Characteristics and Traits Quiz

PDF page 488: Two alleles for a given gene in a diploid organism are expressed and interact to produce physical characteristics. The observable traits expressed by an organism are referred to as its phenotype. An organism’s underlying genetic makeup, consisting of both physically visible and non-expressed alleles, is called its genotype. Mendel’s hybridization experiments demonstrate the difference between phenotype and genotype. When true-breeding plants in which one parent had yellow pods and one had green pods were cross-fertilized, all of the F1 hybrid offspring had yellow pods. That is, the hybrid offspring were phenotypically identical to the true-breeding parent with yellow pods. However, we know that the allele donated by the parent with green pods was not simply lost because it reappeared in some of the F2 offspring. Therefore, the F1 plants must have been genotypically different from the parent with yellow pods. The P1 plants that Mendel used in his experiments were each homozygous for the trait he was studying. Diploid organisms that are homozygous at a given gene, or locus, have two identical alleles for that gene on their homologous chromosomes. Mendel’s parental pea plants always bred true because both of the gametes produced carried the same trait. When P1 plants with contrasting traits were cross-fertilized, all of the offspring were heterozygous for the contrasting trait, meaning that their genotype reflected that they had different alleles for the gene being examined. Dominant and Recessive Alleles Our discussion of homozygous and heterozygous organisms brings us to why the F1 heterozygous offspring were identical to one of the parents, rather than expressing both alleles. In all seven pea-plant characteristics, one of the two contrasting alleles was dominant, and the other was recessive. Mendel called the dominant allele the expressed unit factor; the recessive allele was referred to as the latent unit factor. We now know that these so-called unit factors are actually genes on homologous chromosome pairs. For a gene that is expressed in a dominant and recessive pattern, homozygous dominant and heterozygous organisms will look identical (that is, they will have different genotypes but the same phenotype). The recessive allele will only be observed in homozygous recessive individuals (Table 12.4).

PDF page 486: The characteristics that Mendel evaluated in his pea plants were each expressed as one of two versions, or traits (e.g., green peas versus yellow peas). As we will explore in more detail in later chapters, the physical expression of characteristics is accomplished through the expression of genes (sequences of DNA), carried on chromosomes. The genetic makeup of peas consists of two similar, or homologous (remember this term from Chapter 11), copies of each chromosome, one from each parent. Through meiosis, diploid organisms utilize meiosis to produce haploid (1n) gametes that participate in fertilization. For cases in which a single gene controls a single characteristic, such as pea color, a diploid organism has genetic copies that may or may not encode the same version of the characteristic. These gene variations (e.g., green peas versus yellow peas) are called alleles. Different alleles for a given gene in a diploid organism interact to express physical characteristics such as pea color in plants or hairline appearance in humans. The observable traits of an organism are referred to as its phenotype. The organism’s underlying genetic makeup, i.e., the combination of alleles, is called its genotype. When diploid organisms carry the same alleles for a given trait, they are said to be homozygous for the genotype; when they carry different alleles, they are said to be heterozygous. For a gene whose expression is Mendelian (Section 12.1), homozygous dominant and heterozygous organisms will look identical; that is, they will have different genotypes but the same phenotype. The recessive allele will only be observed in homozygous recessive individuals. However, alleles do not always behave in dominant and recessive patterns. In other words, there are exceptions to Mendel’s model of inheritance. For example, incomplete dominance describes situation in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes (e.g., a pink-flowered offspring is produced from a cross between a red-flowered parent and a white-flowered parent). Codominance describes the simultaneous expression of both of the alleles in the heterozygote (e.g., human blood types). It is also common for more than two alleles of a gene to exist in a population (e.g., variations in sizes of pumpkins). In humans, as in many animals and some plants, females have two X chromosomes, and males have one X chromosome and one Y chromosome. Genes on the X chromosome are X-linked, and males inherit and express only one allele for the gene (e.g., hemophilia, color-blindness). Some alleles can also be lethal, so their phenotype will never be observed. Many human genetic disorders, including albinism, cystic fibrosis, and Huntington’s disease can be explained on the basis of simple Mendelian inheritance patterns created by pedigree analysis. (In later Chapters we will learn how DNA analysis can be used to diagnose genetic disorders). Punnett squares are useful tools that apply the rules of probability and meiosis to predict the possible outcomes of genetic crosses. Test crosses are done to determine whether or not an individual is homozygous or heterozygous by crossing the individual with a homozygous recessive.

PDF page 488: Two alleles for a given gene in a diploid organism are expressed and interact to produce physical characteristics. The observable traits expressed by an organism are referred to as its phenotype. An organism’s underlying genetic makeup, consisting of both physically visible and non-expressed alleles, is called its genotype. Mendel’s hybridization experiments demonstrate the difference between phenotype and genotype. When true-breeding plants in which one parent had yellow pods and one had green pods were cross-fertilized, all of the F1 hybrid offspring had yellow pods. That is, the hybrid offspring were phenotypically identical to the true-breeding parent with yellow pods. However, we know that the allele donated by the parent with green pods was not simply lost because it reappeared in some of the F2 offspring. Therefore, the F1 plants must have been genotypically different from the parent with yellow pods. The P1 plants that Mendel used in his experiments were each homozygous for the trait he was studying. Diploid organisms that are homozygous at a given gene, or locus, have two identical alleles for that gene on their homologous chromosomes. Mendel’s parental pea plants always bred true because both of the gametes produced carried the same trait. When P1 plants with contrasting traits were cross-fertilized, all of the offspring were heterozygous for the contrasting trait, meaning that their genotype reflected that they had different alleles for the gene being examined. Dominant and Recessive Alleles Our discussion of homozygous and heterozygous organisms brings us to why the F1 heterozygous offspring were identical to one of the parents, rather than expressing both alleles. In all seven pea-plant characteristics, one of the two contrasting alleles was dominant, and the other was recessive. Mendel called the dominant allele the expressed unit factor; the recessive allele was referred to as the latent unit factor. We now know that these so-called unit factors are actually genes on homologous chromosome pairs. For a gene that is expressed in a dominant and recessive pattern, homozygous dominant and heterozygous organisms will look identical (that is, they will have different genotypes but the same phenotype). The recessive allele will only be observed in homozygous recessive individuals (Table 12.4).

PDF page 486: The characteristics that Mendel evaluated in his pea plants were each expressed as one of two versions, or traits (e.g., green peas versus yellow peas). As we will explore in more detail in later chapters, the physical expression of characteristics is accomplished through the expression of genes (sequences of DNA), carried on chromosomes. The genetic makeup of peas consists of two similar, or homologous (remember this term from Chapter 11), copies of each chromosome, one from each parent. Through meiosis, diploid organisms utilize meiosis to produce haploid (1n) gametes that participate in fertilization. For cases in which a single gene controls a single characteristic, such as pea color, a diploid organism has genetic copies that may or may not encode the same version of the characteristic. These gene variations (e.g., green peas versus yellow peas) are called alleles. Different alleles for a given gene in a diploid organism interact to express physical characteristics such as pea color in plants or hairline appearance in humans. The observable traits of an organism are referred to as its phenotype. The organism’s underlying genetic makeup, i.e., the combination of alleles, is called its genotype. When diploid organisms carry the same alleles for a given trait, they are said to be homozygous for the genotype; when they carry different alleles, they are said to be heterozygous. For a gene whose expression is Mendelian (Section 12.1), homozygous dominant and heterozygous organisms will look identical; that is, they will have different genotypes but the same phenotype. The recessive allele will only be observed in homozygous recessive individuals. However, alleles do not always behave in dominant and recessive patterns. In other words, there are exceptions to Mendel’s model of inheritance. For example, incomplete dominance describes situation in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes (e.g., a pink-flowered offspring is produced from a cross between a red-flowered parent and a white-flowered parent). Codominance describes the simultaneous expression of both of the alleles in the heterozygote (e.g., human blood types). It is also common for more than two alleles of a gene to exist in a population (e.g., variations in sizes of pumpkins). In humans, as in many animals and some plants, females have two X chromosomes, and males have one X chromosome and one Y chromosome. Genes on the X chromosome are X-linked, and males inherit and express only one allele for the gene (e.g., hemophilia, color-blindness). Some alleles can also be lethal, so their phenotype will never be observed. Many human genetic disorders, including albinism, cystic fibrosis, and Huntington’s disease can be explained on the basis of simple Mendelian inheritance patterns created by pedigree analysis. (In later Chapters we will learn how DNA analysis can be used to diagnose genetic disorders). Punnett squares are useful tools that apply the rules of probability and meiosis to predict the possible outcomes of genetic crosses. Test crosses are done to determine whether or not an individual is homozygous or heterozygous by crossing the individual with a homozygous recessive.

PDF page 486: The characteristics that Mendel evaluated in his pea plants were each expressed as one of two versions, or traits (e.g., green peas versus yellow peas). As we will explore in more detail in later chapters, the physical expression of characteristics is accomplished through the expression of genes (sequences of DNA), carried on chromosomes. The genetic makeup of peas consists of two similar, or homologous (remember this term from Chapter 11), copies of each chromosome, one from each parent. Through meiosis, diploid organisms utilize meiosis to produce haploid (1n) gametes that participate in fertilization. For cases in which a single gene controls a single characteristic, such as pea color, a diploid organism has genetic copies that may or may not encode the same version of the characteristic. These gene variations (e.g., green peas versus yellow peas) are called alleles. Different alleles for a given gene in a diploid organism interact to express physical characteristics such as pea color in plants or hairline appearance in humans. The observable traits of an organism are referred to as its phenotype. The organism’s underlying genetic makeup, i.e., the combination of alleles, is called its genotype. When diploid organisms carry the same alleles for a given trait, they are said to be homozygous for the genotype; when they carry different alleles, they are said to be heterozygous. For a gene whose expression is Mendelian (Section 12.1), homozygous dominant and heterozygous organisms will look identical; that is, they will have different genotypes but the same phenotype. The recessive allele will only be observed in homozygous recessive individuals. However, alleles do not always behave in dominant and recessive patterns. In other words, there are exceptions to Mendel’s model of inheritance. For example, incomplete dominance describes situation in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes (e.g., a pink-flowered offspring is produced from a cross between a red-flowered parent and a white-flowered parent). Codominance describes the simultaneous expression of both of the alleles in the heterozygote (e.g., human blood types). It is also common for more than two alleles of a gene to exist in a population (e.g., variations in sizes of pumpkins). In humans, as in many animals and some plants, females have two X chromosomes, and males have one X chromosome and one Y chromosome. Genes on the X chromosome are X-linked, and males inherit and express only one allele for the gene (e.g., hemophilia, color-blindness). Some alleles can also be lethal, so their phenotype will never be observed. Many human genetic disorders, including albinism, cystic fibrosis, and Huntington’s disease can be explained on the basis of simple Mendelian inheritance patterns created by pedigree analysis. (In later Chapters we will learn how DNA analysis can be used to diagnose genetic disorders). Punnett squares are useful tools that apply the rules of probability and meiosis to predict the possible outcomes of genetic crosses. Test crosses are done to determine whether or not an individual is homozygous or heterozygous by crossing the individual with a homozygous recessive.

PDF page 488: at fertilization to create a diploid zygote. For cases in which a single gene controls a single characteristic, a diploid organism has two genetic copies that may or may not encode the same version of that characteristic. Gene variants that arise by mutation and exist at the same relative locations on homologous chromosomes are called alleles. Mendel examined the inheritance of genes with just two allele forms, but it is common to encounter more than two alleles for any given gene in a natural population.

PDF page 486: The characteristics that Mendel evaluated in his pea plants were each expressed as one of two versions, or traits (e.g., green peas versus yellow peas). As we will explore in more detail in later chapters, the physical expression of characteristics is accomplished through the expression of genes (sequences of DNA), carried on chromosomes. The genetic makeup of peas consists of two similar, or homologous (remember this term from Chapter 11), copies of each chromosome, one from each parent. Through meiosis, diploid organisms utilize meiosis to produce haploid (1n) gametes that participate in fertilization. For cases in which a single gene controls a single characteristic, such as pea color, a diploid organism has genetic copies that may or may not encode the same version of the characteristic. These gene variations (e.g., green peas versus yellow peas) are called alleles. Different alleles for a given gene in a diploid organism interact to express physical characteristics such as pea color in plants or hairline appearance in humans. The observable traits of an organism are referred to as its phenotype. The organism’s underlying genetic makeup, i.e., the combination of alleles, is called its genotype. When diploid organisms carry the same alleles for a given trait, they are said to be homozygous for the genotype; when they carry different alleles, they are said to be heterozygous. For a gene whose expression is Mendelian (Section 12.1), homozygous dominant and heterozygous organisms will look identical; that is, they will have different genotypes but the same phenotype. The recessive allele will only be observed in homozygous recessive individuals. However, alleles do not always behave in dominant and recessive patterns. In other words, there are exceptions to Mendel’s model of inheritance. For example, incomplete dominance describes situation in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes (e.g., a pink-flowered offspring is produced from a cross between a red-flowered parent and a white-flowered parent). Codominance describes the simultaneous expression of both of the alleles in the heterozygote (e.g., human blood types). It is also common for more than two alleles of a gene to exist in a population (e.g., variations in sizes of pumpkins). In humans, as in many animals and some plants, females have two X chromosomes, and males have one X chromosome and one Y chromosome. Genes on the X chromosome are X-linked, and males inherit and express only one allele for the gene (e.g., hemophilia, color-blindness). Some alleles can also be lethal, so their phenotype will never be observed. Many human genetic disorders, including albinism, cystic fibrosis, and Huntington’s disease can be explained on the basis of simple Mendelian inheritance patterns created by pedigree analysis. (In later Chapters we will learn how DNA analysis can be used to diagnose genetic disorders). Punnett squares are useful tools that apply the rules of probability and meiosis to predict the possible outcomes of genetic crosses. Test crosses are done to determine whether or not an individual is homozygous or heterozygous by crossing the individual with a homozygous recessive.

PDF page 494: flowers (C C ). (Note that different genotypic abbreviations are used for Mendelian extensions to distinguish these patterns from simple dominance and recessiveness.) This pattern of inheritance is described as incomplete dominance, denoting

PDF page 486: The characteristics that Mendel evaluated in his pea plants were each expressed as one of two versions, or traits (e.g., green peas versus yellow peas). As we will explore in more detail in later chapters, the physical expression of characteristics is accomplished through the expression of genes (sequences of DNA), carried on chromosomes. The genetic makeup of peas consists of two similar, or homologous (remember this term from Chapter 11), copies of each chromosome, one from each parent. Through meiosis, diploid organisms utilize meiosis to produce haploid (1n) gametes that participate in fertilization. For cases in which a single gene controls a single characteristic, such as pea color, a diploid organism has genetic copies that may or may not encode the same version of the characteristic. These gene variations (e.g., green peas versus yellow peas) are called alleles. Different alleles for a given gene in a diploid organism interact to express physical characteristics such as pea color in plants or hairline appearance in humans. The observable traits of an organism are referred to as its phenotype. The organism’s underlying genetic makeup, i.e., the combination of alleles, is called its genotype. When diploid organisms carry the same alleles for a given trait, they are said to be homozygous for the genotype; when they carry different alleles, they are said to be heterozygous. For a gene whose expression is Mendelian (Section 12.1), homozygous dominant and heterozygous organisms will look identical; that is, they will have different genotypes but the same phenotype. The recessive allele will only be observed in homozygous recessive individuals. However, alleles do not always behave in dominant and recessive patterns. In other words, there are exceptions to Mendel’s model of inheritance. For example, incomplete dominance describes situation in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes (e.g., a pink-flowered offspring is produced from a cross between a red-flowered parent and a white-flowered parent). Codominance describes the simultaneous expression of both of the alleles in the heterozygote (e.g., human blood types). It is also common for more than two alleles of a gene to exist in a population (e.g., variations in sizes of pumpkins). In humans, as in many animals and some plants, females have two X chromosomes, and males have one X chromosome and one Y chromosome. Genes on the X chromosome are X-linked, and males inherit and express only one allele for the gene (e.g., hemophilia, color-blindness). Some alleles can also be lethal, so their phenotype will never be observed. Many human genetic disorders, including albinism, cystic fibrosis, and Huntington’s disease can be explained on the basis of simple Mendelian inheritance patterns created by pedigree analysis. (In later Chapters we will learn how DNA analysis can be used to diagnose genetic disorders). Punnett squares are useful tools that apply the rules of probability and meiosis to predict the possible outcomes of genetic crosses. Test crosses are done to determine whether or not an individual is homozygous or heterozygous by crossing the individual with a homozygous recessive.

PDF page 495: “storebukkebruse”/Flickr) Codominance A variation on incomplete dominance is codominance, in which both alleles for the same characteristic are simultaneously expressed in the heterozygote. An example of codominance is the MN blood groups of humans. The M and N alleles

PDF page 495: “storebukkebruse”/Flickr) Codominance A variation on incomplete dominance is codominance, in which both alleles for the same characteristic are simultaneously expressed in the heterozygote. An example of codominance is the MN blood groups of humans. The M and N alleles

PDF page 486: The characteristics that Mendel evaluated in his pea plants were each expressed as one of two versions, or traits (e.g., green peas versus yellow peas). As we will explore in more detail in later chapters, the physical expression of characteristics is accomplished through the expression of genes (sequences of DNA), carried on chromosomes. The genetic makeup of peas consists of two similar, or homologous (remember this term from Chapter 11), copies of each chromosome, one from each parent. Through meiosis, diploid organisms utilize meiosis to produce haploid (1n) gametes that participate in fertilization. For cases in which a single gene controls a single characteristic, such as pea color, a diploid organism has genetic copies that may or may not encode the same version of the characteristic. These gene variations (e.g., green peas versus yellow peas) are called alleles. Different alleles for a given gene in a diploid organism interact to express physical characteristics such as pea color in plants or hairline appearance in humans. The observable traits of an organism are referred to as its phenotype. The organism’s underlying genetic makeup, i.e., the combination of alleles, is called its genotype. When diploid organisms carry the same alleles for a given trait, they are said to be homozygous for the genotype; when they carry different alleles, they are said to be heterozygous. For a gene whose expression is Mendelian (Section 12.1), homozygous dominant and heterozygous organisms will look identical; that is, they will have different genotypes but the same phenotype. The recessive allele will only be observed in homozygous recessive individuals. However, alleles do not always behave in dominant and recessive patterns. In other words, there are exceptions to Mendel’s model of inheritance. For example, incomplete dominance describes situation in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes (e.g., a pink-flowered offspring is produced from a cross between a red-flowered parent and a white-flowered parent). Codominance describes the simultaneous expression of both of the alleles in the heterozygote (e.g., human blood types). It is also common for more than two alleles of a gene to exist in a population (e.g., variations in sizes of pumpkins). In humans, as in many animals and some plants, females have two X chromosomes, and males have one X chromosome and one Y chromosome. Genes on the X chromosome are X-linked, and males inherit and express only one allele for the gene (e.g., hemophilia, color-blindness). Some alleles can also be lethal, so their phenotype will never be observed. Many human genetic disorders, including albinism, cystic fibrosis, and Huntington’s disease can be explained on the basis of simple Mendelian inheritance patterns created by pedigree analysis. (In later Chapters we will learn how DNA analysis can be used to diagnose genetic disorders). Punnett squares are useful tools that apply the rules of probability and meiosis to predict the possible outcomes of genetic crosses. Test crosses are done to determine whether or not an individual is homozygous or heterozygous by crossing the individual with a homozygous recessive.

PDF page 488: Two alleles for a given gene in a diploid organism are expressed and interact to produce physical characteristics. The observable traits expressed by an organism are referred to as its phenotype. An organism’s underlying genetic makeup, consisting of both physically visible and non-expressed alleles, is called its genotype. Mendel’s hybridization experiments demonstrate the difference between phenotype and genotype. When true-breeding plants in which one parent had yellow pods and one had green pods were cross-fertilized, all of the F1 hybrid offspring had yellow pods. That is, the hybrid offspring were phenotypically identical to the true-breeding parent with yellow pods. However, we know that the allele donated by the parent with green pods was not simply lost because it reappeared in some of the F2 offspring. Therefore, the F1 plants must have been genotypically different from the parent with yellow pods. The P1 plants that Mendel used in his experiments were each homozygous for the trait he was studying. Diploid organisms that are homozygous at a given gene, or locus, have two identical alleles for that gene on their homologous chromosomes. Mendel’s parental pea plants always bred true because both of the gametes produced carried the same trait. When P1 plants with contrasting traits were cross-fertilized, all of the offspring were heterozygous for the contrasting trait, meaning that their genotype reflected that they had different alleles for the gene being examined. Dominant and Recessive Alleles Our discussion of homozygous and heterozygous organisms brings us to why the F1 heterozygous offspring were identical to one of the parents, rather than expressing both alleles. In all seven pea-plant characteristics, one of the two contrasting alleles was dominant, and the other was recessive. Mendel called the dominant allele the expressed unit factor; the recessive allele was referred to as the latent unit factor. We now know that these so-called unit factors are actually genes on homologous chromosome pairs. For a gene that is expressed in a dominant and recessive pattern, homozygous dominant and heterozygous organisms will look identical (that is, they will have different genotypes but the same phenotype). The recessive allele will only be observed in homozygous recessive individuals (Table 12.4).

PDF page 486: The characteristics that Mendel evaluated in his pea plants were each expressed as one of two versions, or traits (e.g., green peas versus yellow peas). As we will explore in more detail in later chapters, the physical expression of characteristics is accomplished through the expression of genes (sequences of DNA), carried on chromosomes. The genetic makeup of peas consists of two similar, or homologous (remember this term from Chapter 11), copies of each chromosome, one from each parent. Through meiosis, diploid organisms utilize meiosis to produce haploid (1n) gametes that participate in fertilization. For cases in which a single gene controls a single characteristic, such as pea color, a diploid organism has genetic copies that may or may not encode the same version of the characteristic. These gene variations (e.g., green peas versus yellow peas) are called alleles. Different alleles for a given gene in a diploid organism interact to express physical characteristics such as pea color in plants or hairline appearance in humans. The observable traits of an organism are referred to as its phenotype. The organism’s underlying genetic makeup, i.e., the combination of alleles, is called its genotype. When diploid organisms carry the same alleles for a given trait, they are said to be homozygous for the genotype; when they carry different alleles, they are said to be heterozygous. For a gene whose expression is Mendelian (Section 12.1), homozygous dominant and heterozygous organisms will look identical; that is, they will have different genotypes but the same phenotype. The recessive allele will only be observed in homozygous recessive individuals. However, alleles do not always behave in dominant and recessive patterns. In other words, there are exceptions to Mendel’s model of inheritance. For example, incomplete dominance describes situation in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes (e.g., a pink-flowered offspring is produced from a cross between a red-flowered parent and a white-flowered parent). Codominance describes the simultaneous expression of both of the alleles in the heterozygote (e.g., human blood types). It is also common for more than two alleles of a gene to exist in a population (e.g., variations in sizes of pumpkins). In humans, as in many animals and some plants, females have two X chromosomes, and males have one X chromosome and one Y chromosome. Genes on the X chromosome are X-linked, and males inherit and express only one allele for the gene (e.g., hemophilia, color-blindness). Some alleles can also be lethal, so their phenotype will never be observed. Many human genetic disorders, including albinism, cystic fibrosis, and Huntington’s disease can be explained on the basis of simple Mendelian inheritance patterns created by pedigree analysis. (In later Chapters we will learn how DNA analysis can be used to diagnose genetic disorders). Punnett squares are useful tools that apply the rules of probability and meiosis to predict the possible outcomes of genetic crosses. Test crosses are done to determine whether or not an individual is homozygous or heterozygous by crossing the individual with a homozygous recessive.

PDF page 488: Two alleles for a given gene in a diploid organism are expressed and interact to produce physical characteristics. The observable traits expressed by an organism are referred to as its phenotype. An organism’s underlying genetic makeup, consisting of both physically visible and non-expressed alleles, is called its genotype. Mendel’s hybridization experiments demonstrate the difference between phenotype and genotype. When true-breeding plants in which one parent had yellow pods and one had green pods were cross-fertilized, all of the F1 hybrid offspring had yellow pods. That is, the hybrid offspring were phenotypically identical to the true-breeding parent with yellow pods. However, we know that the allele donated by the parent with green pods was not simply lost because it reappeared in some of the F2 offspring. Therefore, the F1 plants must have been genotypically different from the parent with yellow pods. The P1 plants that Mendel used in his experiments were each homozygous for the trait he was studying. Diploid organisms that are homozygous at a given gene, or locus, have two identical alleles for that gene on their homologous chromosomes. Mendel’s parental pea plants always bred true because both of the gametes produced carried the same trait. When P1 plants with contrasting traits were cross-fertilized, all of the offspring were heterozygous for the contrasting trait, meaning that their genotype reflected that they had different alleles for the gene being examined. Dominant and Recessive Alleles Our discussion of homozygous and heterozygous organisms brings us to why the F1 heterozygous offspring were identical to one of the parents, rather than expressing both alleles. In all seven pea-plant characteristics, one of the two contrasting alleles was dominant, and the other was recessive. Mendel called the dominant allele the expressed unit factor; the recessive allele was referred to as the latent unit factor. We now know that these so-called unit factors are actually genes on homologous chromosome pairs. For a gene that is expressed in a dominant and recessive pattern, homozygous dominant and heterozygous organisms will look identical (that is, they will have different genotypes but the same phenotype). The recessive allele will only be observed in homozygous recessive individuals (Table 12.4).

PDF page 488: When fertilization occurs between two true-breeding parents that differ in only one characteristic, the process is called a

PDF page 489: 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 F1 and F2 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).

PDF page 490: from different parents. They are grouped together. Because fertilization is a random event, we expect each combination to be equally likely and for the offspring to exhibit a ratio of YY:Yy:yy genotypes of 1:2:1 (Figure 12.4). Furthermore, because the YY and Yy offspring have yellow seeds and are phenotypically identical, applying the sum rule of probability, we expect the offspring to exhibit a phenotypic ratio of 3 yellow:1 green. Indeed, working with large sample sizes, Mendel observed approximately this ratio in every F2 generation resulting from crosses for individual traits. Mendel validated these results by performing an F3 cross in which he self-crossed the dominant- and recessive-expressing F2 plants. When he self-crossed the plants expressing green seeds, all of the offspring had green seeds, confirming that all green seeds had homozygous genotypes of yy. When he self-crossed the F2 plants expressing yellow seeds, he found that one-third of the plants bred true, and two-thirds of the plants segregated at a 3:1 ratio of yellow:green seeds. In this case, the true-breeding plants had homozygous (YY) genotypes, whereas the segregating plants corresponded to the heterozygous (Yy) genotype. When these plants self-fertilized, the outcome was just like the F1 self-fertilizing cross. The Test Cross Distinguishes the Dominant Phenotype Beyond predicting the offspring of a cross between known homozygous or heterozygous parents, Mendel also developed a way to determine whether an organism that expressed a dominant trait was a heterozygote or a homozygote. Called the test cross, this technique is still used by plant and animal breeders. In a test cross, the dominant-expressing organism is crossed with an organism that is homozygous recessive for the same characteristic. If the dominant-expressing organism is a homozygote, then all F1 offspring will be heterozygotes expressing the dominant trait (Figure 12.5). Alternatively, if the dominant expressing organism is a heterozygote, the F1 offspring will exhibit a 1:1 ratio of heterozygotes and recessive homozygotes (Figure 12.5). The test cross further validates Mendel’s postulate that pairs of unit factors segregate equally.

PDF page 490: from different parents. They are grouped together. Because fertilization is a random event, we expect each combination to be equally likely and for the offspring to exhibit a ratio of YY:Yy:yy genotypes of 1:2:1 (Figure 12.4). Furthermore, because the YY and Yy offspring have yellow seeds and are phenotypically identical, applying the sum rule of probability, we expect the offspring to exhibit a phenotypic ratio of 3 yellow:1 green. Indeed, working with large sample sizes, Mendel observed approximately this ratio in every F2 generation resulting from crosses for individual traits. Mendel validated these results by performing an F3 cross in which he self-crossed the dominant- and recessive-expressing F2 plants. When he self-crossed the plants expressing green seeds, all of the offspring had green seeds, confirming that all green seeds had homozygous genotypes of yy. When he self-crossed the F2 plants expressing yellow seeds, he found that one-third of the plants bred true, and two-thirds of the plants segregated at a 3:1 ratio of yellow:green seeds. In this case, the true-breeding plants had homozygous (YY) genotypes, whereas the segregating plants corresponded to the heterozygous (Yy) genotype. When these plants self-fertilized, the outcome was just like the F1 self-fertilizing cross. The Test Cross Distinguishes the Dominant Phenotype Beyond predicting the offspring of a cross between known homozygous or heterozygous parents, Mendel also developed a way to determine whether an organism that expressed a dominant trait was a heterozygote or a homozygote. Called the test cross, this technique is still used by plant and animal breeders. In a test cross, the dominant-expressing organism is crossed with an organism that is homozygous recessive for the same characteristic. If the dominant-expressing organism is a homozygote, then all F1 offspring will be heterozygotes expressing the dominant trait (Figure 12.5). Alternatively, if the dominant expressing organism is a heterozygote, the F1 offspring will exhibit a 1:1 ratio of heterozygotes and recessive homozygotes (Figure 12.5). The test cross further validates Mendel’s postulate that pairs of unit factors segregate equally.