Question

Genetics Problem: Mom is red-green color blind, and Dad has normal color vision. What is the percent chance that a son will be color blind? That a daughter will be color blind? That a daughter will be a carrier? (pp. 485,486 )

Short answer

Answer: There is a 100% chance that both sons and daughters will be colorblind, and a 0% chance that daughters will be carriers of the colorblind gene.

Step-by-step solution

Identifying the Genotypes of the Parents

The mother is red-green color blind, which means she has two mutated X chromosomes (Xc) as women carry two X chromosomes. Mother's genotype: XcXc The father, on the other hand, has normal color vision. Since he is not colorblind, he has one normal X chromosome (X) and one Y chromosome (as all men have). Father's genotype: XY

Creating a Punnett Square

Now that we have the genotypes of the parents, we can create a Punnett Square to determine the possible genotypes of their children. X Y __________________ Xc | XcX XcY | | | |________________| Xc | XcX XcY | | | |________________|

Identifying the Genotypes of the Children and Calculating the Percent Chances

From the Punnett square, we can see the possible genotypes of their children: - XcX (Daughter, colorblind) - XcY (Son, colorblind) - XcX (Daughter, colorblind) - XcY (Son, colorblind) Now let's calculate the percent chances for the son and daughter to be color blind and for the daughter to be a carrier: Percent chance that a son will be color blind: 100% (Both possible sons have the genotype XcY, which is colorblind) Percent chance that a daughter will be color blind: 100% (Both possible daughters have the genotype XcX, which is colorblind) Percent chance that a daughter will be a carrier: 0% (Since both possible daughters are colorblind, none of them are carriers)

Key concepts

Punnett Square

Understand the Punnett Square, and it will become much easier to predict the genetic outcomes for offspring. So, what is a Punnett Square? It's essentially a tool used by geneticists to predict an offspring's possible genotypes based on the parents' genotypes. You draw a grid, placing one parent's potential gametes along the top and the other's along the side.

Each square then represents the union of one gamete from each parent, showing the possible genetic makeup of their future children. In the case of colorblindness—a trait that's X-linked recessive—a Punnett Square is particularly useful because it clearly visualizes how X and Y chromosomes from each parent can combine in their offspring.

When we look at the cross concerning the red-green colorblind mother (XcXc) and the father with normal vision (XY), all the Punnett Square combinations for their children result in XcX (a colorblind daughter) or XcY (a colorblind son). This straightforward representation allows us to see at a glance that all sons will be colorblind and all daughters will also inherit colorblindness, not just being carriers.

Genotype

The term 'genotype' refers to the genetic makeup of an individual, or more simply, the combination of alleles someone possesses. Alleles are versions of a gene, one inherited from each parent. Using the concept of genotype, we explore genetic conditions like colorblindness.

The genetics problem we're examining hinges on understanding the specific genotypes involved in X-linked recessive traits. For instance, the mother's genotype (XcXc) indicates she possesses two recessive alleles for colorblindness, one on each of her X chromosomes, thus expressing the trait. The father's genotype (XY) includes a normal X chromosome and a Y chromosome, which does not bear the allele for colorblindness.

Discussing genotypes helps us understand potential offspring outcomes beyond just physical traits—it delves into the possibility of carrying a trait without expressing it, which is pivotal for comprehending inheritance patterns and the probability of trait manifestation in future generations.

X-linked Recessive Traits

X-linked recessive traits, such as red-green colorblindness, are often misunderstood. How do they work? Well, these traits are carried on the X chromosome and require two copies—one on each X chromosome for females—to express the trait because they are recessive. Males, who have only one X chromosome paired with one Y chromosome, will express the trait if they inherit the recessive allele from their mother.

In our colorblindness example, since the mother has two copies of the colorblindness allele (Xc), any of her X chromosomes passed on to her children come with the colorblindness trait. And since the father provides Y chromosomes to his sons, any son would be colorblind because Y cannot counteract the recessive allele.

Understanding X-linked recessive inheritance makes it clear why, in this scenario, colorblindness is guaranteed for all children of this particular mother and father. Such knowledge is vital for prenatal genetic counseling and understanding the risks of inheriting X-linked recessive conditions.