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What's going on here?

Why did only males inherit the white-eyed trait?

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Sex Chromosomes

Females = XX

Males = XY

X Chromosome = 1,100 genes on it

Y Chromosome = 78 genes

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Note the size of all AUTOSOMES; then note the size of the X vs. Y chromosome.

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Revisiting Morgan's Experiment...

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Morgan's Experiment

When sex chromosomes segregate for females, each gamete has w⁺ on its X chromosomes

When sex chromosomes segregate for males, one gamete has w^--and the other has no allele

The w^-- is masked in the female by the w⁺

In the male, the w^-- has nothing to mask it since the gene is not located on the y chromosome

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Hemizygous

Having only one allele for a trait

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Color Blindness

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Color Blindness

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Transmission of X-linked recessive traits

Colorblind father will transmit the mutant allele to all daughters but to no sons. With homozygous dominant mother, daughters will be carriers.

If a carrier female mates with hemizygous dominant male, there is a 50% chance that each daughter will be carrier; 50% chance that each son will have disorder

If carrier female mates with hemizygous recessive male, 50% chance each child will have disorder. Daughters with normal vision will be carriers.

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Sex-linked Pedigree

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Linked Genes and Genetic Recombination

What should happen if two genes are on the same chromosome?

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Back to Morgan and Drosophila

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Under normal circumstances: genes are on different chromosomes...

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Recombinant Phenotypes

The production of offspring with combinations of traits that differ from those found in either parent

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When two genes are unlinked (on different chromosome)

A testcross reveals…

Offspring only show parental genotypes and recombinant phenotypes

1:1:1:1 ratio

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What will the genotypes of the gametes be after meiosis? Did these genes assort independently?

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When two genes are linked (on the same chromosome)

A testcross reveals…

Offspring only show parental genotypes (1:1 ratio)

Offspring exhibit no recombinant phenotypes

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Morgan’s Experimental Results

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What's going on here?

I thought linked genes would only create parental genotypes!

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Remember the crossing over from Meiosis I?

Sometimes no cross over takes place and gametes only display parental genotypes

However, if cross-over takes place in between the two genes, then gametes may exhibit recombinant genotypes along with parental genotypes

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Recombination usually occurs at a lower frequency. This means that not all gametes experienced crossing over between the two genes at hand.

In this case, crossing over occurred only around 8% of the time.

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Linkage Map

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Create a Linkage Map

Genes A, B, and C are located on the same chromosome. Testcrosses show that the recombination frequency between A and B is 28% and between A and C is 12%. Can you determine the linear order of these genes? Explain.

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