Question

If two black mice are crossed, what is the ratio of black to white mice in the offspring?

1) 10:3
2) 3:10
3) 2:1
4) 1:2

Answer 1

Without additional genetic information on the black mice, one would assume a Mendelian 3:1 phenotypic ratio for black to white mice in the offspring if we consider black to be the dominant trait. This is under the assumption that no lethal alleles are involved and the black mice are heterozygous.

Step-by-step explanation:

Understanding Genetics and Offspring Ratios

When two black mice are crossed, the ratio of black to white mice in the offspring can be determined using Punnett square analysis, a tool that predicts the outcome of a genetic cross. The question assumes that the black color is dominant over the white color. However, the exact ratio depends on the genetic makeup (genotype) of the parents. If both parents are homozygous dominant (BB for black), all offspring will be black, resulting in a 4:0 black to white ratio, which is not an option provided. If both are heterozygous (Bb), the Mendelian genetics principles suggest a 3:1 phenotypic ratio (black to white) in the offspring. This assumes that the black phenotype (B) is dominant and the white phenotype (b) is recessive.

If the cross involves recessive lethal alleles that are expressed in utero, the genotypic ratio can be altered from the standard Mendelian ratio. However, without further information on the genetics of the mice in question, we can only assume the most common ratios. Therefore, the closest answer from the provided options without more genetic context would be a 3:1 ratio, favoring the black phenotype over white if we're dealing with complete dominance and no lethal alleles are involved.

However, it's important to note that without additional context specifying the genotypes of the parents or any information about potential lethal alleles, an exact ratio cannot be confidently provided. Assuming a typical Mendelian scenario with complete dominance and no lethal alleles, option 3 (3:1) would be the logical answer for the ratio of black to white mice in the offspring from the crossing of two black mice.

Answer 2

The ratio of black to white mice in the offspring of two black mice would typically follow Mendelian ratios and would be 3:1 (option 2) if both black mice were heterozygous. However, if a recessive lethal allele were involved, the surviving progeny might display a ratio of 2 black to every 1 white (2:1), assuming heterozygosity and black being dominant.

Step-by-step explanation:

If two black mice are crossed, the ratio of black to white mice in the offspring depends on the underlying genetics. Without specific genetic information, we can turn to Mendelian principles for a basic understanding. For black fur color to be prevalent in mice, it is typically dominant over white fur color. If the question implies that both mice are heterozygous for a black fur allele (B) and a white fur allele (b), the expected genotypic ratio following Mendelian inheritance would be 3 black (BB or Bb) to 1 white (bb). When considering phenotype, this ratio translates to 3 black mice for every white mouse in the offspring. However, if there is a recessive lethal allele involved, as suggested by additional information provided, some offspring may not survive, altering expected ratios.

In cases like the one described for recessive lethal alleles, where heterozygous parents (Aa) mate, and the allele 'a' can cause lethality when in homozygous form (aa), the genotypic ratio observed in the offspring is 2:1 instead of the Mendelian 3:1, because the homozygous recessive offspring do not survive to be counted. Therefore, the true ratio for viable mice would be 1 AA to 2 Aa, with no aa surviving. If black mice are heterozygous and a similar scenario applies, a mating between two such mice may also yield a similar ratio, with 2 black to 1 white offspring, option 3:10 would not apply as it doesn't adhere to typical ratios explained by Mendelian genetics, or common genetic outcomes that include lethal alleles.