Dominant/Recessive Genes

Toby Bradshaw toby at
Wed Mar 2 10:10:15 EST 1994

In article <genecutl-010394200908 at>,
gc <genecutl at> wrote:
>In article <2ktq1h$cej at>, toby at (Toby
>Bradshaw) wrote:
>> In article <1994Feb28.185850.18629 at>,
>> Brian Foley <brianf at> wrote:
>> >Haploid organisms, even if only a small portion 
>> >of their life cycle is haploid, play by different rules.
>> I can't think of a sexually reproducing organism that *doesn't*
>> have a haploid life history stage upon which selection can act.
>> I'm willing to hear about some exceptions, though :)
>There's not much selection on sperm and eggs.

Proof by assertion?  :)

>True, if the sperm
>can't swim they'll be selected against, but there isn't much
>selection apart from that and what selection there is doesn't
>have any effect on their diploid forms (us).

What if the sperm can swim but not as fast as the others?  Are sperm
speed and pollen tube growth rate selectable traits?  Absolutely.
If increased sperm speed or increased pollen tube growth leads to
an increased rate of fertilization, can alleles that increase male
gamete/gametophyte speed be favored in a population *even if the
allele has negative pleiotropic effect on the diploid phase of the
life cycle*?  Yes again.

You can't ignore pleiotropy by asserting that genes and gene products
important in the haploid phase of the life cycle have no relevance to
the the diploid phase.

>> So, how are organisms with primarily haploid life cycles 
>> playing by different rules than organisms with relatively
>> brief haploid stages?  It seems to me to be a difference
>> of degree, not of kind.
>Look at budding yeast as an example of an organism that can
>happily grow as either a haploid or a diploid.  Selection
>operates on both forms in the same way.

No, it doesn't.  Is a deleterious recessive selected against in a
heterozygous diploid?  How about in a haploid?

>The only differnce
>is, if the diploid sporulates to generate four haploids,
>those new haploids will have different genotypes from their
>parent cell.  Haploids that come from haploids and diploids
>that come from diploids are identical to their parents.

Well, there is mutation to consider :)

>In this respect there is a difference.  Is this what was
>meant in the original post?


Toby Bradshaw                       |
Department of Biochemistry          |  Will make genetic linkage maps
and College of Forest Resources     |            for food.
University of Washington, Seattle   |
toby at               |

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