1. Ch 14- Human Genome How many chromosomes does a human cell have?
Karyotype- photograph of chromosomes grouped in order in pairs
Sex chromosomes- 2 chromosomes that determine the sex of the individual
Females have two copies of a large X chromosome
Males have one X and one small Y chromosome
Autosomes- the other 44 chromosomes
Males and females are born 50:50 ratio because sex chromosomes segregate during meiosis
Half of sperm cells carry X chromosome and other half carry Y chromosome

3. Human Traits
Biologists must identify if an inherited trait is controlled by a single gene
Pedigree- chart that shows the relationships within a family that can be used to help identify inherited traits
Genetic counselors analyze pedigrees
So can you look at a pedigree and figure out which family the good looks came from?
Shape of eyes and ears are actually polygenic traits
Many traits influenced by environmental factors such as nutrition and exercise

5. Human Genes
Human genome- complete set of genetic information- includes tens of thousands of genes
DNA sequences on these genes carry information for specifying many characteristics- eye color, protein structure in cells
Scientists spent years studying the genetics of our species- able to identify genes that directly control a single human trait
Some of first genes identified were those that control blood type

6. Blood Types
Number of genes responsible for human blood groups- ABO and Rh blood groups
Rh blood group determined by a single gene with alleles- positive and negative
Positive allele is dominant
ABO blood group has three alleles for this gene, I^A, I^B, and I
A and B are codominant, blood type AB
I allele is recessive, I^AI^A and I^Ai is blood type A
I^BI^B and I^Bi is blood type B
People who are homozygous for I allele (ii) are blood type O
Why is knowing the blood type important?

7. Autosomal Disorders in Humans
Human genes have become known through study of genetic disorders
Phenylketonuria (PKU)- recessive allele, people lack the enzyme that is needed to break down phenylalanine, may build up in tissues during child’s first year and cause mental retardation
Tay- Sachs- recessive allele, lipid accumulation in brain cells, blindness, death early in childhood
Achondroplasia- dominant allele disorder, dwarfism
Huntington’s disease- dominant allele, mental deterioration and uncontrollable movements, symptoms appear in middle age

8. How do the actual DNA sequences in genes affect phenotype so profoundly?
Many genetic disorders are still being worked on, but in both cystic fibrosis and sickle cell disease there is a small change in the DNA of a single gene that affects the structure of a protein, causing the disorder
Cystic fibrosis- recessive allele disorder, excess mucus in lungs, digestive tract, liver. More likely to get infections
Sickle cell disease- characterized by bent and twisted shape of red blood cells, tend to get stuck in capillaries and cause damage to cells, tissues, organs
Hemoglobin in person has different amino acid sequence- just one DNA base is changed
Common among African Americans

9. Sec 2- Human Chromosomes
Chromosomes 21 and 22- smallest human autosomes
First chromosomes whose sequences were determined
Long stretches of repetitive DNA are unstable sites where rearrangements occur
Form of leukemia and neurofibromatosis found on chromosome 22
Gene associated with Lou Gehrig’s disease found on chromosome 21
Genes located together on same chromosome tend to be inherited together
Is there a special pattern of inheritance for genes located on the X chromosome or the Y chromosome?

10. Sex Linked Genes
Sex linked genes- gene located on the X or Y chromosome
Many sex linked genes found on X chromosome
Y chromosome is much smaller than X, contains only a few genes
Colorblindness- an inability to distinguish certain colors, most common is red-green, more common in males
Sex linked disorders are more common in men
X-linked alleles are always expressed in males, because males have only one X chromosome

11. X-Chromosome Inactivation
Hemophillia- another sex linked disorder, a protein necessary for normal blood clotting is missing
Duchenne Muscular Dystrophy- sex linked disorder, results in progressive weakening and loss of skeletal muscle
X-Chromosome Inactivation
Females have 2 X chromosomes, one X chromosome is enough
How does the cell “adjust” to the extra X chromosome in female cells?
In female cells, one X chromosome is randomly switched off-this forms a Barr body
Female cats

12. Chromosomal Disorders
Nondisjunction- homologous chromosomes fail to separate during meisois
May involve autosomes, sex chromosomes, homologous chromosomes
If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result
Trisomy- when two copies of autosomal chromosomes fail to separate during meiosis, individual is born with three copies of chromosomes
Down snydrome- most common form of trisomy, involves three copies of chromosome 21
Produces mild to severe mental retardation, and higher frequency of some birth defects

13. Sex Chromosome Disorders
Turner’s syndrome- females only inherit one X chromosome
Are sterile- unable to reproduce
Sex organs do not develop at puberty
Klinefelter’s syndrome- males inherit an extra X chromosome
Prevent individuals from reproducing
No reported instances of babies born without an X chromosome
Contains genes vital for survival and development of embryo

14. Sec 3- Human Molecular Genetics
Roughly 6 billion base pairs in your DNA
Genetic tests are now available for hundreds of disorders
DNA fingerprinting- form of DNA analysis that analyzes sections of DNA that have little or no known function that vary widely from individual to individual
Used to identify individuals
Used since 1980’s in United States
How has DNA fingerprinting been beneficial?

15. Human Genome Project
Human Genome Project is the ongoing effort in analyze the human DNA sequence
Scientists have analyzed smaller genomes, such as those of viruses and bacteria
Why is this an important breakthrough?
Provides useful clues to some of basic properties of life
May be useful in developing new drugs and treatments for diseases

16. Gene Therapy
The process of changing the gene that causes a genetic disorder
In gene therapy, an absent or faulty gene is replaced by a normal, working gene
Both positive and negative results
What potential ethical problems might occur if humans gain the opportunity to design our bodies?
High School Biology Resources