1.6: The Independent Distribution Act (2023)

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    Oseparation lawit allows us to predict how a single trait associated with a single gene will be inherited. However, in some cases, we may want to predict the inheritance of two traits linked to two different genes. How can we do that

    [Segregation Law Update]

    Oseparation lawstates that each gamete (sperm or egg) produced by an organism receives only one of the two gene copies present in a parent organism and that gene copies are randomly assigned to gametes. For example, if an organism has a genotype ofAA, half of their gametes contain aAallele, and the other half contain aAalleles.

    You can use the link at the beginning of the paragraph for more information on segregation law.

    To make an accurate prediction, we need to know whether or not the two genes are independently inherited. That is, we need to know if they "overlook" each other when classified into gametes, or if they "stick together" and are inherited as a unit.

    SeGregor MendelWhen he asked this question, he discovered that different genes were inherited independently, in what is called aIndependent Assortment Law. In this article, we take a closer look at the Law of Independent Assortment and how it's used to make predictions. We'll also see when and why the law of independent distribution applies (or doesn't!).

    Note: If you don't already know how individual genes are inherited, you can take a look at the inheritance articleseparation lawor theIntroduction to inheritancevideo before diving into this article.

    What is the Independent Assortment Law?

    mendelsIndependent Assortment Lawstates that the alleles of two (or more) different genes are classified independently into gametes. In other words, the allele that a gamete gets for one gene does not affect the allele that gets another gene.

    Example: genes for pea color and shape

    Let's look at a concrete example of the law of independent distribution. Imagine that we are crossing two purebred pea plants: one with round yellow seeds (AARR) and another with wrinkled green seeds (aaaaaa). Since each parent is homozygous, the law of segregation tells us that the gametes produced by the wrinkled green plant are all homozygous.ry, and the gametes of the round and yellow plant are allRY. That's give us F1descendants, that's allRyy.

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    The allele that specifies yellow seed color is dominant over the allele that specifies green seed color, and the allele that specifies round shape is dominant over the allele that specifies wrinkled shape, as indicated by the capital letters and lowercase that will be displayed. This means that the F1The plants are all yellow and round. Because they are heterozygous for two genes that code for F1they are called plantsdihybrid(von-= of,-hybrid= heterocigoto).

    A cross between two dihybrids (or, equivalently, self-pollination of a dihybrid) is denoted as adihybrid cross. When Mendel made this cross and looked at the offspring, he discovered that there were four different categories of pea seeds: yellow and round, yellow and wrinkled, green and round, and green and wrinkled. ThatphenotypicallyThe categories (categories defined by observable characteristics) appeared in a ratio of approximately 9:3:3:1.

    1.6: The Independent Distribution Act (1)

    This ratio was the key that led Mendel to the law of independent distribution. This is because an aspect ratio of 9:3:3:1 is exactly what we would expect if the F1The plant produced four types of gametes (sperm and ova) with equal frequency:Again,Again,years, miAgain. In other words, this is the outcome that we would predict if each gamete were randomly assigned aYojallele, and also random in a separate processRoRallele (creating four equally likely combinations).

    We can confirm the connection between the four types of gametes and the 9:3:3:1 ratio of the above Punnett square. To create the square, we first place the four equally likely types of gametes along each axis. We then connect the gametes to the axes of the boxes in the diagram that represent the fertilization events. The 16 equally likely fertilization events that can occur between gametes are shown in the 16 boxes. The genotypes of the offspring in the boxes correspond to a phenotype ratio of 9:3:3:1, as noted by Mendel.

    [More on Punnett squares with two genes]

    We can draw a Punnett square for a two-gene scenario by following the same basic rules as a monohybrid cross, placing the gametes along the axes and combining them in the squares to represent the fertilization events. But now that there are more gamete types, there should also be more squares in the table: 4 possible maternal gamete types x 4 possible paternal gamete types = 16 total squares.

    Like oneSingle-Gen-Punnett-Quadrat, we place all possible gamete types along the axes and then combine them in the squares where the columns and rows intersect to represent fertilization events (the formation of zygotes or offspring).

    To learn how to use the rules of probability to predict the outcome of a dihybrid cross, read theProbabilities in GeneticsArticle.

    Independent variety vs. binding

    The previous section briefly gives us Mendel's law of independent classification and allows us to see how the law of independent classification leads to a ratio of 9:3:3:1. But what was the alternative option? That is, what would happen if two genesNoDoes the independent assortment follow?

    In extreme cases, the genes for seed color and shape could always have been inherited as a pair. That is, the yellow and round alleles could always have stayed together, as could the green and wrinkled alleles.

    To see how this might work, imagine that the color and shape genes are physically linked and cannot be separated, as illustrated by the boxes around the alleles in the diagram below. This could happen, for example, if the two genes are very, very close together on a chromosome (an idea we'll explore later at the end of the article).

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    1.6: The Independent Distribution Act (2)

    Instead of giving each gamete a color allele and a separate shape allele, the F1dihybrid plants would simply give each gamete a "unit of combination": aAgainpair of alleles or oneAgainAllele pair.

    We can use a Punnett square to predict the results of selfing in this case, as shown above. Whether genes for seed color and shape were always inherited as a unit, orfully linked, a dihybrid cross should produce only two types of offspring, yellow/round and green/wrinkled, in a 3:1 ratio. Mendel's actual results were quite different (the 9:3:3:1 ratio we saw earlier) and told him that the genes sorted independently.

    The reason for the independent assortment.

    To understand why independent sorting occurs, we have to skip half a century and discover that genes are physically located on chromosomes. More specifically, the two copies of a gene carried by an organism (such asYit's ajallele) are found at the same location on both chromosomes of a givenpair of counterparts. Homologous chromosomes are similar but not identical, and an organism inherits one member of the pair from each of its parents.

    The physical basis for the law of independent sorting is in meiosis I of gamete formation, when homologous pairs line up in random orientation in the center of the cell as they prepare to separate. We can get gametes with different combinations of "mother" and "father" homologues (and therefore the alleles of those homologs) because the orientation of each pair is random.

    To see what this means, compare chromosome arrangement 1 (top) and chromosome arrangement 2 (bottom) at the metaphase I stage in the diagram below. In one case, the red "parent" chromosomes fuse, while in the other case they split and mix with the blue "parent" chromosomes. If meiosis occurs many times, as it does in a pea plant, we get both arrangements, and maybeRY,Ry,ry, miryGamete classes - with equal frequency.

    1.6: The Independent Distribution Act (3)

    Genes located on different chromosomes (such asYmiRGenes are classified independently. The genes for seed color and shape are found in real life on chromosomes 1 and 7 of the pea genome.1. Genes that are widely separated on the same chromosome are also sorted independently, thanks to the crossing over, or bit swapping, of homologous chromosomes that occurs early in meiosis I.

    [see a photo]

    1.6: The Independent Distribution Act (4)

    However, there are pairs of genes that do not classify independently. When genes are close together on a chromosome, alleles on the same chromosome are often inherited as a unit. Such genes do not have an independent variety and must beconnected. We'll take a closer look at the genetic link in other articles and videos.

    [see a photo]

    1.6: The Independent Distribution Act (5)

    check your understanding

    Query \(\PageIndex{1}\)

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    Deaf1The dogs are black in color with smooth coats. We know that the black color and the smooth texture dominate the yellow color and the curly texture. Let's call the color genep/band the texture geneair-conditioning, and using upper case for the dominant form of each gene and lower case for the recessive form, we can match the genotypes of the two parent dogsBBC(black and curly) andbbcc(yellow and smooth hairy). When parent dogs are crossed, they produce a smooth black coat F1Dogs that are dihybrids:BBC.

    A cruce between back F1dihybrid dogs produces the Punnett square shown below. the F1The dogs can form four different types of gametes, which are shown along the two ejes del Punnett square. The squares of the table represent fertilization events in which the gametes fuse in the husos. Since all types of gametes are equally likely to be produced (because the genes sort independently, i.e., they do not affect each other's inheritance), all squares in the table represent events with equal probability occurring in 1/16 time.

    1.6: The Independent Distribution Act (6)

    Now we need to find the squares that correspond to the result we are interested in: a puppy with a smooth yellow coat. To have a smooth yellow coat, a puppy must acquire two recessive alleles for coat color (Bed and breakfastgenotype) and at least one dominant allele for coat structure (CCoCCGenotype). If we look at the table and circle the genotypes that meet these requirements, we will see that 3 of the 16 boxes correspond to puppies with smooth yellow coats. So we would expect 3/16 of the F2The pups have a smooth yellow coat.

    Answer: 3/16.

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    Credits and Attributions

    • Khan Academy (CC BY-NC-SA 3.0; all Khan Academy content is available for free atwww.khanacademy.org)

    [Attribution and references]


    This article is a modified derivative of:

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    The modified article is licensed under aCC BY-NC-SA 4.0license.

    Works Cited:

    1. Reid, JB and Ross, J.J. (2011). Mendel's genes: towards complete molecular characterization.Genetic 189(1), 3-10.http://dx.doi.org/10.1534/genetics.111.132118. Retrieved from www.ncbi.nlm.nih.gov/pmc/articles/PMC3176118/.


    Dihybrid. (2015). Inthe free dictionary. Recovered fromhttp://www.thefreedictionary.com/dihybrid.

    Kimball, J.W. (21 Apr 2014). Genetic link and genetic maps. InKimballs Biology. Recovered fromhttps://www.biology-pages.info/L/Linkage.html

    Purves WK, Sadava DEE, Orians GH. and Heller, H.C. (2004). Genetics: Mendel and beyond. InLife: the science of biology(7ª ed., pp. 187-212). Sunderland, MA: Sinauer Associates.

    Raven PH, Johnson GB, Mason KA, Losos J.B. and Singer, S.R. (2014). inheritance pattern. Inbiology(10th edition, AP edition, pp. 221-238). New York, NY: McGraw-Hill.

    Reece, J.B., Urry, L.A., Cain, ML, Wasserman, S.A., Minorsky, P.V. y Jackson, RB (2011). Mendel e a ideia do gene. emCampbell-Biology(August 10, pp. 267-291). San Francisco, CA: Pearson.

    Reid, JB and Ross, J.J. (2011). Mendel's genes: towards complete molecular characterization.Genetic 189(1), 3-10.http://dx.doi.org/10.1534/genetics.111.132118. Retrieved from www.ncbi.nlm.nih.gov/pmc/articles/PMC3176118/.

    The Adapt Project. (August 13, 2014). What are segregation and separate separation laws and why are they so important? InBioBuch. Recovered fromhttps://adapaproject.org/bbk_temp/tiki-index.php?page=Leaf%3A+What+are+segregation+laws+and+independent+rank+and+why+they+are+so+important% 3F.


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