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    despite the law of independent assortment, when two loci are on the same chromosome, the phenotypes of the progeny sometimes do not fit the predicted phenotypes. this outcome can be explained by the phenomenon of

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    The law of independent assortment (article)

    Mendel's law of independent assortment. Dihybrid crosses. 4 x 4 Punnett squares.

    Mendelian genetics

    The law of independent assortment

    Mendel's law of independent assortment. Dihybrid crosses. 4 x 4 Punnett squares.

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    Introduction

    The law of segregation lets us predict how a single feature associated with a single gene is inherited. In some cases, though, we might want to predict the inheritance of two characteristics associated with two different genes. How can we do this? [Refresher on the law of segregation]

    To make an accurate prediction, we need to know whether the two genes are inherited independently or not. That is, we need to know whether they "ignore" one another when they're sorted into gametes, or whether they "stick together" and get inherited as a unit.

    When Gregor Mendel asked this question, he found that different genes were inherited independently of one another, following what's called the law of independent assortment. In this article, we'll take a closer look at the law of independent assortment and how it is used to make predictions. We'll also see when and why the law of independent assortment does (or doesn't!) hold true.

    Note: If you are not yet familiar with how individual genes are inherited, you may want to check out the article on the law of segregation or the introduction to heredity video before you dive into this article.

    What is the law of independent assortment?

    Mendel's law of independent assortment states that the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene.

    Example: Pea color and pea shape genes

    Let's look at a concrete example of the law of independent assortment. Imagine that we cross two pure-breeding pea plants: one with yellow, round seeds (YYRR) and one with green, wrinkled seeds (yyrr). Because each parent is homozygous, the law of segregation tells us that the gametes made by the wrinkled, green plant all are ry, and the gametes made by the round, yellow plant are all RY. That gives us

    \text F_1 F 1 ​

    start text, F, end text, start subscript, 1, end subscript

    offspring that are all RrYy.

    The allele specifying yellow seed color is dominant to the allele specifying green seed color, and the allele specifying round shape is dominant to the allele specifying wrinkled shape, as shown by the capital and lower-case letters. This means that the

    \text F_1 F 1 ​

    start text, F, end text, start subscript, 1, end subscript

    plants are all yellow and round. Because they are heterozygous for two genes, the

    \text F_1 F 1 ​

    start text, F, end text, start subscript, 1, end subscript

    plants are called dihybrids (di- = two, -hybrid = heterozygous).

    A cross between two dihybrids (or, equivalently, self-fertilization of a dihybrid) is known as a dihybrid cross. When Mendel did this cross and looked at the offspring, he found that there were four different categories of pea seeds: yellow and round, yellow and wrinkled, green and round, and green and wrinkled. These phenotypic categories (categories defined by observable traits) appeared in a ratio of approximately

    9:3:3:1 9:3:3:1

    9, colon, 3, colon, 3, colon, 1

    .

    Illustration of the hypothesis that the seed color and seed shape genes assort independently.

    In this diagram, the Y and R alleles of the yellow, round parent and the y and r alleles of the green, wrinkled parent are not inherited as units. Instead, the alleles of the two genes are inherited as independent units.

    P generation: A yellow, round plant (YYRR) is crossed with a green, wrinkled plant (yyrr). Each parental generation can produce only one type of gamete, YR or yr.

    F1 generation: The F1 dihybrid seeds are yellow and round, with a genotype of YyRr. The F1 plants can produce four different types of gametes: YR, Yr, yR, and yr. We can predict the genotypes of the F2 plants by placing these gametes along the top and side axes of a 4X4 Punnett square and filling in the boxes to represent fertilization events.

    F2 generation: Completion of the Punnett square predicts four different phenotypic classes of offspring, yellow/round, yellow/wrinkled, green/round, and green/wrinkled, in a ratio of 9:3:3:1. This is the prediction of the model in which the seed shape and seed color genes assort independently.

    Punnett square: YR Yr yR yr

    YR YYRR YYRr YyRR YyRr

    Yr YYRr YYrr YyRr Yyrr

    yR YyRR YyRr yyRR yyRr

    yr YyRr Yyrr yyRr yyrr

    Plain text = yellow, round phenotype Italic text = yellow, wrinkled phenotype Bold text = green, round phenotype Bold, italic text = green, wrinkled phenotype

    Image credit: "Laws of inheritance: Figure 2," by OpenStax College, Biology, CC BY 4.0.

    This ratio was the key clue that led Mendel to the law of independent assortment. That's because a

    9:3:3:1 9:3:3:1

    9, colon, 3, colon, 3, colon, 1

    Source : www.khanacademy.org

    QUIZ 11 Flashcards

    Start studying QUIZ 11. Learn vocabulary, terms, and more with flashcards, games, and other study tools.

    QUIZ 11

    A particular genetic cross in which the individual in question is crossed with an individual known to be homozygous for a recessive trait is referred to as a

    A.parental cross. B.dihybrid cross.

    C.filial generation mating.

    D.reciprocal cross. E.test cross.

    Click card to see definition 👆

    E. TEST CROSS

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    Despite the law of independent assortment, when two loci are on the same chromosome, the phenotypes of the progeny sometimes do not fit the predicted phenotypes due to

    A.translocation. B.inversions.

    C.chromatid affinities.

    D.linkage.

    E.reciprocal chromosomal exchanges.

    Click card to see definition 👆

    D.linkage.

    Click again to see term 👆

    1/30 Created by Megan_Piffer

    Terms in this set (30)

    A particular genetic cross in which the individual in question is crossed with an individual known to be homozygous for a recessive trait is referred to as a

    A.parental cross. B.dihybrid cross.

    C.filial generation mating.

    D.reciprocal cross. E.test cross. E. TEST CROSS

    Despite the law of independent assortment, when two loci are on the same chromosome, the phenotypes of the progeny sometimes do not fit the predicted phenotypes due to

    A.translocation. B.inversions.

    C.chromatid affinities.

    D.linkage.

    E.reciprocal chromosomal exchanges.

    D.linkage.

    If the same allele has two or more phenotypic effects, it is said to be

    A.codominant. B.a marker. C.linked. D.pleiotropic. E.hemizygous. D.pleiotropic.

    Different forms of a gene are called

    A.traits. B.phenotypes. C.genotypes. D.alleles. E.None of the above D.alleles.

    If Mendel had performed experiments on cattle rather than on peas, the patterns of inheritance would not have been easily detectable, because cattle

    A.reproduce asexually

    B.have small numbers of offspring

    C.do not have observable phenotypes

    D.do not have genotypes

    E.do not have autosomes

    B.have small numbers of offspring

    A mutation at a single locus causes a change in many different characters. This an example of a(n) _______ effect.

    A.polygene B.epigenetic C.cytoplasmic

    D.multiple negativity

    E.pleiotropic E.pleiotropic

    A pea plant with red flowers is test crossed, and one-half of the resulting progeny have red flowers, while the other half has white flowers. Therefore, the genotype of the test-crossed parent was

    A.RR. B.Rr. C.rr. D.either RR or Rr.

    E.This cannot be answered without more information.

    B.Rr.

    Separation of the alleles of a single gene into different gametes is called

    A. synapsis. B.segregation.

    C.independent assortment.

    D.heterozygous separation.

    E.recombination. B.segregation.

    In mice, short hair is dominant to long hair. If a short-haired individual is crossed with a long-haired individual and both long- and short-haired offspring result, one can conclude that

    A. the short-haired individual is homozygous.

    B. the short-haired individual is heterozygous.

    C. the long-haired individual is homozygous.

    D. the long-haired individual is heterozygous.

    E. This cannot be answered without more information.

    B. the short-haired individual is heterozygous.

    A linkage group corresponds to

    A. a group of genes on different chromosomes.

    B. the linear order of chromomeres on a chromosome.

    C. the length of a chromosome.

    D. a group of genes on the same chromosome.

    E. None of the above

    D. a group of genes on the same chromosome.

    The site on the chromosome occupied by a gene is called a(n)

    A. allele. B. region. C. locus. D. type. E. phenotype. C. locus.

    Which of the following methods was not used by Mendel in his study of the genetics of the garden pea?

    A. Maintenance of true-breeding lines

    B. Cross-pollination

    C. Microscopy

    D. Production of hybrid plants

    E. Quantitative analysis of results

    C. Microscopy

    Classical albinism results from a recessive allele. Which of the following is the expected ratio for the progeny when a normally pigmented male with an albino father has children with an albino woman?

    A.3/4 normal; 1/4 albino

    B.3/4 albino; 1/4 normal

    C.1/2 normal; 1/2 albino

    D.All normal E.All albino

    C.1/2 normal; 1/2 albino

    How many autosomes do humans have?

    A.23 pairs B.22 pairs C.1 pair D.45 E.16 B.22 pairs

    Mendel's crossing of spherical-seeded pea plants with wrinkled-seeded pea plants resulted in progeny that all had spherical seeds. This indicates that the wrinkled-seed trait is

    A. codominant. B. dominant. C. recessive. D. Both a and b E. Both a and c C. recessive.

    Y-linked genes include a gene that produces hairy pinnae (the external ear). A male with hairy pinnae will pass this trait

    A. to some of his sons, and occasionally also to a daughter.

    B. only to his sons.

    C. only to his daughters.

    D. only to his grandsons.

    E. to all of his children if the mother is a carrier.

    B. only to his sons.

    Source : quizlet.com

    12.3E: Genetic Linkage and Violation of the Law of Independent Assortment

    12.3E: Genetic Linkage and Violation of the Law of Independent Assortment

    Last updated Aug 15, 2020

    12.3D: Mendel’s Law of Independent Assortment

    12.3F: Epistasis Boundless Boundless

    Genes that are on the same chromosome, or “linked”, do not assort independently, but can be separated by recombination.

    Learning Objectives

    Describe how recombination can separate linked genes

    Key Points

    Two genes close together on the same chromosome tend to be inherited together and are said to be linked.

    Linked genes can be separated by recombination in which homologous chromosomes exchange genetic information during meiosis; this results in parental, or nonrecombinant genotypes, as well as a smaller proportion of recombinant genotypes.

    Geneticists can use the amount of recombination between genes to estimate the distance between them on a chromosome.

    Key Terms

    linkage: the property of genes of being inherited togetherrecombination: the formation of genetic combinations in offspring that are not present in the parents

    Linked Genes Violate the Law of Independent Assortment

    Although all of Mendel’s pea characteristics behaved according to the law of independent assortment, we now know that some allele combinations are not inherited independently of each other. Genes that are located on separate non-homologous chromosomes will always sort independently. However, each chromosome contains hundreds or thousands of genes organized linearly on chromosomes like beads on a string. The segregation of alleles into gametes can be influenced by linkage, in which genes that are located physically close to each other on the same chromosome are more likely to be inherited as a pair. However, because of the process of recombination, or “crossover,” it is possible for two genes on the same chromosome to behave independently, or as if they are not linked. To understand this, let’s consider the biological basis of gene linkage and recombination.

    Figure 12.3E.1 12.3E.1

    : Unlinked genes assort independently: This figure shows all possible combinations of offspring resulting from a dihybrid cross of pea plants that are heterozygous for the tall/dwarf and inflated/constricted alleles.

    Homologous chromosomes possess the same genes in the same linear order. The alleles may differ on homologous chromosome pairs, but the genes to which they correspond do not. In preparation for the first division of meiosis, homologous chromosomes replicate and synapse. Like genes on the homologs align with each other. At this stage, segments of homologous chromosomes exchange linear segments of genetic material. This process is called recombination, or crossover, and it is a common genetic process. Because the genes are aligned during recombination, the gene order is not altered. Instead, the result of recombination is that maternal and paternal alleles are combined onto the same chromosome. Across a given chromosome, several recombination events may occur, causing extensive shuffling of alleles.

    Figure 12.3E.1 12.3E.1

    : Linked genes can be separated by recombination: The process of crossover, or recombination, occurs when two homologous chromosomes align during meiosis and exchange a segment of genetic material. Here, the alleles for gene C were exchanged. The result is two recombinant and two non-recombinant chromosomes.

    When two genes are located in close proximity on the same chromosome, they are considered linked, and their alleles tend to be transmitted through meiosis together. To exemplify this, imagine a dihybrid cross involving flower color and plant height in which the genes are next to each other on the chromosome. If one homologous chromosome has alleles for tall plants and red flowers, and the other chromosome has genes for short plants and yellow flowers, then when the gametes are formed, the tall and red alleles will go together into a gamete and the short and yellow alleles will go into other gametes. These are called the parental genotypes because they have been inherited intact from the parents of the individual producing gametes. But unlike if the genes were on different chromosomes, there will be no gametes with tall and yellow alleles and no gametes with short and red alleles. If you create the Punnett square with these gametes, you will see that the classical Mendelian prediction of a 9:3:3:1 outcome of a dihybrid cross would not apply. As the distance between two genes increases, the probability of one or more crossovers between them increases, and the genes behave more like they are on separate chromosomes. Geneticists have used the proportion of recombinant gametes (the ones not like the parents) as a measure of how far apart genes are on a chromosome. Using this information, they have constructed elaborate maps of genes on chromosomes for well-studied organisms, including humans.

    Mendel’s seminal publication makes no mention of linkage, and many researchers have questioned whether he encountered linkage, but chose not to publish those crosses out of concern that they would invalidate his independent assortment postulate. The garden pea has seven chromosomes and some have suggested that his choice of seven characteristics was not a coincidence. However, even if the genes he examined were not located on separate chromosomes, it is possible that he simply did not observe linkage because of the extensive shuffling effects of recombination.

    Source : bio.libretexts.org

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