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:

50% girl w/ dominant phenotype

25% boy w/ dominant phenotype

25% boy w/ recessive phenotype

XB Xb

XB XB XB XB Xb

Y XBY XbY

Sex-Linkage

With your partner, practice with these crosses:

XDXD x XdYXdXd x XDYXX x XYE

Pedigrees

On a pedigree for a Y-linked trait, fathers with the trait always give it to their sons.

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Pedigrees

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

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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.

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