What would be the expected frequency of agouti brown offspring in the litter?

A) 3/16

B) 1/8

C) 9/16

D) 1/2

E) 1/4

F) not enough information given

Photo of the hair of a guinea pig showing the influence of the agouti gene
Photo of the hair of a guinea pig showing the influence of the agouti gene

The correct answer is F) not enough information given

The color of fur in mammals is determined by a number of genes that interact with each other in various ways. The agouti gene is the allele that determines how various types of melanin pigment, namely eumelanin and pheomelanin are distributed.

The presence of the agouti allele results in a light band on the fur where the melanin has been blocked. When the non-agouti gene is present the hair will always have a solid color.

It is important to note that other genes also influence the hair color, and that agouti is only impacting the distribution of the pigment. The actual pigment color is determined by a B gene, and it can be either dominant (B) which is black, or recessive (b) which is brown.

The only way to predict the genotype and phenotype ratios in the offspring is to know what the parental genotypes are. Thus the expected frequency of agouti brown (cinnamon) offspring will vary depending on parental genes.

The genotype ratio would be 3/16 agouti brown offspring if both the mother and father are genotype AaBb, which is agouti. Another gene present in mice, which is known as gene C acts in an epistatic fashion to influence fur color.

In the recessive homozygous form of cc, this gene blocks any pigment from being expressed. Thus the offspring with these alleles in the genotype are albinos with no melanin showing.

The Agouti gene

The normal agouti gene determines the distribution of pheomelanin and eumelanin in the skin. The pheomelanin is responsible for red or yellow pigment while the eumelanin is responsible for black or brown pigment. The agouti gene influences the color of the hair that mammals will have.

When the agouti gene is present the hair will often look greyish or streaky because of the yellow band that is present along with the dark pigment. The non-agouti gene, on the other hand, results in a solid color to the hair.

The brown color allele is the common wild-type agouti genetic allele, which undergoes transcription at certain times of development of mice.

Scientists discovered that there was another agouti allele in mice that coded for a yellow coat, but this agouti allele was lethal. Mice with this allele do survive if they have the heterozygous condition and are only carrying one allele for yellow since it is a recessive trait.

However, the condition is lethal in the homozygous form. This means that any mice that have this homozygous genotype will die at the embryo stage.

Another frequently studied agouti allele is the variable yellow allele. How this allele is expressed depends on the degree of methylation of the DNA.

Dominance and recessiveness

The normal agouti fur gene in rodents such as mice produces hairs which contain a band of light color pigment present on the hair shaft. The result is alternating bands of light and dark colored fur, which is, in fact, the dominant trait in many species of mammals, including mice.

The agouti allele is, therefore, a dominant trait while the allele for a solid color fur is a recessive trait. If Mendelian rules apply then dominant alleles are always expressed if they are inherited from one or both parents.

This means that even if only one allele is inherited the trait will still be evident in the offspring. Recessive alleles, on the other hand, can only be expressed in the phenotype if they are inherited from both the father and the mother.

Due to these rules of dominance and recessiveness, the dominant traits tend to be more frequently expressed compared with the recessive traits.

In mice, solid fur color is a recessive trait, thus it is expected that it will be less frequently expressed in the population of mice when compared with the agouti fur.

However, other genetic factors such as epistasis and modifier genes can also impact frequencies of genetic alleles. In addition, natural selection will act on a population to select the fittest genotypes in the population.

Dominant alleles are indicated using an upper case letter, while recessive alleles are indicated using a lower case letter. For instance, A = agouti allele, a = solid color allele.

Pigment color

An additional gene is involved in pigment color in the mice, and it is the gene that produced the actual color of the fur. This B gene can produce either a black colored fur or a brown colored fur.

The black is dominant and can be represented by B, while the brown color is recessive and can be represented by b.

If two agouti mice with genotypes AaBb are crossed, then the following would be found in the offspring:

9/16 would be agouti with the genotype AaBb

3/16 would be agouti brown(cinnamon) with the genotype Aabb

3/16 would be solid black with genotype aaBb

1/16 would be solid brown with genotype aabb

The cinnamon fur color is actually a streaked brown (agouti brown), not a solid color since the agouti allele is present in the genotype. There are actually additional genes that also interact to determine the fur color of mice.

Epistasis

In epistasis, one gene can interfere with how another gene is expressed and can influence if the second gene is even phenotypically expressed. This is a type of gene expression that is evident in the color pigment of mice fur.

The additional gene C enables the color of coat pigment to be expressed, while the recessive c blocks the color from being exhibited. The cc has an epistatic effect on the other genes, and so these individuals are albinos and have no pigment expressed.

References

  1. T Dobzhansky, AJF Griffiths, A Robinson (2019). Heredity. Retrieved from Encyclopedia Britannica.
  2. CB Kaelin, X Xu, LZ Hong, VA David, KA McGowan, et al. (2012). Specifying and sustaining pigmentation patterns in domestic and wild cats. Science.
  3. JC Jiménez-Chillarón, et al. (2014). The Agouti and the Axin Fused Mouse Models. Retrieved from sciencedirect.com.
  4. GS Barsch (2001). Agouti. Retrieved from sciencedirect.com.
  5. O Wang, OJ Majzoub (2011). Agouti and agouti-related protein. Retrieved from sciencedirect.com.

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