Cell DivisionMeiosis

Cell DivisionMeiosis

Abnormal Meiosis

Cell DivisionMeiosisAbnormal Meiosis nondisjunction occurs when two homologous chromosomes

fail to separate during meiosis or mitosis. one of the daughter cells will have too many chromosomes,

while another will have too few. the effects of nondisjunction are more devastating in the

production of gametes. nondisjunction occurs during anaphase I or anaphase II

Cell DivisionMeiosis nondisjunction in humans produces:

gametes with 22 and 24 chromosomes. if the gamete with 24 chromosomes joins with a normal

gamete of 23 chromosomes a zygote containing 47 chromosomes is produced. the zygote will have three chromosomes rather than a

pair. this condition is referred to as trisomy.

Cell DivisionMeiosis

if the gamete with 22 chromosomes joins with a normal gamete of 23 chromosomes a zygote containing 45 chromosomes is produced. the zygote will have one chromosome rather than a pair

this condition is referred to as monosomy. once the cells of trisomic or monosomic zygotes begin to

divide, each cell of body will be one plus or one minus a chromosome.

Cell DivisionMeiosis

Cell DivisionMeiosisNondisjunction Disorders Male and Female Syndromes

Down Syndrome trisomy 21

Patau Syndrome trisomy 13

Edward Syndrome trisomy 18

E) Abnormal Meiosis

Down’s Syndrome cause: trisomy of the 21st

chromosome symptoms:

mental retardation, a round full face, enlarged tongue,

large forehead short stature shortened lifespan higher risk of other medical

conditions, such as heart defects(50%), leukemia(10-50 times more common), Alzheimer’s, etc

prevalence: the most common non-lethal non-

disjunction disorder affects about 1 in 800 babies (risk

increases significantly with age of mother)

Patau Syndrome cause: trisomy of the 13th chromosome

least common and most severe of the autosomal trisomies

symptoms: extreme facial deformation (e.g. cyclopia, missing

nose) polydactyly long term neurological disability, heart defects, frequent pneumonia and other

respiratory infections. prevalence: about 1 in 10,000 live births prognosis:

very poor, most embryos do not survive gestation and are spontaneously aborted

of those surviving to term gestation, approximately 82-85% do not survive past 1 month of age, and 85-90% do not survive past 1 year of age (there have only been 5 cases reported in the medical history of patients living beyond 10 years of age)

Edwards Syndrome cause: trisomy of the 18th chromosome symptoms:

low birth weight; a small, abnormally shaped head; small jaw; small

mouth; low-set ears; clenched fists with overlapping fingers. breathing or heart defects normally associated

with premature babies prevalence:

1:3000 live births second most common autosomal trisomy increased risk as a woman's age increases

prognosis: very poor - about half die in utero of liveborn infants, only 50% live to 2 months, and

only 5 - 10% will survive their first year of life. Median life span is 5-15 days.

medical interventions for related medical problems (e.g. heart defects) usually withheld due to poor prognosis

Cell DivisionIII) Meiosis Gender Specific

Turner Syndrome (female) X0

Triplo-X Syndrome (female) XXX or XXXX

Klinefelter Syndrome (male) XXY or XXXY

Jacob’s Syndrome (male) XYY

E) Abnormal Meiosis

Turner’s Syndrome cause: sex chromosomes undergo non-disjunction,

causes a monosomic female female only has one X chromosome instead of the

normal two it is the only known viable monosomy in humans does not occur in males because the embryo cannot

survive without at least one X chromosome (females born as XXX are healthy and cannot be

distinguished from normal XX females except by karyotype)

symptoms: (females only) sexually underdeveloped, tend to be short and have thick, widened necks sterility, congenital heart disease and hypothyroidism

prevalence: 1 in 5,000 births most Turner’s embryos resulting in miscarriages

Klinefelter Syndrome cause: again, a non-disjunction of

the sex chromosomes, but this time producing a trisomy at birth, the child will have primary male

sex characteristics and will appear male at puberty, he will begin producing high

levels of female sex hormones (e.g. estrogen)

(males with an extra Y chromosome used to be thought of as destined to be criminals; now we know them typically to be only taller than average)

symptoms: (males) though he has male sex organs, they are

small and the man is sterile he will have breast enlargement and other

female body characteristics X-linked recessive conditions occur less

frequently than in normal males prevalence: 1 in 1000 births

Cell DivisionMeiosis the chances of nondisjunction disorder increases with age

chances of having a child with Down’s Syndrome conceiving between 20 and 24 years, 1 in 1490 conceiving at age 40, 1 in 106 conceiving at age 49, 1 in 11

E) Abnormal Meiosis

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Karyotypes

Cell DivisionMeiosis- RecallKaryotype a chart of chromosomes. obtained by mixing a small sample of tissue with a chemical that

stimulates mitotic division. division is the stopped during metaphase. chromosomes are stained a picture is taken and chromosomes are paired up with their

homologue. homologue chromosomes are similar in size, length,

centromere location and banding pattern. they are organized in decreasing size with the sex

chromosome placed at the end.

Cell DivisionMeiosis

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Normal Male

E) Karyotype

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Jacobs Syndrome

Cell DivisionMeiosis Turner Syndrome

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Down Syndrome