Sex-LINKED GENETICS
DIHYBRID CROSS vs. Sex-linked Dihybrid Cross Two traits on DIFFERENT chromosomes Sex-linked Two traits on SAME chromosome
What is a sex-linked trait? Sex-linked traits are due to genes located on sex chromosomes. As we know, male sex chromosomes are XY while female sex chromosomes are XX. The X chromosome contains over 1000 genes while the Y chromosome contains as few as 26. Therefore, many sex- linked traits are discussed in terms of the X-chromosomes.
Carriers Because females have two copies of the X chromosome, it is possible to have certain traits “hidden” by a dominant copy. However, because males only have one X chromosome, the observable phenotype is obvious and identifies the genotype.
Carriers When a female contains a recessive allele that is hidden by the dominant allele, we call them carriers. A carrier maintains the ability to pass on a trait even if they do not express/show it.
Examples of X-linked genes Other than determining sex, genes on the X chromosome are responsible for traits. Some examples are: Hemophilia Red-green color blindness High blood pressure Muscular dystrophy Fragile-X syndrome
Punnett Practice Determine the probability of the offspring being a carrier when: The woman is a carrier and her partner does not have the trait. -- What % do the boys have of receiving the trait? What % of the girls?
If the man has an X-recessive trait and his partner does not have it (nor is a carrier). What % of boys will have it? Carrier? What % of girls will have it? Carrier?
If the women is a carrier and the man has the trait If the women is a carrier and the man has the trait. What % of boys will have the disease? Carrier? What % of girls will have it? Carrier?
If the woman has the trait and the man does not have the trait If the woman has the trait and the man does not have the trait. What % of boys will have the disease? Carrier? What % of girls will have it? Carrier?
Practice All Punnetts Find the genotype ratios for a homozygous dominant x heterozygous Find the phenotype ratios for a heterozygous x heterozygous What % of male offspring will be hemophilliacs if the mother is a carrier and the father is a hemophilliac? Determine the parent if a unknown dominant parent produces 100% heterzygous offspring? What are the chances of a)male and b)female offspring to be infected (but not carriers) if a mother is a carrier and the father is not infected? What is the chance of a recessive lethal when crossing a heterozygous with a homozygous dominant?
http://www2.edc.org/weblabs/Punnett/punnettsquar es.html