Human Genes and Chromosomes Chromosomes 21 and 22 are the smallest human autosomes Chromosome 22 has about 43 million DNA base pairs Chromosome 21 has about 32 million base pairs Chromosome 22 has about 545 different genes Genetic disorders tat occur on chromosome 22 are leukemia and neurofi ID: 784643
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Slide1
Chapter 14.2
Human Chromosomes
Slide2Human Genes and Chromosomes
Chromosomes 21 and 22 are the smallest human autosomes. Chromosome 22 has about 43 million DNA base pairs. Chromosome 21 has about 32 million base pairs.
Chromosome 22 has about 545 different genes. Genetic disorders tat occur on chromosome 22 are leukemia and neurofibromatosis (a tumor causing disease). There is also a long stretch of repeating DNA that does not code for proteins.
Chromosome 21 contains about 225 genes. One gene is associated with ALS (Lou Gehrig’s disease). It also has segments that do not code for proteins.
Slide3Sex-Linked Genes
Genes that are located on the X or Y chromosome are called sex-linked genes.
There are many sex linked genes on the X chromosome – like muscular dystrophy, melanoma, hemophilia, and colorblindness.
The Y chromosome is much smaller and has fewer genes.
Slide4Colorblindness
Colorblindness is linked to genes on the X chromosome. Males have colorblindness more often than females. The reason for this is because males only have 1 X chromosome. If any recessive alleles are found in a male on the X chromosome they will be expressed.
In females in order for a recessive allele to be expressed it must be found on both X chromosomes.
Slide5Hemophilia
This is another X link disorder.
This controls blood clotting. Hemophilia does not allow blood to clot. There are more males found with hemophilia than females because the same rules apply as with color blindness.
Slide6Duchenne Muscular Dystrophy
This is a sex-linked disorder that makes weakening and loss of skeletal muscle.
This again follows inheritance similar to color blindness and hemophilia.
Slide7X-Chromosome Inactivation
In female cells one X chromosome is randomly switched off. This turned off X chromosome forms a Barr body – a dense region in the nucleus.
It happens in mammals. In cats, a gene that controls fur color is located on the X chromosome. 1 X chromosome may have an allele for black spots and the other an allele for orange spots. Depending on which X is inactivated it will give large patches of black and orange. Calico cats can only truly be female.
Slide8Chromosomal Disorders
The most common error in separating chromosomes during meiosis is nondisjunction. This is where the pairs do not separate correctly. This will give gamete cells the wrong number of chromosomes.
Slide9Down Syndrome
If 2 copies of autosomal chromosomes do not separate during meiosis an individual with 3 copies of a chromosome may be born. This is called trisomy.
The most common form of trisomy in humans is 3 copies of chromosome 21. This results in Down Syndrome. This produces an individual with mild to severe mental retardation.
Slide10Sex Chromosome Disorders
There are 2 disorders that take place with the wrong number of sex chromosomes.
Turner’s Syndrome – This female only inherits 1 X chromosome. These women will be sterile, not able to reproduce.
Klinefelter’s Syndrome – this is when a male gets an extra X chromosome. These individuals also are sterile.