Lecture 3: Mendelian and Non-Mendelian Genetics II

Question 1

Genetic Anticipation

Answer 1

An increase in severity and/or earlier onset of a phenotype in successive generations. This is caused by trinucleotide repeats throughout the genome.

Question 2

Mosaicism

Answer 2

The presence of more than one genetically distinct cell line within an individual. Two Types: 1. Somatic Mosaicism 2. Gonadal Mosaicism

Question 3

Somatic Mosaicism

Answer 3

Usually caused by a post-zygotic mutation. Example: Tumors. One cell mutates and the rest of the daughter cells with also have the same mutation. The surrounding tissues remain the same.

Question 4

Gonadal Mosaicism

Answer 4

Presence of a mutation in all or some germ line (egg or sperm) cells, but not in the rest of the body. The individual with gonadal mosaicism is unaffected with the condition.

Question 5

Genomic Imprinting

Answer 5

The epigenetic modification of the maternal and paternal genetic contributions to the zygote.

Question 6

What are the effects of genomic imprinting?

Answer 6

Differences in gene expression and subsequent phenotype. Differences in phenotype if a patient has a uniparental disomy or a heterozygous deletion or mutation for an imprinted region of a chromosome or gene.

Question 7

What is the relevance of methylation?

Answer 7

Plays an important role in the imprinting process. Is done selectively before and/or around the time of fertilization. Confers transcriptional silencing. Is reversible on passage through the germ-line.

Question 8

Prader-Willi Syndrome

Answer 8

Results from lack of expression from genes in the critical region that are normally only expressed from the paternal allele. Hypotonia, ID, and hyperphagia. (Hint: Prader, no fader)

Uniparental disomy.

Question 9

Angelman Syndrome

Answer 9

Results from a lack of expression from genes in the critical region that are normally only expressed from the maternal allele. Severe ID, movement disorder, and seizures.

Question 10

Uniparental Disomy

Answer 10

The presence of two homologous chromosomes inherited from only one parent. (One parent has contributed two copies of all or part of a chromosome, the other hasn’t at all)

Question 11

Heterodisomy

Answer 11

A type of uniparental disomy. When the contributing parent provides one copy of each homologous chromosome. Non-disjunction in Meiosis I.

Question 12

Isodisomy

Answer 12

A type of uniparental disomy. When the contributing parent provides two identical copies of the same chromosome. Non-disjunction in Meiosis II.

Question 13

When does genomic imprinting take place?

Answer 13

Selective methylation takes place before and/or around the time of fertilization.

Question 14

What is the mechanism used for genomic imprinting?

Answer 14

Methylation of DNA.

Question 15

What is the effect of DNA methylation?

Answer 15

Confers transcriptional silencing (by reducing/eliminating gene expression from the allele).

Question 16

True or False: Genomic imprinting is unable to be transmitted through mitosis in somatic cells.

Answer 16

False. Imprinting is stably transmitted through somatic cells undergoing mitosis.

Question 17

True or False? Imprinting is non-reversible in the germline.

Answer 17

False. It is reversible on passage through the germ line. (i.e. If an allele is maternally imprinted, this must be removed and changed in the gametes of a male offspring.)

Question 18

What is the purpose of genomic imprinting?

Answer 18

To change gene expression, and subsequently, phenotype.

Question 19

Prader-Willi Syndrome Hint: prader, no fader.

Answer 19

Results from the lack of expression from genes in the critical region that are normally only expressed from the paternal allele. Characterized by hypotonia, ID, and hyperphagia.

Question 20

Angelman Syndrome

Answer 20

Caused by a lack of expression from genes in the critical region that are normally only expressed from the maternal allele. Characterized by severe ID, movement disorder, and seizures.