Chromosomes, chromosome abnormalities and birth defects Flashcards Preview

MCD - Genetics > Chromosomes, chromosome abnormalities and birth defects > Flashcards

Flashcards in Chromosomes, chromosome abnormalities and birth defects Deck (30):

Definition of congenital abnormalities

Abnormalities present at birth that may be due to genetics or infection.


Definition and example of malformation

Primary structural defect - atrial septal defect, cleft lip


Definition and example of disruption

Secondary abnormal structure of organ or tissue - amniotic band wrapping around blood vessel cutting off blood supply to an organ


Definition and example of deformation

Abnormal mechanical force distorting a structure - club foot, hp dislocation


Definition and example of syndrome

Consistent pattern of abnormalities with a specific underlying cause - Down syndrome


Definition and example of sequence

Multiple abnormalities initiated by primary factor - Reduced amniotic fluid leads to Potter's sequence


Definition and example of dysplasia

Abnormal organisation of cells into tissue - thanatophoric dysplasia


Definition and example of association

Non-random occurrence of abnormalities not explained by syndrome. Cause usually unknown - VATER association


Definition of teratogen

An agent that interferes with normal embryonic or foetal development


Whole-chromosome factors of congenital defects

Numerical - loss or gain of chromosomes
Structural - Translocations, deletions, insertions, inversions
Mosaicism - Normal development of mutated chromosomes alongside normal chromosomes (stage at which mutation occurs affects degree of expression e.g. early mutation = obvious effects)


Effect of age on genetic risk

Increased age of either or both parents results in increased risk of a child with genetic abnormalities


Normal human karyotype

46 chromosomes in somatic cells
23 chromosomes in germ cells
Males have XY as sex-determining pair
Females have XX as sex-determining pair


Process of chromosome banding

Bands labelled according to chromosome number
Short (p) or long (c) arm
Numbered out from centromere.
Used to identify different chromosomes


Nomenclature of karyotyping

- Write total number of chromosomes
- Then a comma
- Then sex chromosomes
e.g. 46,XY or 46,XX


Definition of aneuploidy

An abnormality in the number of chromosomes, caused by the loss or gain of one or more chromosomes


The 3 types of aneuploidy, their definition and whether they can be tolerated or not

Monosomy - loss of one chromosome, nearly always fatal
Trisomy - gain of one chromosome, can be tolerated
Tetrasomy - gain of two chromosomes, can be tolerated


Definition of chromosome translocation

The transfer of genetic material from one chromosome to another when a chromosome break occurs during mitosis or meiosis


The two types of translocation



Features and consequences of balanced translocation

- Even exchange of genetic material between the two chromosomes
- No genetic data lost
- Chromosomes likely to remain the same length
- Little to no physiological effect seen


Features and consequences of unbalanced translocation

- Uneven exchange of genetic material
- Loss or gain of genetic data
- Chromosomes unlikely to remain the same length
- More likely to cause disease


Explain copy number variant/variation

- Phenomenon where sections of the genome are repeated
- Number of repeats in the genome varies between individuals
- Specifically large scale duplication or deletion events that cause variation


Aim of meiosis

To create 4 haploid cells each containing 23 chromosomes


Non-disjunction definition

The failure of the chromosomes to separate correctly in meiosis I or II


Consequences of non-disjunction

- Creates diploid and empty germ cells
- Hence when a diploid and a haploid germ cell combine, a trisomal zygote is created, diploid-diploid = tetrasomal, haploid-empty = monosomal.


The three chromosome aberrations that lead to Down syndrome

- Trisomy 21
- Robertsonian translocation
- Mosaicism


Clinical features of Down syndrome

Newborn period - sever hypotonia, sleepy, excess nuchal skin
Craniofacial - macroglossia, small ears, epicanthic folds, upward sloping palpebral fissures (gaps between upper and lower eyelids)
Limbs - single palmar crease (should be 3, they have 2) wide gap between first and second toes
Cardiac - atrial and ventricular septal defects
General - Short stature, duodenal atresia (abnormal closure/absence of lumen of duodenum)
Low IQ but advanced social skills


Dosage compensation definition

A genetic mechanism in which alleles of a gene automatically regulate the amount of useful product produced such that heterozygous individuals produce the same amount of product as homozygous dominant individuals.


Mechanisms of dosage compensation

- One of each X chromosome in female cells is inactivated. Selection of chromosome to be inactivated is random
- Increased (2x) expression of X chromosome genes in males
- Decreased (0.5x) expression of X chromosome genes in hermaphrodites


Mechanism of sex determination

The SRY gene causes development of testes. Can be translocated to X chromosome in SRY recombination.


Consequences of SRY recombination

SRY gene now on X chromosome
- XX males develop testes but are sterile as other genes on Y chromosome needed for spermatogenesis
- XY females have mutations in SRY gene that lead to sterile phenotypically female individuals