Question

Pigment in mouse fur is only produced when the \(C\) allele is present. Individuals of the \(c c\) genotype are white. If color is present, it may be determined by the \(A, a\) alleles. \(A A\) or \(A a\) results in agouti color, while a results in black coats. (a) What \(F_{1}\) and \(F_{2}\) genotypic and phenotypic ratios are obtained from a cross between \(A A C C\) and aacc mice? (b) In three crosses between agouti females whose genotypes were unknown and males of the aacc genotype, the following phenotypic ratios were obtained:

Short answer

Answer: The genotypes of the three agouti females are: (1) A_C_ (2) A_Cc (3) AaCC

Step-by-step solution

Perform the cross between AACCaacc mice

Use Punnett squares to calculate the genotypes of offspring from AA CC x aa cc parents. CAA x CcAa => 100% offspring are AaCc as result F1 genotypic ratio: 100% Aa Cc (they would be heterozygous at both loci)

Identify the F1 phenotypic ratio

Determine which phenotype corresponds to Aa Cc genotype. Aa Cc => Agouti and colored F1 phenotypic ratio: 100% Agouti-colored mice

Perform a self-cross among F1 mice to get F2 generation

Self-cross F1 mice to get the offspring's genotypes. AaCc x AaCc => Punnett square would result in 16 possible combinations of alleles

Identify F2 genotypic and phenotypic ratios

Group the genotypes and find their corresponding phenotypes: - 1 AACC => Agouti - 2 AaCC => Agouti - 1 AAcc => Agouti - 1aaCC => Black - 2aaCc => Black - 1aacc => White - 4 AaCc => Agouti - 4 Aa cc => White F2 genotypic ratio: 1 AACC : 2 AaCC : 1 AAcc : 1 aaCC : 2 aa Cc : 1 aacc : 4 Aa Cc : 4 Aa cc F2 phenotypic ratio: 9 Agouti : 3 Black : 4 White (b) Unknown agouti female genotypes #Analysis#: Learning the three agouti females' genotypes while knowing that the phenotypic ratios resulted from crosses with aacca-male can help determine the genotypes.

Cross agouti females and aacc males

Use the info given by the crosses and phenotypic ratios to infer the agouti females' genotypes. Cross results: (1) 5 agouti: 4 black: 3 white (2) 4 agouti: 4 black: 0 white (3) 12 agouti: 0 black: 4 white

Deduce the genotype possibilities of each female

Check each female's genotype by comparing the phenotypic ratios of their offspring to the expected results. (1) A-C-aacc: 2:2:1 Cross would result => 1 AC: 1 ACc: 2 ac: 2 ac: 1 cc This cross matches with the given phenotypic ratio (2) A-Cc-aacc = no cc offspring Cross would result => 1 AC: 1 ACc: 1 Ac: 1 Ac: 0 cc This cross matches with the given phenotypic ratio (3) Aa CC x aacc: 3Agouti:1White Cross would result => 1 AAc: 1 AAc: 1 AAc: 1 cc This cross matches with the given phenotypic ratio So the genotypes of the agouti females are: (1) A_C_ (2) A_Cc (3) AaCC

Key concepts

Punnett Square

The Punnett square is an essential tool in genetics used to predict the possible genotypes of offspring from a particular cross. It's named after Reginald Punnett, who devised the approach to visualize the outcome of breeding experiments. For example, when crossing two mice, one with the genotype AACCaacc, you can use a Punnett square by listing the possible gametes of each parent on the top and side of a grid, then combining them to see all potential genotypes of their offspring. The resulting genotypes for the F1 generation from an AACCaacc cross would all be AaCc, showing heterozygosity at both loci.

The Punnett square offers a straightforward visual representation of how alleles combine and segregate during reproduction. It is crucial because it allows researchers and students to easily determine the probability of offsprings inheriting a particular combination of alleles.

Phenotypic Ratios

Phenotypic ratios refer to the relative numbers of offspring with different physical traits — or phenotypes — resulting from a genetic cross. These ratios provide valuable information about dominance and can indicate which traits are likely to appear in a population. In the mouse example, after crossing two AACCaacc parents, all F1 offspring display the agouti-colored phenotype, giving us an F1 phenotypic ratio of 100% agouti-colored mice.

For the F2 generation, a variety of phenotypes appear based on the combination of alleles they inherit. The F2 phenotypic ratio in this case is 9 agouti : 3 black : 4 white. By understanding how these phenotypic ratios come about, students can predict the likelihood of a certain trait appearing in the next generation of a genetic cross.

Genotypic Ratios

Genotypic ratios describe the expected number of offspring with different genotypes that can arise from a genetic cross. These ratios help to explain the underlying genetics behind the phenotypic ratios. For the F1 generation of mice in the AACCaacc cross, the ratio is simple: since all offspring are AaCc, the ratio is 100%. However, in the F2 generation, produced by crossing two F1 individuals (AaCc x AaCc), the genotypic ratio becomes much more complex with 16 possible combinations of alleles.

This complexity leads to an F2 genotypic ratio of 1 AACC : 2 AaCC : 1 AAcc : 1 aaCC : 2 aa Cc : 1 aacc : 4 Aa Cc : 4 Aa cc. Understanding these ratios is essential for grasping how genetic variation is maintained in a population and how certain traits get passed from parents to offspring.

Inheritance Patterns

Inheritance patterns dictate how traits are transmitted from parent to offspring and can be quite diverse. In this exercise, we see an example of simple Mendelian inheritance, where traits are determined by single genes with dominant and recessive alleles. In the case of mouse coat color, the presence of the C allele is necessary to produce pigment. If an organism is homozygous recessive for this gene (cc genotype), it will have white fur regardless of the alleles for the A gene.

Patterns like complete dominance, co-dominance, and incomplete dominance play a fundamental role in determining phenotypes from genotypes. Dominant alleles mask the effect of recessive ones, which is why the cc genotype results in white fur in mice. By studying inheritance in controlled crosses, scientists have developed laws that explain how traits are passed down and can predict the outcomes of genetic crosses.