Section 7-1 “Human genetics”

KEY CONCEPT The chromosomes on which genes are located can affect the expression of traits

In chapter 6 you learned how some genetic traits depend on dominant and recessive alleles. But…many factors affect phenotype, including the specific chromosome upon which a gene is located. Gene expression is often related to whether a gene is located on an autosome or on a sex chromosome.

Much of what has been learned about human genes comes from studies of genetic disorders. Many genetic disorders are caused by recessive alleles on autosomes. Two copies of the recessive allele must be present for a person to have the disorder. These disorders often appear in offspring of parents who are both heterozygotes.

Disorders Caused by Recessive Alleles People who have one dominant allele and one recessive, disorder-causing allele, do not have the disorder, but can pass it on because they are carriers of the disorder.

Reminder: What does the term dominant and recessive mean?

EXAMPLE Cystic Fibrosis is a severe RECESSIVE disorder that mainly affects the sweat glands and the mucus glands. A person who is HOMOZYGOUS (cc) for the recessive allele will have the disease. Someone who is HETEROZYGOUS (Cc) will not have the disease, but is a carrier of the disease

A heterozygous for a recessive disorder is a carrier Mendel’s rules of inheritance apply to autosomal genetic disorders. A heterozygous for a recessive disorder is a carrier (dominant)

Cystic Fibrosis Cystic fibrosis (CF) is caused by a defective gene which tells the body to produce abnormally thick and sticky fluid, called mucus. This mucus builds up in the breathing passages of the lungs and the pancreas, the organ that helps to break down and absorb food.

Cystic Fibrosis This collection of sticky mucus results in life- threatening lung infections and serious digestion problems. The disease may also affect the sweat glands and a man's reproductive system.

Cystic Fibrosis Millions of Americans carry the defective CF gene, but do not have any symptoms. That's because a person with CF must inherit two defective CF genes - - one from each parent. An estimated 1 in 29 Caucasian Americans have the CF gene. The disease is the most common, deadly, inherited disorder affecting Caucasians in the United States. It's more common among those of Northern or Central European descent.

Disorders caused by Dominant Alleles far less common Example: Huntington’s Disease American doctor George Huntington first described the disorder in 1872. Huntington's disease is caused by a genetic defect on chromosome #4. The defect causes a part of DNA, called a CAG repeat, to occur many more times than it is supposed to. Normally, this section of DNA is repeated 10 to 35 times. But in persons with Huntington's disease, it is repeated 36 to 120 times.

Males and females can differ in sex-linked traits Genes on sex chromosomes are called sex-linked genes Y chromosome genes in mammals are responsible for male characteristics X chromosome genes in mammals affect many traits

Our chromosomes: Recall … In humans the diploid number of chromosomes is 46 (23 pairs) 22 pairs of homologous (matching) chromosomes called autosomes The 23rd pair are sex chromosomes

Have 2 X chromosomes that are alike Males are XY Sex Chromosomes: Females are XX Have 2 X chromosomes that are alike Males are XY Have 1 X and 1 Y chromosome The Y chromosome is smaller (codes for male reproductive system)

Males have an XY genotype All of a male’s sex-linked genes are expressed BECAUSE Males have no second copies of sex-linked genes

Expression of sex-linked genes in females is similar to the expression of autosomal genes: two copies of each gene can affect phenotype. However, one X chromosome in each cell is randomly turned off by a process called X chromosome inactivation

Females have an XX genotype Expression of sex-linked genes is similar to autosomal genes in females X chromosome inactivation randomly “turns off” one X chromosome

There are many X-linked traits like: Muscular Dystrophy Color-blindness These are more prevalent in males because they do not have another normal X to cover up the bad one

Color blindness Color blindness occurs when there is a problem with the color-sensing materials (pigments) in certain nerve cells of the eye. These cells are called cones. They are found in the retina, the light-sensitive layer of tissue at the back of the inner eye. If you are missing just one pigment, you might have trouble telling the difference between red and green. This is the most common type of color blindness. Other times, people have trouble seeing blue-yellow colors. People with blue-yellow color blindness almost always have problems identify reds and greens, too.

The most severe form of color blindness is achromatopsia The most severe form of color blindness is achromatopsia. A person with this rare condition cannot see any color. Most color blindness is due to a genetic problem : X- linked recessive About 1 in 10 men have some form of color blindness. Very few women are color blind.

The most common inherited form of color blindness (red-green) is caused by a common X-linked recessive gene. If you have a common form of color blindness caused by an X-linked recessive gene, your mother must be a carrier of the gene or be color deficient herself.

The most common inherited form of color blindness (red-green) is caused by a common X-linked recessive gene. If you have a common form of color blindness caused by an X-linked recessive gene, your mother must be a carrier of the gene or be color deficient herself.

Fathers with this inherited form of red-green color blindness pass the X-linked gene to their daughters but not to their sons, because a son cannot receive X-linked genetic material from his father.

A daughter who inherits the color-deficient gene from her father will be only a carrier unless her mother also has the color-deficient gene. If a daughter inherits the X-linked trait from both her father and her mother, then she will be color blind as well as a carrier.

Summarize The chromosome on which the genes are located can affect the expression of traits Two alleles of autosomal genes interact to produce phenotypes Genes on the sex chromosomes are expressed differently in males and females In humans males are XY and females XX Males have only one copy of each gene found on the X chromosome so those genes are expressed in their phenotypes.