Cytogenetics & Nondisjunction

Humans contain _ chromosomes

46

How many pairs of homologous autosomes do humans have?

22

How many sex chromosomes do humans have?

2

Stained and photographed preparation of metaphase chromosomes arranged according to their size and position of centromeres

Karyotype

each chromosome is hybridized with different fluorescent probe

Chromosome painting

Chromosomes are classified according to the relative position of their _____

centromeres

Metacentric

located in middle of chromosome

Submetacentric

closer to one end of chromosome

Acrocentric

near one end of chromosome

Acentric

no centromere

Dicentric

two centromeres

What type of chromosomes are genetically unstable?

acentric and dicentric

Each chromosome in karyotype is divided into ____ regions (arms) separated by the ______

two, centromere

p

short arm (petit)

q

long arm

p and q arms are divided into ______ and _____ based on pattern of staining

numbered bands and interband regions

One copy of X is ____ _____ in all somatic cells.

randomly inactivated

Females are _____ ______ for genes on the X chromosome

genetic mosaics

Barr body

inactive X chromosome in the nucleus of interphase cells

_____ _____ equalizes the number of active copies of X-linked genes in females and males

Dosage compensation

Y chromosome is largely

heterochromatic

_____ is condensed inactive chromatin

Heterochromatin

Pseudoautosomal region

region of shared X-Y homology

TDF

Testis determining factor

Master sex controller gene that encodes testis determining factor (TDF) for male development

SRY

the set of alleles at two or more loci present in a particular chromosome

Haplotype

balanced chromosome abnormality

Euploid

unbalanced set of chromosomes

Aneuploid

loss of a single chromosome copy

Monosomic

extra copies of single chromosomes

Polysomic

_____ or ____ of most human autosomes is unviable.

Monosomy, trisomy

What are the three exceptions of trisomies that are viable?

trisomies of 13, 18, 21

a genetic disorder due to trisomy 21

Down Syndrome

Frequency of Down Syndrome increases with ____ age

mother’s

abnormal pairing of trisomic chromosomes in cell division

Trivalent

extra chromosome in trisomy is unpaired in cell division

Univalent

Trisomy-X =

47, XXX (female)

Double-Y

47, XYY (male)

Klinefelter Syndrome

47, XXY (male, sterile)

Turner Syndrome

45, X (female, sterile)

Aneuplody results from _____

nondisjunction

a failure of chromosomes to separate and move to opposite poles of the division spindle

nondisjunction

The rate of nondisjunction can be increased by _____ _ ___ _____.

chemicals in the environment

missing chromosome segment; deficiency

Deletions

Deletions often result from _____ ______-_____.

unequal crossing-over

Large deletions are often ______.

lethal

genetics rearrangements in which chromosome segment present in multiple copies

Duplication

repeated segments are adjacent

Tandem duplications

Tandem duplications often result from _____ _____-____ due to mispairing of homologous chromosomes during meiotic recombination

unequal crossing-over

genetic rearrangements in which the order of genes in a chromosome segment is reversed

Inversions

Inversions do not alter ____ ____ ____.

the genetic content

In an inversion heterozygote, chromosomes twist into a ____ in the region in which the gene order is inverted

loop

Paracentric inversion does not include ______.

centromere

Crossing-over within a paracentric inversion loop during recombination produces one ______ (no centromere) and one _____ (two centromeres)
chromosome

acentric, dicentric

Pericentric inversion includes ______.

centromere

Crossing-over within a _____ _____ ____ during homologous recombination results in ______ ___ ______ of genetic information

pericentric inversion loop; duplications and deletions

A chromosomal aberration resulting from the interchange of parts between nonhomologous chromosomes

Translocation

____ _____ may be elevated or decreased in translocated gene

Gene expression

In ____ _____, one pair of chromosomes interchanged their segments and one pair is normal

heterozygous translocation

In ____ ____, both pairs interchanged their segments

homozygous translocation

Synapsis involving heterozygous reciprocal translocation results in pairing of four pairs of sister chromatids

quadrivalent

fusion of two acrocentric chromosomes in the centromere region

Robertsonian translocation

Translocation results in apparent loss of one chromosome in ____ analysis

karyotype

Genetic information is ____ in the tips of the translocated acrocentric chromosomes

lost

When chromosome 21 is one of the acrocentrics in a Robertsonian translocation, the rearrangement leads to a

familial type of Down Syndrome

The heterozygous carrier is _____ _____, but a high risk of Down syndrome results from aberrant segregation in meiosis

phenotypically normal