25 Questions
What is a key factor in determining the patterns shown by single-gene disorders in pedigrees?
Whether the phenotype is dominant or recessive
What is the main difference between autosomal and X-linked disorders?
Autosomal disorders affect both males and females equally, while X-linked disorders affect males more severely
What is the term used to describe a phenotype that is expressed in both homozygotes and heterozygotes for a mutant allele?
Dominant
Why do females have a double complement of X-linked genes?
Because they have two X chromosomes
What is the term used to describe a disorder that occurs when a severe, debilitating genetic disorder has normal fitness because the onset of the disease is well past the usual reproductive age?
None of the above
What is the purpose of X-inactivation in females?
To compensate for the double complement of X-linked genes in females
What is a person called when they have a pair of identical alleles at a locus encoded in nuclear DNA?
Homozygous
What is the term used to describe a genotype in which two different mutant alleles of the same gene are present?
Compound Heterozygous
What is the term used to describe a male who has an abnormal allele for a gene located on the X chromosome and there is no other copy of the gene?
Hemizygous
Why are the terms homozygous, heterozygous, and hemizygous not used to describe genotypes at mitochondrial loci?
Because mitochondrial DNA molecules are present in tens to thousands of copies per cell
What is the term used to describe single-gene traits caused by mutations in genes in the nuclear genome?
Mendelian Inheritance
What is the usual first step in establishing the pattern of transmission of a single-gene disorder in a family?
To obtain information about the family history of the patient and summarize the details in the form of a pedigree
What type of disorders result from a combination of multiple genetic and environmental causes?
Multifactorial Disorders
What is the term used to describe the family member through whom a family with a genetic disorder is first brought to the attention of the geneticist?
Proband
What is the term used to describe disorders in which entire chromosomes (or large segments of them) are missing, duplicated, or otherwise altered?
Chromosomal Disorders
What type of disorders are often termed mendelian conditions, or single-gene disorders?
Disorders in which single genes are altered
What is the term used to describe the study of the characteristics of garden peas by Gregor Mendel?
Mendelian Inheritance
What is the term used to describe well-known examples of single-gene disorders?
Cystic Fibrosis, Sickle Cell Disease, and Hemophilia
What is the characteristic of a pure dominant disease?
Homozygotes and heterozygotes are both affected equally.
What is the term for the expression of two different alleles for a locus?
Codominant expression
What is the characteristic of an incompletely dominant disease?
Homozygotes are more severely affected than heterozygotes.
What is the characteristic of a recessive disease?
The disease is expressed only in homozygotes.
What is the probability that a gene will have any phenotypic expression at all?
Penetrance
What is the term for the phenomenon where some individuals with the appropriate genotype completely fail to express the phenotype?
Reduced penetrance
Why do heterozygotes typically not develop recessive diseases caused by mutations that impair or eliminate enzyme function?
Because they have one normal allele and one abnormal allele.
Study Notes
Patterns of Single-Gene Inheritance
- Genetic diseases can be classified into three types: single-gene, chromosomal, and complex
- Single-gene traits are caused by mutations in genes in the nuclear genome and occur in fixed proportions among offspring of specific matings
- Mendelian inheritance is the study of single-gene traits and their patterns in pedigrees
Types of Genetic Diseases
- Single-gene disorders: caused by mutations in genes, often termed Mendelian conditions (e.g., cystic fibrosis, sickle cell disease, and hemophilia)
- Chromosomal disorders: result from entire chromosomes or large segments being missing, duplicated, or altered (e.g., Down syndrome and Turner syndrome)
- Multifactorial disorders: result from a combination of multiple genetic and environmental causes
Mendelian Inheritance
- Patterns of single-gene inheritance depend on two main factors:
- Whether the phenotype is dominant (expressed when one chromosome carries the mutant allele) or recessive (expressed only when both chromosomes carry mutant alleles)
- Chromosomal location of the gene locus (autosome or sex chromosome)
- There are four basic patterns of single-gene inheritance: autosomal dominant, autosomal recessive, X-linked dominant, and X-linked recessive
Autosomal and X-Linked Inheritance
- Autosomal disorders affect males and females equally
- X-linked disorders affect males and females differently due to hemizygosity in males and heterozygosity in females
- Females can be heterozygous or homozygous at X-linked loci, while males are never heterozygous
Dominant Inheritance
- A phenotype expressed in both homozygotes and heterozygotes is inherited as a dominant
- Dominant disorders occur regardless of normal gene product made from the remaining normal allele
- A person with a pair of identical alleles is homozygous, while a person with different alleles is heterozygous
Genotype and Phenotype
- Compound heterozygote: a genotype with two different mutant alleles of the same gene
- Hemizygous: a male with an abnormal allele for a gene located on the X chromosome and no other copy of the gene
- Mitochondrial DNA: special case where terms homozygous, heterozygous, and hemizygous are not used to describe genotypes at mitochondrial loci
Pedigrees
- Pedigrees are graphical representations of family trees used to establish patterns of transmission in families
- The extended family depicted in a pedigree is a kindred
- Proband: the member through whom a family with a genetic disorder is first brought to the attention of the geneticist
Dominant Inheritance (continued)
- Pure dominant disease: homozygotes and heterozygotes are affected equally (rare in medical genetics)
- Codominant: phenotypic expression of two different alleles for a locus (e.g., ABO blood group system)
- Incompletely dominant: disease is more severe in homozygotes than in heterozygotes
Recessive Inheritance
- A phenotype expressed only in homozygotes (or male hemizygotes for X-linked traits) is recessive
- Most recessive disorders are due to mutations that reduce or eliminate the function of the gene product (loss-of-function mutations)
- Examples of recessive diseases include those caused by mutations that impair or eliminate enzyme function
This quiz covers the basics of single-gene inheritance, including genotype and phenotype, pedigrees, and Mendelian inheritance. It also explores factors affecting pedigree patterns and correlating genotype and phenotype.
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