Chapter 15 Chromosomal Basis of Inheritance

Discovery 1900 – cytology and genetics converge: correlation between chromosomes and Mendelian genetics. Mendelian genes have certain loci on chromosomes that undergo segregation and independent assortment

Thomas Hunt Morgan 1 st to associate a specific gene with a _____________. Worked with Drosophila melanogaster (__________) – Hundreds of offspring, can be bred every two weeks – Only four chromosome pairs; 3 ___________ and one _____ pair (F-XX M-XY)

Terms Wild type: most common in a population (+) Mutant type: alternative to wild type

Example Red eyes (+) X White eyes Red is dominant to white Only males had white eyes Morgan concluded that the gene must be on the X chromosome and not the Y Females: X R X W  _______ Males: X W Y  _________ Sex linked genes: genes located on a sex chromosome.

Linked Genes Genes located on the same chromosome that tend to be inherited together. Ex. Drosophila body color and wing size b+ gray b black vg+ normal vg vestigial b+ b vg+ vg X b b vg vg Expected: 4 phenotypes in equal ratios Found: More gray normal and black vestigial than gray vestigial and black normal Conclusion: Body color and wing shape are inherited together on the same chromosome (found more parental phenotypes than nonparental)

Genetic Recombination Parental types: match a parent Recombinant types: don’t match a parent (when ½ of the offspring are recombinants, there is a 50% frequency of recombination). 50% recombination is observed for any 2 genes located on different chromosomes (due to independent assortment of alleles at Metaphase I of meiosis). Recombination frequency = #/total * 100%

Crossing Over If genes on the same chromosome, we would expect them to always be linked. However, we saw that this is not true (we saw some gray vestigial and black normal). This incomplete linkage can be accounted for by crossing over in meiosis I. (SHOW MEIOSIS)

Using recombination for genetic mapping Alfred H. Sturtevan (student of Morgan’s) created a genetic map. Recombinant frequencies reflects distances on chromosomes: greater distances = higher chance of crossing over Ex. Body color (b), wing size (vg) and cinnabar (cn) Recombination frequency: cn and b is 9%, cn and vg is 9.5% and b and vg is 17% Order on chromosomes must be: Map unit = 1% recombination or 1 centimorgan Maximum value = 50% recombination, so far apart they could be on different chromosomes.

Types of Maps Linkage map: sequence of genes, not actual locations Cytological map: locate genes with respect to chromosomal features (staining patterns) DNA nucleotide map

Sex chromosomes Mammals: Males are _____ females are ____ Males: in meiosis there is little crossing over between homologous chromosomes because they are different (X and Y). Development: embryo starts differentiating at about 2 months: Y chromosome has ____: sex determining region of the Y discovered in In the absence of ____, gonads develop into ____________. (many other genes responsible)

Sex linked genes Humans: usually X linked Fathers pass X linked genes to daughters, not sons Mothers pass X linked genes to both sons and daughters If recessive: female must be homozygous to express trait. Males are ____________ (only one X); any male receiving a trait from mom will express it. Consequences: more __________ than _______________ are affected, although both can be affected.

Human Disorders Duchenne Muscular Dystrophy: (1/3500 _________ in the US) Progressive weakening of muscles, loss of coordination, rarely live past 20’s (absence of muscle protein dystrophin, gene on X chromosome) Hemophilia: on X, absence of protein required for _____________________, excessive _____________ (used to ________________ from small _______). Royal Family: starting with Queen Victoria, spread to Prussia, Spain and Russia (Anastasia).

X Inactivation Females inactivate one X, which condenses into a Barr body inside the nucleus and most genes are not expressed. Mary Lyon: discovered the Barr body assignment is random, females are a mosaic of the two types of cells (active X from mom and active X from dad) Ex. Calico Cat: heterozygous for black and orange