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Chapter 2: Problem 65

A sex-linked recessive allele \(c\) produces a red-green color blindness in humans. A normal woman whose father was color blind marries a color-blind man. a. What genotypes are possible for the mother of the color-blind man? b. What are the chances that the first child from this marriage will be a color-blind boy? c. Of the girls produced by these parents, what proportion can be expected to be color blind? d. Of all the children (sex unspecified) of these parents, what proportion can be expected to have normal color vision?

Short Answer

Expert verified
a. XCXc or XcXc; b. 25%; c. 25%; d. 50%

Step by step solution

01

Determine Possible Genotypes for the Mother of the Color-Blind Man

Since a sex-linked recessive allele causes color blindness, only females can be carriers without expressing the trait. The color-blind man's mother could have been either XCXc (carrier) or XcXc (color blind), because her son must have received the Xc allele from her.
02

Determine the Genotype of the Woman

The woman is normal but her father was color blind. Thus, her genotype must be XCXc, where XC denotes a normal allele and Xc denotes a color-blind allele.
03

Determine the Cross Between Parents

The cross is between a woman with XCXc (carrier) and a man with XcY (color blind).
04

Set Up a Punnett Square

Set up a Punnett square: - The woman's possible alleles are XC and Xc. - The man's possible alleles are Xc and Y. The square will show the combinations: | | Xc | Y | |:---:|:---:|:---:| |XC| XCXc | XCY | |Xc| XcXc | XcY |
05

Calculate the Probability for a Color-Blind Boy

From the Punnett square, the probability of a color-blind boy (XcY) is 25% or 1 in 4.
06

Calculate the Proportion of Color-Blind Girls

From the Punnett square, color-blind girls (XcXc) make up 25% or 1 in 4 of all girls.
07

Calculate the Proportion of Children with Normal Color Vision

Normal color vision includes XCXc and XCY. From the Punnett square, these two genotypes occur 50% or 1 in 2 of the time.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Color Blindness
Color blindness is a condition where individuals have trouble distinguishing between certain colors, commonly red and green. It results from a defect in color perception. Most cases of color blindness arise from a genetic factor linked to the X chromosome. This type of inheritance is known as sex-linked, specifically affecting more males because they have only one X chromosome. This means if a male inherits an X chromosome with the gene for color blindness, he will exhibit the trait since males have an XY chromosome pair. In contrast, females have two X chromosomes, so they can be carriers without being color blind themselves. Understanding the inheritance pattern of color blindness helps us predict how this trait can be passed down across generations.
Genotype
The term 'genotype' refers to the genetic makeup of an individual, specifically the combination of alleles they possess. In sex-linked cases such as color blindness, we denote the genotypes using 'X' to represent the X chromosome and subscripts to indicate whether the allele is normal (X_C) or causes color blindness (X_c). For example:
  • Normal Female: X_CX_C or X_CX_c (carrier)
  • Color-Blind Female: X_cX_c
  • Normal Male: X_CY
  • Color-Blind Male: X_cY
In genetic questions, understanding the genotype is crucial for predicting offspring characteristics. Always take note of parental genotypes to calculate possible outcomes in children.
Punnett Square
A Punnett square is a tool used by geneticists to predict the possible genetic outcomes of offspring from two parents. It鈥檚 like a tic-tac-toe grid where you place possible gametes from each parent on the top and side. By filling in the squares, you get all possible combinations of parental genes. If the mother's genotype is X_CX_c and the father's genotype is X_cY, we set up the square to predict potential offspring.
  • Columns 1 & 2: The mother's possible alleles X_C and X_c
  • Rows 1 & 2: The father's possible alleles X_c and Y
Through this grid, you can calculate probabilities for different traits, such as the likelihood of producing a color-blind child.
Recessive Allele
A recessive allele is an allele that expresses its trait only when two copies are present. In other words, it鈥檚 "hidden" when paired with a dominant allele. For color blindness, the allele on the X chromosome is recessive. To better understand:
  • A color-blind male has one X_c allele and displays color blindness because there is no corresponding dominant allele on the Y chromosome.
  • A female requires two copies of X_c (X_cX_c) to express color blindness. If she has one X_C (normal) allele (X_CX_c), she will be a carrier, not color-blind due to the presence of the dominant X_C allele.
Understanding the behavior of recessive alleles allows us to predict carriers and affected individuals in pedigree analysis.

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