The Human Genome Human Chromosomes Biologists analyze chromosomes

The Human Genome

Human Chromosomes Biologists analyze chromosomes by photographing cells in mitosis and then grouping the chromosomes in homologous pairs. (karyotype) Ø Two of our 46 chromosomes are known as sex chromosomes. Ø XX female Ø XY male Ø The other 44 chromosomes are called autosomal chromosomes or autosomes. Ø

Karyotype

Pedigrees Ø If the allele is on a sex chromosome… Ø Carrier females who have one allele for the trait do not express the trait (X*X) Ø Males express the trait if they have just one allele. (X*Y)

Pedigrees can be used to… Ø Determine whether a trait is inherited Ø Show a trait is passed from one generation to the next Ø Determine whether an allele is dominant or recessive

Polygenic Traits Ø Traits that are controlled by many genes (shape of eyes or ears) * Phenotype is only partly determined by genotype * Environment can influence genes: a person with genes to be tall may not grow to their full potential without proper nutrition.

Polygenic Trait – skin color

Disorders Caused by Recessive Alleles Cystic Fibrosis: excess mucus in lungs, digestive tract, liver; childhood death unless treated. Ø PKU: people lack the enzyme needed to break down phenylalanine, an amino acid found in milk and other foods. Ø Phenylalanine can build up in tissues and cause mental retardation. Newborns can be tested and put on a low phenylalanine diet. Ø

Disorders Caused by Dominant Alleles Ø Achondroplasia: a form of dwarfism Ø Huntington’s Disease: loss of muscle control and mental function until death. Symptoms do not show up until a person is in their thirties or forties.

Sickle Cell Disease Ø Sickle Cell Disease: Mutation in the hemoglobin gene. Ø Hemoglobin is a protein that carries oxygen in red blood cells. Ø Causes bent-shaped red blood cells that get stuck in capillaries, preventing blood from moving through. Ø Caused by a change (mutation) in one DNA base.

Ø People who are heterozygous for sickle cell are generally healthy. Ø They produce normal and abnormal hemoglobin (alleles are co-dominant).

Sex-Linked Genes Ø Genes found on the sex chromosomes are said to be sex-linked genes. Ø Many sex-linked genes are found on the X chromosome. (X*X)

Colorblindness Ø The colorblindness gene is found on the X chromosome Ø Males have just one X chromosome so all X-linked alleles are expressed in males, even if they are recessive. Ø Females must have two copies of a recessive allele to show the phenotype.

Hemophilia Ø Also an X-linked disorder (recessive) Ø A protein needed for blood clotting is missing people can bleed to death from cuts or suffer internal bleeding from bruises. Ø People can be treated with injections of clotting proteins.

Hemophilia – “The Royal Disease”

X- Chromosome Inactivation Ø In order to deal with having two X chromosomes, in any given cell one X chromosome in a female forms a Barr Body. Ø The formation if the Barr Body causes the genes on one of the X chromosomes in a female cell to be switched off (one X chromosome is inactive).

Ø The reason X inactivation happens is so females do not have twice as many X chromosome gene products as males.

Example Ø If you see a cat with three colors (calico: white, orange, and black) it is a female. Ø The black and orange alleles for the fur color gene are on the X chromosome. Ø In a female, One X chromosome might have the allele for orange and the other X chromosome could have the allele for black. Ø Males only have one X chromosome so can have only one color of spots.