Chapter 14 Human Heredity

Human Chromosomes

A picture of chromosomes arranged in this way (previous page) is know as a karyotype. This karyotype is a typical human body cell. Its number of chromosomes, 46, helps identify it. Pair # 23 are the sex chromosomes. This example has 2 X chromosomes, which makes this person a female.

The other 22 sets, or 44 chromosomes, are known as autosomal chromosomes, or autosomes for short. A quick way to write the chromosomes in a human cell is to put 46XX for a female and 46XY for a male.

Pedigree Charts Pedigree charts show the relationships within a family and can be used to show how traits are passed from one generation to the next.

Human Genes Blood Group Genes: The Rh blood group is determined by a single gene with two alleles: positive and negative. Persons with Rh + /Rh + or Rh + /Rh - are positive. Persons with Rh - /Rh - are negative.

The ABO blood group is more complicated. There are three alleles for this gene. They are I A, I B, and i. Alleles I A and I B are codominant. These alleles produce molecules called antigens on the surface of red blood cells. People with I A and I B alleles produce antigens for both, making them blood type AB.

The i allele is recessive. Individuals with I A I A or I A i produce only the A antigen, making them blood type A. Those with I B I B or I B i are type B. Those who are homozygous for the i allele (ii) produce no antigens and are said to have blood type O.

Phenotype (Blood Type Genotype Antigen on Red Blood Cell Safe Transfusions To From Section 14-1 Figure 14-4 Blood Groups

Recessive Alleles Persons that inherit recessive alleles from their parents sometimes end up with terrible conditions. Albinism: Lack of pigment in skin, hair, eyes. Cystic Fibrosis: Excess mucus in lungs. Galactosemia: Accumulates sugar in tissues which causes retardation, eye/liver damage.

PKU: Accumulates phenylalanine in tissues which causes retardation. Tay-Sachs disease: Lipid accumulation in brain; causes mental deficiency, blindness, and death in early childhood.

caused by Section 14-1 include Concept Map Autosomol Disorders Recessive alleles Dominant alleles Codominant alleles Albinism Galactosemia Tay-Sachs disease Huntington’s disease Sickle cell disease Cystic fibrosis Phenylketonuria Achondroplas ia Hypercholes- terolemia

Dominant Alleles You only need one allele present to display these diseases. Dwarfism: small body and features. Huntington’s disease: Mental deterioration and uncontrolled movements. Hypercholesterolemia: Excess cholesterol Codominant Alleles: Sickle Cell disease, which causes red blood cells to lose shape.

caused by Section 14-1 include Concept Map Autosomol Disorders Recessive alleles Dominant alleles Codominant alleles Albinism Galactosemia Tay-Sachs disease Huntington’s disease Sickle cell disease Cystic fibrosis PhenylketonuriaAchondroplasia Hypercholes- terolemia

Sex-Linked Genes Genes located on the X and Y chromosome are said to be sex-linked. More than 100 genetic disorders have been found on the X chromosome. The Y chromosome is so much smaller and only has a few genes on it.

Colorblindness: There are 3 genes associated with colorblindness on the X chromosome. Since males only have one X chromosome, all X-linked disorders are expressed in males. Red-green colorblindness is found in 1 out of 10 males in the U.S. while only 1 out of 100 females has colorblindness.

Father (normal vision) Colorbli nd Norm al visio n Mother (carrier ) Daughter (normal vision) Son (normal vision) Daught er (carrier) Son (colorbli nd) Section 14-2 Male Fema le Figure Colorblindness

Hemophilia: A disease that keeps blood from clotting properly. Blood clotting is controlled by two genes on the X chromosome. 1 in 10,000 males is born with a form of hemophilia.

Duchenne Muscular Dystrophy: a sex-linked disorder that results in a progressive weakening and loss of skeletal muscle. In the U.S., 1 out 3000 males is born with this condition. It is caused by a defective version of the gene that codes for muscle protein.

Chromosomal Disorders Nondisjunction: When chromosomes fail to separate during meiosis. This causes abnormal numbers of chromosomes to find their way into the gametes, which results in a disorder of chromosome number.

Trisomy: Ending up with 3 copies of a chromosome instead of 2. If a person has 3 copies of chromosome 21, this person has the condition known as Down Syndrome. This occurs in 1 out of 800 babies born. The main symptoms are mild to moderate mental retardation.

Turner’s Syndrome: A female only inherits one X chromosome (45X). These persons are sterile and they cannot reproduce. Klinefelter’s Syndrome: Males get an extra X chromosome (47XXY).