Chromosomal Anomalies Flashcards by Esme Cosham

What is aneuploidy?

Loss or gain of whole chromosome

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What is aneuploidy usually caused by?

Nondisjunction - where pairs of homologous chromosomes or sister chromatids fail to separate during meiosis I or II (or during mitosis)

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What are the 2 common types of aneuploidy?

Monosomy and trisomy

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What is monosomy?

When an organism has only one copy of a chromosome that should be present in 2 copies (2n-1)

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What is trisomy?

When an organism has a third copy of a chromosome that should be present in 2 copies (2n+1)

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What is the most common trisomy?

Down Syndrome - Trisomy 21: 47,XX,+21)

Caused by non-disjunction in meiosis I

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What is Edward’s syndrome?

Trisomy 18: 47,XX,+18

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What is Palau syndrome?

Trisomy 13: 47,XX,+13

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What do human autosomal monosomes result in?

Death - baby won’t make it to birth

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What do human autosomal trisomies result in?

Tend to prevent embryo from developing to birth but an extra copy of the smaller chromosomes can allow individual to survive

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Why are aneuploidies involving sex chromosomes less harmful?

Human cells can shut down extra X chromosomes in X inactivation

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What is triploidy?

Rare chromosomal abnormality where foetuses are born with an extra set of chromosomes in their cells

69 not 46 chromosomes per cell

Lethal - miscarriages

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What happens during nondisjunction during meiosis I?

Homologues don’t separate which can lead to production of aneuploid gametes

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What happens during nondisjunction during meiosis II?

Sister chromatids (instead of homologous chromosomes) don’t separate

Results in 2 normal gametes, 1 with no genetic information and one with double genetic information

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What is Turner syndrome?

Sex chromosome anomaly –> monosomy

Partial or complete loss of 2nd sex chromosome

45, X

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Results of Turner syndrome?

99% lost during pregnancy
1 in 3000 live female births

Short stature, wide carrying angle of arms, 1ary amenorrhoea (ovaries inovulate before birth), congenital heart disease 20%

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What is Klinefelter syndrome?

Sex chromosome anomaly –> trisomy

Causes male to be born with extra sex chromosome (X is most common)

47, XXY

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Results of Klinefelter syndrome?

1 in 850 live male births

Infertility, poorly developed 2ary sex characteristics, tall, osteoporosis

What are sex chromosome anomalies caused by?

Abnormal gametes with unusual complements of sex chromosomes

Each spermatogonium in the testis at age 15 is the result of how many previous cell divisions?

What does spermatogonium divide into?

Divides into 2

One maintains stock of spermatogonia and continues to divide

Other divides into 4 spermatozoa

How many cell divisions have occurred by age 25 to produce a particular sperm?

310

What increases as age increases in relation to sperm?

The number of cell divisions that have had to occur to produce a spermatozoa

Why do mutations increase with paternal age?

Higher mutation rates in males are related to greater number of germ cell divisions

What occurs during meiosis I in relation to oocytes?

1st polar body formed and is completed at ovulation

In oogenesis, how many daughter cells develop into mature oocytes?

1/4 - the rest form polar bodies

What occurs during meiosis II in relation to oocytes?

2nd polar body formed and is completed at fertilisation Zygote produced

What can damage to the repair mechanism of the 1ary oocyte result in?

Increased chance nondisjunction

What is mitotic nondisjunction?

After fertilisation of a normal egg, the embryo undergoes abnormal separation of chromosomes in one of its cells Results in a foetus with two (or more) populations of cells

What does proportion of abnormal cells in an embryo due to mitotic nondisjunction depend on?

When the nondisjunction occurred

What is a chromosomal rearrangement?

Big chunks of chromosomes, but not entire chromosomes, are affected

What is a duplication?

Part of a chromosome is copied

What is a deletion?

Part of a chromosome is removed

What is an inversion?

Chromosomal region is flipped around so that it points in opposite direction

What is a translocation (reciprocal and non-reciprocal)?

A piece of a chromosome gets attached to another (non-homologous) Reciprocal - two chromosomes swapping segments Non-reciprocal - one chunk of a chromosome moving to another

How can Down Syndrome also be caused by chromosomal rearrangement?

Can occur when a large piece of chromosome 21 moves to another chromosome and is passed on to offspring, along with a regular chromosome 21

What is Centric Fusion Robertsonian?

Breakage of 2 acrocentric chromosomes at (or close to) their centromeres Subsequent fusion of their long arms and short arms are lost

What is an acrocentric chromosome?

A chromosome in which the centromere is located quite near one end of the chromosome

What 3 different patterns of chromosomes can cause Down Syndrome?

1. Trisomy 21 - 95% have 3 separate copies of chromosome 21 2. Robertsonian translocation - 4% 3. Mosaicism - 1% have mosaicism with normal and trisomy 21 cell lines, occurs post-zygotically

What is result of Down syndrome caused by mosaicism?

Milder features due to presence of normal cells

What is an acquired cytogenetic abnormality?

A genetic change associate with a neoplastic or cancer disease process

What is translocation during 90% of cases of chronic myelogenous leukaemia (CML)

A translocation between chromosomes 9 and 22 (46,XX,t)

What occurs during the neoplasm follicular lymphoma?

bcl-2 oncogene is translocated from its usual site on chromosome 18 to new site on chromosome 14 Antibody genes expressed in B lymphocytes --> gene now continually expressed when it is normally suppressed Cells keep growing and form cancer of B cells

What is a neoplasm?

A new and abnormal growth of tissue in a part of the body, especially as a characteristic of cancer