Question

If two fruit flies, heterozygous for genes of one allelic pair, were bred together and had 200 offspring ... (a) about how many would have the dominant phenotype? (b) of these offspring, some will be homozygous dominant and some heterozygous. How is it possible to establish which is which?

Short answer

(a) About 150 offspring would have the dominant phenotype, since 75% of the offspring show this phenotype (25% AA and 50% Aa). (b) To establish which offspring are homozygous dominant (AA) or heterozygous (Aa), perform a test cross by breeding the offspring with an organism that has a homozygous recessive genotype (aa). If the offspring show a mix of dominant and recessive phenotypes, the unknown genotype is heterozygous (Aa); if only dominant phenotype appears, it is homozygous dominant (AA).

Step-by-step solution

Determine the Parental Genetic Makeup

The genetic makeup of both parent fruit flies is given as heterozygous, represented by Aa.

Creating a Punnett Square

To predict the cross results between two heterozygous fruit flies, we can build a Punnett Square. A Punnett Square is a tool used in genetics to determine possible offspring genotypes and phenotypes. Let's set up the Punnett Square: A a ----------- A | AA Aa ----------- a | Aa aa -----------

Ratio of Genotypic Distribution

Observing the Punnett Square, we can determine the genotypes' ratio: - 25% AA (Homozygous dominant) - 50% Aa (Heterozygous) - 25% aa (Homozygous recessive) This is also known as a classic 1:2:1 ratio.

Calculate the Number of Offspring with Dominant Phenotypes

Now let's answer the questions given: (a) To calculate the number of offspring with the dominant phenotype (A), we must look at both the homozygous dominant (AA) and heterozygous (Aa) genotypes. Since the dominant phenotype appears in 25% AA and 50% Aa, summing these ratios will give the percentage of offspring with the dominant phenotype. Percentage of dominant phenotype = 25% + 50% = 75% Now, multiply the percentage by the total number of offspring (200). Number of dominant offspring = 75% * 200 = 150 So, about 150 offspring would have the dominant phenotype. (b) To determine which offspring are homozygous dominant (AA) and which are heterozygous (Aa), we can use a test cross.

Test Cross to Determine Homozygous and Heterozygous Offspring

A test cross involves crossing an organism with an unknown genotype with an organism that is homozygous recessive (aa) for the trait. If the offspring display a mix of dominant and recessive phenotypes, the unknown genotype is heterozygous (Aa). However, if only the dominant phenotype is displayed, the unknown genotype is homozygous dominant (AA).

Key concepts

Genetic Cross

A genetic cross refers to the mating of two organisms to analyze the inheritance patterns of certain traits. This is frequently performed in a controlled scientific setting to predict the likely genotypes and phenotypes of the offspring. The simplest form of genetic cross, a monohybrid cross, considers one characteristic controlled by a single pair of genes. For example, in the exercise with the fruit flies, the cross between two heterozygous flies involves one gene with two alleles (A and a), representing a monohybrid cross. The resulting patterns of inheritance are often visualized using a Punnett square, a graphical representation used by geneticists to calculate the probability of an offspring inheriting specific alleles.

In the exercise provided, the Punnett square is used to determine the potential outcomes of the genetic cross between heterozygous fruit flies, and it simplifies the process of understanding how dominant and recessive traits are passed on to the offspring.

Dominant and Recessive Alleles

Alleles are different forms of a gene that determine distinct traits that can be passed from parents to offspring. Dominant alleles, typically represented by capital letters (e.g., A), are those that express their trait even if only one copy is present in the genotype. Conversely, recessive alleles, represented by lowercase letters (a), only express their traits when two copies are present (homozygous recessive). When considering the exercise with the fruit flies, despite heterozygous flies carrying both dominant (A) and recessive (a) alleles, the dominant trait would prevail in the phenotype.

It's important to note that dominance does not indicate superiority or higher frequency in a population, but simply refers to the allele's ability to mask the expression of the alternate (recessive) allele in a heterozygous organism. The Punnett square demonstrates how these alleles interact and predict the likelihood of each genotype and phenotype in the offspring.

Test Cross

A test cross is an essential method in genetics used to determine an organism's genotype when its phenotype expresses the dominant trait. It involves mating the organism in question (with an unknown genotype) with a homozygous recessive organism for the trait under study. The outcome of the offspring phenotypes can reveal information about the parent's genotype.

In the exercise, to differentiate between homozygous dominant (AA) and heterozygous (Aa) offspring, scientists could use a test cross involving the fruit fly with the unknown dominant phenotype and another fly that is homozygous recessive (aa) for the gene of interest. If any of the resulting offspring exhibit the recessive phenotype, the test subject must be heterozygous. If all offspring have the dominant phenotype, the test subject is likely homozygous dominant. This test cross serves as a simple yet powerful tool in understanding inheritance patterns.

Phenotype and Genotype

Phenotype refers to the observable characteristics of an organism, such as its physical appearance, behavior, or biochemical properties. The genotype, however, relates to the organism's specific genetic makeup, which includes all the alleles inherited from its parents. While phenotypes can be seen, genotypes must be inferred unless genetic testing is done.

In our fruit fly scenario, both homozygous dominant (AA) and heterozygous (Aa) flies have the same dominant phenotype because the allele 'A' masks the presence of the 'a' allele. To know the genotype of an individual with a dominant phenotype, one would have to carry out a genetic test or a test cross, as described earlier. Understanding the distinction between phenotype and genotype is crucial when analyzing the outcomes of genetic crosses and predicting the inheritance patterns of various traits.