Concept explainers
Campomelic dysplasia (CMD1) is a congenital human syndrome featuring malformation of bone and cartilage. It is caused by an autosomal dominant mutation of a gene located on chromosome 17. Consider the following observations in sequence, and in each case, draw whatever appropriate conclusions are warranted.
- (a) Of those with the syndrome who are karyotypically 46, XY, approximately 75 percent are sex reversed, exhibiting a wide range of female characteristics.
- (b) The nonmutant form of the gene, called SOX9, is expressed in the developing gonad of the XY male, but not the XX female.
- (c) The SOX9 gene shares 71 percent amino acid coding sequence homology with the Y-linked SRY gene.
- (d) CMD1 patients who exhibit a 46, XX karyotype develop as females, with no gonadal abnormalities.
HINT: This problem asks you to apply the information presented in this chapter to a real-life example. The key to its solution is knowing that some genes are activated and produce their normal product as a result of expression of products of other genes found on different chromosomes.
(a)
To determine: The explanation for the sex reversal in Campomelic dysplasia (CMD1).
Introduction: The mutation is the change in the nucleotide sequence of the gene, which result in either the formation of a defective protein or no protein at all. The mutation can also alter the regulation of certain genes leading to their hyperactivity or hypoactivity.
Explanation of Solution
The individuals with Campomelic dysplasia contain an autosomal dominant mutation in chromosome number 17. The disease causes the malformation of bone and cartilage. The karyotype of the patients with this disorder is 46 chromosomes with X and Y sex chromosomes. The individual thus cannot show typical sexual differentiation. This is because gene CMD1 plays an important role in controlling sexual differentiation. The mutation in the gene can interfere with the transcription, translation, or the receptor functioning of genes responsible for male sex differentiation. Therefore, the individual can exhibit some female characteristics.
(b)
To determine: The reason for the expression of the non-mutant form of SOX9 in gonads of male and not females
Introduction: Genetic abnormality can take place when there is any alteration in the gene expression or the nucleotide composition. The typical effect of genetic change is the decrease in the fertility of the affected individual.
Explanation of Solution
The non-mutant gene SOX9 is present in an individual without Campomelic dysplasia. This finding indicates that the gene SOX9 is responsible for the development of the gonads in males. This gene is not found in the females having XX chromosomes, and so the gene does not play any significant role in the sex differentiation in females.
(c)
To determine: The reason for 71% homology between SOX9 gene and Y-linked SRY gene.
Introduction: Homology is the degree of similarity between the genes, proteins, or other biomolecules. The homology can be attributed to the similar ancestry or the same point of origin of the molecules or structures.
Explanation of Solution
The SOX9 gene shares high similarity or homology (71%) with the SRY gene. The high degree of similarity can account for the similar functions shared between the genes. As the Y-linked SRY gene plays a role in male sex differentiation, the similarity in the genes can explain the function of SOX9 gene in the development of undifferentiated gonadal tissue into testes.
Thus, the high degree of similarity between the Y-linked SRY gene and SOX9 gene indicates the similarity in the function and role in male sex differentiation.
(d)
To determine: The reason for the CMD1 female patients exhibiting 46, XX karyotype with no gonadal abnormalities.
Introduction: Karyotype is the representation of the genetic composition in an individual. It is expressed as the total number of chromosomes followed by the composition of sex chromosomes and genetic abnormality if it is present.
Explanation of Solution
The individual with Campomelic dysplasia with the karyotype 46, XX will develop into a female and will not exhibit any gonadal abnormalities. This is because the SOX9 gene that gets mutated and hampers the sexual differentiation is only in males and not in females. The SOX9 gene does not play any role in the female sex differentiation. Therefore, the alterations in this gene will not cause any gonadal abnormalities in females due to the absence of its function in female gonad development.
Thus, the female with CMD1 will not have any gonadal abnormalities as the SOX9 gene that gets mutated does not play a role in sexual differentiation in females.
Want to see more full solutions like this?
Chapter 7 Solutions
Concepts of Genetics (12th Edition)
- Tay–Sachs disease is caused by loss-of-function mutations ina gene on chromosome 15 that encodes a lysosomal enzyme.Tay–Sachs is inherited as an autosomal recessive condition.Among Ashkenazi Jews of Central European ancestry, about1 in 3600 children is born with the disease. What fraction ofthe individuals in this population are carriers?arrow_forwardAssuming no other abnormalities, classify each of the following human sex chromosome complements as to whether or not individuals with that complement will be morphologically male or female. Three human sex chromosome complements will be classified as male and three will be classified as female. XXY XYY XXX XO XY XXarrow_forwardWhen a female melanotic fly is crossed with a normal male, the progeny are produced: 123 normal females, 125 melanotic females, and 124 normal males. In subsequent crosses between melanotic females and normal males, melanotic females are frequently obtained, but never any melanotic males. Provide a possible explanation for the inhertiacne of the melanotic mutation (Hint: The cross produces twice as many female progeny as male progeny)arrow_forward
- In a karyotype of a man whose wife has had several miscarriages, it was discovered that he had onlv 45 chromosomes. However, he displayed no obvious phenotypic defects. Furthermore, one of the chromosomes of pair 15 was abnormally long. Which of the following terms best describes the condition of this man? a. monosomy b,. trisomy c. tetrasomy d. translocation carrier e. edward syndormearrow_forwardTwo phenotypically unaffected parents produce two children with familial Down syndrome. Regarding chromosome 14 and 21, what are the chromosomal composition of the parents?arrow_forwardShown below is a pedigree for Phenylketonuria (PKU), an autosomal recessive metabolic disorder. The characteristic feature of PKU is severe mental retardation A) What is the probability that individual II-1 is heterozygous for this gene? B) What is the probability that individual III-4 is heterozygous for this gene? C) If individuals III-3 and III-4 were to marry, what is the probability that their child would express PKU?arrow_forward
- Phenylketonuria (PKU) is a disease that results from a recessive gene.Suppose that two unaffected parents produce a child with PKU. a. What is the probability that a sperm from the father will contain the PKU allele?b. What is the probability that an egg from the mother will contain the PKU allele?c. What is the probability that their next child will have PKU?d. What is the probability that their next child will be heterozygous for the PKU gene?arrow_forwardPhenylketonuria (PKU) is a disease that results from a recessive gene. Suppose that two unaffected parents produce a child with PKU. a. What is the probability that a sperm from the father will contain the PKU allele? b. What is the probability that an egg from the mother will contain the PKU allele? c. What is the probability that their next child will have PKU? d. What is the probability that their next child will be heterozygous for the PKU gene?arrow_forwardThe autosomal (not X-linked) gene for brachydactyly, short fingers, is dominant to normal finger length. Assume that a female with brachydactyly in the heterozygous condition is married to a man with normal fingers. What is the probability that(a) their first child will have brachydactyly?(b) their first two children will have brachydactyly?(c) their first child will be a brachydactylous girl?arrow_forward
- . Human geneticists interested in the effects of abnormalities in chromosome number often karyotypetissue obtained from spontaneous abortions. About35% of these samples show autosomal trisomies,but only about 3% of the samples display autosomalmonosomies. Based on the kinds of errors that cangive rise to aneuploidy, would you expect that thefrequencies of autosomal trisomy and autosomalmonosomy should be more equal? Why or why not?If you think the frequencies should be more equal,how can you explain the large excess of trisomiesas opposed to monosomies?arrow_forwardHuman sex chromosomes are XX for females and XY for males. a. With respect to an X-linked gene, how many different types of gametes can a male produce? b. If a female is homozygous for an X-linked allele, how many different types of gametes can she produce with respect to this allele? c. If a female is heterozygous for an X-linked allele, how many different types of gametes can she produce with respect to this allele?arrow_forwardPhenylketonuria and alkaptonuria are both autosomal recessive diseases. If a person with PKU marries a person with AKU, what will the phenotype of their children be?arrow_forward
- Human Heredity: Principles and Issues (MindTap Co...BiologyISBN:9781305251052Author:Michael CummingsPublisher:Cengage LearningHuman Biology (MindTap Course List)BiologyISBN:9781305112100Author:Cecie Starr, Beverly McMillanPublisher:Cengage LearningBiology Today and Tomorrow without Physiology (Mi...BiologyISBN:9781305117396Author:Cecie Starr, Christine Evers, Lisa StarrPublisher:Cengage Learning