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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

    Law of Segregation

    Law of Segregation definition, importance, and examples, on Biology Online, the world’s most comprehensive dictionary of biology terms and topics.

    Dictionary > Law of Segregation

    Law of Segregation

    Law of Segregation n.

    Definition: two members of a pair of alleles separate during gamete formation

    Table of Contents

    Mendel’s Laws of Inheritance

    The father of genetics, Gregor Mendel, reported his findings in 1860 that were initially unpopular during his time but eventually gained traction and became so widely accepted that his findings paved the way for the founding of the science of genetics. Three different laws of inheritance were formulated based on his experimenting with pea plant reproduction. His experiments explained the transfer of genetic traits from one generation to the next. These laws have significantly expanded the understanding of genetic inheritance and resulted in new experimental methods becoming developed.

    Three Different Mendel inheritance laws are as follows:

    Law of SegregationLaw of Independent AssortmentLaw of Dominance

    Law of Segregation – Biology Definition

    Question: What is the law of segregation?

    Answer: It is also called the first law of inheritance. The law of segregation states that:

    ‘‘The two copies of each genetic factor segregate during the development of gametes, to ensure that each parent’s offspring attains one factor.’’

    OR

    ‘‘During the development of the gamete, each gene is segregated in such a way that the gamete consists of just one allele for that gene.’’

    The copies of a gene are segregated when any individual produces gametes so that each gamete accepts only one copy. One allele is received by a gamete.

    The exact proof of this was later discovered as the process of meiosis was understood. In meiosis, the mother’s and the father’s genes are separated, and so the character alleles are separated into two distinct gametes.

    Difference between allele and gene

    A gene is an essential part of the DNA that defines a specific trait; an allele is a specific form of a gene. The expression of traits is the key role of the genes. Alleles are important for the variations in which the trait can be expressed.

    Mendel, who had no knowledge of chromosomes, proposed that the determining factors of inheritance are discrete “unit factors” (now called genes) that maintain their integrity from the time that the zygote is formed through the time that it matures and produces its own gametes. During gamete formation, the members of these paired “unit factors” segregate from one another and enter into separate gametes.

    What is segregation?

    Segregation is the separation of allele pairs (different traits of the same gene) during meiosis so that they can transfer specifically to separate gametes.

    Figure 1: Maternal and paternal alleles segregating during meiosis. Credit: BIL 250 – Lecture 2.

    Law of Segregation (biology definition): one of the Mendelian Laws of Inheritance stating that the two members of a pair of alleles separate during gamete formation. Consequently, each gamete contains only one member of every pair of genes. Synonym: Law of Purity of Gametes. Compare: Law of Independent Assortment, the Law of Dominance, and the Law of Unit Characters.

    ‌Why is Mendel’s Law of segregation defined as the purity law of gametes?

    In genetics, the Law of Segregation shows that because a gamete carries either a recessive or a dominant allele but not both the alleles at the same time. This is the reason how this law is also known as the law of purity of gametes.

    The segregation law is Mendel’s first law. It states that during meiosis alleles segregate. The fundamental principles of this law are posited as follows:

    There can be more than one type of allele for a gene.

    During the process of meiosis, when gametes are formed, the allele pairs segregate, i.e. they separate.

    For the determination of a Mendelian trait, two alleles are involved — one is recessive and the other is dominant.

    Even without influencing each other, they stay on together in pure form. They don’t mix or blend. Therefore the segregation law is also known as the law of purity of gametes for this reason. During the formation of gametes, the segregation of two alleles of a gene usually occurs because of the segregation of homologous chromosomes during meiosis. Tetrads (where each tetrad consists of four chromatids of a homologous pair that form by synapsis) separate during anaphase I, and then sister chromatids of homologous chromosomes separate during anaphase II.

    A gamete is a cell, which is involved in fertilization.  The egg and sperm are the female and male gametes in humans, respectively. Human eggs contain only one type of sex chromosome, i.e. X chromosome. Human sperm cells contain either X or Y chromosome. This determines the sex of the offspring. Under the segregation law, for any characteristic, including the dominant or recessive trait, a gamete will receive one of the two alleles.

    Example

    The alleles for a Mendelian trait may either be dominant or recessive and may be passed down from parent to child (animal or plant). In plants, for example, the color trait of the flower will depend on the type of allele inherited by the offspring. Each parent plant transfers one of the alleles to their offspring. And these sets of alleles in the offspring will depend on the chromosomes of the two gametes uniting at fertilization. These two sets of chromosomes randomly segregated during gamete formation (wherein meiosis is a part of the process).

    Source : www.biologyonline.com

    Laws of Inheritance

    Boundless Biology

    Mendel's Experiments and Heredity

    Laws of Inheritance

    Laws of Inheritance Mendel's Laws of Heredity

    Mendel formed the Laws of Heredity (the Law of Segregation and the Law of Independent Assortment) from his pea plant experiments.

    LEARNING OBJECTIVES

    Discuss the methods Mendel utilized in his research that led to his success in understanding the process of inheritance

    KEY TAKEAWAYS

    Key Points

    By crossing purple and white pea plants, Mendel found the offspring were purple rather than mixed, indicating one color was dominant over the other.

    Mendel's Law of Segregation states individuals possess two alleles and a parent passes only one allele to his/her offspring.

    Mendel's Law of Independent Assortment states the inheritance of one pair of factors ( genes ) is independent of the inheritance of the other pair.

    If the two alleles are identical, the individual is called homozygous for the trait; if the two alleles are different, the individual is called heterozygous.

    Mendel cross-bred dihybrids and found that traits were inherited independently of each other.

    Key Terms

    homozygous: of an organism in which both copies of a given gene have the same alleleheterozygous: of an organism which has two different alleles of a given geneallele: one of a number of alternative forms of the same gene occupying a given position on a chromosome

    Introduction

    Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent organisms to their children; it underlies much of genetics. The tenets were initially derived from the work of Gregor Mendel published in 1865 and 1866, which was "re-discovered" in 1900; they were initially very controversial, but they soon became the core of classical genetics.

    The laws of inheritance were derived by Gregor Mendel, a 19th century monk conducting hybridization experiments in garden peas (Pisum sativum). Between 1856 and 1863, he cultivated and tested some 28,000 pea plants. From these experiments, he deduced two generalizations that later became known as Mendel's Laws of Heredity or Mendelian inheritance. He described these laws in a two part paper, "Experiments on Plant Hybridization", which was published in 1866.

    Mendel's Laws

    Mendel discovered that by crossing true-breeding white flower and true-breeding purple flower plants, the result was a hybrid offspring. Rather than being a mix of the two colors, the offspring was purple flowered. He then conceived the idea of heredity units, which he called "factors", one of which is a recessive characteristic and the other dominant. Mendel said that factors, later called genes, normally occur in pairs in ordinary body cells, yet segregate during the formation of sex cells. Each member of the pair becomes part of the separate sex cell. The dominant gene, such as the purple flower in Mendel's plants, will hide the recessive gene, the white flower. After Mendel self-fertilized the F1 generation and obtained an F2 generation with a 3:1 ratio, he correctly theorized that genes can be paired in three different ways for each trait: AA, aa, and Aa. The capital A represents the dominant factor while the lowercase a represents the recessive.

    Mendel's Pea Plants: In one of his experiments on inheritance patterns, Mendel crossed plants that were true-breeding for violet flower color with plants true-breeding for white flower color (the P generation). The resulting hybrids in the F1 generation all had violet flowers. In the F2 generation, approximately three-quarters of the plants had violet flowers, and one-quarter had white flowers.

    Mendel stated that each individual has two alleles for each trait, one from each parent. Thus, he formed the "first rule", the Law of Segregation, which states individuals possess two alleles and a parent passes only one allele to his/her offspring. One allele is given by the female parent and the other is given by the male parent. The two factors may or may not contain the same information. If the two alleles are identical, the individual is called homozygous for the trait. If the two alleles are different, the individual is called heterozygous. The presence of an allele does not promise that the trait will be expressed in the individual that possesses it. In heterozygous individuals, the only allele that is expressed is the dominant. The recessive allele is present, but its expression is hidden. The genotype of an individual is made up of the many alleles it possesses. An individual's physical appearance, or phenotype, is determined by its alleles as well as by its environment.

    Mendel also analyzed the pattern of inheritance of seven pairs of contrasting traits in the domestic pea plant. He did this by cross-breeding dihybrids; that is, plants that were heterozygous for the alleles controlling two different traits. Mendel then crossed these dihybrids. If it is inevitable that round seeds must always be yellow and wrinkled seeds must be green, then he would have expected that this would produce a typical monohybrid cross: 75 percent round-yellow; 25 percent wrinkled-green. But, in fact, his mating generated seeds that showed all possible combinations of the color and texture traits. He found 9/16 of the offspring were round-yellow, 3/16 were round-green, 3/16 were wrinkled-yellow, and 1/16 were wrinkled-green. Finding in every case that each of his seven traits was inherited independently of the others, he formed his "second rule", the Law of Independent Assortment, which states the inheritance of one pair of factors (genes) is independent of the inheritance of the other pair. Today we know that this rule holds only if the genes are on separate chromosomes

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