In 1905, the French biologist Lucien Cuénot discovered an interesting mutant mou
ID: 226364 • Letter: I
Question
In 1905, the French biologist Lucien Cuénot discovered an interesting mutant mouse displaying a yellow coat color. He bred the mouse and its progeny for several generations but could never obtain a pure breeding stock of yellow mice! Normal mice have a coat color called Agouti which appears gray. When he crossed pure breeding gray Agouti mice with the yellow mouse, half the progeny had the gray Agouti coat and half had the light yellow coat. When he crossed the F1 yellow mice with each other, he found that 1/3 of the F2 progeny had the gray Agouti coat and 2/3 had the yellow coat. When he crossed the F2 yellow progeny with pure breeding Agouti mice, he found again that they produced 50% gray Agouti mice and 50% yellow mice.
Based on these results, you would conclude that relative to the Agouti coat color, the Yellow coat color is:
A. Completely Dominant
B. Incompletely Dominant
C. Co-dominant
D. Incompletely Penetrant
E. Recessive
Cuénot was never able to produce a pure breeding strain of yellow mice even after trying several crosses for several years. Provide a one-sentence explanation for why Cuénot was unable to obtain yellow mice but not a pure breeding stock of yellow mice.
Explanation / Answer
(A) It is complete dominant. As per definition, dominant allele shows phenotype in homozygous.
Genes are located on chromosomes and consist of DNA. They are passed from parents to their offspring through reproduction. Genes also contain information about a specific characteristic or trait and can either be dominant or recessive. Each gene has a designated place on every chromosome, called a locus. All copies of a gene are not identical and alternative forms of a gene is called alleles that lead to the alternative or different form of one trait. Alleles are helpful in identification of the two members of a gene pair, which produce opposite contrasting phenotypes, e.g., b is an allele of B and vice versa. When the two alleles of a gene are identical, the individual is homozygous for that trail, and on the other hand, if there are two different alleles, the individual is heterozygous. A homozygous pair can be either dominant (AA, BB) or recessive (aa, bb). Heterozygous pairs are made up of one dominant and one recessive allele (Aa, Bb). In heterozygous individuals, only one allele, the dominant one, is able to express itself, while the other allele, the recessive, is hidden but still present. The dominant genes are denoted by upper case letters and recessive genes are denoted by lower case letters. The word genotype was created to identify genes of an individual and phenotype for the external appearance of the trait and genes.Phenotype and genotype are terms used to describe the difference between the visible expressions of the trait versus the actual gene makeup. An individual, which expresses a dominant trait may carry a recessive allele, but the recessive expression is hidden by its dominant partner. Mendel's observations from these experiments can be summarized in two principles:
The principle of segregation states that for any particular trait, the pair of alleles of each parent separate and only one allele passes from each parent on to an offspring. Which allele in one parent's pair of alleles is inherited is a matter of chance. We now know that this segregation of alleles occurs during the process of sex cell formation, i.e., during meiosis. The principle of independent assortment states that the different pairs of alleles are passed to offspring independently of each other. The result is that new combinations of genes are possible in the offspring that are not present in either parent. For example, the inheritance of the ability to produce purple flowers instead of white ones in a pea plant does not make it more likely that it will also inherit the ability to produce yellow pea seeds in contrast to green ones. Similarly, the principle of independent assortment also explains why the human inheritance of a particular eye colour donot increase or decrease the likelihood of having six fingers on each hand. In the present day, we know this is due to the fact that the genes for independently assorted traits are located on different chromosomes.