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Ch. 3 - Cell Division and Chromosome Heredity
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 3 - Cell Division and Chromosome HeredityProblem 17a
Chapter 3, Problem 17a

In cats, tortoiseshell coat color appears in females. A tortoiseshell coat has patches of dark brown fur and patches of orange fur that each in total cover about half the body but have a unique pattern in each female. Male cats can be either dark brown or orange, but a male cat with tortoiseshell coat is rarely produced. Two sample crosses between males and females from pure-breeding lines produced the tortoiseshell females shown.


Cross I     P: dark brown male × orange female
           F₁: orange males and tortoiseshell females
Cross II    P: orange male × dark brown female
          F₁: dark brown males and tortoiseshell females


Explain the inheritance of dark brown, orange, and tortoiseshell coat colors in cats.

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1
Step 1: Understand the genetic basis of coat color in cats. Coat color in cats is determined by a gene located on the X chromosome. The gene has two alleles: one for orange fur (Xᴼ) and one for dark brown fur (Xᴮ). Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
Step 2: Explain the inheritance pattern in females. Female cats can inherit one Xᴼ allele and one Xᴮ allele, resulting in a tortoiseshell coat due to X-chromosome inactivation. This process randomly silences one X chromosome in each cell, leading to patches of orange and dark brown fur.
Step 3: Analyze Cross I. In Cross I, a dark brown male (XᴮY) is crossed with an orange female (XᴼXᴼ). The offspring inherit one X chromosome from the mother and one sex chromosome from the father. Female offspring (XᴼXᴮ) are tortoiseshell, while male offspring (XᴼY) are orange.
Step 4: Analyze Cross II. In Cross II, an orange male (XᴼY) is crossed with a dark brown female (XᴮXᴮ). The offspring inherit one X chromosome from the mother and one sex chromosome from the father. Female offspring (XᴮXᴼ) are tortoiseshell, while male offspring (XᴮY) are dark brown.
Step 5: Summarize the rarity of tortoiseshell males. A tortoiseshell male would require two X chromosomes (XᴮXᴼY), which is possible only in cases of chromosomal abnormalities such as Klinefelter syndrome (XXY). This explains why tortoiseshell males are rarely produced.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Sex-Linked Inheritance

In cats, coat color is influenced by genes located on the X chromosome. Since females have two X chromosomes (XX), they can express two different color alleles, leading to the tortoiseshell pattern. Males, having one X and one Y chromosome (XY), can only express one color allele, which is why tortoiseshell males are rare.
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Codominance

The tortoiseshell coat color in cats is an example of codominance, where both alleles (dark brown and orange) are expressed simultaneously in the phenotype. This results in a unique pattern of patches rather than a blend of colors, allowing for distinct areas of dark and orange fur on tortoiseshell females.
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Genotype and Phenotype

The genotype refers to the genetic makeup of an organism, while the phenotype is the observable physical traits. In the case of tortoiseshell cats, the genotype of females includes both color alleles, resulting in the tortoiseshell phenotype, whereas males can only exhibit one color due to their single X chromosome.
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Related Practice
Textbook Question

A wild-type male and a wild-type female Drosophila with red eyes and full wings are crossed. Their progeny are shown below.

What is/are the genotype(s) of females with purple eye? Of males with purple eye and miniature wing?

Textbook Question

A woman with severe discoloration of her tooth enamel has four children with a man who has normal tooth enamel. Two of the children, a boy (B) and a girl (G), have discolored enamel. Each has a mate with normal tooth enamel and produces several children. G has six children—four boys and two girls. Two of her boys and one of her girls have discolored enamel. B has seven children—four girls and three boys. All four of his daughters have discolored enamel, but all his boys have normal enamel. Explain the inheritance of this condition.

Textbook Question

In a large metropolitan hospital, cells from newborn babies are collected and examined microscopically over a 5-year period. Among approximately 7500 newborn males, six have one Barr body in the nuclei of their somatic cells. All other newborn males have no Barr bodies. Among 7500 female infants, four have two Barr bodies in each nucleus, two have no Barr bodies, and the rest have one. What is the cause of the unusual number of Barr bodies in a small number of male and female infants?

Textbook Question

In cats, tortoiseshell coat color appears in females. A tortoiseshell coat has patches of dark brown fur and patches of orange fur that each in total cover about half the body but have a unique pattern in each female. Male cats can be either dark brown or orange, but a male cat with tortoiseshell coat is rarely produced. Two sample crosses between males and females from pure-breeding lines produced the tortoiseshell females shown.


Cross I     P: dark brown male × orange female

           F₁: orange males and tortoiseshell females

Cross II    P: orange male × dark brown female

           F₁: dark brown males and tortoiseshell females


Why are tortoiseshell cats female?

Textbook Question

In cats, tortoiseshell coat color appears in females. A tortoiseshell coat has patches of dark brown fur and patches of orange fur that each in total cover about half the body but have a unique pattern in each female. Male cats can be either dark brown or orange, but a male cat with tortoiseshell coat is rarely produced. Two sample crosses between males and females from pure-breeding lines produced the tortoiseshell females shown.


Cross I     P: dark brown male × orange female

           F₁: orange males and tortoiseshell females

Cross II    P: orange male × dark brown female

           F₁: dark brown males and tortoiseshell females


The genetics service of a large veterinary hospital gets referrals for three or four male tortoiseshell cats every year. These cats are invariably sterile and have underdeveloped testes. How are these tortoiseshell male cats produced? Why do you think they are sterile?

Textbook Question

The gene causing Coffin–Lowry syndrome (OMIM 303600) was recently identified and mapped on the human X chromosome. Coffin–Lowry syndrome is a rare disorder affecting brain morphology and development. It also produces skeletal and growth abnormalities, as well as abnormalities of motor control. Coffin–Lowry syndrome affects males who inherit a mutation of the X-linked gene. Most carrier females show no symptoms of the disease but a few carriers do. These carrier females are always less severely affected than males. Offer an explanation for this finding.