Hello, My name is Dr. Carole Sargent.
I'm a member of the University of Cambridge, Department of Pathology.
My background includes genome analysis on the
human sex chromosomes, and sequencing of the pig X and Y chromosomes.
Today we're going to think about why the X chromosome behaves differently in terms of
its genetics, and the impact that X chromosome inactivation has on disease outcomes.
Before we consider the X chromosome,
it's useful to recap on the principles of Mendelian genetics.
Mendel's experiments with pea plants used statistics to
analyse the phenotypic outcomes in the offspring of crosses.
His crosses used true-breeding plants,
which were selected for easily-scorable phenotypes.
In this example, we'll consider pea colour.
In his first generation,
only one of the traits was observed in the offspring.
All the peas were yellow.
If the plants from the F1 generation were crossed with each other,
the second (or F2) generation showed one in four plants produced
green peas, and the rest produced yellow.
Important observations were that it doesn't matter
which parent is green and which parent is yellow.
He also concluded that the green phenotype is recessive to the yellow phenotype.
The ratios are achieved in the offspring of the F2 generation,
assuming that the adults have two copies of the gene for the trait.