SEX LINKED TRAITS

Why aren’t women bald? A man and a woman have a child together. Even though the man has a full head of hair at the age of 70, his son goes bald at 40. X

Explaining Baldness To fully understand what’s going on here we need to look back one generation X X

Sex Chromosomes First we should look again at how sex is determined genetically XY XX X Y X X XY XX XY 50% Male and 50% Female

Sex Chromosomes The X chromosome is substantially larger than the Y This allows it to carry more genes Some genes exist only on the X chromosome and not the Y

Genotype in our Bald People X X X XBYXBY XBYXBY XBXBXBXB XBXBXBXB XbYXbY XBXbXBXb XbYXbY Male pattern baldness is SEX LINKED

Punnett Squares in Sex Linked Traits XBXbXBXb XBYXBY XBXB XbXb XBXB Y XBXbXBXb XBYXBY XBXBXBXB XbYXbY

Genotype and Phenotype in Sex Linkage The X b allele which results in baldness is recessive In men, however, there is no additional X chromosome to hold the dominant allele This results in sex linked traits affecting men much more often than they effect women Women who are heterozygous for the sex linked trait are called CARRIERS, and are phenotypically normal

Practice Is it possible to have a female with male pattern baldness? What P generation genotypes would result in this outcome? Hemophilia (lack of blood clotting) is also a well known sex linked trait. Explain using a Punnett grid why the sons of hemophiliac males are much less likely to have hemophilia than the grandsons.

Genetics of a Family The P and F1 generations are occasionally not enough An expanded view of more of an organism’s relatives is needed Creating these expanded views can sometimes be labour intensive A simplified view of a family, giving the maximum information is required The figures produced are called PEDIGREES

Example Pedigree X X X XBYXBY XBYXBY XBXBXBXB XBXBXBXB XbYXbY XBXbXBXb XbYXbY

Practice A man with colour blindness (a sex linked trait) has five children with a unaffected woman. His three sons do not have colour blindness. Both of his daughters, however, are carriers. Each of these daughters have children with unaffected men One daughter has two boys, one of which is colour blind. The other daughter has a boy and a girl, who when tested genetically, was shown to be a carrier for colour blindness. Give a pedigree using this information

Pedigree Construction

Combining Pedigrees with Other Traits Pedigrees often involve probability Can’t know certainly that certain offspring will have certain genotypes, but we can determine a probability

Practice – Finish these questions for next class A woman (Gertrude) has seven daughters and four of them have bald sons, while the other three only have sons with hair. Gertrude’s husband died when he was very young, and it is unknown whether he would have gone bald or not. Is Gertrude right in blaming her dead husband for her grandsons’ baldness? A woman who is homozygous for normal vision has children with a man who is colour blind (sex linked trait). Determine the possible genotypes of their children? The following chart shows the family history of the disorder “Woolly hair”. Woolly hair is a recessive trait, using the pedigree provided determine if woolly hair is sex linked or not.