SexLINKED GENETICS DIHYBRID CROSS vs Sex linked Dihybrid

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 sexlinked 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. 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. 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: //www 2. edc. org/weblabs/Punnett/punnettsquare s. html