Human Chromosomes

Human Chromosomes The 46 human chromosomes contain 6 billion base pairs (bp) of DNA. Genes make up only 2 percent of the DNA. Average human gene is about 3,000 bp. The largest gene is more than 2 million bp.

Genetic Disorders Recessive alleles – phenotype expressed in homozygous recessive individuals only. Dominate alleles – phenotype expressed if the dominate allele is present (homozygous and heterozygous dominant). Codominant alleles – both alleles are dominant.

Human Genes and Chromosomes Chromosome 22 – approx. 43 million bp; 545 different genes. Genetic disorders include a form of leukemia and neurofibromatosis. Also contains long stretches of repetitive DNA that are unstable sites where rearrangements can occur (breakage).

Chromosome 21 – approx. 32 million bp; about 225 genes. Genetic disorders include amyotrophic lateral sclerosis (ALS). Also contains many regions with no genes at all. Many genes that are located close together on the same chromosome are linked, meaning that they tend to be inherited together. May be separated by crossing-over during meiosis.

Chromosomal Disorders Errors in chromosomal separation can occur during meiosis. Nondisjunction – homologous chromosomes fail to separate. If this occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result.

Nondisjunction Section 14-2 Homologous chromosomes fail to separate Meiosis I: Nondisjunction Meiosis II

Down Syndrome Trisomy 21 – an individual is born with three copies of autosomal chromosome 21. In U.S., occurs in about 1 in 800 babies born. Produces mild to severe mental retardation and an increased susceptibility to many diseases and a higher frequency of birth defects.

Sex Chromosome Disorders Turner’s syndrome (karyotype 45,X) – female that inherits only one X chromosome. The women are sterile and their sex organs do not develop at puberty. Klinefelter’s syndrome (karyotype 47,XXY) – Males with an extra chromosome X. (Y chromosome has sex-determining region.) Cannot reproduce. XXXY and XXXXY have been reported.

Super Male or XYY syndrome • Affected individuals are usually very tall and thin. • Many experience severe acne during adolescence. • Additional symptoms may include antisocial or behavioral problems and learning disabilities. Intelligence is usually normal, although IQ, on average, is 10 to 15 points lower than siblings.

The Human Genome Karyotyping To analyze chromosomes, cells are photographed during mitosis when chromosomes are fully condensed and easy to see. Chromosomes are cut out of photographs and lined up with homologous chromosome and in order of largest to smallest.

Sex chromosome = one of two chromosomes that determine an individual’s sex; females have two X chromosomes; males have one X chromosome and one Y chromosome. Autosomes = chromosome that is not a sex chromosome. 46,XX = female 46,XY = male

Polygenic traits Trait controlled by more than one gene. Examples: height, skin color, and eye color.

None of the genes for a polygenic trait are dominant. Each gene has an active allele and an inactive allele. Active alleles have an additive effect on the phenotype. Inactive alleles do not affect the phenotype. Because of the additive effects, a continuous range of phenotypes is possible.

Sex-Linked Genes Genes located on the X or Y chromosomes. Males have one X chromosomes. All X-linked alleles are expressed in males, even if they are recessive. Because males pass their X chromosomes along to their daughters, X-linked genes move from fathers to daughters and then to sons of the daughters.

Colorblindness Three human genes associated with color vision are located on the X chromosome. In males, a defective version of any one of these genes produces colorblindness. The most common form, red-green colorblindness, is found in about 1 in 10 males and 1 in 100 females.

Figure 14-13 Colorblindness Section 14-2 Father (normal vision) Normal vision Colorblind Male Female Daughter (normal vision) Son (normal vision) Mother (carrier) Daughter (carrier) Son (colorblind)

Hemophilia X-linked disorder. Two genes that help control blood clotting are found on the X chromosome. Recessive allele of either gene causes hemophilia. A protein necessary for normal blood clotting is missing. Occurs in about 1 in 10,000 males. Treated by injections of clotting proteins.

Duchenne Muscular Dystrophy Sex-linked disorder that results in the progressive weakening and loss of skeletal muscles. In the U.S., 1 in 3,000 males affected. Caused by a defective version of the gene that codes for a muscle protein.

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

Blood Types: A, B, AB, O Rh factor: +, -

Blood Types Multiple alleles & Codominance Section 14-1 Safe Transfusions Antigen on Red Blood Cell Phenotype (Blood Type Genotype From To

Transfusion Reaction • Treatment: • transfusion is stopped • corticosteroids • antihistamines • acetominophen (Tylenol) • Symptoms: • fever • chills • rash • flank pain or back pain • bloody urine • fainting or dizziness

Figure 14-3 A Pedigree Section 14-1 A circle represents a female. A square represents a male. A vertical line and a bracket connect the parents to their children. A horizontal line connecting a male and female represents a marriage. A half-shaded circle or square indicates that a person is a carrier of the trait. A circle or square that is not shaded indicates that a person neither expresses the trait nor is a carrier of the trait. A completely shaded circle or square indicates that a person expresses the trait. A pedigree chart shows the relationships within a family.

From Gene to Molecule Cystic fibrosis – caused by the deletion of three bases in the DNA of the gene resulting in an amino acid (phenylalanine) missing from the protein. The protein produced by the gene is a chloride ion pump. The deletion of phenylalanine prevents the protein from being transported to the cell membrane.

Figure 14-8 The Cause of Cystic Fibrosis Section 14-1 Chromosome # 7 CFTR gene The most common allele that causes cystic fibrosis is missing 3 DNA bases. As a result, the amino acid phenylalanine is missing from the CFTR protein. Normal CFTR is a chloride ion channel in cell membranes. Abnormal CFTR cannot be transported to the cell membrane. The cells in the person’s airways are unable to transport chloride ions. As a result, the airways become clogged with a thick mucus.

Sickle cell disease – characterized by sickle-shaped red blood cells that are more rigid than normal cells and tend to get stuck in capillaries leading to damage to cells, tissues, and organs. The normal allele differs from the sickle cell allele by one DNA base leading to a substitution of valine for glutamic acid.

Sickle cell disease and malaria: In parts of the world where malaria is a major health threat, the presence of the sickle cell allele in heterozygous individuals confers resistance to malaria.

X-Chromosome Inactivation In female cells, one X chromosome is randomly switched off. The turned-off chromosome forms a dense region in the nucleus called a Barr body. Also occurs in cats and other mammals (ex, white cat with orange and black spots).

Go Online Internet Links from the authors on DNA samples Career links on geneticists More information on the Human Genome Project Interactive test For links on pedigrees, go to www.SciLinks.org and enter the Web Code as follows: cbn-4141.

Human Chromosomes