Margaret M. Loos
I.
Objectives
:
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1. To see what can go wrong with chromosomes in general.
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2. To learn about sex chromosome abnormalities in particular.
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3. To become aware of sex linked and sex limited characteristics.
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4. To recognize abnormal karyotypes.
II.
Information
:
The X and Y chromosomes are of different size, and each has areas without a comparable section on the other. If a trait is coded for by a gene or genes in this area of the X chromosome it is said to be sex-linked. Color-blindness and hemophilia are transmitted genetically as sex-linked traits. If a trait is carried on the Y chromosome without a homologue on the X it is said to be sex limited.
III.
Task
:
Predict the offspring in cases of sex.linked disorders. These are all recessive alleles. Differentiate between male and females and their own genotypes.
IV. Sex chromosomes show more abnormalities than most of the autosomal chromosomes. Changes can occur in the phenotype of the individual that affect him or her both sexually and physically, often limiting his or her ability to become a parent and even to earn a living. In an article on sex chromosome disorders in the New England Journal of Medicine (March 25, 1978) Dr. Park S. Gerald measures the incidence of such disorders as 1/380 males and 1/1100 females.
Doctors have two ways to ascertain if there are sex chromosome abnormalities.
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1. Examination for Barr bodies and F bodies. Barr bodies can be seen as dark spots near the nuclear membrane. There is always one less Barr body than the number of X chromosomes. The F body appears as a bright spot with a florescent staining method. The number of F bodies is equal to the number of Y chromosomes.
Chromosome examination is done by karyotyping methods
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A. The main dysfunction in sex chromosomes (and frequently in autosomal chromosomes as well ) is the failure of the pairs to separate during meiotic reduction division. This is called non-disfunction. It occurs more in oogenesis than in spermatogenesis. Can you relate this to when the gametes are produced in females and when they complete meiosis.
What genotypes could this produce?
Task A Normal
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Non-disjunction in Female
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In Male
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XX and XY
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XX or O and XY
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XX and XY or O
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When 3 chromOsomes occur in the zygote, the condition is called a
trisomy
(like XXX). When the non-disjuntion causes only one chromosome of a pair in the zygOte it is called a
monosomy
. Monosomies, other than the XO in the sex chromosomes, seem to be unfit to survive. Trisomies do survive but often exhibit phenotypic and genetic problems.
In the above offspring:.
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1. The YO does not survive. An X chromosome is a prerequisite for life.
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2. The XO female, Turner’s syndrome.
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____
a. How many Barr bodies, how many chromosomes?
These are females,with rudimentary or nO ovaries, underdeveloped breasts, short stature, and with a peculiar webbing of the neck and often sub-normal in intelligence. They cannot reproduce and occur about 1/25,000 births.
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3. The XXX—Super female:
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____
a. How many Barr bodies? F bodies, chromosomes? These females can reproduce normal males and females. The possible genotypes for eggs would be XX and X. Only about 1/6 Of their offspring have chromosome abnormalities so XX code doesn’t seem to function well. Some of these females do nOt menstruate.
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4. The XXY male, Klinefelter’s syndrome, 6/1,000 males.
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____
a. Barr bodies? F bodies? chromosomes? These males do not look abnormal until puberty when underdeveloped gonads, long limbs, and often, development of breasts become evident. Most are sterile and many are mentally defective.
Task B.:
Predict the testosterone level, sperm production, bodily hair, musculature and voice of XXY.
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5. The XYY genotype also exists and it is believed in a ratiO of 1/300 males. These males are usually taller than average, have barely normal IQ’s and often suffer persistence.
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6. Tetro X (XXXX) females occur. How many Barr bodies? Chromosomes? They exhibit same characteristics as XXX.
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7. Klinefelter with XXXY, XXXXY genotypes also occur. Barr bodies? F bodies? Chromosomes?
Assignment:
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A
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1. Research one of the above or
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2. Research testicular feminization or
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3. Research new findings in hemophilia research.
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B Everyone examine these karyotypes, paste in notebook and describe problems Of each.