Monohybrid crossover - an overview (2023)

In the progeny of monohybrid crosses or the F2 generation, Mendel repeatedly observed a phenotypic ratio of three plants with the dominant phenotype to one plant with the recessive phenotype (3:1) in the F2 generation.

Outside of:Brenner's Encyclopedia of Genetics (2nd edition), 2001

Related Terms:

  • alleles
  • gametes
  • Genotyping
  • nested gene
  • cyclization
  • phenotype
  • Mutation
  • dominant inheritance
  • codominância
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Reproduction, Reproduction and Inheritance

JK INGLIS B.Sc., B.A., Dip.Ed., M.I. Biol., emIntroduction to laboratory animal science and technology, 1980


There are cases where amonohybrid crosswill produce a homozygous recessive genotype that will not survive. Pure Yellow (AjAj) Rats die before birth. The presence of this lethal condition reduces the 3:1 ratio of F2up to 2:1. Refer toAbb. 68to demonstrate this condition.

Monohybrid crossover - an overview (1)

(Video) Mendel's P, F1, and F2 Generations Explained

FEIGE. 68. Monohybrid inheritance with lethality

Genes don't always separate as if separate entities have no effect on other genes. Genes interact in the most complex way. A good demonstration of this situation can be seen in the example shown in FIG.Abb. 69.

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gene transmission

Leon E. Rosenberg, Diane Drobnis Rosenberg, inHuman genes and genomes, 2012

The Law of Genetic Segregation

Mendel performed a large series of experiments calledmonohybrid crosses, over several years, as described inFigure 5.4. He did this with each pair of phenotypes shown in FIG.Figure 5.3, but we will use the seed color (pea color) as an example. intersection at whichparental generation(P1), purebred plants producing yellow peas with purebred plants producing green peas, Mendel observed hybrid offspring in which all the peas were yellow (theF1Generation). Plants of this F1generation were allowed to self-pollinate, and the next generation peas (F2) were counted and scored. Of the more than 8,000 peas collected, 6,022 were yellow and 2,001 were green - a nearly perfect ratio of 3 yellow to 1 green. Using each of the other six characters, Mendel achieved the same result - self-fertilization of the only character observed in F1rendered both parent figures in F2in a ratio of 3:1.

Monohybrid crossover - an overview (2)

(Video) Punnett Squares - Basic Introduction

FIGURE 5.4. Visual summary of a typical monohybrid cross performed by Mendel. In the parental cross (P) he used purebred plants that produce yellow or green peas. Only yellow plants are producedYgametes; only green plantsjgametes. On the cross, F.1, they all had peasEor hisesame genotype and all were phenotypically yellow. So he crossed aloneEVegetables and Preserved, in F2green, yellow, and green peas in a ratio of three yellow to one green (as given in the Punnett square below). This type of experiment led to his principle of genetic segregation and the first use of the words "dominant" and "recessive".

These findings were not compatible with the idea of ​​mixing. Every parental character has been restored intact in F2, instead of getting "lost" in the F1. Mendel argued that yellow peas in P1were not identical to the yellow peas in the F1why the p1Yellows were true breeds and the F1It wasn't yellow. He suggested that the feature found in the F1was dominant and that the disappearing feature in F1but reappeared in F2it was recessive. But what accounted for the reproducible 3:1 ratio?

Mendel proposed - surprisingly presciently - that each plant carried two copies of an inheritance unit for each trait, one male, one female. He further suggested that each unit occurs in alternative shapes that lead to the distinguishing characteristics he studied (yellow-green, round-wrinkled, etc.). Today we call it "units” “Gene" and be "alternative forms” “allelesHe further suggested that the two alleles found in the cells of a mature plant separate (separate) during germ cell formation and come together at fertilization, one from each parent. Mendel went in search of inheritance rights. This was the first: the law ofGen-Segregation.

The law explains the 3:1 ratio in F2as follows (Figure 5.4), with the visually accessiblePunnett Square(a graph used to predict the outcome of a given cross or breeding experiment). Purebred yellow pea plants (P1) have two copies of the dominant allele, denotedY; plants that produce only green peas have two copies of the recessive allele, denotedj. (Capital letters usually represent the dominant allele, lowercase letters the recessive.) Gametes of these P1Plants (YY and yy, denoted as "homozygous") are bothYorj. At fertilization, all are zygotesE(heterozygous). goodYdominates, all plants are yellow. When these plants self-fertilize, males and females produce gametes that are bothYorj. . . . no f2, so they are 1/4 of the offspringJJ, 1/4 areE, 1/4 aree,and 1/4 arejj. Given theYis dominant, and thatEEeare equivalent, the ratio between green and yellow peas is 3:1.

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transmission genetics

J. R. Fabian, emEncyclopedia of Genetics, 2001

(Video) Monohybrid Cross

Monohybrid cross and test cross

Mendel's cross-hybridization studies included purebred plants that differed in a single contrasting trait. Purebred homozygous parental lines have been crossed and the offspring of that cross are designated F1hybrids or monohybrids. in the f1By generation, all hybrids resembled the parent with the dominant trait. The genotype of these monohybrid or heterozygous plants can be represented as a genotypeaa, where the uppercase letter represents the dominant allele and the lowercase letter represents the recessive allele. the f1The hybrid plants were then self-pollinated (aa×aa) and this cross is known asmonohybrid cross. In the offspring of monohybrid crosses or F2Generation observed Mendel in the F2Generation. Mendel predicted that plants with an F-dominant phenotype2generation were of mixed genotypes, with some having a homozygous dominant genotypeAAand others who have a heterozygous genotypeaa. Determine the genotypes of plants with dominant phenotypes in F2Mendel generation invented the test cross.

The testcross takes an organism with a dominant phenotype but unknown genotype and crosses it with a homozygous recessive individual with a known genotype.ah. In a test cross with a plant of the genotypeAAAll offspring have the dominant phenotype and the heterozygous genotypeaa. However, if a plant with genotypeaais used in a test cross, then the genotypes of 50% of the offspring will have the genotypeaaand show the dominant trait. The other 50% have the recessive phenotype because they have the homozygous recessive genotypeah. Mendel's test-cross method is still used today in plant and animal breeding processes to determine the genotype of plants with dominant phenotypes.

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Leis de Mendel

R. Lewis, emBrenner's Encyclopedia of Genetics (2nd edition), 2013

The first generation of hybrids and beyond

The fifth section of Mendel's article repeatedly shows that dominant and recessive forms of each trait occur in a 3:1 ratio in the progeny of hybrids that have been crossed with each other. The numbers speak for themselvesTisch 3. Mendel exhibited the classic 3:1 phenotypic ratio of amonohybrid cross(a trait present in two forms, or alleles), although the terms 'phenotype' (an individual's appearance) and 'genotype' (the gene variants present) have not yet been used. This observation became known years later as Mendel's first law or the law of segregation (illustration 1). The ratios that Mendel recorded were actually the result of meiosis, the type of cell division that produces gametes. When a sperm or egg forms, the pairs of chromosomes (homologous pairs) whose DNA has been replicated separate. Likewise, the pairs of genes that make up the chromosomes separate and are distributed to different gametes. The part of meiosis that determines the combinations of genes that enter gametes and eventually express themselves in organisms is called metaphase, when the chromosomes line up in the middle of the cell.

Tisch 3. "First generation hybrid" experiments show a dominant to recessive phenotypic ratio of 3:1

To experimentNo totalDominantrecessiveRelationship
same way7324547418502,96:1
seed color8023602220013.01:1
Farbe der Samenschale9297052243,15:1
immature pod color5804281522,82:1
flower position8586512073.14:1
stem length10647872772,84:1
(Video) Ch7.1 - Monohybrid Crosses

Monohybrid crossover - an overview (3)

illustration 1. Mendel derived what would come to be known as his first law, the law of segregation, by crossing plants grown for "true" tall plants with plants grown for short, parental or P1Generation. All plants of the first branch (F1) generation were great. allow the F1Plants for self-pollination produced an F2Plant generation where tall plants outnumbered them three to one. Through new crossings of the F2plants for short plants, Mendel derived the genotypic ratio in F2Generation from one small to two large non-pedigree hybrids to one large purebred hybrid.

Reprinted with permission from Lewis R (2010)Human Genetics: Concepts and Applications, 9ª ed., Nova York: McGraw-Hill.

Mendel tracked crosses beyond the third generation and found that individuals who appeared to be dominant among the hybrids' offspring were "double important," meaning that they belonged to two species. He wrote: "...of the forms that possess the dominant character in the first generation, two-thirds have the hybrid character, while a third remain constant with the dominant character". produced the dominant phenotype. The second type, when crossed with hybrids, produced both dominant and recessive phenotypes. Plants that didn't grow properly outnumbered other plants two to one.

Today we refer to dominant-looking plants that are homozygous "constantly" as dominant. You have two copies of the dominant allele. Hybrids, called heterozygotes, have a dominant and a recessive allele. Individuals who express the recessive trait constitute the homozygous recessive class and are also true breeders. That is, when crossed with each other, they produce only homozygous recessive individuals. A monohybrid cross results in a 3:1 phenotypic ratio (dominant to recessive) and a 1:2:1 genotypic ratio (dominant homozygote to heterozygote to recessive homozygote).

Mendel performed crosses for each of the seven traits over four to six generations, each time crossing the individuals that were "genuine bred" (the homozygote dominant and the homozygote recessive) and the hybrids themselves. When he did this repeatedly, the proportion of hybrids decreased by 50% with each generation. In the 10th generation, only two hybrids would remain for every 1023 individuals of each homozygous class.

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(Video) Genetics - Mendelian Experiments - Monohybrid and Dihybrid Crosses - Lesson 3 | Don't Memorise


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