Human Genetics

Human GeneticsHeredity

The passing-down of traits from parent to child through genes, which are located in chromosomes. Such traits are said to be "inherited."

Human GeneticsPredicting Disorders

Record keeping helps scientists use pedigree analysis to study inheritance patterns, determine phenotypes, and ascertain genotypes.

Human GeneticsPedigree

A diagram that shows the pattern of inheritance of a gene in a family. These diagrams can also be called a family tree of inheritance.

Human GeneticsPedigree symbols

Karyotype

A picture of the chromosomes in a cell that is used to check for abnormalities.

Human GeneticsSex-Linked Traits

The sex chromosomes, X and Y, carry genes for many characters other than gender.

A sex-linked gene is located on either an X or a Y chromosome.

Traits that are not expressed equally in both sexes are commonly sex-linked traits.

Colorblindness is an example of a sex-linked trait that is expressed more in males than in females.

Genetic DisordersGenetic Disorder

A genetic disorder is a disease that is caused by an abnormality in an individual's DNA. Abnormalities can range from a small mutation in a single gene to the addition or subtraction of an entire chromosome or set of chromosomes.

These harmful effects are produced by inherited mutations (defective alleles).

Genetic DisordersTypes of Genetic Disorders

Autosomal Recessive Genetic Disorders

Autosomal Dominant Genetic Disorders

Sex-Linked Genetic Disorders

Nondisjunction Genetic Disorders

Genetic DisordersAutosomal

An autosome is a non-sex chromosome. It is a pair of chromosomes that is the same in both sexes of a species. For example, in humans, there are 22 pairs of autosomal chromosomes (chromosomes 1 through 22). The X and Y chromosomes are not autosomal (chromosome 23).

Genetic DisordersAutosomal Recessive Genetic Disorders

Cystic Fibrosis

Phenylketonuria

Galactosemia

Genetic DisordersCystic Fibrosis (1:3,900)

Cystic fibrosis is a genetic disorder that affects the respiratory and digestive systems.

People with cystic fibrosis inherit a defective gene on chromosome 7.

The protein produced by this gene normally helps salt (sodium chloride) move in and out of cells. If the protein doesn't work correctly, that movement is blocked and an abnormally thick sticky mucous is produced on the outside of the cell. The cells most seriously affected by this are the lung cells. This mucous clogs the airways in the lungs, and increases the risk of infection by bacteria.

Genetic DisordersPhenylketonuria (1:18,000)

Phenylketonuria is a rare metabolic disorder that affects the way the body breaks down protein.

PKU is caused by a mutation in a gene on chromosome 12.

The gene codes for a protein called PAH (phenylalanine hydroxylase), an enzyme in the liver. This enzyme breaks down the amino acid phenylalanine into other products the body needs. When this gene is mutated, the shape of the PAH enzyme changes and it is unable to properly break down phenylalanine. Phenylalanine builds up in the blood and poisons nerve cells (neurons) in the brain.

Genetic DisordersGalactosemia (1:55,000)

Galactosemia is a rare disorder that affects the body's ability to break down a food sugar called galactose.

Galactosemia is caused by a mutation in a gene on chromosome 9.

People with galactosemia are missing an enzyme called GATL (galactose-1-phosphate uridyl transferase), which normally converts galactose into glucose. Without this enzyme, harmful amounts of galactose build up in the blood.

Genetic DisordersAutosomal Dominant Genetic Disorders

Breast/Ovarian Cancer

Huntington’s Disease

Colon Cancer

Genetic DisordersBreast/Ovarian Cancer (5-10% of patients)

Most people who develop breast or ovarian cancer have no history of the disease in their family. In fact, only 5 to 10 percent of all breast and ovarian cancers are caused by inherited genetic factors. These rare cases typically result from inherited mutations on either chromosome 17 or chromosome 13.

Men who inherit the defective genes are also more likely to develop breast and/or prostrate cancer. (Yes, men can get breast cancer.)

Genetic DisordersHuntington’s Disease (1:30,000)

Huntington's Disease (HD) is a brain disorder that affects a person's ability to think, talk, and move.

HD is caused by a mutation in a gene on chromosome 4.

Normally, the coding region of this gene contains the DNA sequence "CAG" repeated again and again. The number of times this triplet is repeated varies from person to person, ranging from 10 to 26 times.

People with HD have an abnormally high number of these CAG triplets, approximately 40 or more. Somehow the brain cells of HD patients accumulate clumps of protein that become toxic, resulting in cell death. Some patients lose more than 25% of their brain cells before they die.

Genetic DisordersColon Cancer (~80% of patients)

People who have a history of colon cancer in their family are at greater risk of getting the disease themselves. The risk increases when a relative has had the disease before age 50. These families are considered high-risk, because they may have inherited one rare genetic condition: FAP (familial adenomatous polyposis).

FAP is caused by mutations in a gene on chromosome 5.

Genetic DisordersSex-Linked Genetic Disorders

Klinefelter syndrome

Turner syndrome

Fragile-X syndrome

Genetic DisordersKlinefelter Syndrome (1:1,000)

Klinefelter syndrome is a disorder that affects only males. Males normally have an X chromosome and a Y chromosome (XY). But males who have Klinefelter syndrome have an extra X chromosome (XXY), giving them a total of 47 instead of the normal 46 chromosomes.

People with this disorder develop as males with subtle characteristics that become apparent during puberty. They are often tall and usually don't develop secondary sex characteristics, such as facial hair or underarm and pubic hair.

Genetic DisordersTurner Syndrome (1:3,000)

Turner syndrome is caused by a missing or incomplete X chromosome.

People who have Turner syndrome develop as females. The genes affected are involved in growth and sexual development, which is why girls with the disorder are shorter than normal and have abnormal sexual characteristics.

Genetic DisordersFragile X Syndrome (1:3,600 males and 1:6,000

females)

Fragile X syndrome is the most common form of inherited mental retardation.

A change or mutation in a gene on the X chromosome causes the fragile X syndrome. In females, these are two X's; in males they are and X and Y. Gene responsible for fragile X syndrome is called the FMR1 (fragile X mental retardation 1) gene.

Genetic DisordersNondisjunction Genetic Disorders

Patau syndrome/Trisomy 13

Edwards syndrome/Trisomy 18

Downs syndrome/Trisomy 21

Genetic DisordersNondisjunction

An event that can occur during meiosis where the chromosomes don't separate properly. It results in the developing egg or sperm cell having too many or too few chromosomes.

Genetic DisordersPatau syndrome/Trisomy 13 (1:10,000)

Trisomy 13, also called Patau syndrome, is a genetic disorder associated with the presence of extra material from chromosome 13.

• Cleft lip or Palate

• Close-Set Eyes (eyes may actually fuse together into one)

• Decreased Muscle Tone

• Extra Fingers or Toes

• Hernias

• Hole, Split, or Cleft in the Iris

• Low-Set Ears

• Severe Mental Retardation

• Scalp defects (absent skin)

• Skeletal (limb) Abnormalities

• Small Eyes

• Small Head

• Small Lower Jaw

Genetic DisordersEdwards syndrome/Trisomy 18 (1:3,000)

Trisomy 18 is a relatively common syndrome. It is three times more common in girls than boys. The syndrome is caused by the presence of an extra material from chromosome 18.

• Clenched Hands

• Crossed Legs (preferred position)

• Heart Disease

• Kidney Problems

• Low Birth Weight

• Hole, Split, or Cleft in the Iris

• Low-Set Ears

• Severe Mental Retardation

• Hernias

• Underdeveloped fingernails

• Unusual Shaped Chest

• Small Head

• Small Lower Jaw

Genetic DisordersDowns syndrome/Trisomy 21 (1:800)

Down syndrome occurs when there is an extra copy of chromosome 21. This form of Down syndrome is called Trisomy 21. The extra chromosome causes problems with the way the body and brain develop. Down syndrome is the most common single cause of human birth defects.

• Decreased Muscle Tone

• Excessive Skin (Usually on the nape of the neck)

• Flattened Nose

• Separated Sutures

• Single Crease in Palm

• Small Ears

• Small mouth

• Upward slanting Eyes

• Wide, Short Hands

• Moderate Mental Retardation

• Heart Defects

• Hearing Problems

• Cataracts

Genetic DisordersNondisjunction

Nondisjunction can occur with any chromosome, but often result in miscarriage. For example, Trisomy 16 is the most common trisomy in humans, occurring in more than 1% of pregnancies. This condition, however, usually results in spontaneous miscarriage in the first trimester.