Overview of Mendelian Genetics Flashcards Preview

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Flashcards in Overview of Mendelian Genetics Deck (39):
1

What is the distribution of causes of congenital malformations?

60% unknown
20% multifactorial
7.5% single gene defect
6% chromosomal abnormalities
The rest is external maternal factors

2

What are single gene defect?

Often termed mendelian because they follow inheritance patterns predicted by his classic experiments

3

What are the four categories of single-gene disorders?

>50% Autosomal dominant
~36% Autosomal recessive
<10% X-linked
Mitochondrial
Y-linked

4

Define locus.

The position of a gene on a chromosome

5

Define allele.

The alternative versions of a gene that may occupy a given locus

6

Define polymorphism.

The occurrence in a population of two or more alternative genotypes (may or may not be deleterious)

7

Define genotype.

The genetic constitution of an individual

8

Define phenotype.

The observable expression of a genotype as a morphological, biochemical or molecular trait

9

Define homozygote.

Individual with a pair of identical alleles at a given locus

10

Define heterozygote.

Individual with different alleles at a given locus

11

Define compound heterozygote.

Individual with 2 different mutant alleles at a given locus

12

Define double heterozygote.

Individual with 1 mutant allele at each of 2 different loci

13

Define autosomal dominant dominance.

Heterozygotes are phenotypically affected

14

What are the criteria of autosomal dominance?

1) phenotype appears in every generation
2) offspring have 50% chance of inheritance
3) phenotypically normal individuals do not transmit the phenotype
4) no difference in male/female transmission

15

Examples of autosomal dominant diseases.

Marfan's disease
Huntington's disease
Familial hypercholesterolemia
Achondroplasia
Neurofibromatosis
Non-insulin dependent diabetes

16

3 exceptions to autosomal dominance.

1) New mutation in gamete of phenotypically normal parent
2) Variable pentrance
3) Variable expressivity

These phenomena may mask other things

17

Define penetrance.

The probability that a gene will have any detectable phenotypic expression at all

18

Conditions of penetrance.

An all or none concept
Failure accounts for skipping of generations
Reported as %age of individuals to carry mutant genotype which express the mutant phenotype

19

Example of reduced penetrance.

Split-Hand Deformity

70%penetrance * 50% dominant risk = 35% overall risk

20

Define Expressivity.

The degree of expression of the phenotype

21

Conditions of expressivity.

Severity of disease manifestation can very considerably
Environmental factors play a role
Unrelated genes may affect function of the mutant gene product

22

Define autosomal recessive dominance.

Only homozygotes are phenotypically affected

23

4 criteria of autosomal recessive dominance.

1) Affected phenotype is apparent only in sibs
2) Recurrent risk for each sib is 25%
3) Consanguineous parents increases likelihood of inheritance
4) Males and females equally likely to be affected

24

Examples of autosomal recessive diseases.

Cystic Fibrosis
Sickle cell anemia
Tay-Sachs disease
Phenylketonuria (PKU)

25

Conditions of autosomal recessive dominance.

Complete absence of phenotype except in sibs
Both parents are carriers of 1 copy of the defective gene

26

Define X-linked recessive dominance.

"mother to son" transmission

27

3 criteria of X-linked recessive dominance.

1) Incidence of phenotype is much higher in males than females
2) Mutant gene is transmitted from father to all daughters/no sons
3) Heterozygous females are usually unaffected but may show some variable expression of the trait (manifesting heterozygote)

28

What are some X-linked dominant diseases?

Females are affected but males affected more severly, often lethal

Examples include Ornithine Transcarbomoylase Deficiency and Hypophosphatemic Rickets

29

Since males have only one X chromosome they are _____ for X genes.

Hemizygous

30

Why do female carriers of X-linked recessive traits sometimes exhibit phenotypes?

Due to unfavorable lyonization

31

What are some examples of X-linked recessive diseases?

Hemophilia A
Duchenne Muscular Dystrophy
Glucose-6-Phosphate Dehydrogenase Deficiency
Red/Green Color Blindness

32

Why do women not experience twice the gene products in X-linked disorders?

They undergo "dosage compensation" at the morula stage of development.

This was described by Mary Lyons in 1961.

33

What is the Lyon Hypothesis?

All females are mosaics with respect to their X chromosomes. Inactivated X chromosomes are referred to as Barr Bodies.

34

What is Somatic Cell Mosaicism?

Mutation occurs in somatic cell and results in clone of mutant cells derived from original mutant.

35

What is the classic example of Somatic Cell Mosaicism?

Cancer

36

Is there such thing as Y-linked genes?

Yes, several X-linked genes have Y-linked counterparts.
Other Y-linked genes are required for production of testes and sperm.

37

What are the 3 criteria for Mitochondrial Disorders?

1) Purely maternal inheritance
2) Sons and daughters are equally affected
3) Wide variability of expression

38

Diseases linked to mitochondrial disorders.

Leber's hereditary optic atrophy (LHON)
Mitochondrial encephalomyopathy lactic acidosis, and stroke like episodes (MELAS)
Myoclonus epilepsy with ragged red fibers (MERRF)

39

Define mitochondrial disorders.

Mother to everybody transmission