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    which inheritance pattern results when parents are crossed for pure traits and the resulting offspring have traits that appear to blend?

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    Which inheritance pattern results when parents are crossed for pure traits and the resulting offspring have traits that appear to blend?

    Incomplete dominance is generally the "cause" of "blended" phenotypes. Clearly there are other more complicated origins, but hopefully this suffices.

    Which inheritance pattern results when parents are crossed for pure traits and the resulting offspring have traits that appear to blend?

    Biology Genetics & Inheritance Genetics Overview

    1 Answer

    Al E. Nov 21, 2017

    Incomplete dominance is generally the "cause" of "blended" phenotypes. Clearly there are other more complicated origins, but hopefully this suffices.

    http://legacy.hopkinsville.kctcs.edu

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    Gregor Mendel and the Principles of Inheritance

    By experimenting with pea plant breeding, Gregor Mendel developed three principles of inheritance that described the transmission of genetic traits before anyone knew exactly what genes were. Mendel's insight provided a great expansion of the understanding of genetic inheritance, and led to the development of new experimental methods.

    Gregor Mendel and the Principles of Inheritance

    By: Ilona Miko, Ph.D. (Write Science Right) © 2008 Nature Education

    Citation: Miko, I. (2008) Gregor Mendel and the principles of inheritance. Nature Education 1(1):134

    Gregor Mendel's principles of inheritance form the cornerstone of modern genetics. So just what are they?

    Aa Aa Aa

    Ever wonder why you are the only one in your family with your grandfather's nose? The way in which traits are passed from one generation to the next-and sometimes skip generations-was first explained by Gregor Mendel. By experimenting with pea plant breeding, Mendel developed three principles of inheritance that described the transmission of genetic traits, before anyone knew genes existed. Mendel's insight greatly expanded the understanding of genetic inheritance, and led to the development of new experimental methods.

    Figure 1 Figure Detail

    Traits are passed down in families in different patterns. Pedigrees can illustrate these patterns by following the history of specific characteristics, or phenotypes, as they appear in a family. For example, the pedigree in Figure 1 shows a family in which a grandmother (generation I) has passed down a characteristic (shown in solid red) through the family tree. The inheritance pattern of this characteristic is considered dominant, because it is observable in every generation. Thus, every individual who carries the genetic code for this characteristic will show evidence of the characteristic. In contrast, Figure 2 shows a different pattern of inheritance, in which a characteristic disappears in one generation, only to reappear in a subsequent one. This pattern of inheritance, in which the parents do not show the phenotype but some of the children do, is considered recessive. But where did our knowledge of dominance and recessivity first come from?

    Gregor Mendel’s Courage and Persistence

    Figure 3 Figure 2

    Our modern understanding of how traits may be inherited through generations comes from the principles proposed by Gregor Mendel in 1865. However, Mendel didn't discover these foundational principles of inheritance by studying human beings, but rather by studying , or the common pea plant. Indeed, after eight years of tedious experiments with these plants, and—by his own admission—"some courage" to persist with them, Mendel proposed three foundational principles of inheritance. These principles eventually assisted clinicians in human disease research; for example, within just a couple of years of the rediscovery of Mendel's work, Archibald Garrod applied Mendel's principles to his study of alkaptonuria. Today, whether you are talking about pea plants or human beings, genetic traits that follow the rules of inheritance that Mendel proposed are called Mendelian.

    Mendel was curious about how traits were transferred from one generation to the next, so he set out to understand the principles of heredity in the mid-1860s. Peas were a good model system, because he could easily control their fertilization by transferring pollen with a small paintbrush. This pollen could come from the same flower (self-fertilization), or it could come from another plant's flowers (cross-fertilization). First, Mendel observed plant forms and their offspring for two years as they self-fertilized, or "selfed," and ensured that their outward, measurable characteristics remained constant in each generation. During this time, Mendel observed seven different characteristics in the pea plants, and each of these characteristics had two forms (Figure 3). The characteristics included height (tall or short), pod shape (inflated or constricted), seed shape (smooth or winkled), pea color (green or yellow), and so on. In the years Mendel spent letting the plants self, he verified the purity of his plants by confirming, for example, that tall plants had only tall children and grandchildren and so forth. Because the seven pea plant characteristics tracked by Mendel were consistent in generation after generation of self-fertilization, these parental lines of peas could be considered pure-breeders (or, in modern terminology, homozygous for the traits of interest). Mendel and his assistants eventually developed 22 varieties of pea plants with combinations of these consistent characteristics.

    Mendel not only crossed pure-breeding parents, but he also crossed hybrid generations and crossed the hybrid progeny back to both parental lines. These crosses (which, in modern terminology, are referred to as F1, F1 reciprocal, F2, B1, and B2) are the classic crosses to generate genetically hybrid generations.

    Understanding Dominant Traits

    Before Mendel's experiments, most people believed that traits in offspring resulted from a blending of the traits of each parent. However, when Mendel cross-pollinated one variety of purebred plant with another, these crosses would yield offspring that looked like either one of the parent plants, not a blend of the two. For example, when Mendel cross-fertilized plants with wrinkled seeds to those with smooth seeds, he did not get progeny with semi-wrinkly seeds. Instead, the progeny from this cross had only smooth seeds. In general, if the progeny of crosses between purebred plants looked like only one of the parents with regard to a specific trait, Mendel called the expressed parental trait the dominant trait. From this simple observation, Mendel proposed his first principle, the principle of uniformity; this principle states that all the progeny of a cross like this (where the parents differ by only one trait) will appear identical. Exceptions to the principle of uniformity include the phenomena of penetrance, expressivity, and sex-linkage, which were discovered after Mendel's time.

    Source : www.nature.com

    Incomplete dominance, codominance & multiple alleles (article)

    In the real world, genes often come in many versions (alleles). Alleles aren't always fully dominant or recessive to one another, but may instead display codominance or incomplete dominance.

    Introduction

    Gregor Mendel knew how to keep things simple. In Mendel's work on pea plants, each gene came in just two different versions, or alleles, and these alleles had a nice, clear-cut dominance relationship (with the dominant allele fully overriding the recessive allele to determine the plant's appearance).

    Today, we know that not all alleles behave quite as straightforwardly as in Mendel’s experiments. For example, in real life:

    Allele pairs may have a variety of dominance relationships (that is, one allele of the pair may not completely “hide” the other in the heterozygote).

    There are often many different alleles of a gene in a population.

    In these cases, an organism's genotype, or set of alleles, still determines its phenotype, or observable features. However, a variety of alleles may interact with one another in different ways to specify phenotype.

    As a side note, we're probably lucky that Mendel's pea genes didn't show these complexities. If they had, it’s possible that Mendel would not have understood his results, and wouldn't have figured out the core principles of inheritance—which are key in helping us understand the special cases!

    Incomplete dominance

    Mendel’s results were groundbreaking partly because they contradicted the (then-popular) idea that parents' traits were permanently blended in their offspring. In some cases, however, the phenotype of a heterozygous organism can actually be a blend between the phenotypes of its homozygous parents.

    For example, in the snapdragon, Antirrhinum majus, a cross between a homozygous white-flowered plant (

    C^WC^W C W C W

    C, start superscript, W, end superscript, C, start superscript, W, end superscript

    ) and a homozygous red-flowered plant (

    C^RC^R C R C R

    C, start superscript, R, end superscript, C, start superscript, R, end superscript

    ) will produce offspring with pink flowers (

    C^RC^W C R C W

    C, start superscript, R, end superscript, C, start superscript, W, end superscript

    ). This type of relationship between alleles, with a heterozygote phenotype intermediate between the two homozygote phenotypes, is called incomplete dominance.

    Diagram of a cross between

    C^WC^W C W C W

    C, start superscript, W, end superscript, C, start superscript, W, end superscript

    (white) and C^RC^R C R C R

    C, start superscript, R, end superscript, C, start superscript, R, end superscript

    (red) snapdragon plants. The F1 plants are pink and of genotype

    C^RC^W C R C W

    C, start superscript, R, end superscript, C, start superscript, W, end superscript

    .

    We can still use Mendel's model to predict the results of crosses for alleles that show incomplete dominance. For example, self-fertilization of a pink plant would produce a genotype ratio of

    1 1 1 C^RC^R C R C R

    C, start superscript, R, end superscript, C, start superscript, R, end superscript

    : : colon 2 2 2 C^RC^W C R C W

    C, start superscript, R, end superscript, C, start superscript, W, end superscript

    : : colon 1 1 1 C^WC^W C W C W

    C, start superscript, W, end superscript, C, start superscript, W, end superscript

    and a phenotype ratio of

    1:2:1 1:2:1

    1, colon, 2, colon, 1

    red:pink:white. Alleles are still inherited according to Mendel's basic rules, even when they show incomplete dominance.

    Self-fertilization of pink

    C^RC^W C R C W

    C, start superscript, R, end superscript, C, start superscript, W, end superscript

    plants produce red, pink, and white offspring in a ratio of 1:2:1.

    Codominance

    Closely related to incomplete dominance is codominance, in which both alleles are simultaneously expressed in the heterozygote.

    We can see an example of codominance in the MN blood groups of humans (less famous than the ABO blood groups, but still important!). A person's MN blood type is determined by his or her alleles of a certain gene. An

    L^M L M

    L, start superscript, M, end superscript

    allele specifies production of an M marker displayed on the surface of red blood cells, while an

    L^N L N

    L, start superscript, N, end superscript

    allele specifies production of a slighly different N marker.

    Homozygotes ( L^ML^M L M L M

    L, start superscript, M, end superscript, L, start superscript, M, end superscript

    and L^NL^N L N L N

    L, start superscript, N, end superscript, L, start superscript, N, end superscript

    ) have only M or an N markers, respectively, on the surface of their red blood cells. However, heterozygotes (

    Source : www.khanacademy.org

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