How do you explain phenotypic ratios?
Video TranscriptWhat is the expected phenotypic ratio of a dihybrid cross between two heterozygous individuals? (A) Nine to three to three to one. (B) 12 to three to one. (C) 16 to zero to zero to zero. (D) Nine to six to one. Show This question asked for the expected phenotypic ratio of a dihybrid cross between two heterozygous individuals. There’s a lot of terminology involved in this question, so let’s review a few terms first. A dihybrid cross refers to a mating experiment involving two different genes. This is easy to remember because the root word di- means two. Dihybrid crosses can be done using a Punnett square with 16 boxes. Heterozygous refers to an individual that has two different alleles for a gene in their genotype. So the heterozygous individuals in this example would have the genotype uppercase Y lowercase y uppercase F lowercase f. There are two different letters, Y and F, to represent two different genes in our dihybrid cross. You may recall that a genotype is the genetic makeup, or alleles, of an organism. So our four letters here uppercase Y lowercase y uppercase F lowercase f is an example of a genotype that is heterozygous. This question is wanting the phenotypic ratio. And the phenotype refers to the observable traits of an organism determined by the genotype. In this example, uppercase Y will represent the allele for yellow fur, while lowercase y will represent the allele for green fur. And uppercase F will represent the allele for a fuchsia tail, while lowercase f represents the allele for a red tail. In order to answer this question, we can construct a theoretical dihybrid cross with our two heterozygous individuals. We can use the FOIL method to identify the possible gametes from the parents’ genotype. FOIL stands for first, outside, inside, last. The gametes for the first combination will be uppercase Y uppercase F. The gametes for the outside combination will be uppercase Y lowercase f. The gametes for the inside combination will be lowercase y uppercase F. And the gametes for the last combination will be lowercase y lowercase f. These will be the same for both parents as both parents have the same genotype. When we construct a dihybrid Punnett square, the four gamete possibilities are placed across the top of the Punnett square for parent one and down the left side of the Punnett square for parent two. Now, we fill in each square with the possible gametes from parent one at the top and parent two on the left side so that each square has four letters, two alleles for each trait. For example, the first square will be all uppercase, YYFF, while this square will be heterozygous for both traits: uppercase Y lowercase y uppercase F lowercase f. Once all of the genotypes are filled in, we can start determining phenotypes. Any genotypes with an uppercase Y will have yellow fur. And any genotype with an uppercase F will have a fuchsia tail. We can represent that by filling in those squares like so, with the top half of the square representing the fur color and the bottom half representing the tail color. Any individuals with two lowercase y’s will have a green fur color. And any individuals with two lowercase f’s will have a red tail color. Out of the 16 theoretical individuals, nine exhibit the yellow and fuchsia phenotype. Three exhibit the green and fuchsia phenotype. Three exhibit the yellow and red phenotype. And one exhibits the green and red phenotype. Therefore, the ratio of phenotypes is nine to three to three to one. So the correct answer to our question about the expected phenotypic ratio of a dihybrid cross between two heterozygous individuals is (A) nine to three to three to one. For a variety of reasons, the phenotypic ratios observed from real crosses rarely match the exact ratios expected based on a Punnett Square or other prediction techniques. There are many possible explanations for deviations from expected ratios. Sometimes
these deviations are due to sampling effects, in other words, the random selection of a non-representative subset of individuals for observation. On the other hand, it may be because certain genotypes have a less than 100% survival rate. For example, Drosophila crosses sometimes give unexpected results because the more mutant alleles a zygote has the less likely it is to survive to become an adult. Genotypes that cause death for embryos or larvae are underrepresented
when adult flies are counted. A statistical procedure called the
chi-square (χ2) test can be used to help a
geneticist decide whether the deviation between observed and expected ratios is due to sampling effects, or whether the difference is so large that some other explanation must be sought by re-examining the assumptions used to calculate the expected ratio. The procedure for performing a chi-square test is covered in the labs. What does a 1 2 1 phenotypic ratio mean?A cross of two F1 hybrids, heterozygous for a single trait that displays incomplete dominance is predicted to give a 1:2:1 ratio among both the genotypes and phenotypes of the offspring.
What does the 9 3 3 1 ratio for phenotypes mean?The 9:3:3:1 ratio simply means that nine are wild-type meaning they are normal; six exhibit one mutant and one normal character, three are normal for one trait the other three are normal for the opposite trait; one has both mutant phenotypes.
What does a 2 1 phenotypic ratio mean?When a phenotypic ratio of 2 : 1 is observed, there is probably a lethal allele. Individuals homozygous for the lethal allele (tt in this case) do not survive embryonic development, and are not born.
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