Human Chromosomes Human genetics

Human Genes and Chromosomes  Only about 2% of the DNA in your chromosomes functions as genes (transcribed into RNA).  Chromosomes 21 and 22 are the smallest human autosomes and the first chromosomes who’s gene sequences were discovered

Chromosome 22  Contains 545 different genes  Contains alleles that cause a form of leukemia and neurofibromatosis (tumor causing disease)  Long stretches of DNA that do not code for proteins (unstable)

Chromosome 21  Contains about 225 genes  Contains gene for ALS or Lou Gehrig’s disease  Has many regions that have no genes at all

Sex-Linked Genes  There is a special pattern of inheritance of genes located on the X and Y chromosomes  Genes located on the X and Y chromosomes are called sex-linked genes.  Most are found on the X chromosome (more than 100 genetic disorders)  Y is smaller and has few genes

X and Y Chromosome

Colorblindness  3 human genes are associated with vision on the X chromosome. A defect in any one of those can cause colorblindness. (can’t determine certain colors)  Red-green colorblindness is most common and is found in 1 in 10 males in US.  Males have just 1 X chromosome. Therefore, all sex-linked alleles are expressed in males, even if they are recessive.

Colorblindness  In order for a recessive allele (colorblindness) to be expressed in females, there must be 2 copies of the allele, (one on each X chromosome) present.

Hemophilia  2 genes on the X chromosome that help control blood clotting  A recessive allele in either of those 2 genes can produce the disorder.  Hemophilia  a protein necessary for normal blood clotting is missing (1 in 10,000 males)  Can bleed to death from minor cuts and may suffer internal bleeding from bumps and bruises  Can be treated by receiving injections of normal clotting proteins

Duchenne Muscular Dystrophy  Duchenne Muscular Dystrophy is a sex- linked disorder that results in the progressive weakening and loss of skeletal muscle. (1 out of 3000 males)  Caused by a defective version of the gene that codes for a muscle protein

X-chromosome Inactivation  Because females have 2 X chromosomes, how does the cell adjust to that?  One X is randomly “switched off”. That turned off chromosome forms a dense region in the nucleus known as the Barr body

Example  In cats, the gene that controls the color of coat spots in located on the X chromosome. One X may have an allele for orange spots and the other may have an allele for black spots. In cells in some parts of the body, one X chromosome is switched off. In other parts, the other X is switched off. The result is the cat’s fur has a mixture of black and orange spots.

X Chromosome Inactivation in Cats

Chromosomal Disorders  Most of the time, meiosis works very well, but every once in a while, something goes wrong. The most common error that occurs in meiosis is non-disjunction. This is when the chromosomes fail to separate.  If non disjunction occurs, abnormal numbers of chromosomes may find their way into gametes and a disorder of chromosome numbers may result.

Non-disjunction

Down Syndrome  Trisomy 21, which means you have 3 copies of chromosome 21.  1 in 800 babies in US  Produces mild to severe mental retardation  Less effective immune system

Sex Chromosome Disorders  Nondisjunction of the x chromosome in females can lead to Turner’s syndrome. Females have only 1 X chromosome. Women with this disorder are sterile because their sex organs don’t develop at puberty. (Karyotype 45,X)  Nondisjunction of the X chromosome in males can lead to Klinefelter’s Syndrome. Males have 2 X chromosomes and a Y chromosome. Males become sterile. (Karyotype 47, XXY)  If regions of the Y chromosome is absent, then the embryo develops as a female.

Karyotypes (Down)

Karyotype (Patau)

Karyotype (Klinefelter’s)

Kayrotype