Chapter 14.1 Human Heredity

Human Chromosomes Human chromosomes are fully condensed and easy to see during mitosis. Scientists will photograph chromosomes during this time to study them. Karyotypes will be made with the chromosomes. This is a picture of chromosomes arranged and grouped together. Two of the 46 human chromosomes are sex chromosomes because they determine an individuals sex. Female have 2 X chromosomes. Males have 1 X and 1 Y chromosome. The rest of the 44 chromosomes are called autosomes – they code for other genes.

All human egg cells carry a single X chromosome (they come from mom). Half of sperm cells carry an X and half carry a Y. The male parent is the one that determines if the child will be a girl or boy.

Pedigree Charts A pedigree chart shows relationships in a family that shows how a certain trait is passed through that family. Genetic counselor use pedigree charts to help predict the genotypes of family members.

Genes and the Environment Many phenotypes are only partly controlled by genotypes. Environment is important in determining how the trait is expressed. For example if maximum height is 6ft but the child has really bad nutrition they may not reach their maximum height. Only genes are inherited, environment is not.

Blood Group Genes There are 2 sets of genes that control blood type. The first is the ABO blood group and the second is the Rh blood group. The Rh blood group is determined by a single gene with 2 possible alleles – Rh+ and Rh-. Anyone who has Rh+/Rh+ or Rh+/Rh- will have a phenotype of Rh+. The ABO blood group has 3 alleles for this gene: IA, IB, and i. IA and IB are codominant. If someone has a genotype of IA and IB they will have AB blood type. The i allele is recessive to the other 2. So IA/IA or IA/i will be type A. IB/IB or IB/i will have type B blood. Only ii will produce type O. When talking about blood type both ABO blood group and Rh will be mentioned at the same time.

Recessive Alleles PKU – these individuals lack the enzyme needed to break down phenylalanine – an amino acid found in milk and other foods. If untreated a newborn will collect phenylalanine and become mentally retarded. This is an autosomal recessive allele. Other recessive disorders are Tay- Sachs disease – this results in nervous system breakdown and death.

Dominant Alleles Some genetic disorders are caused by dominant alleles. Dwarfism is caused by a dominant allele. Huntington disease is also caused by a dominant allele – this leased to loss of muscle control and mental function until death occurs. People with Huntington disease have no symptoms until they are in their 30s or older.

Codominant Alleles Sickle cell anemia is a codominant allele.

From Gene to Molecule In cystic fibrosis and sickle cell anemia, a small change in DNA of a single gene affect the structure of the protein causing a severe genetic disorder.

Cystic Fibrosis This disease is caused by a recessive allele on chromosome 7. Children with CF have severe digestive problems and produce a thick, heavy mucus that clogs lungs and breathing passageways. In cystic fibrosis there is a very small genetic change. Usually it is caused by a deletion of 3 bases in the middle of a gene. This removes 1 amino acid. But now a protein that allows chloride ions to pass across membranes do onot allow the transport of chloride. To be affected an individual must have 2 recessive alleles.

Sickle Cell Disease This is a common disorder found in African Americans. In this disease the red blood cell changes to a sickle shape. These cells are more rigid than normal cells and get stuck in capillaries so it causes problems. This can cause physical weakness, and damage to the brain, heart, and spleen. In sickle cell just one base changes and this changes one amino acid. It makes it so that when there is low oxygen the blood cells stick together and form long chains. The reason it is found mostly in those with African descent is because people who are heterozygous for sickle cell will be resistant to malaria.