Sex-Linked Traits
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Transcript of Sex-Linked Traits
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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