25 Questions
What is the approximate size of the mitochondrial genome?
16.5 kb
What is the characteristic of mitochondrial DNA inheritance in humans?
Maternal inheritance
What is the term for the presence of a single type of mitochondrial DNA molecule within an individual?
Homoplasmy
What is a characteristic of diseases resulting from mitochondrial DNA mutations?
Maternal inheritance with variable expressivity
How many genes are encoded in the mitochondrial genome?
37 genes
What is a feature of the mitochondrial genome that differs from the nuclear genome?
All of the above
What is the main factor that determines the pattern of single-gene disorders in pedigrees?
Whether the phenotype is dominant or recessive and the chromosomal location of the gene locus
What is the characteristic of males in relation to X-linked genes?
They are hemizygous with respect to X-linked genes
What is the difference between autosomal and X-linked disorders in terms of clinical expression?
Autosomal disorders affect males and females equally, while X-linked disorders affect males more severely
What is the characteristic of mitochondrial inheritance?
It is inherited solely through the maternal line
What is the term for the phenomenon where a single gene has multiple effects on the phenotype?
Pleiotropy
What is the term for the mixture of different mitochondrial DNA types within a cell?
Heteroplasmy
What is the characteristic of X-linked recessive disorders in females?
They are rarely seen among females
What is the term for a female carrier of a recessive X-linked allele who has phenotypic expression of the disease?
Manifesting heterozygote
What is the pattern of inheritance of X-linked recessive disorders?
Expressed in all males who receive it but only in homozygous females
Why is it unusual for a female to be homozygous for an X-linked recessive disorder?
Because the parents are not consanguineous
What is the characteristic of muscle from DMD carriers?
It exhibits both positive and negative patches of dystrophin immunostaining
What is the difference between X-linked dominant and recessive inheritance patterns?
The phenotype in heterozygous females
What is a characteristic of X-linked recessive inheritance patterns?
The incidence of the trait is much higher in males than in females.
What happens to the daughters of an affected male in X-linked recessive inheritance?
All of their sons have a 50% chance of inheriting it.
What is a key feature of X-linked dominant inheritance?
Lack of male-to-male transmission of the condition.
What can be observed in a pedigree demonstrating X-linked dominant inheritance?
All the daughters and none of the sons of affected males are affected.
Why are isolated cases of X-linked recessive disorders often due to new mutations?
Because the mutant allele is not transmitted from father to son.
What is the term for the expression of a condition in heterozygotes?
Dominance.
What is a characteristic of X-linked recessive disorders?
Heterozygous females are usually unaffected, but some may express the condition with variable severity.
Study Notes
Patterns of Single-Gene Inheritance
- Four basic patterns of single-gene inheritance depend on whether the phenotype is dominant or recessive and the chromosomal location of the gene locus (autosome or sex chromosome)
X-Linked Inheritance
- X-linked disorders affect males more severely than females due to hemizygosity in males and heterozygosity in females
- Females can be heterozygous or homozygous at X-linked loci, while males are never heterozygous for alleles at X-linked loci
- X inactivation leads to random inactivation of one X chromosome in females, affecting expression of X-linked genes
Mitochondrial Genome
- Mitochondrial DNA (mtDNA) is a circular chromosome located inside mitochondrial organelles, not in the nucleus
- mtDNA contains 37 genes encoding 13 polypeptides, 2 types of ribosomal RNA, and 22 transfer RNAs
- Rearrangements and point mutations in mtDNA can cause human disease, often involving the central nervous and musculoskeletal systems
- Mitochondrial genome has three unusual features: replicative segregation, homoplasmy and heteroplasmy, and maternal inheritance
Maternal Inheritance of mtDNA
- mtDNA is inherited solely from the mother due to elimination of sperm mitochondria from the embryo
- All children of a female with a mtDNA mutation will inherit the mutation, while none of the offspring of a male carrying the same mutation will inherit it
Expression of X-Linked Genes
- Recessive and dominant patterns of X-linked inheritance are distinguished based on the phenotype in heterozygous females
- X-linked "dominant" and "recessive" patterns depend on the expression of the phenotype in carriers
X-Linked Recessive Inheritance
- X-linked recessive disorders are typically expressed in all males who receive the mutated allele and only in females who are homozygous for the mutation
- X-linked recessive disorders are generally restricted to males and rarely seen among females
- Hemophilia A is a classic X-linked recessive disorder
Manifesting Heterozygotes and Unbalanced Inactivation
- Female carriers of X-linked recessive alleles may occasionally express the disease, known as manifesting heterozygotes
- Manifesting heterozygotes have been described for many X-linked recessive disorders, including color blindness and hemophilia A
Characteristics of X-Linked Recessive Inheritance
- Incidence of the trait is higher in males than females
- Heterozygous females are usually unaffected, but some may express the condition with variable severity
- The gene responsible is transmitted from an affected man through all his daughters
- Any of his daughters' sons has a 50% chance of inheriting it
This quiz covers advanced topics in single-gene inheritance, including X-linked inheritance, dosage compensation, and pseudoautosomal inheritance. It also explores mosaicism, unstable repeat expansions, and mitochondrial genome mutations.
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