Karyotype and genetic disease. The number of chromosome per species is fixed. Each species has a characteristic number of chromosomes which is constant for that species: Examples: Human: 46 chromosomes Mouse: 40 chromosomes Onion: 16 chromosomes.
Size of the arms depends on the position of the centromere
Centromere in the center
Centromere close to the end
Examination of the chromosomes:
Chromosomes are isolated, stained, and examined under the microscope.
A picture of the chromosomes is taken through the microscope.
The picture of the chromosomes is cut up and chromosomes are arranged as homologous pairs in descending order of size.
Chromosomes are numbered:
each chromosome has been assigned a number based on its size;
the largest chromosome is chromosome 1.
The 23rd pair of chromosomes are sex chromosomes which determine the gender:
Female: two X chromosomes
Male: an X and a Y chromosome.
22 numbered pairs of human chromosomes, called autosomes (autosomal chromosomes)
A pair of homologues chromosomes may not separate during meiosis I
sister chromosomes fail to separate properly during meiosis II:
Result: formation of gametes with too many chromosomes or too few (aneuploid);
Baby is born with genetic disease
Nondisjunction of chromosome 21 during egg formation:
Down syndrome(trisomy 21), individual has 3 copies of chromosome 21 (47 chromosomes)
Mental retardation and distinct physical traits;
Probability increases with age of the mother
Under 20 years: risk about 1:1700
20-30 years: risk about 1:1400
30-35 years: risk about 1:750
45 years: risk about 1:16
Loss of a chromosome is always lethal
XX do not separate: resulting gametes: XX and O
If an XX gamete combines with an X gamete, the resulting zygote is a female XXX, usually sterile (XXX Syndrome);
If an XX gamete combines with a Y gamete, the resulting zygote is XXY, infertile male with enhanced female characteristics (breast) and diminished mental capacities (Klinefelter syndrome)
If a O gamete combines with a Y gamete, the resulting zygote is nonviable
If a O gamete combines with an X gamete, the resulting zygote is XO, sterile female, does not mature sexually (Turner syndrome)
Nondisjunction of XY chromosomes: YY gamete
If a YY gamete combines with an X gamete, the resulting zygote is XYY, fertile male, with learning difficulties and violent tendencies