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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:
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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: 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.
Pedigrees • For a recessive X-linked trait, it will skip generations and usually only men will have the trait.
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.
