Punnett Squares

7 Key excerpts on "Punnett Squares"

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  Genetics For Dummies

  • Rene Fester Kratz, Lisa Spock(Authors)
  • 2023(Publication Date)
  • For Dummies

    (Publisher)

  Independent assortment means that every offspring also has the same opportunity to inherit any allele at any other locus (although this rule does have some excep- tions, which are described in Chapter 4). Predicting with Punnetts Mendel’s discoveries opened the door to the science of genetics. One British geneticist who continued Mendel’s studies was Reginald C. Punnett. Although Punnett made several important contributions to genetics, he is almost certainly best remembered for his invention of the Punnett square, a useful tool for tracking the movement of factors (alleles) through the generations of a cross. When you know the genotype of parents in a cross, a Punnett square can help you predict the numbers and types of phenotypes you will see in their offspring. You can see how Punnett Squares work in Figure 3-3. To track the alleles in a cross, make a square with the same number of columns and rows as the number of alleles in the parents. For a single gene, that would be two columns and two rows: 1. Along the outside edges of the square, write the symbols for the gametes that each parent can produce. Remember to segregate the alleles; each gamete only gets one copy of each gene. 46 PART 1 The Lowdown on Genetics: Just the Basics 2. Fill in the boxes of the square by combining the alleles from the gametes you write along the edges. By filling in the square, you predict all the possible combinations of gametes that could occur in the next generation. In order to get results that match the prediction from a Punnett square, you need to observe lots of offspring just like Mendel did in his experiments. For example, I can use a Punnett square to predict that two human parents are likely to produce half female children and half male children, but I won’t see this result if the couple has only one child.

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  Babies by Design

  The Ethics of Genetic Choice

  • Ronald M. Green(Author)
  • 2008(Publication Date)
  • Yale University Press

    (Publisher)

  Drawing Lines Reginald Crundall Punnett, a British geneticist who worked during the first half of the twentieth century, spent much of his life studying sweet peas and domestic fowl. His name lives on not so much for that work but for a visual aid he invented that geneticists still use to com-municate parents’ chances of passing on a classic Mendelian trait (or disease) to their children. A Punnett Square consists of a large rectan-gle subdivided into four equal compartments, two above, two below. A mother carries two versions of a genetic trait, one on each of her two chromosomes. These versions are inscribed above the top two com-partments, with the dominant trait—the one likely to manifest itself in the offspring if even a single copy is transmitted—to the left. The fa-ther’s two versions are inscribed next to the two left compartments, with the dominant one above the other. Filling each box in the square with the corresponding maternal and paternal versions of a trait makes it easy to see the possible combinations of dominant or recessive gene variants that could crop up in any four of the parents’ offspring. Figure 1 is a classic Punnett Square for a genetic trait inherited in Mendelian fashion, in this case eye color. Here the mother and father are both hybrid for brown eyes; that is, each has a gene for brown and blue on the two chromosomes, but their eyes are brown because the 53 chapter 3 trait that is dominant prevails in a hybrid mix. “Brown” is capitalized in the Punnett Square to show its dominance. The square reveals that out of every four children these parents have, the odds are that one child will have brown eyes, with two copies of that gene, one from each par-ent; one child will have blue eyes; and two children will be brown-eyed hybrids like their parents. Do geneticists naturally think in Punnett Squares? I doubt it.

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  Biology for AP® Courses

  • Julianne Zedalis, John Eggebrecht(Authors)
  • 2018(Publication Date)
  • Openstax

    (Publisher)

  The Punnett Square Approach for a Monohybrid Cross When fertilization occurs between two true-breeding parents that differ in only one characteristic, the process is called a Chapter 12 | Mendel's Experiments and Heredity 479 monohybrid cross, and the resulting offspring are monohybrids. Mendel performed seven monohybrid crosses involving contrasting traits for each characteristic. On the basis of his results in F 1 and F 2 generations, Mendel postulated that each parent in the monohybrid cross contributed one of two paired unit factors to each offspring, and every possible combination of unit factors was equally likely. To demonstrate a monohybrid cross, consider the case of true-breeding pea plants with yellow versus green pea seeds. The dominant seed color is yellow; therefore, the parental genotypes were YY for the plants with yellow seeds and yy for the plants with green seeds, respectively. A Punnett square, devised by the British geneticist Reginald Punnett, can be drawn that applies the rules of probability to predict the possible outcomes of a genetic cross or mating and their expected frequencies. To prepare a Punnett square, all possible combinations of the parental alleles are listed along the top (for one parent) and side (for the other parent) of a grid, representing their meiotic segregation into haploid gametes. Then the combinations of egg and sperm are made in the boxes in the table to show which alleles are combining. Each box then represents the diploid genotype of a zygote, or fertilized egg, that could result from this mating. Because each possibility is equally likely, genotypic ratios can be determined from a Punnett square. If the pattern of inheritance (dominant or recessive) is known, the phenotypic ratios can be inferred as well. For a monohybrid cross of two true-breeding parents, each parent contributes one type of allele. In this case, only one genotype is possible. All offspring are Yy and have yellow seeds (Figure 12.4).

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  Visualizing Human Biology

  • Kathleen A. Ireland(Author)
  • 2018(Publication Date)
  • Wiley

    (Publisher)

  Punnett Squares predict phenotypic ratios Punnett Squares predict the phenotypic ratios that Mendel observed in his pea plant experiments. Crossing a homozygous dominant individual and a homozygous recessive individual yields 100% heterozygous offspring, regardless of the trait. All of the offspring will express the dominant trait. Self-pollinating these heterozygotes yields three phenotypically dominant offspring and one phenotypically recessive individual (who has a homozygous recessive genotype). The same Punnett square can be used to represent flower color in peas or attached earlobes in humans. It is amazing that Mendel accurately explained this using his heritable unit theory without any knowledge of genes or chromo- somes. Even with inheritance patterns of codominance or incomplete dominance, Punnett Squares predict the proportions of potential gen- otypes of the offspring. The phenotypic expression of those genes may not yield the typical 3:1 or 9:3:3:1 ratios expected by Mendel, but the genotype ratios remain the same. Concept Check 1. What does the phenotype of a heterozygous individual show? 2. What are multifactorial traits? Codominant traits? 3. What can be learned from a Punnett square? autosomal Any chromosome other than the sex chromosomes, X and Y. asymptomatic Without symptoms. Genetic Theory Is Put to Practical Use 475 Some Traits Are Sex Linked As you know, humans have one pair of chromosomes, called the sex chromosomes, that do not match in terms of size or content. The sex chromosomes include the large X chromosome and the smaller Y. These two determine gender. If a Y chromosome is present during development (XY), the fetus will become a male. If there is no Y present (XX), the fetus becomes a female. Females have two copies of every gene on the X chromosome Since only one copy of each allele is needed during normal growth and development, one X chromosome is ran- domly shut down.

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  DAT Prep Plus 2023-2024

  2 Practice Tests + Proven Strategies + Online

  • (Author)
  • 2023(Publication Date)
  • Kaplan Test Prep

    (Publisher)

  The parental genotypes are arranged around a grid. Because the genotype of each progeny will be the sum of the alleles donated by the parental gametes, their genotypes can be determined by looking at the intersections on the grid. A Punnett square indicates all the potential progeny genotypes, and the relative frequencies of the different genotypes and phenotypes can be easily calculated. Figure 8.1 When the F1 generation from our monohybrid cross is self-crossed (i.e., Pp × Pp), the F2 progeny are more genotypically and phenotypically diverse than their parents. Because the F1 plants are heterozygous, they will donate a dominant, P, allele to half of their descendants and a recessive, p, allele to the other half. One-fourth (25 percent) of the F2 plants will have the homozygous dominant, PP, genotype, 50 percent will have the heterozygous, Pp, genotype, and 25 percent will have the homozygous recessive, pp, genotype. Because the homozygous dominant and heterozygous genotypes both produce the dominant phenotype purple flowers, 75 percent of the F2 plants will have purple flowers, and 25 percent will have white flowers. This is a standard pattern of Mendelian inheritance. Its hallmarks are the disappearance of the silent (recessive) phenotype in the F1 generation and its subsequent reappearance in 25 percent of the individuals in the F2 generation. If we were to take a closer look at the physical characteristics of the plants themselves, we would find that the 1:2:1 genotypic ratio produces a 3:1 phenotypic ratio. Figure 8.2 Testcross Mendel also developed the testcross, a diagnostic tool used to determine the genotype of an organism. Only with a recessive phenotype can genotype be predicted with 100 percent accuracy. If the dominant phenotype is expressed, the genotype can be either homozygous dominant or heterozygous. Thus, homozygous recessive organisms always breed true

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  Introduction to Genetics

  11th Hour

  • Sandra Pennington(Author)
  • 2009(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Stratton's parents were first cousins, a mating that increases the likelihood of producing homozygous recessive children (see Chapter 3). This points 10 one of the common modern uses of genetic information, genelic counseling, where knowledge of the genetic basis for diseases can help members of affected families know the likelihood that they or their children will inherit the disease. Topic 7: Mendel and Meiosis 17 Table 1.2 Results for Five Matings of Specific Plants PROGENY PHENOTYPES MATING PARENTAL PHENOTYPES WHITE SQUARE RED SQUARE WHITE ROUND REO ROUND 1 red square x white square 0 761 0 254 2 red round x white square 210 202 196 199 3 red square x red round 49 158 54 153 4 red square x white square 225 231 76 80 s red square x white round 206 200 0 0 DEMONSTRATION PROBLEM Question: Flowers of a panicular plant can be either red or white, and be square or round. Each of these traiLS is determined by a single gene. The results for five matings of specific plants (not necessarily !.rue breeders) are shown in Table 1.2. Which alleles are dominant and which are recessive? What are the most probable genotypes for the parents in each cross? Answer: The best approach is to consider each character separately. The square allele is domi- nant to the round allele, which is recessive. This can be seell in crosst:s 1,4, and 5, where square crossed with square produces the phenotypic ratio three square to one round, so the square parents must be heterozygous. In the square-with-round cross (5), only square-flower progeny were produced; therefore, both parents were homozygous and all of the progeny are heterozygous, with square flowers. Red is dominant to whitc. This is easiest to see in crosses I and 3. For the genotypes, R = square, r = round, W = red, w = white. The best approach here is to consider each character separately and assign the recessive homozygous genotypes first, then decide which of the dominant parental phenotypes must be heterozygous.

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  Molecular & Cell Biology For Dummies

  • Rene Fester Kratz(Author)
  • 2020(Publication Date)
  • For Dummies

    (Publisher)

  Gametes combine randomly during sexual reproduction. In other words, certain sperm don’t seek out certain eggs — it’s just a sort of reproductive free-for-all to see who gets lucky. Because fertilization is random, inheritance follows the rules of probability.

  You can use the rules of probability to predict the outcome of a cross in the same way that you use a Punnett square to make predictions. Here are the rules of probability that apply to problems of inheritance:

  • The rule of multiplication says that if two events are independent of each other, then the probability that they will both occur at the same time is the product of their individual probabilities. You can use the rule of multiplication to predict the outcome of a cross, such as a monohybrid cross (Tt × Tt). For example, you can answer the question “What is the chance that a homozygous recessive offspring (tt) will be produced from a monohybrid cross?” by completing the following steps:
    1. Determine the probability of each gamete type that the parents can produce. The parents are heterozygous (Tt), so half their gametes will receive the recessive allele and half the gametes will receive the dominant allele. The probability that each parent would donate each type of allele is one-half (½).
    2. Figure out how many ways this cross could produce the offspring in the question. In order for a homozygous recessive offspring to be produced, each parent would have to donate a recessive allele. So, these parents could produce a homozygous recessive offspring in only one way.
    3. Multiply the independent probabilities together. The probability that one parent would donate the recessive allele is independent of the probability that the other parent would donate the recessive allele. (After all, they’re both making gametes independently in their own bodies.) So, the total probability of creating a homozygous recessive offspring (tt) in this cross is the probability that one parent would donate the recessive allele (½) times the probability that the other parent would donate the recessive allele (½), which equals ¼. The rules of probability predict that one out of four offspring from this cross would be homozygous recessive. If you compare this result to the lower Punnett square in Figure 15-1

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