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Animation: Crosses of Two Characters in "MendAliens"

by Pearson
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Let’s explore the inheritance of two characters in MendAliens, a mythical alien species. We will look at eye color and skin color. Let’s start by crossing a black-eyed, green-skinned MendAlien with an orange-eyed, white-skinned MendAlien. The F1 offspring all have black eyes and green skin, so we know that the allele for black eyes is dominant to the allele for orange eyes, and the allele for green skin is dominant to the allele for white skin. Now let’s make the F1 by F1 cross. If we look at just eye color in the F2 generation, there are 293 individuals with black eyes plus another 96 individuals with black eyes, for a total of 389 individuals with black eyes. There are 104 individuals with orange eyes plus another 34 individuals with orange eyes, for a total of 138 individuals with orange eyes. This is a ratio of approximately 3 black-eyed individuals to 1 orange-eyed individual, the expected ratio in the F2 generation when F1 hybrids are crossed. Similarly, there are approximately 3 green-skinned individuals for every 1 white-skinned individual. Now let's look at both eye color and skin color in the F2 generation to determine if these characters are inherited as a package or inherited independently. The F2 phenotypic ratio is approximately 9 black eyes, green skin to 3 black eyes, white skin to 3 orange eyes, green skin to 1 orange eyes, white skin. The 9 to 3 to 3 to 1 ratio indicates that this is a dihybrid cross--a cross between F1 dihybrids, individuals who are heterozygous for the two characters being followed in the cross. The 9 to 3 to 3 to 1 ratio in this dihybrid cross shows that the genes for eye color and skin color assort independently. Since this is a dihybrid cross, the parents must have been true-breeding, so we can now assign alleles to the individuals and their gametes. Let’s use capital B for the black-eyes allele and small b for the orange-eyes allele, and capital G for the green-skin allele and small g for the white-skin allele. The parent with black eyes and green skin is homozygous dominant for both characters (capital B, capital B, capital G, capital G), and the parent with orange eyes and white skin is homozygous recessive for both characters (small b, small b, small g, small g). The parental gametes are therefore capital B capital G and small b small g. The F1 MendAliens are heterozygous for both characters (capital B, small b, capital G, small g). They have black eyes and green skin because the black-eyes allele (capital B) is dominant to the orange-eyes allele (small b), and the green-skin allele (capital G) is dominant to the white-skin allele (small g). Here are the results of the F1 by F1 cross. The Punnett square for the F2 generation predicts a ratio of 9 MendAliens with black eyes and green skin to 3 MendAliens with black eyes and white skin to 3 MendAliens with orange eyes and green skin to 1 MendAlien with orange eyes and white skin. The observed F2 phenotypic ratio closely matches the expected ratio of 9 to 3 to 3 to 1. We can conclude that the genes for eye color and skin color assort independently. If an unknown MendAlien has the dominant traits of black eyes and green skin, how can we figure out its genotype? We can perform a testcross with an individual that exhibits both recessive traits--orange eyes and white skin. This diagram illustrates the results of one such test cross. In this case, there are four different types of offspring in a 1 to 1 to 1 to 1 ratio, so the MendAlien with black eyes and green skin must have all four possible alleles --the black eyes allele (capital B), the orange eyes allele (small b), the green skin allele (capital G), and the white skin allele (small g). In other words, our mystery MendAlien is heterozygous for both eye color and skin color.
Let’s explore the inheritance of two characters in MendAliens, a mythical alien species. We will look at eye color and skin color. Let’s start by crossing a black-eyed, green-skinned MendAlien with an orange-eyed, white-skinned MendAlien. The F1 offspring all have black eyes and green skin, so we know that the allele for black eyes is dominant to the allele for orange eyes, and the allele for green skin is dominant to the allele for white skin. Now let’s make the F1 by F1 cross. If we look at just eye color in the F2 generation, there are 293 individuals with black eyes plus another 96 individuals with black eyes, for a total of 389 individuals with black eyes. There are 104 individuals with orange eyes plus another 34 individuals with orange eyes, for a total of 138 individuals with orange eyes. This is a ratio of approximately 3 black-eyed individuals to 1 orange-eyed individual, the expected ratio in the F2 generation when F1 hybrids are crossed. Similarly, there are approximately 3 green-skinned individuals for every 1 white-skinned individual. Now let's look at both eye color and skin color in the F2 generation to determine if these characters are inherited as a package or inherited independently. The F2 phenotypic ratio is approximately 9 black eyes, green skin to 3 black eyes, white skin to 3 orange eyes, green skin to 1 orange eyes, white skin. The 9 to 3 to 3 to 1 ratio indicates that this is a dihybrid cross--a cross between F1 dihybrids, individuals who are heterozygous for the two characters being followed in the cross. The 9 to 3 to 3 to 1 ratio in this dihybrid cross shows that the genes for eye color and skin color assort independently. Since this is a dihybrid cross, the parents must have been true-breeding, so we can now assign alleles to the individuals and their gametes. Let’s use capital B for the black-eyes allele and small b for the orange-eyes allele, and capital G for the green-skin allele and small g for the white-skin allele. The parent with black eyes and green skin is homozygous dominant for both characters (capital B, capital B, capital G, capital G), and the parent with orange eyes and white skin is homozygous recessive for both characters (small b, small b, small g, small g). The parental gametes are therefore capital B capital G and small b small g. The F1 MendAliens are heterozygous for both characters (capital B, small b, capital G, small g). They have black eyes and green skin because the black-eyes allele (capital B) is dominant to the orange-eyes allele (small b), and the green-skin allele (capital G) is dominant to the white-skin allele (small g). Here are the results of the F1 by F1 cross. The Punnett square for the F2 generation predicts a ratio of 9 MendAliens with black eyes and green skin to 3 MendAliens with black eyes and white skin to 3 MendAliens with orange eyes and green skin to 1 MendAlien with orange eyes and white skin. The observed F2 phenotypic ratio closely matches the expected ratio of 9 to 3 to 3 to 1. We can conclude that the genes for eye color and skin color assort independently. If an unknown MendAlien has the dominant traits of black eyes and green skin, how can we figure out its genotype? We can perform a testcross with an individual that exhibits both recessive traits--orange eyes and white skin. This diagram illustrates the results of one such test cross. In this case, there are four different types of offspring in a 1 to 1 to 1 to 1 ratio, so the MendAlien with black eyes and green skin must have all four possible alleles --the black eyes allele (capital B), the orange eyes allele (small b), the green skin allele (capital G), and the white skin allele (small g). In other words, our mystery MendAlien is heterozygous for both eye color and skin color.