Genetic Diseases Lecture 10

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Questions and Answers

What is the primary classification of human diseases?

Genetically determined, Environmentally determined, and Infectious diseases

Single gene disorders, Multifactorial disorders, and Chromosomal disorders

Infectious, Autoimmune, and Developmental diseases

Genetically determined, Environmentally determined, and Genetically and environmentally determined diseases (correct)

What is the main characteristic of chromosomal disorders?

They arise from chromosomal alteration numeric or structural abnormalities (correct)

They are multifactorial diseases

They are caused by environmental factors

They are caused by single gene mutations

What is the euploid number of chromosomes in humans?

46 (correct)

What is polyploidy?

Chromosome numbers such as 3n and 4n Signup and view all the answers

What is aneuploidy?

Any number that is not an exact multiple of the haploid number Signup and view all the answers

What is mosaicism?

The presence of two or more populations of cells with different chromosomal complement Signup and view all the answers

Why is autosomal mosaicism rare?

Because it is usually lethal Signup and view all the answers

What is the usual outcome of polyploid zygotes?

Spontaneous abortion Signup and view all the answers

What is the result of chromosomal breakage followed by loss or rearrangement of the broken part?

Structural abnormalities Signup and view all the answers

What is the purpose of cytogenetic mapping?

To identify chromosomal abnormalities Signup and view all the answers

What does the notation '2q34' represent in chromosomal mapping?

Chromosome 2, long arm, region 3, band 4 Signup and view all the answers

What is the characteristic of a balanced reciprocal translocation?

No net gain or loss of genetic material Signup and view all the answers

What is indicated by the notation '46,XX,t(2;5)(q31;p14)'?

A reciprocal translocation involving chromosomes 2 and 5 Signup and view all the answers

What is the outcome of a balanced reciprocal translocation during gametogenesis?

Formation of abnormal gametes with an abnormal number of chromosomes Signup and view all the answers

What is a characteristic of a centric fusion translocation?

Involves the transfer of segments between two acrocentric chromosomes Signup and view all the answers

What is the consequence of a centric fusion translocation?

The carrier has 45 chromosomes Signup and view all the answers

What is the most common cause of death in childhood for individuals with congenital malformations?

Cardiovascular abnormalities Signup and view all the answers

What is the characteristic of the genitalia in affected girls during adolescence?

Minimal development Signup and view all the answers

What is the term for a permanent change in the DNA sequence of nucleotide arrangement?

Gene mutation Signup and view all the answers

Which type of cells can transmit mutant genes to progeny?

Germ cells Signup and view all the answers

What is the term for the arrangement of genes on each chromosome?

Genetic locus Signup and view all the answers

What is the term for a person with two identical alleles of a gene?

Homozygote Signup and view all the answers

What is the characteristic of a dominant trait in a heterozygote?

Only expressed in heterozygotes Signup and view all the answers

How many patterns of inheritance are followed by single-gene disorders?

Three Signup and view all the answers

What is a characteristic of autosomal dominant disorders?

A single mutant allele from an affected parent is transmitted to an offspring regardless of sex. Signup and view all the answers

What is the risk of an affected child in each pregnancy for autosomal recessive disorders?

One in four Signup and view all the answers

Which of the following is an example of an autosomal dominant disease?

Marfan Syndrome Signup and view all the answers

What is the age of onset for many autosomal dominant disorders?

Delayed Signup and view all the answers

What is a characteristic of autosomal recessive disorders?

They are manifested only when both members of the gene pair are mutated. Signup and view all the answers

What is the likelihood of a disorder being the product of a close relative marriage?

Low if the mutant gene occurs with a low frequency in the population. Signup and view all the answers

What type of proteins are usually affected in autosomal dominant disorders?

Structural proteins such as collagen Signup and view all the answers

What is a characteristic of X-linked disorders?

They are associated with the X chromosome. Signup and view all the answers

What is the probability that a daughter of a carrier mother will be a carrier of the mutant gene?

50% Signup and view all the answers

Why do sons of a carrier mother not inherit the disorder?

Because the Y chromosome is unaffected Signup and view all the answers

What is the role of Factor VIII in the blood?

It is involved in the clotting of blood Signup and view all the answers

Why do affected sons transmit the defective gene to all of their daughters?

Because the X chromosome is involved Signup and view all the answers

What is the pattern of inheritance of X-linked recessive disorders?

X-linked recessive Signup and view all the answers

What is an example of an X-linked recessive disorder?

Hemophilia A Signup and view all the answers

Study Notes

Classification of Human Diseases

Human diseases can be classified into three categories: genetically determined, environmentally determined, and genetically and environmentally determined diseases.

Classification of Genetic Diseases

Genetic diseases can be classified into three categories: chromosomal disorders, single gene disorders, and multifactorial disorders.

Chromosomal Disorders (Cytogenic)

Chromosomal disorders arise from chromosomal alterations, either numeric or structural abnormalities, affecting autosomes or sex chromosomes.
They are a major category of genetic diseases, responsible for early abortions, congenital malformations, and mental retardation.

Numeric Abnormalities

Euploid: the normal number of chromosomes for a species, which is 46 (diploid) in humans, with the exception of unfertilized egg and sperm, which is 23 (haploid).
Polyploid: chromosome numbers such as 3n and 4n, which generally result in spontaneous abortion.
Aneuploid: any number that is not an exact multiple of n, including mosaicism.
Mosaicism: the presence of two or more populations of cells with different complements of chromosomes in the same individual, common in sex chromosomes, but rare in autosomal chromosomes due to early fetal death and spontaneous abortion.

Structural Abnormalities

Result from chromosomal breakage followed by loss or rearrangement of the broken part.
Diagnosed using cytogenetic mapping, which divides the short arm (p) and the long arm (q) of a chromosome into numbered regions and bands.
Types of structural abnormalities:
- Translocation: the transfer of a part of one chromosome to another chromosome, of two types:
  - Balanced reciprocal translocation: the exchange of chromosomal material between two chromosomes, with no net gain or loss of genetic material (e.g., 46,XX,t(2;5)(q31;p14)).
  - Centric fusion translocation (robertsonian): a special pattern of translocation involving two acrocentric chromosomes, typically resulting in one very large chromosome and one extremely small one, with the short fragments lost, and the carrier having 45 chromosomes.
- Congenital malformations, such as horseshoe kidney, and cardiovascular abnormalities, which are the most common cause of death in childhood.
- In adolescence, affected girls fail to develop normal secondary sex characteristics, with the genitalia remaining infantile, breast development minimal, and little pubic hair appearing.

Single Gene Disorders (Mendelian Disorder)

Include uncommon conditions, such as storage diseases and inborn errors of metabolism, resulting from single-gene mutations of large effect.
Follow the well-known Mendelian patterns of inheritance.
Mutation: a permanent change in the DNA, which can be present in:
- Somatic cells, which are not transmitted to the progeny, but are important in the causation of cancers and some congenital malformations.
- Germ cells, which can transmit to the progeny and give rise to inherited diseases.
The genes on each chromosome are arranged in pairs, with each gene occupying a specific location or locus, and can be:
- Homozygous, where the two members of a gene pair are identical.
- Heterozygous, where the two members of a gene pair are different, and the trait is expressed in the heterozygote (dominant) or only in the homozygote (recessive).

Transmission Patterns of Single-Gene Disorders

Mutations involving single genes follow one of three patterns of inheritance:
- Autosomal dominant.
- Autosomal recessive.
- X-linked.

Autosomal Dominant Disorders

Manifested in the heterozygous state, where a single mutant allele from an affected parent is transmitted to an offspring, regardless of sex.
Both males and females are affected, and both can transmit the condition.
When an affected person marries an unaffected one, every child has one chance in two of having the disease.
The gene products of autosomal dominant disorders usually are regulatory proteins of metabolism or key components of structural proteins, such as collagen.
Examples: Marfan Syndrome, Ehlers-Danlos Syndromes.

Autosomal Recessive Disorders

Manifested only when both members of the gene pair are mutated.
Both parents may be unaffected but are carriers of the defective gene.
If the mutant gene occurs with a low frequency in the population, there is a strong likelihood that the disorder is the product of a close relative marriage.
Autosomal recessive disorders affect both sexes.
The occurrence risk in each pregnancy is one in four for an affected child, two in four for a carrier child, and one in four for a normal (non-carrier, unaffected) homozygous child.
The age of onset is frequently early in life.
The symptomatology tends to be more uniform than with autosomal dominant disorders.
The disorders are characteristically caused by deficiencies in enzymes, rather than abnormalities in structural proteins.
Examples: phenylketonuria, lysosomal storage diseases, and Albinism.

X-Linked Disorders

Sex-linked disorders are almost always associated with the X, or female, chromosome.
No Y-linked diseases are as yet known.
The inheritance pattern is predominantly recessive.
Female heterozygotes rarely experience the effects of a defective gene.
The common pattern of inheritance is one in which an unaffected mother carries one normal and one mutant allele on the X chromosome, transmitting the defective gene to her sons, and her daughters have a 50% chance of being carriers of the mutant gene.
When the affected son procreates, he transmits the defective gene to all of his daughters, who become carriers of the mutant gene.
Because the genes of the Y chromosome are unaffected, the affected male does not transmit the defect to any of his sons, who will not be carriers or transmit the disorder to their children.
Example: hemophilia A, a haemorrhagic disorder in which blood does not clot normally, due to a lack of normal Factor VIII.

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Description

This quiz covers the classification of human diseases into genetically determined, environmentally determined, and genetically and environmentally determined diseases. It also explores the classification of genetic diseases, including chromosomal disorders and single gene disorders.