Human chromosomes There are 46 chromosomes (23 homologous pairs) in each somatic cell 22 pairs of autosomes 1 pair of sex chromosomes XX = Female OR XY = Male Karyotype - chromosomes are arranged according to shape and size

Karyotype

Nondisjunction and chromosomal disorders Nondisjunction – failure of chromosomes to separate and segregate into daughter cells Nondisjunction may occur during meiosis 1 or meiosis 2 Abnormal number of chromosomes may result

Abnormal meiosis Sometimes mistakes occur during meiosis –Segments of chromosome (block of genes) can be lost or added – chromosome mutation / aberration –Cell may be missing a chromosome or have an extra chromosome (aneuploidy – incorrect number of chromosomes) – Plant cells have more than 2n sets of chromosomes in nucleus (3n, 4n – called polyploidy) Examples of aneuploidy –Down’s syndrome (3 x chromosome 21- trisomy) –Turner’s syndrome (female has 1 X chromosome – monosomy) –Klinefelter’s syndrome (3 x sex chromosome – XXY) 4

How Down’s syndrome happens Trisomy – chromosome 21 Chromosomes of homologous pair 21 do not separate during meiosis One gamete (♀) may have two chromosomes for chromosome 21 and the other (♀) does not have a chromosome 21. Fertilisation: 2 x 21 (♀) + 1 x 21 (♂) = 3 x chromosomes 21 Extra chromosome 21 (trisomy) – offspring/baby This process is called non-disjunction (failure to separate) 5

The diagram below shows an abnormality during a certain phase of meiosis with the number 21 chromosome pair (labelled A). 1. What is the specific purpose of this phase of the cell division? 2. What specific genetic condition does this abnormality (shown by A) cause? 3.Give TWO symptoms of this genetic disorder. 4.People with this condition are often sterile. Explain why you think this is so. 6

Karyotype Karyotype of a person with a genetic disorder 7

How trisomy and monosomy with pair 23 happen Sex chromosomes - Trisomy and Monosomy Chromosomes of homologous pair 23 do not separate during meiosis One gamete (♀) may have two chromosomes for chromosome 23 and the other (♀) does not have a chromosome 23. Fertilisation: 2 chromosomes 23 (♀) + 1 chromosome (♂) = 3 chromosome 23 Extra chromosome 23 (trisomy - XXY) AND Fertilisation: 0 (♀) + 1 (♂) = 1 X chromosome 23 Only ONE X chromosome (monosomy - X0) 8

How Polyploidy happens Polyploidy – Plant cells with more than the diploid set of Chromosomes Chromosomes of all the homologous pairs of one organism do not separate during meiosis One gamete (♀) may have two sets of chromosomes. Fertilisation: 2n (♀) + n (♂) = 3n (triploid) OR Self pollination and Fertilisation: 2n + 2n = 4n (tetraploid) 9

Normal monosomic gametes Normal meiosis MEIOSIS I MEIOSIS II Results of crossing- over not shown Replicate DNA

MEIOSIS I MEIOSIS II Replicate DNA Nondisjunction during meiosis I Non-disjunction Disomic gametesNullisomic gametes

MEIOSIS I MEIOSIS II Replicate DNA Nondisjunction during meiosis II Non-disjunction DisomicNullisomicMonosomic gametes

EXAMPLES OF NON-DISJUNCTION

AN EXTRA COPY OF CHROMOSOME 21 CAUSES DOWN SYNDROME This condition is called trisomy 21. Person with this condition suffers from the Down syndrome. Characteristic facial features: Round faceRound face Flattened nose bridgeFlattened nose bridge Small, irregular teethSmall, irregular teeth Short statureShort stature Heart defectsHeart defects

Klinefelter's syndrome, 47, XXY It is the most common sex chromosome disorder and the second most common condition caused by the presence of extra chromosomes Symptoms: -Language impairment -Lanky, youthful build or rounded body type -Low levels of Testosterone and small testicles / Infertile

Turner’s Syndrome (X) Common symptoms: Short stature swelling of the hands and feet Broad chest and widely spaced nipples Low hairline Low-set ears Reproductive sterility Increased weight, obesity Small fingernails Characteristic facial features Webbed neck

1 in 1,250 births 47 chromosomes XY or XX #21 Trisomy Nondisjunction Down Syndrome

Nondisjunction Telematics Life Sciences