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    Monohybrid genetic cross to determine dominant trait

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    This is the genetic problem that I need to understand how to complete.

    Sand lizards in white sands new mexico:
    If for each species a lizard that was homozygous for the dark Mc1r allele had been crossed with a lizard that was homozygous for the light Mc1r allele the resulting F1 generation would have obtained the percentage of light skinned offspring shown below in coloum 2. If they had gone on to cross 2 offspring from the F1 generation, the resulting F2 generation would have obtained the results shown in column 3 below.
    for each of the 2 species state whether the light allele is dominant or recessive to the dark Mc1r allele. Draw mating diagrams to support your answer.It needs to show the crosses and the genotype and phenotype ratios that result in the F1 anf F2 generations.

    Lizard species % of light skinned lizards % of lizards in F2
    in F1 generation generation
    Sceloporus undulatus 100% 75%

    Aspidoscelis inornata 0% 25%

    Thanks in advance for any help you can offer. Bit confused on how to lay out the diagrams.

    © BrainMass Inc. brainmass.com December 24, 2021, 9:05 pm ad1c9bdddf

    SOLUTION This solution is FREE courtesy of BrainMass!

    This cross represents a basic monohybrid cross. You are examining one trait, lizard color in this mating. In order to determine which characteristic, light or dark, is dominant you must first understand the terms dominant and recessive. Each organism contains two copies of every gene, usually refered to as alleles of that gene (one copy came from the mother, one from the father). These two alleles interact to determine what the organisms phenotype will be (what they will look like). The lizards that began the cross (the parental generation) were both said to be homozygous. This means that their two alleles are identical. The light colored lizard will have two light alleles while the dark colored lizard will have two dark alleles. When these lizards are crossed the light lizard will give a light colored allele to the offspring and the dark lizard will give a dark colored allele to the offspring. The resulting F1 generation is then said to be heterozygous, their two alleles are not the same. By examining what color these F1 lizards are you can determine which trait is dominant. When there are two different alleles in the genotype the one that overpowers the other and is seen in the phenotype is considered the dominant trait. So if all the F1 lizards are light then the light allele is the dominant one, if they are all dark than the dark allele is dominant.
    This can also be seen by diagraming out a series of punent squares. These boxes represent how an organism passes its genes on from one generation to the other. The first step in doing the diagram is to assign representative letters to the alleles.

    For this diagram we will say the light allele is Mc1r-L and the dark allele is Mc1r-D. The parents were both homozygous so they will have two copies of the same gene

    P1 = Mc1r-L, Mc1r-L (light phenotype) P2 - Mc1r-D, Mc1r-D (dark phenotypes)

    Parental cross = Mc1r-L, Mc1r-L x Mc1r-D, Mc1r-D (you use an x in between the genotypes to designate a mating)

    To diagram a punnet square you draw a box with 4 squares. On the outside of the box go the possible alleles that each parent could have given to the offspring. Inside the box go the possible offspring genotypes after the egg and sperm combined (alleles on the outside of the boxes come together) The punnet square for the parental cross should look like this:
    Mc1r-D Mc1r-D
    Mc1r-L Mc1r-L, Mc1r-D Mc1r-L, Mc1r-D
    Mc1r-L Mc1r-L, Mc1r-D Mc1r-L, Mc1r-D

    Along the top are the possible alleles from the dark parent, along the side are the possible alleles from the light parent, because these parents were homozygous they only have one type of allele to give. Inside the boxes are the combination of alleles that the offspring can get. In this case you can see that 4/4 boxes, or 100%, of offspring are Mc1r-L, Mc1r-D. When you look at the information in the problem you see that 100% of F1 offspring were light colored. This indicates that the Mc1r-L allele was the dominant one because it overpowered the Mc1r-D allele in the heterozygote.

    To look at the F2 generation you will take two offspring from F1 and cross them using another punnet square as diagramed below

    F1 cross: Mc1r-L, Mc1r-D x Mc1r-L, Mc1r-D

    Mc1r-L Mc1r-D
    Mc1r-L Mc1r-L, Mc1r-L Mc1r-L, Mc1r-D
    Mc1r-D Mc1r-L, Mc1r-D Mc1r-D, Mc1r-D

    This time when you draw the square each individual of the cross has one of two alleles to give so they are different. When you combine the alleles in the boxes you get a variety of outcomes for the F2 generation. In a monohybrid cross those outcomes will have a 1:2:1 genotype ratio and a 3:1 phenotype ratio. There is 1 homozygous individual for the light allele (Mc1r-L, Mc1r-L), 2 heterozygous individuals (Mc1r-L, Mc1r-D), and 1 homozgous individual for the dark allele (Mc1r-D, Mc1r-D). For phenotypes we know the Mc1r-L, Mc1r-L will be light colored because they only have light alleles and the Mc1r-D, Mc1r-D will be dark colored because they only have dark alleles. The 2 heterozygous individuals Mc1r-L, Mc1r-D will be colored based on which was dominant. We determined in the F1 generation that light was dominant so the heterozygoutes will be light colored. When you count the boxes then you have 3 that are light colored (1 homozygous and 2 heterozygous) and 1 that is dark colored. This is the 3:1 ratio.

    The other way that can be written is 3 out of the 4 boxes were light, ¾ is also 75%, and 1 out of the 4 boxes was dark, ¼ is also 25%. When you look at the data provided in the problem 75% of the F2 offspring were light colored, matching what you see in the punnet square.

    As a note I used Mc1r-L and Mc1r-D as allele designations because at the start of the problem we did not know which was dominant or recessive. Often this can be simplified to a single letter representing the gene with the uppercase form being the dominant trait and the lowercase form being the recessive. If we were to use those designations we could say the light allele is represented by L and the dark by l.
    Homozygous light would then have the genotype LL
    Homozygous dark would have the genotype ll
    Heterozygous would have the genotype Ll
    The diagrams can be written with either designation as long as the punnet squares are diagramed correctly.
    This should help you write out the diagrams yourself, using the punnet squares and explanation provided.
    Good luck :)

    This content was COPIED from BrainMass.com - View the original, and get the already-completed solution here!

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