Download Ch 14: The Human Genome

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Ch 14: The Human Genome
14-1 Human Heredity
A. Human Chromosomes
1. In order to learn more about humans, biologists often use a karyotype to analyze human chromosomes.
2. Karyotype- a picture of a cell’s chromosomes grouped in homologous pairs
3. Humans have 46 chromosomes
4. Two of these, X and Y, are sex chromosomes
a. Males have one X and one Y chromosome (XY)
b. Females have two X chromosomes (XX)
5. Autosomes- the other 44 chromosomes
6.
All human egg cells carry a single X chromosome (eggs/ovum in females)
7.
Sperm cells carry either X or Y chromosomes
a. ½ of all sperm cells carry an X chromosome and ½ carry a Y chromosome.
b. This ensures that just about ½ of the zygotes will be female and ½ will be male.
B. Human Traits
1. To study human inheritance, biologists use pedigree charts
2. Pedigree chart- shows relationships within a family
3. From this, biologists may determine the genotypes of family members.
4. It is difficult to link an observed human trait with a specific gene.
a. Many human traits are polygenic, or controlled by many genes
b. The environment also affects some traits
C. Human Genes
1. The genes controlling blood type were among the first human genes to be identified
2. A number of genes are responsible for blood groups, but the to best known are the ABO blood groups
and the Rh blood group
a. Red blood cells carry two antigens A and B
b. Antigens are molecules the immune system can recognize
i. A person who has only antigen B has type B blood
ii. A person who has only antigen A has type A blood
iii. A person who has both antigens has type AB blood
iv. A person who does not have either antigen has type O blood
v. A single gene with three alleles determine the ABO blood types
c. Red blood cells can also have the Rh antigen
i. People with the Rh antigen are Rh positive
ii. Those without it are Rh negative
D. From Gene to Molecule
1. There are many human genetic disorders
2. Include PKU and Tay-Sachs disease are caused by recessive alleles.
a. Individuals must inherit a recessive allele from each parent
b. Other disorders, such as Huntington disease, are caused by a dominant allele
c. Other disorders, like sickle cell disease, are caused by a codominant allele
3. Scientists are starting to learn which changes in the DNA sequence cause certain genetic disorders
4.
Sometimes, a small change in the DNA of a single gene affects the structure of a protein and causes a
serious genetic disorder
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5. This is the case with cystic fibrosis and sickle cell disease
14-2 Human Chromosomes
A. Human Genes and Chromosomes
1. Chromosomes 21 and 22 are the smallest human autosomes
a. These two were the first to be sequences
b. Chromosome 21 contains roughly 32 million base pairs
c. Amyotrophic lateral sclerosis (ALS) Lou Gehrig’s disease-is located on chromosome 21
2. Genes located close together on the same chromosome are linked, meaning that they tend to be inherited
together
a. Linked genes may be separated by crossing over during meiosis
B. Sex Linked Genes
1. Sex linked- genes on the X and Y chromosome
2. They are inherited in a different pattern that are genes on autosomes
3.
Males have just one X chromosome. Thus all X-linked alleles are expressed in males, even if they
are recessive.
4. Some examples of sex-linked disorders are color-blindness and hemophilia
a. Red-green colored blindness is found in about 1 in 10 males in America but only 1 in 100
females
b. Hemophelia( protein needed for normal blood clotting): about 1 in 10,000 males
C. Chromosomal Disorders
1. Most of the time, the mechanisms that separate chromosomes in meiosis work well, but sometimes
errors happen.
2. The most common error during meiosis is nondisjunction
3. nondisjunction- failure of chromosomes to separate properly during meiosis
4. If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a
disorder of chromosome numbers may result.
a. Down syndrome is an example of auosomal nondisjunction. In this disorder, there is an extra
copy of chromosome 21
b. Nondisjunction can also occur in sex chromosomes.
a. Turner’s syndrome- happens when a female only inherits one X chromosome
i. Women with Turner’s syndrome are sterile
b. Klinefelter’s syndrome- in males, have an extra X chromosome (XXY)
i. The extra X chromosome interferes with meiosis and usually prevents these
individuals from reproducing
14-3 Human Molecular Genetics
A. Human DNA Analysis
1. Biologists use molecular biology techniques to read, analyze, and change the DNA code of human
genes.
2. DNA analysis techniques can be used in different ways
a. DNA analysis can be used to test parents for recessive alleles that code for genetic disorders
b. DNA fingerprinting is a DNA analysis technique that can be used to identify individuals
B. Human Genome Project
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1.
The Human Genome Project is an ongoing effort to analyze the human DNA sequence
2. In 1990, the project began and in 2000 the sequencing of the human genome was complete.
3. Now the project goal is to analyze these sequences and look for genes
4. To do this they look at the promoter sequences
5. These are sequences that bind to RNA polymerase
C. Gene Therapy
1. Information about the human genome can be used to cure genetic disorders by gene therapy
2. In one method of gene therapy, a virus is used to deliver the normal gene into cells to correct the genetic
defects.
3. The virus is changed so that it cannot cause disease
4. The normal gene is attached to the DNA of the virus
5. The inserted gene can make proteins that correct the genetic defect
D. Ethical Issues in Human Genetics
1. There are risks and problems with gene therapy
2. Having the power to manipulate human DNA doesn’t make it right
3. People in a society are responsible for making sure that the tools made available by science are used
wisely
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