Bio 1000: Genetic Linking and Recombination Part 3

PEDIGREE

PEDIGREE is the analysis made by interviewing and testing living members of a family and reconstructing the genotypes and phenotypes of past generations from family reconstruction. It determines the absence and presence of the trait of interest.

• Females are circles
• Males are squares
• Solid circle/square = presence of trait

Sex-linked recessive traits appear more likely on males than females. WHY? Males need to receive only one copy of the allele on the X chromosome inherited from the mothers to develop while females must receive two copies of recessive allele to express the gene (one from each parent). Female parents who carry the mutant allele and can pass it to offspring, but have no sign of it are called CARRIERS

INACTIVATION OF ONE X CHROMOSOME

Females have twice as many copies of genes carried on the X chromosome on males, but it unlikely that it is used twice as much. Theoretically, products from genes on the X chromosome can be equalized in males and females if :

1. Expression of genes on the single male X chromosome is doubled
2. Expression of genes on both female X chromosome is halved
3. Expression of one X chromosome is turned off

The third option seems to be the hypothetical correct option. Inactivation occurs by a condensation process that folds and packs the chromatin of one of the 2 X chromosomes into a tightly coiled state. The inactive and condensed X structure can be seen within the nucleus in cells of females called BARR BODY. Inactivation occurs in embryonic development and is completely random in the selection of the X chromosome.