A cross in which only one characteristic is tracked is a monohybrid (MAHN-oh-HIE-brid) cross. The offspring of a monohybrid cross are called monohybrids. A cross between a pea plant that is true-breeding for producing purple flowers and one that is true-breeding for producing white flowers is an example of a monohybrid cross. Biologists use a diagram called a Punnett (PUHN-uht) square, such as the one shown in Figure 9-7, to aid them in predicting the probable distribution of inherited traits in the offspring. The following examples show how a Punnett square can be used to predict the outcome of different types of crosses.
Example 1: Homozygous X Homozygous
Figure 9-7 shows a cross between a pea plant homozygous for purple flower color (PP) and a pea plant homozygous for white flower color (pp). The alleles carried in gametes of the homozygous dominant parent are represented by P's on the left side of the Punnett square. The alleles carried in gametes of the homozygous recessive parent are represented by p's across the top of the Punnett square. Each box within the Punnett square is filled in with the letters, or alleles, that are above it and at left of it outside the square. The combinations of alleles in the four boxes indicate the possible genotypes that can result from the cross. The predicted genotype is Pp in every case. Thus, there is a 100 percent probability that the offspring will have the genotype Pp and thus the phe-notype purple flower color.
Example 2: Homozygous X Heterozygous
Figure 9-8 shows a cross between a guinea pig that is homozygous dominant for the trait of black coat color (BB) and a guinea pig that is heterozygous for this trait (Bb). The letter b stands for the recessive allele. Genotype bb results in a brown coat. Notice that two possible genotypes can result from this cross: BB or Bb. The probability of an offspring having the genotype BB is 2/4, or 50 percent. The probability of an offspring having the genotype Bb is also 2/4, or 50 percent. You could expect about 50 percent of the offspring resulting from this cross to be homozygous dominant for the black coat and about 50 percent to be heterozygous dominant for a black coat. The probable phenotype is black coat color in every case; thus, 4/4, or 100 percent, of the offspring are expected to have a black coat. What would happen if the homozygous guinea pig were homozygous recessive for coat color? The homozygote would have the genotype bb. Crossing a bb guinea pig with a Bb guinea pig is likely to produce about 50 percent Bb offspring and about 50 percent bb offspring.
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