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4.2.1: Monohybrid Crosses and Segregation

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    25731
  • True Breeding Lines

    Mendel used true-breeding lines of pea plants, which are in-bred populations of plants or animals in which all parents and their offspring (over many generations) have the same phenotypes with respect to a particular trait. True breeding lines are useful, because they are typically assumed to be homozygous for the alleles that affect the trait of interest. When two individuals that are homozygous for the same alleles are crossed, all of their offspring will all also be homozygous. The continuation of such crosses constitutes a true breeding line or strain. A large variety of different strains, each with a different, true breeding character, can be collected and maintained for genetic research.

     

    Monohybrid Crosses

    A monohybrid cross is one in which both parents are heterozygous (or a hybrid) for a single (mono) trait. The trait might be petal color in pea plants. When conducting crosses, the first generation is called P (or P0), the second generation is F1 (F is for filial), and the next generation is F2.

    Figure \(\PageIndex{1}\): (a) A true-breeding line (b) A monohybrid cross produced by mating two different pure-breeding lines. (Original-Deholos-CC:AN)

    Using monohybrid crosses, Mendel observed that although different alleles could influence a single trait, they remained indivisible and could be inherited separately. Additionally, the allele could be present but invisible in one generation, only to reappear in the next generation.

    Definition: Mendel's First Law

    The Law of Segregation states that during gamete formation, the two alleles at a gene locus segregate from each other; each gamete has an equal probability of containing either allele.

     

    Punnett Squares

    Given the genotypes of any two parents, we can predict the genotypes of gametes that will be produced during meiosis. Using that information, we can predict all of the possible genotypes of the offspring. Furthermore, if we also know the dominance relationships for all of the alleles, we can predict the phenotypes of the offspring. A convenient method for calculating the expected genotype and phenotype ratios from a cross was invented by Reginald Punnett. A Punnett square is a matrix in which all of the possible gametes produced by one parent are listed along one axis, and the gametes from the other parent are listed along the other axis. Each possible combination of gametes is listed at the intersection of each row and column. Punnett squares can also be used to calculate the frequency of types offspring that are expected.

    parent big P cross little p can make gametes big P or little p which label the top and side of the box. The 4 squares in the box are then big P big P, big P little p, big P little P, and little p little p
    Figure \(\PageIndex{1}\): Top: A Punnett square shows the possible offspring of a monohybrid cross between Pp and Pp parents. Bottom Video: How to set up and complete the square (Copyright CC BY Leacock)

    Query \(\PageIndex{1}\)

     

    Test Crosses

    Knowing the genotypes of an individual is usually an important part of a genetic experiment. However, genotypes cannot be observed directly; they must be inferred based on phenotypes. Because of dominance, it is often not possible to distinguish between a heterozygote and a homozgyote based on phenotype alone. To determine the genotype of a specific individual, a test cross can be performed, in which the individual with an uncertain genotype is crossed with an individual that is homozygous recessive for all of the loci being tested.

    For example, if you were given a pea plant with purple flowers it might be a homozygote (AA) or a heterozygote (Aa). You could cross this purple-flowered plant to a white-flowered plant, because you know the genotype of the plant is homozygous recessive aa. Depending on the genotype of the purple-flowered parent, you will observe different phenotypic ratios in the F1 generation. If the purple-flowered parent was a homozygote, all of the F1 progeny will be purple. If the purple-flowered parent was a heterozygote, the F1 progeny should segregate purple-flowered and white-flowered plants in a 1:1 ratio.

    Video \(\PageIndex{2}\): How to use a test cross (Copyright CC BY Leacock)

    Query \(\PageIndex{2}\)

    Contributors and Attributions

     

    Molecular basis of dominant and recessive alleles

    What determines whether alleles are dominant or recessive?

    The alleles have different DNA sequences. Because the sequence of DNA contains information to make products, different sequences can lead to different products. One advantage of diploid species is that there are two copies of every sequence. If one sequence makes a "faulty" or non-functional product, it would be called a loss-of-function allele. However, there is likely another sequence that produces a "correct" or functional product. For most genes a single wild-type (usually the normal / functional) allele is capable of producing enough product for the cell, resulting in a dominant phenotype. However, if both copies of the gene are loss-of-function alleles, there will not be any functional protein and the recessive phenotype will be observed.

    An example from pea plants

    Mendel studied pea plants in which the peas could be round or wrinkled, with wrinkled being the recessive characteristic. The gene that determines this trait (originally represented as R) has since been identified and named SBE1 (for starch-branching enzyme). The wild-type sequence encodes an enzyme that catalyzes a chemical reaction of carbohydrates to form branched chains of starch in plants. When SBE1 protein is present and functional, the peas produce branched starch and exhibit a round shape. When SBE1 protein is absent, the amount of branched starch is reduced, but the levels of disaccharides are higher, resulting in increased water absorption. Later, this water will be lost and the pea will become wrinkled.

    Molecular studies of DNA in round (RR) and wrinkled (rr) plants revealed an insertion of about 800 base pairs in an exon of the SBE1 gene in the rr plants (Bhattacharyya et al, 1990). The insertion is derived from a transposon and disrupts the SBE1 protein-coding sequence. Therefore, the RR plants have two copies of a functional SBE1 allele (SBE1+/+) to make functional SBE1 enzyme, while rr plants have two copies of the SBE1 gene in which the sequence is disrupted (SBE1-/-) and cannot produce any functional SBE1 enzyme.

    So what happens in a heterozygous Rr (SBE1+/-) plant? These plants have one chromosome with the SBE1+ allele and one chromosome with the SBE1- allele. The SBE1- allele is still transcribed, but does not code for the functional protein during translation. The SBE1+ allele is transcribed and translated into a functional enzyme, starch-branching occurs, and the peas have the round phenotype. Although the total amount of SBE1 enzyme is less than homozygous dominant peas, it is sufficient for the round phenotype and therefore dominant. In this case, molecular discoveries 125 years after Mendel's work reveal the reason for the dominance of the R allele over the r allele.

    References

    Bhattacharyya MK, Smith AM, Ellis TH, Hedley C, Martin C. The wrinkled-seed character of pea described by Mendel is caused by a transposon-like insertion in a gene encoding starch-branching enzyme. Cell. 1990 Jan 12;60(1):115-22. doi: 10.1016/0092-8674(90)90721-p. PMID: 2153053.