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Questions and Answers
Which of the following diseases is associated with X-linked dominant inheritance?
Which of the following diseases is associated with X-linked dominant inheritance?
- Cleft lip
- Rett's syndrome (correct)
- Hirschsprung disease
- Diabetes mellitus
In X-linked recessive inheritance, a son can be unaffected while his sister is a carrier.
In X-linked recessive inheritance, a son can be unaffected while his sister is a carrier.
True (A)
What type of inheritance involves traits influenced by both genetic and environmental factors?
What type of inheritance involves traits influenced by both genetic and environmental factors?
Multifactorial inheritance
Conditions like neural tube defects and diabetes mellitus are examples of __________ inheritance.
Conditions like neural tube defects and diabetes mellitus are examples of __________ inheritance.
Match the following conditions with their type of inheritance:
Match the following conditions with their type of inheritance:
Who coined the term 'genetics'?
Who coined the term 'genetics'?
Each human cell contains 48 chromosomes.
Each human cell contains 48 chromosomes.
What determines genetic sex in humans?
What determines genetic sex in humans?
A genetic female has __ sex chromosomes.
A genetic female has __ sex chromosomes.
Which of the following is a type of genetic disorder?
Which of the following is a type of genetic disorder?
Homozygotes for the dominant gene often survive in utero.
Homozygotes for the dominant gene often survive in utero.
What inheritance pattern results when one out of two homologous genes is sufficient to manifest a disease?
What inheritance pattern results when one out of two homologous genes is sufficient to manifest a disease?
Match the type of genetic disorder with its description:
Match the type of genetic disorder with its description:
What is the inheritance pattern of mitochondrial gene mutations?
What is the inheritance pattern of mitochondrial gene mutations?
Sons of an affected female with a mitochondrial disorder can transmit the disease to their offspring.
Sons of an affected female with a mitochondrial disorder can transmit the disease to their offspring.
What condition is characterized by the absence of chromosome 15 from paternal origin?
What condition is characterized by the absence of chromosome 15 from paternal origin?
A child with both copies of a chromosome from the same parent is said to have __________.
A child with both copies of a chromosome from the same parent is said to have __________.
Match the genetic syndromes with their corresponding chromosomes:
Match the genetic syndromes with their corresponding chromosomes:
Which of the following increases the risk of Down syndrome?
Which of the following increases the risk of Down syndrome?
Down syndrome is caused only by nondisjunction during the formation of maternal gametes.
Down syndrome is caused only by nondisjunction during the formation of maternal gametes.
What percentage of Down syndrome cases are due to Trisomy 21?
What percentage of Down syndrome cases are due to Trisomy 21?
The term __________ refers to a mixture of normal and abnormal mitochondrial genomes in a cell.
The term __________ refers to a mixture of normal and abnormal mitochondrial genomes in a cell.
Which type of uniparental disomy features identical chromosomes?
Which type of uniparental disomy features identical chromosomes?
Which of the following diseases is inherited in an autosomal dominant manner?
Which of the following diseases is inherited in an autosomal dominant manner?
In autosomal recessive inheritance, males and females are affected equally.
In autosomal recessive inheritance, males and females are affected equally.
What is the recurrence risk for parents who have previously had an affected child with an autosomal recessive disorder?
What is the recurrence risk for parents who have previously had an affected child with an autosomal recessive disorder?
Diseases such as hemophilia A and B are inherited in an ______ manner.
Diseases such as hemophilia A and B are inherited in an ______ manner.
Match the following inherited conditions with their appropriate inheritance pattern:
Match the following inherited conditions with their appropriate inheritance pattern:
An affected male with an X-linked dominant disorder can pass the disorder to which of the following?
An affected male with an X-linked dominant disorder can pass the disorder to which of the following?
X-inactivation in females results in the expression of both X chromosomes.
X-inactivation in females results in the expression of both X chromosomes.
Name one disease that follows an autosomal recessive inheritance pattern.
Name one disease that follows an autosomal recessive inheritance pattern.
If a female is a carrier for an X-linked recessive disease, she has a ______ chance of having an affected son.
If a female is a carrier for an X-linked recessive disease, she has a ______ chance of having an affected son.
Which of the following is a characteristic of autosomal recessive inheritance?
Which of the following is a characteristic of autosomal recessive inheritance?
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Study Notes
Genetics: Introduction
- Genetics deals with the study of genes and their inheritance from parents to offspring.
- Human cells contain 46 chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes (XY or XX).
- Chromosomes have a short arm (p) and a long arm (q) connected by the centromere.
- Genetic sex is determined by the combination of sex chromosomes:
- One X and one Y chromosome = Genetic male
- Two X chromosomes = Genetic female
Genetic Disorders
- Chromosomal disorders: Caused by alterations in chromosomes, such as deletions, duplications, or numerical abnormalities.
- Single gene disorders: Caused by alterations in a single gene, following Mendelian inheritance patterns.
- Multifactorial disorders: Result from combined genetic and environmental factors.
Autosomal Dominant Inheritance
- One copy of the dominant gene is sufficient to manifest the disease (heterozygous state expression).
- Homozygotes for the dominant gene often die prenatally.
- Characteristics:
- Affected parent transmits the disease to 50% of offspring.
- Males and females are equally affected and can both transmit the disorder.
Autosomal Recessive Inheritance
- Both copies of the gene must be altered for the disease to manifest (homozygous state expression).
- Heterozygotes are carriers but do not express the disease.
- Characteristics:
- Transmitted in a horizontal pattern (only one generation).
- Affects males and females equally.
- Often involves consanguineous marriages.
- Most inborn errors of metabolism are inherited recessively.
X-Linked Recessive Inheritance
- Females are carriers, while males are primarily affected.
- Affected males transmit the disease to daughters (carriers).
- Male-to-male transmission is not observed.
- Examples: Hemophilia A & B, Duchenne's muscular dystrophy, fragile X syndrome.
X-Linked Dominant Inheritance
- Males are more severely affected than females.
- Affected males transmit the disease only to daughters.
- Examples: Vitamin D resistant rickets, Rett's syndrome.
X-Inactivation (Lyon Hypothesis)
- One X chromosome in each female cell is randomly inactivated during early development.
- This balances X-linked gene expression between males and females.
Mitochondrial Disorders
- Mitochondria, and their DNA, are inherited only from the mother.
- Mutations in mitochondrial DNA can lead to phenotypic defects.
- Variable clinical manifestations due to heteroplasmy (mixture of normal and abnormal mitochondrial genomes).
- Both sexes can be affected, all offspring of an affected female will be affected, and daughters will transmit the disease.
Genomic Imprinting
- The phenotypic expression of some genes is determined by the parent of origin.
- One parent's copy of a specific gene is silenced, while the other parent's copy is expressed.
- Examples:
- Prader-Willi syndrome (paternal chromosome 15 inactivation).
- Angelman syndrome (maternal chromosome 15 inactivation).
Uniparental Disomy (UPD)
- Offspring inherits two copies of the same chromosome from one parent.
- Can be isodisomy (identical chromosomes) or heterodisomy (different chromosomes).
- Examples:
- Beckwith-Wiedemann syndrome (chromosome 11).
- Silver-Russell syndrome (chromosome 7).
Down Syndrome (Trisomy 21)
- Most common chromosomal disorder.
- Occurs in 1 per 1000 live births.
- Extra chromosome 21 can be maternal or paternal origin.
- Advanced maternal age increases the risk.
- Cytogenetics:
- Trisomy 21 (95% of cases): Extra copy of chromosome 21 due to nondisjunction in meiosis I.
- Translocations (4% of cases): Chromosome 21 is translocated with chromosomes 13, 14, or 15.
- Mosaicism (1% of cases): Trisomy 21 only present in some cells.
Examples of Diseases Following Different Inheritance Patterns
- Autosomal Dominant (AD): Huntington's disease, Marfan's syndrome, neurofibromatosis, familial hypercholesterolemia.
- Autosomal Recessive (AR): Cystic fibrosis, sickle cell anemia, phenylketonuria.
- X-Linked Recessive: Hemophilia A & B, Duchenne's muscular dystrophy, color blindness.
- X-Linked Dominant: Vitamin D resistant rickets, Rett's syndrome.
- Multifactorial/Polygenic: Neural tube defects, cleft lip/palate, diabetes mellitus.
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