SEX LINKED GENES
The sex chromosomes are XX and XY in humans. Some alleles are located on the X chromosome. But very few on the Y chromosome. Remember that people who are females have the genotype XX and people who are males have XY.
The Y chromosome carries very little information, only enough to influence the embryo on the path towards male primary and secondary characteristics.
Meanwhile, many RECESSIVE ALLELES are located on the X chromosome, including
1. hemophilia, the inability to clot blood
2. colourblindness inability to tell the difference between red and green colour
3. baldness
These characteristics are far more common in malea than females because males have only one X. And if that X contains the recessive allele, they will show the phenotype. Meanwhile, females may have the recessive and their dominant genes on their other X will protect them. For example
An example of using a punnet square to calculate probability is here:
Some things to note about sex linkage: Signs of sex linked alleles are:
1. more males get the phenotype, but females can be carriers
2. males can inherit from mother but not father (because males get their X from mom)
3. females have an extra X and this protects them. Females must be homozygous to show the sex linked phenotype.
Further reading on sex linkage here http://learn.genetics.utah.edu/content/pigeons/sexlinkage/
FYI if you were born a male and you want to know if you're colourblind: Test yourself here: But careful, if you are a genetically male, you might find out you are colour blind! Apparently most people find out they are colourblind from their science teacher !
http://colorvisiontesting.com/home.html
Basic practice worksheet going over main concepts
