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Medical Genetics > Sheet 4 > Flashcards

Sheet 4 Flashcards

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1
Q

What happens if nondisjunction occurs with X or Y chromosomes?

A

Sexual aneuploidies

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2
Q

Which gene is smaller: X or Y? What does this mean?

A

Y; that there are genes in the X chromosome that are absent in the Y chromosome

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3
Q

What do we call the sex chromosomes?

A

Hemizygous genes: Only one allele for a certain gene

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4
Q

What are Psuedoautosomal regions?

A

Look like autosomal regions where there are 2 versions of the same gene (2 alleles) {Some regions of X are shared on Y}

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5
Q

What can you find on the Y chromosome?

A

1) SRY (Sex determining Region Y)
2) AZFa
3) AZFb
4) AZFc

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6
Q

What is the SRY region responsible for?

A

The development of male anatomical features

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7
Q

Explain what causes the case in which the karyotype is: 46, XY, but the primary sexual organs indicate a female (ovaries, not testes).

A

There’s a deletion in the SRY region.

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8
Q

What are the AZFa, AZFb, and AZFc regions responsible for?

A

The formation of sperm

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9
Q

If a male presents with infertility (low or zero sperm count = azoospermic), what should you check for?

A

Mutations in the 3 AZF regions on the Y chromosome.

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10
Q

True or false:

All the genes carried on sex chromosomes are sex-related.

A

False; not all are sex-related (some genes are not related to a sex trait)

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11
Q

What are some non-sex related genes found on the sex chromosomes?

A

1) Hemophilia gene

2) DMD (Duchenne Muscular Dystrophy) gene

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12
Q

Where is the hemophilia gene found?

A

Bottom of the Q arm of the X chromosome

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13
Q

Overall, we have - genes on X chromosome, - genes on Y chromosome.

A

900-1600; 70-200

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14
Q

What causes chromosomal disorders?

A

1) Numerical abnormalities (a missing or an extra chromosome)
2) Structural abnormalities (i.e., deletion, duplication, inversion or translocation).

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15
Q

What are some diseases caused by numerical abnormalities?

A

1) Klinefelter Syndrome (KS)
2) Turner Syndrome
(Both sex chromosome aneuploidies)

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16
Q

What are some diseases caused by structural abnormalities?

A

1) Cri-du-chat (cry of the cat) Syndrome

2) Certain cancers such as Chronic Myelogenous Leukemia (CML)

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17
Q

What is the karyotype for Klinefelter Syndrome (KS)?

A

47, XXY (MALES only)

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18
Q

Individuals with KS develop __(female/male) primary sexual organs. Why?

A

Male; regardless of how many X chromosomes they have, there’s still a Y chromosome and an SRY region.

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19
Q

What are the characteristics of Klinefelter Syndrome?

A

1) Secondary female characteristics:
Female fat distribution (hips) with breast development (gynecomastia)
2) Underdeveloped primary male sexual organs (testicular atrophy/small testes = absent sperm) = they’re infertile males.
3) Coarse/reduced body hair
4) Taller than average
5) Evidence of mental retardation that may or may not be present
6) Evidence of osteoporosis

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20
Q

What is the karyotype for Turner syndrome?

A

45, XO (missing X chromosome)

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21
Q

What kind of patients does Turner syndrome produce?

A

Sterile females

22
Q

Which syndrome is the ONLY VIABLE monosomy?

A

Turner syndrome

23
Q

Why are patients of Turner syndrome ALWAYS female?

A

Because there’s neither a Y

chromosome nor an SRY region.

24
Q

What are the characteristics of Turner syndrome?

A

1) Skin brown spots (nevi)
2) Underdeveloped breasts with a wide distance between the nipples
3) Webbed neck
4) 20 cm shorter than average
5) Rudimentary ovaries and underdeveloped gonads = infertility
6) No menstrual cycle
7) Abnormal elbow position
8) Secondary non-significant features (small fingernails and shortened metacarpals)

25
Q

Which is less deleterious (less damaging): Trisomy or monosomy?

A

Trisomy

26
Q

What causes Cri-du-chat syndrome?

A

A deletion mutation in the p arm of chromosome 5

27
Q

What are the main characteristics of Cri-du-chat syndrome?

A

1) Small head (microcephaly)
2) Small chin
3) Small nasal bridge
4) Unusually round face
5) Eyes are far from each other with a fold of skin over them

28
Q

What are some clinical symptoms of Cri-du-chat syndrome?

A

1) Heart defects
2) Hearing or sight problems
3) Motor problems: muscular/skeletal defects, poor muscle tone, and walking difficulties.
4) Hyperactivity and aggression
5) Severe mental retardation

29
Q

Most Cri-du-chat patients die within:

A

The 1st year of age but nowadays, with the advanced healthcare system and management, they make it to older ages.

30
Q

What are the most common causes of death in Cri-du-chat syndrome?

A

1) Pneumonia
2) Aspiration pneumonia
3) RDS
4) CHD

31
Q

CML can develop from:

A

1) Stem cells
OR
2) Myeloid stem cells (precursors for WBCs)

32
Q

What are the 4 types of leukemias?

A

1) Acute mylogenous leukemia
2) Acute lymphoblastic leukemia
3) Chronic mylogenous leukemia
4) Chronic lymphoblastic leukemia

33
Q

In CML, myeloid cells grow __(fast/slowly), and it is more common in __(adults/children)

A

Slowly; adults

34
Q

What causes CML?

A

Reciprocal translocation between chromosome 9 and chromosome 22 (Philadelphia chromosome)

35
Q

What is the exact mechanism regarding the translocation in CML?

A

The ABL gene that induces the cell cycle becomes under a stronger promoter which is the BCR promoter. The BCR-ABL transcript is translated into a mutant tyrosine kinase that results in a protein that is continuously activated. Therefore, there are many more unregulated cell divisions and, thus, a higher chance of developing cancer.

36
Q

What are the 2 scenarios regarding fragments of reciprocal translocation?

A 
1) Acentric (does not contain the centromere) fragment is exchanged with another acentric fragment = both
centromeres are preserved = mitotically stable chromosomes
2) Centric fragment is exchanged with an acentric fragment → acentric + dicentric chromosomes → mitotically unstable chromosomes
{#1: Acentric + acentric = stable
#2: Centric + acentric = unstable = dicentric + acentric chromosomes}
37
Q

What does the P arm of the acrocentric chromosomes contain?

A

1) The proximal heterochromatic region (highly repetitive non-coding DNA)
2) A satellite region (noncoding distal heterochromatic region)
3) A thin connecting region of euchromatin (the stalk) composed of tandem rRNA genes.

38
Q

What is a Robertsonian translocation?

A

Removing the p arms of both non-homologous acrocentric chromosomes then fusing the q arms of both chromosomes together to get one chromosome carrying two q arms of two acrocentric chromosomes.

39
Q

Robertsonian translocation occurs exclusively after:

A

Breaks in the short arms of the human acrocentric chromosomes

40
Q

What is the result of a Robertsonian translocation?

A

Centric + acentric fragments =
1) A dicentric chromosome that is stable in mitosis
+
2) An acentric chromosome that is lost in mitosis (unstable in mitosis) without any effect on the phenotype

41
Q

Why don’t Robertsonian translocations affect the phenotype?

A

Because the only genes lost are rRNA genes of the p arm; other acrocentric chromosomes can compensate this loss.

42
Q

Why are dicentric chromosomes stable in Robertsonian translocations?

A

Because the break occurs close to the centromere, and the two fused centromeres are so close to each other that they can function as a single centromere

43
Q

What are the products of Robertsonian/reciprocal translocations on gametes?

A

Unbalanced, resulting in monosomy or trisomy

44
Q

What does the risk of a reciprocal translocation carrier having a child with each of the possible meiotic outcomes depend on?

A

1) Its frequency on the gametes

2) The likelihood of a conceptus with that abnormality developing to term

45
Q

What is the karyotype of a Robertsonian translocation between chromosomes 14 and 21?

A

45, XY, t(21q:14q)

46
Q

Why do we have 45 chromosomes in Robertsonian translocations?

A

Because the (q arms) of both acrocentric chromosomes fuse together resulting in one viable dicentric chromosome. The other acentric chromosome is lost; so instead of 46 chromosomes the individual has 45 chromosomes

47
Q

Which translocation gives partial trisomy/monosomy gametes?

A

Reciprocal translocation

48
Q

Which translocation gives full trisomy/monosomy gametes?

A

Robertsonian translocation

49
Q

What are the ONLY viable trisomies?

A

1) Trisomy 21 (Down Syndrome)
2) Trisomy 18 (Edward Syndrome)
3) Trisomy 13 (Patau Syndrome)

50
Q

What is the ONLY viable monosomy?

A

Turner Syndrome

Medical Genetics (7 decks)

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