The Chromosomal Basis of Inheritance Chapter 15

The importance of chromosomes In 1902, the chromosomal theory of inheritance began to take form, stating: genes have specific locations (loci) on chromosomes, and you randomly get one chromosome from each parent.

Fruit flies Fruit flies are Drosophila melanogaster They have 4 pairs of chromosomes They use the same XX, XY sex determination as people They breed quickly (every 2 weeks, hundreds of offspring) Wild type is the phenotype most common in natural populations

Linked genes Genes on the same chromosome tend to be inherited together “linked genes”

Genetic Recombination Genetic recombination is the production of offspring with combinations of traits different from those found in either parent. 1. Independent Assortment 2. Crossing Over

Linkage Mapping A genetic map is an ordered list of the locations of genes on a chromosome.

Linkage Mapping Based on a linkage map, one can assume: the farther apart 2 genes are, the more likely a crossover will occur between them, therefore the recombination frequency is higher.

Linkage Mapping A linkage map is a genetic map based on recombination frequencies. Units are called map units and show the distance between genes. 1 map unit = a 1% chance of recombination. If two genes are 50 map units apart, how likely is recombination?

Cytogenic Map A cytogenic map locates genes on chromosomes with respect to visible features like stained bands.

Chromosomal Abnormalities: X inactivation in females Sometimes, the second X is inactivated in females, and is called a Barr body. She is still normal and fertile, but sometimes has patchy skin.

Abnormal Chromosome Number Nondisjunction is when chromosomes do not separate correctly during meiosis. This causes an abnormal chromosome number, called aneuploidy Trisomy is when you have 3 chromosomes instead of 2 (2n + 1) Monosomy is when you have 1 chromosome instead of 2 (2n – 1) Polyploidy is having more than one complete set of chromosomes

Down Syndrome Trisomy 21 is Down Syndrome, where the child has 3 copies of chromosome 21 (instead of 2) Different facial features Shorter stature Mental disabilities Prone to leukemia and Alzheimer’s Sexually underdeveloped or sterile Reduced life expectancy (improving)

Aneuploidy of Sex chromosomes XXY is Klinefelter Syndrome: male sex organs, small testes, sterile XYY: taller than average XXX: trisomy X, healthy and normal XO: Turner Syndrome, Monosomy X, sterile, appear female, no secondary sex characteristics develop without hormone therapy

Alterations of chromosome structure Deletion: chromosomal fragment is deleted Duplication: a chromosomal fragment is doubled Inversion: chromosomal fragment gets reversed Translocation: chromosomal fragments get switched around

Other Diseases Cri du chat: deletion from chromosome 5, mentally disabled, meows, die early Chronic myelogenous leukemia: 22 and 9 trade pieces, cancer of white blood cells results

2 exceptions to this chapter 1. Genomic imprinting: a zygote expresses only one allele (either mom’s or dad’s) instead of both. 2. Inheritance of organelle genes: we have DNA in our mitochondria, and it comes only from mom