Download Introduction Chapter 12 Week 10 Chromosomes and Human Genetics

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Introduction
Chapter 12 Week 10
Chromosomes and Human Genetics
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Introduction:
Chromosomes of encode heritable characteristics. These characteristics involve every function in a the human
system from digestion to assimilation of food to growth and development and how we may respond to our
environment about us. Chapter 12 explains the terms of genetics necessary to understand how genetic disorders may
arise when either whole genes or alleles of a single gene are not present and accounted for (table 12.1). Some times
unbalanced translocation of alleles later in life, due to possible environmental factors, may result cancers while
balanced translocated chromosome may go undetected. Such an unbalanced translocation is Philadelphia syndrome
(22 to 9 translocation). Other genetic disorders arise from mistakes either in the egg or sperm during meiosis. A
karyotype of a person exhibiting three number 21 chromosome has a condition described as Down syndrome (page
209 of text).
Learning Goals for Chapter 12:
1. Understand the key terms
2. Know the difference between the autosomes and sex chromosomes
3. Describe sex determination in humans
4. Describe changes that are part of human sexual maturation.
5. Describe the role of the placenta, umbilicalous, and amnion
6. Describe development of sperm and eggs.
7. Understand pedigree charts (pg 202 of text)
8. Understand how sex linked traits transmit from one generation
to the next generation
9. Identify abnormalities of chromosomal structure and their resulting
physiological responses.
Chromosomes and Human Genetics
Chapter 12
----------------------------------------------------------------------------------------I. CHROMOSOMAL BASIS OF INHERITANCE
A. Introduction
1. Genes are units of information about heritable traits
2. Diploid organisms possess pairs of homologous chromosomes, which are alike in
length, shape, and gene sequence
3. Alleles are slightly different molecular forms of the same gene, which are shuffled
during meiosis.
4. Crossing over between homologous chromosomes results in new combination of
alleles on the chromosomes.
5. A chromosome’s structure may change due to
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a. Deletion
1. A loss of a chromosome segment as when a terminal segment is lost
1) Resulted from viruses, chemical or irradiation breaking a chromosomal
region
2) i.e. loss of a piece of chromosome 5 leading to a disorder called
cri-du-chat with symptoms of crying and mental retardation
3) Example 2: loss of a piece of a single chromosome ( 16 to 30 alleles from
human number 7 chromosome) leading to Williams syndrome
b. Duplication
1) Gene sequence is in excess of normal amount
a) Chromosome regions that code for polypeptides of
hemoglobin -- not harmful
2) In fragile X syndrome there are regions of abnormal constriction caused
by repeated sequences in the X chromosome resulting mental impaired
males
c. Inversion & Translocation
1) An inversion alters the position and sequence of the genes so that
genes order is reversed
2) A translocation occurs when a part of one chromosome is transferred to
a nonhomologous chromosome
a) Example: Translocation between chromosome 9 and 22 leads to an
abnormality associated with chronic myelogenous leukemia. This
cancer is typically found in older individuals.
B. Autosome and Sex Chromosomes
1. Two kinds of chromosomes in human cells
a. Most types of chromosomes in both sexes are referred to autosomes
(44 chromosomes in humans)
b. Sex chromosomes determine gender
1) Human females have two X chromosomes
a) The X chromosome codes for sexual traits as well as many genes for
nonsexual traits
2) Human males have X and one Y
a) The Y chromosome carries male determining gene
c. Total chromosome compliment in humans 46
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C. Karyotype Analysis
1. Chromosomes are visualized in lab preparations are called Karyotypes
2. Each chromosome has distinct size, length, centromere location, and banding patterns
III. Changes in Chromosome Number
A. Categories of Change
1. Aneuploidy
a. gametes or cells of an affected individual end up with one extra or one less
chromosome than is normal
2. Polyploidy
a. Three or more of each type of chromosomes in gametes or cells
1) Common in plants but fatal in humans
B. Mechanisms of Change
1. Chromosome number can change during mitotic or meiotic cell division or
during the fertilization process
2. Tetraploid germ cells can result if cytoplasmic division does not follow normal
DNA replication and mitosis
3. Nondisjunction at anaphase I or anaphase II frequently results in a change in
chromosome numbers
a. If a gamete with an extra chromosome (n + 1) joins a normal gamete at
fertilization, the diploid cell will be 2n + 1; this condition is called trisomy
b. If an abnormal gamete is missing a chromosome, the zygote will
be 2n - 1 monosomy.
IV. A Change in Autosomal Numbers
A. Down Syndrome
1. Trisomy 21
a. Symptoms: mental retardation and 40% have heart defects
2. Occurs more frequently in children born to women over the age of 35 years
V. A Change in Sex Chromosomal Numbers
A. Turner Syndrome
1. Involves females whose cells have only on X chromosome (XO)
2. Affected individuals are sterile and have other phenotypic problems such as
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premature aging and shorter life expectancy
3. About 75% of the cases are due to nondisjunction in the father
a) About 98% of XO zygotes spontaneously abort.
B. Klinefelter Syndrome
1. Nondisjunction resulting in an extra X chromosome in the cells (XXY) of these
affected males
2. About 67% of these result from nondisjunction in the mother, 33 % in the father
3. Symptoms: Slight mental retardation, sterility and body feminization
C. XYY Condition
1. The extra Y chromo in these males does not affect fertility
a. Maybe taller than average and are slightly mentally retarded
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