Human ChromosomesHuman ChromosomesHuman geneticsHuman genetics

Human Genes and ChromosomesHuman Genes and Chromosomes

Only about 2% of the DNA in your Only about 2% of the DNA in your chromosomes functions as genes chromosomes functions as genes (transcribed into RNA).(transcribed into RNA).

Chromosomes 21 and 22 are the smallest Chromosomes 21 and 22 are the smallest human autosomes and the first human autosomes and the first chromosomes who’s gene sequences chromosomes who’s gene sequences were discoveredwere discovered

Chromosome 22Chromosome 22

Contains 545 different Contains 545 different genesgenes

Contains alleles that Contains alleles that cause a form of cause a form of leukemia and leukemia and neurofibromatosis neurofibromatosis (tumor causing (tumor causing disease)disease)

Long stretches of Long stretches of DNA that do not code DNA that do not code for proteins (unstable)for proteins (unstable)

Chromosome 21Chromosome 21

Contains about 225 Contains about 225 genesgenes

Contains gene for Contains gene for ALS or Lou Gehrig’s ALS or Lou Gehrig’s diseasedisease

Has many regions Has many regions that have no genes at that have no genes at allall

Sex-Linked GenesSex-Linked Genes

There is a special pattern of inheritance of There is a special pattern of inheritance of genes located on the X and Y genes located on the X and Y chromosomeschromosomes

Genes located on the X and Y Genes located on the X and Y chromosomes are called chromosomes are called sex-linked sex-linked genesgenes..

Most are found on the X chromosome Most are found on the X chromosome (more than 100 genetic disorders)(more than 100 genetic disorders)

Y is smaller and has few genesY is smaller and has few genes

X and Y ChromosomeX and Y Chromosome

ColorblindnessColorblindness

3 human genes are associated with vision on 3 human genes are associated with vision on the X chromosome. A defect in any one of those the X chromosome. A defect in any one of those can cause can cause colorblindness. colorblindness. (can’t determine (can’t determine certain colors)certain colors)

Red-green colorblindness is most common and Red-green colorblindness is most common and is found in 1 in 10 males in US.is found in 1 in 10 males in US.

Males have just 1 X chromosome. Therefore, Males have just 1 X chromosome. Therefore, all sex-linked alleles are expressed in males, all sex-linked alleles are expressed in males, even if they are recessive.even if they are recessive.

ColorblindnessColorblindness

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

HemophiliaHemophilia

2 genes on the X chromosome that help control 2 genes on the X chromosome that help control blood clottingblood clotting

A recessive allele in either of those 2 genes can A recessive allele in either of those 2 genes can produce the disorder.produce the disorder.

HemophiliaHemophilia a protein necessary for normal a protein necessary for normal blood clotting is missing (1 in 10,000 males)blood clotting is missing (1 in 10,000 males)

Can bleed to death from minor cuts and may Can bleed to death from minor cuts and may suffer internal bleeding from bumps and bruisessuffer internal bleeding from bumps and bruises

Can be treated by receiving injections of normal Can be treated by receiving injections of normal clotting proteinsclotting proteins

Duchenne Muscular DystrophyDuchenne Muscular Dystrophy

Duchenne Muscular DystrophyDuchenne Muscular Dystrophy is a sex- is a sex-linked disorder that results in the linked disorder that results in the progressive weakening and loss of progressive weakening and loss of skeletal muscle. (1 out of 3000 males)skeletal muscle. (1 out of 3000 males)

Caused by a defective version of the gene Caused by a defective version of the gene that codes for a muscle proteinthat codes for a muscle protein

X-chromosome InactivationX-chromosome Inactivation

Because females have 2 X chromosomes, Because females have 2 X chromosomes, how does the cell adjust to that?how does the cell adjust to that?

One X is randomly “switched off”. That One X is randomly “switched off”. That turned off chromosome forms a dense turned off chromosome forms a dense region in the nucleus known as the Barr region in the nucleus known as the Barr bodybody

ExampleExample

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

X Chromosome Inactivation in CatsX Chromosome Inactivation in Cats

Chromosomal DisordersChromosomal Disorders

Most of the time, meiosis works very well, Most of the time, meiosis works very well, but every once in a while, something goes but every once in a while, something goes wrong. The most common error that wrong. The most common error that occurs in meiosis is occurs in meiosis is non-disjunctionnon-disjunction. This . This is when the chromosomes fail to separate.is when the chromosomes fail to separate.

If non disjunction occurs, abnormal If non disjunction occurs, abnormal numbers of chromosomes may find their numbers of chromosomes may find their way into gametes and a disorder of way into gametes and a disorder of chromosome numbers may result.chromosome numbers may result.

Non-disjunctionNon-disjunction

Down SyndromeDown Syndrome

Trisomy 21, which Trisomy 21, which means you have 3 means you have 3 copies of copies of chromosome 21. chromosome 21.

1 in 800 babies in US1 in 800 babies in US Produces mild to Produces mild to

severe mental severe mental retardationretardation

Less effective Less effective immune systemimmune system

Sex Chromosome DisordersSex Chromosome Disorders

Nondisjunction of the x chromosome in females Nondisjunction of the x chromosome in females can lead to can lead to Turner’s syndrome.Turner’s syndrome. Females have Females have only 1 X chromosome. Women with this only 1 X chromosome. Women with this disorder are sterile because their sex organs disorder are sterile because their sex organs don’t develop at puberty. (Karyotype 45,X)don’t develop at puberty. (Karyotype 45,X)

Nondisjunction of the X chromosome in males Nondisjunction of the X chromosome in males can lead to can lead to Klinefelter’s Syndrome.Klinefelter’s Syndrome. Males have Males have 2 X chromosomes and a Y chromosome. Males 2 X chromosomes and a Y chromosome. Males become sterile. (Karyotype 47, XXY)become sterile. (Karyotype 47, XXY)

If regions of the Y chromosome is absent, then If regions of the Y chromosome is absent, then the embryo develops as a female. the embryo develops as a female.

Karyotypes (Down)Karyotypes (Down)

Karyotype (Patau)Karyotype (Patau)

Karyotype (Klinefelter’s)Karyotype (Klinefelter’s)

KayrotypeKayrotype