Please pick up the notes, yeti sheet, and x-linked traits sheet.

Chromosomes and Inheritance

Chromosome Theory of Inheritance A basic principle that states genes are located on chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns. Supported by: Mendel’s three laws. Post-Mendelian Developments Gene linkage. Crossing-over

Sex Determination In the early 1900’s Thomas Hunt Morgan discovered (while studying fruit flies (also known as Drosophila) that one pair of chromosomes was different in males from that of females. Gametes produced by females have only x chromosomes. Gametes produced by males have an x and a y chromosome. 50% Of the offspring will be female and 50% will be male.

Sex Linkage Due to the different shape in the x and y chromosomes. Morgan hypothesized that more genes could be carried on the x chromosome. The presence of a gene on a sex chromosome is sex linkage. X-linked genes Y-linked genes

Sex Linkage For example: Morgan noticed that while most fruit flies have red eyes some males have white eyes. He proposed that white eyes was an x-linked trait. Do a Punnett square between a white eyed male and a homozygous red-eyed female.(Where R = red eyes)

Linkage Groups There are many more genes than there are chromosomes. The genes on one chromosome form a linkage group. Therefore two genes on the same chromosome are said to be linked genes. Linked genes tend to be inherited together.

Linkage Groups Morgan proved this by making dihybrid crosses of Drosophila with genotypes of GgLl (where G for gray body is dominant to g for black body and L for long wings is dominant to l for short wings). If these genes assorted independently the phenotypic ratio should be 9:3:3:1, instead they produced a phenotypic ratio of 3:1, as if it were a heterozygous monohybrid cross.

Linkage Groups While linked genes tend to stay together, this doesn’t always occur. Genes may become separated during crossing over. The farther apart the genes are on a chromosome, the more likely it is that they will assort independently.

Chromosome Mapping Scientists can conduct experiments to determine the frequency that certain traits occur together. This information is used to create a chromosome map.

Genetic Traits and Disorders Multiple Allele Traits are controlled by 3 or more alleles. Example: Human blood types are controlled by 2 of 3 different alleles IA IB and i. Polygenic Traits – Traits that are controlled by multiple genes. Example: Eye color, skin color, height. Some can be modified by environmental factors as well.

Mutations Mutation – A change in the DNA of an organism. Mutations may occur in the entire chromosome or a single nucleotide. Germ cell mutations occur in the gametes. Somatic mutations occur in body cells. Which mutations can be passed to offspring? Germ Cell

Mutations Lethal Mutations – Mutations that cause death (many times before birth). Some mutations are beneficial phenotypically to the organism. These mutations give organisms an evolutionary advantage.

Chromosome Mutations Can be a change in the structure of a chromosome or the deletion of an entire chromosome. A deletion is the loss of a piece of a chromosome due to breakage. An inversion is when a segment breaks of and reattaches to the same chromosome inverted.

Chromosome Mutations Translocation – A piece of the chromosome breaks off and reattaches to another nonhomologous chromosome. Nondisjunction – The failure of a chromosome to separate from its homolog during meiosis.

Gene Mutations Involve large segments of DNA or a single nucleotide within a codon. Point mutation – The substitution, addition, or removal of a single nucleotide. Substitution – One nucleotide in a codon is replaced by another nucleotide, resulting in a new codon. If the substitution codes for the same amino acid then there will be no affect. If it codes for something different problems will arise.

Gene Mutations An example of a genetic disorder arising from a substitution is sickle cell anemia. An adenine is substituted for a thymine in a single codon. This results in a defective form of hemoglobin. Prominent in African-Americans (1 in 500) Approximately 1 in 10 are heterozygous for the sickle cell allele.

Gene Mutations A single nucleotide may also be deleted or added. This results in a frame shift mutation. The missing or additional nucleotide throws all of the succeeding codons off.