TERMS/ DEFINITIONS
HAPLOID: (n) refers to half the full set of chromosomes. In humans the haploid
number is 23 and is found only in the egg and sperm.
GAMETES: another name for the sex cells – egg and sperm- which undergo a
process called gametogenesis to reduce the chromosome number to half
HOMOLOGOUS CHROMOSOMES: paired chromosomes that are similar in shape,
size, gene arrangement and position of centromere. One of the pair came from
the egg ( mother) the other came from the sperm( father). An individual’s
appearance is determined by the manner in which the genes from homologous
chromosomes interact.
TETRAD: results when a pair of double stranded homologous chromosomes
comes together during prophase I. A tetrad has 4 chromatids and 2 centromeres.
SYNAPSIS: the term given to the pairing or coming together of homologous
chromosomes during late prophase I of meiosis.
SEGREGATION: the term used to describe the separation of homologous
chromosomes during anaphase I of meiosis.
CROSSING OVER: the exchange of pieces of genetic material between 2
homologous chromosomes which may occur during prophase I or metaphase I
when chromatids of homologous chromosomes are so close together they
intertwine. Segments may become week and break off becoming reattached to its
homologous chromosome.
GAMETOGENESIS: the formation/ maturation of sex cells that occurs during
meiosis. There are 2 types of gametogenesis- oogenesis (maturation of eggs in the
ovary) and spermatogenesis ( maturation of sperm in the testes).
SEX CHROMOSOMES: the 23rd pair of chromosomes( last pair) in the cell. They
determine the sex of the individual because they are different in males compared
to females. A female has 2 rod shaped homologous chromosomes (XX). A male
has one rod shaped (X) and one hook shaped ( Y) chromosome. The sex
chromosomes in males are not the same size and do not carry the same amount
of genetic information.
AUTOSOMES: the name given to all the other chromosomes in the cell except the
sex chromosomes.There are 22 pairs of autosomes.
NONDISJUNCTION: the term given to the failure of homologous chromosomes to
separate properly during anaphase I of meiosis. This will result in the daughter
cell receiving one too many chromosomes (24 instead of 23) or one too few
chromosomes( 22 instead of 23)
TRISOMY: a condition in which a zygote has 3 copies of one of a particular
chromosome instead of 2. It occurs as a result of nondisjunction. For example
trisomy 21 is a condition in which there are 3 instead of 2 of the 21rst
chromosomes. The child will have an extra chromosome ( 47 chromosomes
instead of 46) in all his/her body cells. A sex cell with 24 chromosomes combined
with a normal sex cell with 23 chromosomes to produce a zygote with
47chromosomes.
MONOSOMY: a disorder in which a sex cell with only 22 chromosomes is
combined with a normal sex cell with 23 chromosomes. The resulting zygote will
have 45 instead of 46 chromosomes in every body cell. One of the homologous
pairs of chromosomes is missing its homologue.
KARYOTYPE: picture of chromosomes arranged in homologous pairs. A karyotype
is used to detect irregularities in chromosome number.
DOWN’S SYNDROME: a.k.a. trisomy 21 is a disorder in which the 21 pair of
chromosomes failed to separate properly. The individual receives three copies of
chromosome 21 instead of the normal 2. An individual with Down’s Syndrome
will usually have some degree of mental retardation, a rounded full face,
enlarged, creased tongue, large forehead, and short stature. Often times people
with Down’s syndrome do not develop fully sexually and may be sterile. Down’s
syndrome occurs once in every 600 births. Women over 40 increase their risk of
having a Down’s Syndrome baby by 25 times.( one in 40 births)
TURNER’S SYNDROME: a monosomy disorder in which a female is born with only
one X chromosome instead of two due to nondisjunction occurring in the sex
chromosomes of the father. A girl born with Turner’s syndrome will have a total
of 45 chromosomes instead of 46. The chromosome that is lacking is the X
chromosome. One in 3000 females are born with Turner’s syndrome. Turner’s
syndrome females do not fully develop sexually which will result in sterility. She
will also be short in stature with a thick/ broad neck.
KLINEFELTER’S SYNDROME:( XXY) a trisomy disorder in which an individual is
born with 2 X chromosomes and a Y instead of two sex chromosomes. It is due to
nondisjunction of the 23rd pair of chromosomes in the egg or the sperm. The
individual is a male but will produce higher amounts of female sex hormones after
puberty and this will result in sterility. One in 500 male babies will have
Klinefelter’s syndrome. A male born with Klinefelter’s has an extra X
chromosome- a total of 47 chromosomes instead of 46. He may be below normal
intelligence and have long arms and legs in proportion to the rest of his body.
SUPERMALE: ( XYY) a male born with one X chromosome and 2 Y chromosomes,
produces more male hormones than is normal. It is reported that supermales may
show more aggressive/ violent tendancies. They are usually tall and lanky in
height.
AMNIOCENTESIS: a technique which involves drawing amniotic fluid from the sac
surrounding the developing fetus with a syringe. The fluid contains genetic
material from the developing fetus which is then stained with special dyes so that
the chromosomes become more visible and a karyotype can then be preformed
to check for chromosome abnormalities. The test cannot be performed before the
11th week of pregnancy. It is safer to wait until the 14th week.
CHORIONIC VILLI SAMPLING: a technique which involves drawing cells from the
outer membrane( chorion) surrounding the embryo to detect any genetic
abnormalities. It has the advantage of being able to be performed as early as the
8th week of pregnancy.
MULTIPLE MARKER SCREEN: ( MMS) a more recent test of a mother’s blood
taken at 15 – 18 weeks of pregnancy to identify women who may be at a higher
risk of having a baby with a genetic disorder.
ULTRASOUND: a test that uses energy from sound waves bouncing off the fetus
to make its image. It can be used to locate the position of the fetus within the
uterus and confirm the age of the baby and physical formation, including the sex
of the baby.