Albinism
http://www.cbc.umn.edu/iac/facts.htm#genetics
Understanding Genetics:
An individual's development is directed by the genetic information that he or she has
received from both parents. This genetic information is carried in every cell of the human
body on structures called chromosomes Human cells contain 46 chromosomes which are
present in 23 separate pairs. The only exception to this rule involves the reproductive
cells. Sperm cells and egg cells contain only 23 chromosomes, one from each of the 23
pairs. We receive one of each of our chromosome pairs from our mother through her egg
cell, and one of each of our chromosome pairs from our father through his sperm cell.
Figure 4 and Figure 5 are pictures of human chromosomes. The first 22 pairs of
chromosomes are called autosomes. They are numbered according to size, with
chromosome #1 being the largest chromosome and chromosome 22 the smallest
chromosome. These are the non-sex chromosomes. The 23rd pair is the sex chromosome
pair. Females have two X chromosomes, and males have one X and one Y chromosome.
Each chromosomes is composed of many hundreds or thousands of genes. Genes are
biochemical blueprints which code for products needed to produce and maintain human
life. Genes account for physical traits as well as cellular chemical reactions, which direct
the production and maintenance of our body systems. As we have two of each of the
autosomes, one from each parent, we have two of each of the genes located on the
autosomes, one from each parent. Females have two copies of every gene located on the
X chromosome, while males have one copy of the genes on the X chromosome and one
copy of the genes on the Y chromosome.
Autosomal Recessive Inheritance:
The term autosomal means the gene responsible for the condition or trait is located on
one of the non-sex chromosomes or autosomes. This means that both males and females
have an equal chance of inheriting these genes and showing the trait. The term recessive
refers to the way in which the gene is expressed. In the case of albinism, the gene
responsible for the condition is a blueprint coding for the production of one of the
products needed to make pigment.
If a person carries one gene of a pair that has an altered blueprint and the other of the pair
has an unaltered blueprint, then the effects of the altered blueprint do not show. This
person is an unaffected carrier. Carriers make enough of the gene product to produce
pigment. Therefore, people who carry only one gene responsible for albinism do not
know they are carriers. If, however, a person carries two copies of an altered gene, then
the product of that gene cannot be made correctly. Such persons have albinism because
they cannot produce pigment.
When two people who carriers for the same gene have a child together, then the child has
one out of four chances of getting two copies of the albinism gene and having albinism.
The child has one out of four chances of getting the two copies of the normal gene and
having normal pigment and not being a carrier. The child has two out of four chances of
getting one normal gene and one albinism gene and having normal pigment but being a
carrier. See Diagram 6.
Meiosis
http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page2.html
Summary of chromosome characteristics
 Diploid set for humans; 2n = 46
 Autosomes; homologous chromosomes, one from each parent (humans = 22 sets
of 2)
 Sex chromosomes (humans have 1 set)
1. Female-sex chromosomes are homologous (XX)
2. Male-sex chromosomes are non-homologous (XY)
Karyotyping
Karyotype
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A pictorial display of metaphase chromosomes from a mitotic cell
Homologous chromosomes- pairs
Spectral Karyotyping- a new method
Ploidy: Number of sets of chromosomes in a cell
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Haploid (n)-- one set chromosomes
Diploid (2n)-- two sets chromosomes
Most plant and animal adults are diploid (2n)
Eggs and sperm are haploid (n)
Comparing Meiosis and Mitosis
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Chromosome behavior
1. Mitosis: Homologous chromosomes independent
2. Meiosis: Homologous chromosomes pair forming bivalents until anaphase
I
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Chromosome number- reduction in meiosis
1. mitosis- identical daughter cells
2. meiosis- daughter cells haploid
Genetic identity of progeny:
1. Mitosis: identical daughter cells
2. Meiosis: daughter cells have new assortment of parental chromosomes
3. Meiosis: chromatids not identical, crossing ove
Meiotic errors
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Nondisjunction- homologues don't separate in meiosis 1
1. results in aneuploidy
2. usually embryo lethal
3. Trisomy 21, exception leading to Downs syndrome
4. Sex chromosomes
1. Turner syndrome: monosomy X
2. Klinefelter syndroms: XXY
Translocation and deletion: transfer of a piece of one chromosome to another or
loss of fragment of a chromosome.
Mitosis, Meiosis, and Ploidy
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Mitosis can proceed independent of ploidy of cell, homologous chromosomes
behave independently
Meiosis can only proceed if the nucleus contains an even number of chromosomes
(diploid, tetraploid).
Trisomy 21 does not prevent meiosis