Transcript document
Sex-Linked Traits Homework Test Monday 4/5 Objectives Be able to construct and interpret a monohybrid cross for a sex-linked gene Distinguish between autosomal and sex chromosomes Be able to recognize sex-linkage in a pedigree Chromosomes There are two broad categories of chromosome: Sex Chromosomes: A chromosome that men and women have different amounts of. (X and Y in humans.) Autosomal Chromosomes: All the other chromosomes BESIDES the sex chromosomes. (Chromosomes #1-22 in humans) Chromosomes So far, we’ve only studied autosomal traits, because a gene on a sex chromosome is a bit more complicated. Why might it be harder to figure out genotypes when the gene is on a sex chromosome? Sex-Linkage Sex-Linked Traits: The gene for the trait is located on either the X or Y chromosome. When writing alleles that are sex-linked, we use a convention like that more complicated one from back at the beginning: we write the X or Y normally, and make the allele itself a superscript. This can produce some interesting effects. For instance, say there’s a disease. We’ll use the alleles A and a. Sex-Linkage If the A gene is on the X chromosome, then genotypes can have one of these alleles: XA, Xa, and Y. If there’s no allele, the Y becomes sort of like the “free square” in the middle of a bingo board. It doesn’t affect the phenotype at all. So a man who is XAY will have the dominant version of the trait, and a man who is XaY will have the recessive version of the trait. He doesn’t have two little-a’s, true, but there’s also no dominant allele to “drown out” the recessive allele. Sex-Linkage Meanwhile, if the gene is on the Y chromosome, we could have the alleles X, YA, or Ya. Would each of these people have the dominant phenotype or the recessive? XX XYA XYa Sex-Linkage Meanwhile, if the gene is on the Y chromosome, we could have the alleles X, YA, or Ya. Would each of these people have the dominant phenotype or the recessive? XX - Will not have the trait, whatever it is. XYA - Dominant phenotype XYa - Recessive phenotype Sex-Linkage How about phenotypes for these people with a gene on the X-chromosome? XBXB XBXb XbXb XBY XbY Sex-Linkage How about phenotypes for these people with a gene on the X-chromosome? XBXB - Dominant XBXb - Dominant XbXb - Recessive XBY - Dominant XbY - Recessive Sex-Linkage Punnett Squares for sex-linked traits work like normal, except that you use the superscripts. A Punnett Square for a cross of XBXb with XBY looks like this: Sex-Linkage Punnett Squares for sex-linked traits work like normal, except that you use the superscripts. A Punnett Square for a cross of XBXb with XBY looks like this: XB Xb XB XB XB XB Xb Y XBY XbY 50% girl w/ dominant phenotype 25% boy w/ dominant phenotype 25% boy w/ recessive phenotype Sex-Linkage With your partner, practice with these crosses: XDXD x XdY XdXd x XDY XX x XYE Pedigrees On a pedigree for a Y-linked trait, fathers with the trait always give it to their sons. QuickTime™ and a decompressor are needed to see this picture. Pedigrees For a recessive X-linked trait, it will skip generations and usually only men will have the trait. QuickTime™ and a decompressor are needed to see this picture. Pedigrees X-linked dominant traits are very hard to tell apart from simple dominance. The only helpful hint is that fathers with the trait always give it to their daughters. QuickTime™ and a decompressor are needed to see this picture.