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A couple was referred for genetic counseling because they wanted to know the chances of having a child with dwarfism. Both the man and the woman had achondroplasia (MIM 100800), the most common form of short-limbed dwarfism. The couple knew that this condition is inherited as an autosomal dominant trait, but they were unsure what kind of physical manifestations a child would have if it inherited both mutant alleles. They were each heterozygous for the FGFR3 (MIM 134934) allele that causes achondroplasia. Normally, the protein encoded by this gene interacts with growth factors outside the cell and receives signals that control growth and development. In achrodroplasia, a mutation alters the activity of the receptor, resulting in a characteristic form of dwarfism. Because both the normal and mutant forms of the FGFR3 protein act before birth, no treatment for achrondroplasia is available.
The parents each carry one normal allele and one mutant allele of FGRF3, and they wanted information on their chances of having a homozygous child. The counsellor briefly reviewed the
Because achondroplasia is an autosomal dominant condition, a heterozygote has a 1-in-2, or 50%, chance of passing this trait to his or her offspring. However, about 75% of those with achondroplasia have parents of average size who do not carry the mutant allele. In these cases, achondroplasia is due to a new mutation. In the couple being counseled, each individual is heterozygous, and they are at risk for having a homozygous child with two copies of the mutated gene. Infants with homozygous achondroplasia are either stillborn or die shortly after birth. The counselor recommended prenatal diagnosis via ultrasounds at various stages of development. In addition, a DNA test is available to detect the homozygous condition prenatally.
What is the chance that this couple will have a child with two copies of the dominant mutant gene? What is the chance that the child will have normal height?
To determine: The chances of the couple having a child with two copies of the dominant mutant gene in the given case study.
Introduction: Autosomal genetic defects are inherited from one generation to next in two patterns, either in autosomal recessive form or in autosomal dominant form. For a disease that is autosomal recessive, presence of two copies of the defective alleles (homozygous condition) is required for the development of disease. In case of autosomal dominant disease, presence of even a single copy of the defective allele (heterozygous condition) can cause development of the disease.
Explanation of Solution
Information given in the case study is as follows:
- Achondroplasia is an autosomal dominant defect.
- Both the parents carry heterozygous allelic combination for the disease.
- Phenotypic effects of the disease are short stature, shortened arms and legs, abnormal facial features, and many more.
The parents wanted to know the chances of their child having homozygous dominant state for the disease. For this, genotype of the parents can be assumed as:
A represents gene for achondroplasia and a representing a healthy gene.
Both the parents are having heterozygous genotype for the disease, so their genotype will be Aa and the gametes produced by them will be having genotype as A and a.
Possible genotypes of the child produced from these parents can be found as:
| Gametes | A | a |
| A | AA | Aa |
| a | Aa | aa |
It is clear that only 25% (1 out of 4) chance is there that the child will have two copies of the dominant allele (AA) for the disease.
To determine: The chances of having a child with normal height in the given case study.
Explanation of Solution
Information given in the case study is as follows:
- Achondroplasia is an autosomal dominant defect.
- Both the parents carry heterozygous allelic combination for the disease.
- Phenotypic effects of the disease are short stature, shortened arms and legs, abnormal facial features and many more.
The parents in the given case study want to know the chances of their child having homozygous dominant state for the disease. For this, genotype of the parents can be assumed as:
A represents gene for achondroplasia and a representing a healthy gene.
Both the parents are having heterozygous genotype for the disease, so their genotype will be Aa and the gametes produced by them will be having genotype as A and a.
Possible genotypes of the child produced from these parents can be found as:
| A | a | |
| A | AA | Aa |
| a | Aa | aa |
Achondroplasia is a heterozygous dominant disease. So, from the possible genotypes of the child, AA and Aa will give dwarfism phenotypically, whereas only aa genotype will give a normal height phenotypically.
Thus, it can be seen that the child from the parents in the given case study will only have 25% chance of having normal height (possible only in aa genotype).
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Chapter 10 Solutions
EP HUMAN HERDITY:PRIN.+ISSUES-MINDTAP
- A couple was referred for genetic counseling because they wanted to know the chances of having a child with dwarfism. Both the man and the woman had achondroplasia (MIM 100800), the most common form of short-limbed dwarfism. The couple knew that this condition is inherited as an autosomal dominant trait, but they were unsure what kind of physical manifestations a child would have if it inherited both mutant alleles. They were each heterozygous for the FGFR3 (MIM 134934) allele that causes achondroplasia. Normally, the protein encoded by this gene interacts with growth factors outside the cell and receives signals that control growth and development. In achrodroplasia, a mutation alters the activity of the receptor, resulting in a characteristic form of dwarfism. Because both the normal and mutant forms of the FGFR3 protein act before birth, no treatment for achrondroplasia is available. The parents each carry one normal allele and one mutant allele of FGRF3, and they wanted information on their chances of having a homozygous child. The counsellor briefly reviewed the phenotypic features of individuals with achondroplasia. These include facial features (large head with prominent forehead; small, flat nasal bridge; and prominent jaw), very short stature, and shortening of the arms and legs. Physical examination and skeletal X-ray films are used to diagnose this condition. Final adult height is approximately 4 feet. Because achondroplasia is an autosomal dominant condition, a heterozygote has a 1-in-2, or 50%, chance of passing this trait to his or her offspring. However, about 75% of those with achondroplasia have parents of average size who do not carry the mutant allele. In these cases, achondroplasia is due to a new mutation. In the couple being counseled, each individual is heterozygous, and they are at risk for having a homozygous child with two copies of the mutated gene. Infants with homozygous achondroplasia are either stillborn or die shortly after birth. The counselor recommended prenatal diagnosis via ultrasounds at various stages of development. In addition, a DNA test is available to detect the homozygous condition prenatally. Should the parents be concerned about the heterozygous condition as well as the homozygous mutant condition?arrow_forwardFamilial retinoblastoma, a rare autosomal dominant defect, arose in a large family that had no prior history of the disease. Consider the following pedigree (the darkly colored symbols represent affected individuals): a. Circle the individual(s) in which the mutation most likely occurred. b. Is the person who is the source of the mutation affected by retinoblastoma? Justify your answer. c. Assuming that the mutant allele is fully penetrant, what is the chance that an affected individual will have an affected child?arrow_forwardMike was referred for genetic counseling because he was concerned about his extensive family history of colon cancer. That family history was highly suggestive of hereditary nonpolyposis colon cancer (HNPCC). This predisposition is inherited as an autosomal dominant trait, and those who carry the mutant allele have a 75% chance of developing colon cancer by age 65. Mike was counseled about the inheritance of this condition, the associated cancers, and the possibility of genetic testing (on an affected family member). Mikes aunt elected to be tested for one of the genes that may be altered in this condition and discovered that she did have an altered MSH2 gene. Other family members are in the process of being tested for this mutation. Once a family member is tested for the mutant allele, is it hard for other family members to remain unaware of their own fate, even if they did not want this information? How could family dynamics help or hurt this situation?arrow_forward
- Mike was referred for genetic counseling because he was concerned about his extensive family history of colon cancer. That family history was highly suggestive of hereditary nonpolyposis colon cancer (HNPCC). This predisposition is inherited as an autosomal dominant trait, and those who carry the mutant allele have a 75% chance of developing colon cancer by age 65. Mike was counseled about the inheritance of this condition, the associated cancers, and the possibility of genetic testing (on an affected family member). Mikes aunt elected to be tested for one of the genes that may be altered in this condition and discovered that she did have an altered MSH2 gene. Other family members are in the process of being tested for this mutation. Is colon cancer treatable? What are the common treatments, and how effective are they?arrow_forwardMike was referred for genetic counseling because he was concerned about his extensive family history of colon cancer. That family history was highly suggestive of hereditary nonpolyposis colon cancer (HNPCC). This predisposition is inherited as an autosomal dominant trait, and those who carry the mutant allele have a 75% chance of developing colon cancer by age 65. Mike was counseled about the inheritance of this condition, the associated cancers, and the possibility of genetic testing (on an affected family member). Mikes aunt elected to be tested for one of the genes that may be altered in this condition and discovered that she did have an altered MSH2 gene. Other family members are in the process of being tested for this mutation. Seventy-five percent of people who carry the mutant allele will get colon cancer by age 65. This is an example of incomplete penetrance. What could cause this?arrow_forwardA pedigree analysis was performed on the family of a man with schizophrenia. Based on the known concordance statistics, would his MZ twin be at high risk for the disease? Would the twins risk decrease if he were raised in an environment different from that of his schizophrenic brother?arrow_forward
- A woman knows that her mother is a carrier of Kartagener’s syndrome (an autosomal recessive disorder). The woman does not know if either she or her husband are carriers. The couple wants to have a child, but is worried about whether or not they could have a child with Kartagener’s syndrome. Should the couple seek the advice of a genetic counselor? In other words, is there a chance they could have an affected child? If there is a chance, please make sure your answer includes the specific parental genotypes necessary to make this possible.arrow_forwardUsing the pedigree chart attached: Above is a pedigree for colorblindness. Based on the pedigree, is the disease dominant or recessive and is it sex-linked or autosomal? Why? Furthermore, what is the probability that 18 on this chart is affected but the condition, and what is the probability that 18 is a carrier? Why? Are the probability of being a carrier and an affected individual different? Why?arrow_forwardThe attached image is a pedigree of a family with a history of sickle cell anemia (the individuals with the filled-in symbols have the disease and no new mutations are occurring in any individual). Sickle cell anemia is inherited in an autosomal recessive manner. What is the probability that the individual with the question mark (?) will get the disease? a) 1/4 b) 1/2 c) 2/3 d) 1arrow_forward
- A couple who are about to get married learn from studying their family histories that, in both their families, theirunaffected grandparents had siblings with cystic fibrosis(a rare autosomal recessive disease).a. If the couple marries and has a child, what is theprobability that the child will have cystic fibrosis?b. If they have four children, what is the chance that thechildren will have the precise Mendelian ratio of 3:1 fornormal:cystic fibrosis?c. If their first child has cystic fibrosis, what is theprobability that their next three children will be normal?arrow_forwardImagine that you are a genetic counselor, and a couple planning to start a family comes to you for information. Charles wasmarried once before, and he and his first wife had a child withcystic fibrosis. The brother of his current wife, Elaine, died ofcystic fibrosis. What is the probability that Charles and Elainewill have a baby with cystic fibrosis? (Neither Charles, Elaine,nor their parents have cystic fibrosis.)arrow_forwardBelow is a pedigree chart for a family that has a history of Alkaptonuria. Individuals infected with this condition can have darkened skin, brown urine, and can suffer from joint damage and other complications. Given this pedigree answer the following questions. Given the data in the pedigree chart is this genetic condition autosomal dominant or autosomal recessive? What are the genotypes for #1, #2, and #3? If either of the 4th generation "aa" females were to mate with a homozygous dominant male would any of their offspring illustrate the phenotype? Why or why not?arrow_forward
Human Heredity: Principles and Issues (MindTap Co...BiologyISBN:9781305251052Author:Michael CummingsPublisher:Cengage Learning