Genetics

Question 1

What is the gneeral onset of chromosomal, single gene, and multifactorial disorders?

Answer 1

Chromosomal: in utero/at birth

Single gene: at/near birth

Multifactorial: increasing after puberty and into adulthood

Question 2

What is the law of segregation?

Answer 2

Every individual possesses a pair of alleles (assuming diploidy) for any particular trait and that each parent passes a randomly selected copy (allele) of only one of these to its offspring

Question 3

What is the law of independent assortment?

Answer 3

Separate genes for separate traits are passed indepently of one another from parents to offspring.

Question 4

What is a polymorphism?

Answer 4

existence of multiple alleles for a particular gene (>1% freq. for each allele)

Question 5

What is codominance?

Answer 5

The expression of each allele can be detected (neither allele is dominant)

Question 6

What does it mean to be:

Metacentric?

Submetacentric?

Acrocentric?

Answer 6

Metacentric: having the centromere in the middle

Sumetacentric: having the centromere somewhat distant from the middle

Acrocentric: having the centromere near the telomere
(short arms have distinctive structure, and contain mostly satellite sequences - short, simple sequence repetitive DNA - and multiple copies of the genes for ribosomal RNA)

Question 7

How are short and long arms of chromosomes labeled?

Answer 7

Short = p

Long = q

Question 8

Euploidy

Answer 8

Exact multiple of the number of chromosomes in a normal gamete

Question 9

Aneuploidy

Answer 9

At least one set of chromosomes incomplete (usually a loss or gain of a single chromosome)

Question 10

Polyploidy

Answer 10

More than two complete sets of chromosomes

Question 11

What causes aneuploidy?

Answer 11

Aneuploidy arises during mitosis or meiosis through nondisjunction:
-faliure of chromosomes to separate normally

Question 12

What is Klinefelter Syndrome?

Answer 12

47, XXY

Associated with tall stature, postpubertal testicular failure and azoospermia (no sperm)

Question 13

What is XYY Syndrome?

Answer 13

47, XYY

Tall staure and an increased incidence of speech delay

Question 14

What is Turner Syndrome?

Answer 14

45, X

Short stature, amenorrhea, infertility and somatic abnormalities

Question 15

What is XXX syndrome?

Answer 15

47, XXX

tall stature

Question 16

Trisomy 21

Answer 16

47, XY or XX, +21

Down Syndrome:

Head and facial abnormalities
growth retardation
variable mental retardation

Question 17

Chromosomal Duplication

Answer 17

Extra copy of part of a chromosome leading to partial trisomy

Question 18

Chromosomal inversion

Answer 18

Breakage and rejoining of a chromosome segment in reverse order

pericetnric: if centromere is included
paracentric: if centromere not included

Question 19

Ring chromosome

Answer 19

deletion of telomeres and fusion of the broken ends to form ring

• very unstable during mitosis and frequently lost

Question 20

Chromosomal Insertion

Answer 20

Segment of one chromosome inserted intoa non-homologous chromosome

Question 21

Isochromosome

Answer 21

chromosome having one arm duplicated and the other arm lost; arises when the centromere divides abnormally and can involve eitehr the short (ISO p) or long (ISO q) arm

Question 22

Chromosomal Translocation

Answer 22

exchange of chromosome segments between non-homologous chromosomes

• Two main types are:

Reciprocal translocations: Material is simply exchanged between two Ch.
Robertsonian translocations: Fusion at centromere between two acrocentric chromosomes, with the loss of the short arms and the satellites

Question 23

What are features that characterize autosomal recessive inheritance on a pedigree?

Answer 23

1. Pedigree shows horizontal transmission; siblings are affected but not their parents or other relatives (1/4 siblings on average)
2. Only homozygous or compound heterozygous individuals express trait
3. Males and females equally affected
4. If both parents affected; all children affected
5. Pedigrees often show consanguineous matings
6. In most cases, if 1 parent is affected and other is not, all children will be normal; unless normal parent is a carrier

Question 24

What are features that characterize Autosomal dominant inheritance?

Answer 24

1. Trait is transmitted vertically from one generation to another without skipping generations
2. Both males and females are affected and transmit trait with equal probability
3. each affected individual has one affected parent and unaffected family members do not transmit trait

Question 25

What can lead to autosomal dominance?

Answer 25

1. Haploinsufficiency
2. Dominant Negative Effect
3. Gain of Function

Question 26

What is Haploinsufficiency?

Answer 26

Dominance is due to the fact that half the normal amount of protein is insufficient to maintain normal function

Question 27

What is Dominant Negative Effect?

Answer 27

The product of the mutant allele interferes with the product of another, normal, allele, resulting in overall adverse outcome

Question 28

What is Gain-of-Function?

Answer 28

Dominant disorders can result from a change in the level of activity of a protein, or its expression in the wrong tissue or at the wrong time

Question 29

What are features that characterize X-linked recessive disorders?

Answer 29

1. Males are affected much more than females
2. All the daughters of an affected father receive the mutant allele and are carriers, and will pass the trait onto half their sons - Daughters are obligate heterozygotes
3. Father to son transmission is never seen
4. Affected females have affected fathers and affected or carrier mothers

Question 30

What are features that characterize X-linked dominant disorders?

Answer 30

1. The trait does not skip generations
2. Mothers transmit the trait to about half the children, both daughters and sons (assuming heterozyguos mom)
3. Fathers transmit the trait to all daughters but no sons
4. Within the population, females are affected about twice as often as males

Question 31

What is locus heterogeneity?

Answer 31

Multiple mutant proteins in the same pathway cause the same disease

Question 32

What is allelic heterogeneity?

Answer 32

Different mutations in the same gene cuse the same disease

Question 33

What is penetrance?

Answer 33

The likelihood taht the disease allele will result in the disease

• If the mutation is not fully penetrant, an individual may carry the mutant gene, but may be asymptomatic

Question 34

What does it mean to be a germline mosaic?

Answer 34

Only some of the cells in that person are mutant and depending on which cells/tissues have the mutation, the individual could be clinically normal, yet be able to pass it to their proginy

Question 35

What is pleiotropy?

Answer 35

Diverse effects of a single gene on several organs

• frequently show variable expressivity (which differs from penetrance)

Question 36

How does penetrance differ from variable expressivity?

Answer 36

Penetrance = All or nothing

Variable Expressivity = Different expressions of the disorder due to the gene’s effects on the organs