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Chapter 17

HMB265H1 Chapter 17: HMB265 Lecture 17 READINGS - Changes in Chromosome Number

Human Biology
Course Code
Maria Papaconstantinou

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HMB265 Lecture 17 READINGS - Changes in Chromosome Number
(Pg. 325-339)
9.4 Aberrations in Chromosome Number: Aneuploidy
- Diploid individuals have 2n chromosomes
- N = number of chromosomes in gametes
- All chromosomes in haploid gametes are different from one another
Aneuploid: Individuals whose chromosome number is not an exact multiple of the haploid
number (n) for the species
- Monosomic: Normally diploid spp lacking 1 chromosome (2n - 1)
- Trisomic: Normally diploid spp w/ single additional chromosome (2n + 1)
Creates genetic imbalance = deleterious to organism
Autosomal aneuploidy is usually lethal
- Monosomy for any autosome usually lethal
- Trisomy for autosome usually very deleterious
- Baby aborted spontaneously early in pregnancy
- Developmental abnormalities = early death
- Trisomy 21 = Down syndrome
- Chromosome 21 very small autosome
- Mental retardation, skeletal abnormalities, increased susceptibility to
- 3 copies of part of chromosome 21 (not whole ting)
Most organisms tolerate aneuploidy for sex chromosomes
X chromosome in human
- One of the longest human chromosomes
- X chromosome aneuploidy
- XXY males
- XO females
- XXX females
- People usually survive well compared to aneuploids for autosomes
- = tolerance for X-chromosome aneuploidy
- X-chromosome inactivation equalizes expression of X-linked genes in individuals
w/ different numbers of X-chromosomes
- EX: In XX mammals:

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- Have X-chromosome inactivation
- Represses expression of most genes on one of 2 chromosomes
- If X-chromosome aneuploid has more than 2 X chromosomes (XXX), only 1 activated X
- = Amount of protein generated by X-linked genes in X chromosome aneuploids same as
normal XX or XY individuals
- SUMMARY = Only genes on 1 X chromosome actually expressed
- Genes that escape X inactivation are in the pseudoautosomal (PAR) regions of X and Y
Still have consequences for X-chromosomes aneuploidies
- XXY men = Klinefelter syndrome
- XO women = Turner syndrome
- Infertility, skeletal abnormalities
- Abnormalities from abnormal dosage of 30 PAR genes in somatic cells
- Encodes protein for bone development
- XO females have 1 less copy of these genes than normal females
- XXY males have 1 more copy than normal males
Also have infertility
- Abnormal dosage in germ-line cells of X-linked genes outside PAR region
- Germ-line cells undergo reverse of X inactivation = X-chromosome reactivation
- X reactivation occurs in oogonia
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