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Linked Genes and GeneMapping

Ch 10/11

Background and Review

• Gregor Mendel was not aware thatchromosomes carried our genetic information.

• Gregor Mendel discovered– Alternate versions of a gene is called an allele– An organism inherits two alleles. One from each

parent– There is a dominant and recessive allele– Law of segregation– Law of independent assortment

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Background and Review

• Walter Sutton (1902) ChromosomeTheory of inheritance.– Genes are located on the chromosomes.– Each gene occupies a specific place on a

chromosome (locus)– The chromosomes undergo segregation and

independent assortment.

Linked Genes• Thomas H. Morgan (Early 1900’s) Used

Drosophila melanogaster (fruit fly) in orderto study inheritance.

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Linked Genes

• Morgan discoveredlinked genes

• linked genes Genes located on thesame chromosometend to be inheritedtogether.

• Linked genes do notassort independentlyduring meiosis.

Linked Genes

• Genes can be– Not linked– Completely linked– Partially linked

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Results for Genes which are NOTLinked and assort independently

Possible gametecombinations are GW, Gw,gW and gw for this parent

Possible gametecombination is gw for thisparent

Expected resultswhen the genesassortindependently(not linked)

Results for Genes which are NOTLinked and assort independently

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Results for genes which arecompletely linked

Completely linked genes do not assortindependently during meiosis

We should see the same phenotype as theparents.1:1 (Gray with normal:Black with small wings)phenotypic ratio.

Body color andwing size arecompletely linked

Actual resultsdue to linkageof body colorand wing size(linked genes)

Results for genes which are partially linked• The results Morgan observed were due to

partially linked genes• About 17% of the offspring were a

recombination of the genes (recombinants).

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Crossing Over

• The resultsMorganobserved weredue to crossingover.

• Crossing overunlinks linkedgenes.

Sex Chromosomes

• We have 22 pairs ofautosomes and onepair of sexchromosomes.

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Sex Chromosomes

• The sex chromosomes are referred to as Xand Y.– The longer chromosome is the X and the shorter is

the Y.– Male XY– Female XX

• The sperm cell determines the sex of a child

Sex Chromosomes

X X Y Y

X

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Sex-Linked Genes

• Sex-linked genes Genes located onsex chromosomes.

• The X chromosome carries themajority of the genes.

Sex-Linked Genes

• Example genotypes and phenotypes– XRY Red eyed male– XrY White eyed male– XRXr Red eyed female– XrXr White eyed female

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Sex-Linked Gene• The Y chromosome carries very few

genes.• The X chromosome carries the majority of

the sex linked genes.• For males ALL alleles on the X

chromosome are expressed even if theyare recessive.

• For females there must be two copies(One on each X chromosome) of therecessive allele in order for it to beexpressed.

Sex-Linked Genes

Phenotype All red eyes Phenotype 3:1(Red:White)

• Morgan noticed that only the males had white eyes.• Morgan hypothesized that the gene for eye color was

located on the X chromosome.

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Sex-Linked Genetic Disorders• Colorblindness

– Sex-linked recessive gene on the Xchromosome.

– Difficulty distinguishing between red and green.• Hemophilia

– Sex-linked recessive gene on the Xchromosome

– Missing a protein necessary for normal bloodclotting

– Can bleed to death from minor cuts or bruising.

Practice Problems

• G = gray body color, g= yellow body color– Cross a Gray carrier female with a Gray male.

Draw a punnet square.– What are the phenotypic ratios for the males?

Females? All offspring?• M = normal wings, m=miniature wings

– Cross a normal carrier female with a miniaturewing male. Draw a punnett square.

– What are the phenotypic ratios for the males?Females? All of the offspring?