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Introduction I
PloidyDegree of repetition of the basic number of
chromosomes Diploidy
Chromosomes repeat 2X Remember, in humans, you have one copy of a
chromosome from the maternal father and one from the maternal mother
Euploidy “True” ploidy
In the case of somatic human cells, euploidy occurs when the cell is diploid.
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Aneuploidy
“Not True” ploidy Having too many or too
few chromosomes.
Non-Disjunction An event that occurs
occasionally during meiosis in which a pair of chromosomes fail to separate so that the resulting germ (sex) cell has either too many or too few chromosomes
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Turner Syndrome 2n - 1
45 XO 44 autosomes + 1 X
chromosome There is the absence of a sex
chromosome
Female in appearance but their female sex organs do not develop at puberty and they are sterile
Short in stature
Shows normal intelligence but some cognitive functions are defective
1 in 5000 female births show Turner Syndrome
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Klinefelter Syndrome
2n + 1 47 XXY
44 autosomes + 3 sex chromosomes There is an extra X
chromosome
Male in appearance and they too are sterile
Female type pubic hair pattern
May have breast development
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XYY vs. XXX
XXYMales that are usually
fertile
Predisposition to be more violent in behavior
Gametes never YY or XY--meiosis is normal
After age of 35, extra Y chromosome often degenerates and is not passed onto offspring
XXXFertile females
May be some mental disabilities
Eggs will produce only X after meiosis--not XX
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Down Syndrome—Trisomy 21
Caused by an extra copy of chromosome #21 There are three copies of
chromosome #21 in their somatic cells
0.15 percent of all live births
Growth failure and mental retardation
Big toes widely spaced
Congenital heart disease
Mean life expectancy is about 17 years and only 8 % survive past age 40
Edward’s SyndroneTrisomy 18
Ear deformities
Heart defects
Spasticity and other damage
Death is typical by the age of 1 Some may survive
longer
Introduction I
The changes in the shape of the chromosome may be due to either of the followingTranslocationDeletionsFragile sites
Translocation
A chromosome fragment breaking off and attaching to a non-homologous chromosome Reciporcal translocation
Two non-homologous pairs exchange genetic information
Can result in deletion and/or duplication of genes
Translocation Down Syndrome
4% of Down Syndrome cases
Individuals actually have 46 chromosomes
One of copies of chromosome #14 has combined with chromosome #21The large arm of
chromosome #21 has been translocated to the large arm of another chromosome--usually chromosome #14
Deletion
The loss of part of a chromosome
The abnormal chromosome is known as a deletion
Sometimes chromosomes break and fail to rejoin
Cri du Chat Syndrome Part of the short arm of
chromosome #5 is deletedBreakage point
varies from case to case
Infants normally have a small head with altered featuresMoon face
Infants have a distinctive cry that sounds like a cat mewing
Infants normally survive childhood
Exhibit severe mental retardation
Fragile Sites
Weak points at specific locations in chromatids
Appears to be a place where part of a chromatid appears to be attached to the rest of the chromosome by a thin thread of DNAHave been identified on the X chromosome
and certain autosomes
Fragile X Syndrome
Fragile site occurs near the tip of the X chromosome
Where nucleotide triplet CGG is repeated many more times than normal
Most common cause of mental retardation
Genetic Screening & Genetic Counseling
Genetic ScreeningIdentifies individuals
who might carry a serious genetic disease Screening of newborns
is the first step in preventative medicine
Genetic CounselingProvide couples,
concerned about the risk of abnormality in their children, medical and genetic information
Pedigrees
DefinitionA family tree that shows the transmission
of genetic traits within a family over several generations.
Pedigree AnalysisUseful in detecting autosomal dominant
mutations, autosomal recessive mutations, X linked recessive mutations and defects due to genomic imprintingGenomic Imprinting
Expressions of a gene based on its parental origin