherited as adominant trait. The songwriter Woody Guthrie died of Huntington's disease. If hewas heterozygous for the trait and his wife did not carry it, what was the probabilitythat their son would develop Huntington's disease?

2.

2/33/41/21/41/3

If both parents are heterozygous for a trait, what is the probability that a givenoffspring of theirs will display the recessive trait? Assume the gene for the trait isnot sex-linked.

3.

1/2 BB, 1/2 bb1/4 bb, 3/4 Bball Bb1/4 bb, 1/2 Bb, 1/4 BB1/3 BB, 2/3 Bb

In fruit flies, brown body color (B) is dominant to black body color (b). A brown fruitfly is crossed with a black fruit fly. Both flies are purebred. What will the offspring'sgenotypes be?

4.

This cannot be predicted based on the information provided.50%75%25%100%

In humans, dry earwax is recessive to wet earwax. Consider a woman with dryearwax and her male partner who has wet earwax. If he is heterozygous for thetrait of wet earwax, what is the probability that their child will have wet earwax?

5.

yellow-roundyellow-wrinkledgreen-wrinkledgreen-roundany type

In peas, yellow color and round shape are dominant to green color and wrinkledshape, which are recessive. If you have a round, yellow pea and want to know itsgenotype, what kind of pea should you breed it with?

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Laws of Probability

The Punnett Square

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Genetics in the Lab


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Model Organism

In Mendel's Words


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SCIENCE ON THE WEB







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