Question

Two black female mice are crossed with a brown male. In several litters, female I produced 9 black offspring and 7 brown; female II produced 57 black offspring. What deductions can you make about the inheritance of black and brown coat color in mice? What are the genotypes of the parents in this case?

Short answer

The black coat color in mice is dominant and brown coat color is recessive. From the data, Female I has a heterozygous genotype 'Bb', Female II has a homozygous dominant genotype 'BB', and the brown male has a homozygous recessive genotype 'bb'. This inheritance pattern explains the observed offspring phenotypes.

Step-by-step solution

Understand the problem and define the genotypes

To solve this problem, we need to use the information about the offspring's phenotypes to deduce the genotypes of the parents. Coat color in mice can be inherited in various ways, but we will assume a simple Mendelian inheritance model, where only one gene determines the coat color. Let's use 'B' for the dominant allele (black) and 'b' for the recessive allele (brown).

Analyze the data from Female I

Female I is black, and she crossed with a brown male. She produced 9 black offspring and 7 brown offspring. Since the brown coat color is recessive, the brown male must have the genotype 'bb'. Since the black female produced both black and brown offspring, she must have a heterozygous genotype 'Bb'.

Analyze the data from Female II

Female II is also black, and she was crossed with the same brown male (genotype 'bb'). She produced 57 black offspring. Since no brown offspring were produced, we can deduce that Female II must have a homozygous dominant genotype 'BB'. This is because if she had a heterozygous 'Bb' genotype, brown offspring could have been produced.

Deduce the inheritance pattern of coat color

Based on our analysis, we can deduce that the black coat color is dominant and brown coat color is recessive. A mouse with at least one dominant 'B' allele will have a black coat color, while a mouse with two recessive 'b' alleles will have a brown coat color.

Determine the genotypes of the parent mice

After analyzing the data, we can conclude that: - Female I has a heterozygous genotype 'Bb' (black coat color) - Female II has a homozygous dominant genotype 'BB' (black coat color) - The brown male has a homozygous recessive genotype 'bb' (brown coat color) These genotypes explain the observed phenotypes of the offspring produced in the crosses described in the exercise.

Key concepts

Genotype

Genotype refers to the genetic makeup of an organism, the blueprint that determines an organism's characteristics. This blueprint is comprised of alleles, which are various forms of a gene that arise by mutation and are found at the same place on a chromosome. In our exercise, we described the genotype using letters: 'B' represents the allele for black coat color, while 'b' signifies the allele for brown coat color in mice.

Organisms have two alleles for each gene— one inherited from each parent. The combination of these two alleles determines the genotype. For instance, the brown male mouse in our problem has the genotype 'bb', meaning he has two copies of the recessive allele. The genotype plays a crucial role in Mendelian inheritance, serving as the genetic foundation that will determine the phenotype, or physical expression, of the trait.

Phenotype

Phenotype, on the other hand, is the observable physical or biochemical characteristics of an organism, as determined by both genotype and environment. In the context of our exercise, the phenotype would be the color of the mice's coats — black or brown. Unlike the genotype, the phenotype is what we can see and measure directly.

It's important to note that while the genotype sets the potential for a trait, the phenotype also involves the influence of environmental factors, which can sometimes modify the expression of genetic traits. In the mice example, the gene for the black coat color (B) is expressed in the phenotype only if the allele is present in the genotype, giving us a direct link between the genotype 'Bb' or 'BB' and the phenotype 'black coat'.

Dominant and Recessive Alleles

Genes can exist in different forms, or alleles, as we've seen with the 'B' and 'b' alleles for coat color. One critical aspect of Mendelian inheritance is the concept of dominant and recessive alleles.

Dominant Alleles

A dominant allele is one that expresses its trait even in the presence of a different allele. In the mice, the B allele for black coat color is dominant. This means that if an organism has at least one B allele (BB or Bb genotypes), it will have a black coat.

Recessive Alleles

A recessive allele, like 'b' for brown coat color, is only expressed phenotypically when two copies are present (bb genotype). If paired with a dominant allele, the trait from the recessive allele will not be visible. The concept of dominance and recessiveness explains why the brown male, despite mating with black females, could produce brown offspring only if the female was heterozygous Bb.

Mendel's Laws

Mendel's laws of inheritance are key principles that capture the essence of genetic transmission from one generation to the next. Key to understanding our textbook exercise is the Law of Segregation and the Law of Independent Assortment.

The Law of Segregation

This law states that during the formation of gametes (eggs and sperm), the two alleles responsible for a trait separate from each other, so that each gamete carries only one allele for the trait. Our black mice female I being 'Bb' would create gametes with either 'B' or 'b', as observed by the mix of black and brown offspring.

The Law of Independent Assortment

According to this law, alleles for different traits are distributed to sex cells (&gametes) independently of one another. This principle explains how different traits can combine in a variety of ways. Our exercise focuses specifically on coat color, but if considering multiple traits (like coat color and eye color), this law becomes particularly relevant.

Understanding Mendel's laws helps explain why the offspring from our textbook problem displayed the patterns of color that they did, influenced by the inheritance of dominant and recessive alleles from their parents.