Biology 1001A | 2012
Lecture 12 Inheritance in Populations
- Which of the following is a true characteristic of dominant alleles?
a. They are the most evolutionarily ﬁt alleles.
b. They are the most common alleles in a population
c. They inhibit the expression of recessive alleles
d. They determine the phenotype of heterozygotes
- Answer: Only d
- If you mated a pure-breeding black pig with a pure-breeding brown pig, what do you
think the piglets will look like?
- Answer: all black piglets
- the allele for black colour must be dominant to the allele for brown colour
- if two F1s were crossed, what would the resulting piglets look like?
- Answer: Some would be black and some would be brown in Mendel’s 3:1 ratio
Why is it dominant?
- why is the black allele dominant over brown?
- skin and hair pigments are created in specialized cells called melanocytes
- two kinds of melanin can be made, one in each colour
- to melanin is shipped in melanosomes to the skin cells, resulting in the pigmentation of
the skin and hair
- when MC1R is “on”, cyclic AMP (cAMP) levels in the cell are high and the melanocyte
- The as both kinds of melanosomes
- the black allele is insensitive to hormone control and cAMP is always high
- when both of these alleles are together,
- two alleles (black and brown) in the nucleus get transcribed, spliced, exported out of
the nucleus, ﬁnd ribosomes, taken t o the ER and those proteins get packaged into
vesicles and put into the Golgi and get sent to the cell membrane for export
- pig cell has both receptors functioning at the same time
- it doesn’t matter what the brow allele is doing because the black allele keeps the black
melanin level high at all times
- the black allele never interferes with the brown allele in any way
- this is one mechanism that will be dominant because it is always on, as opposed to the
other allele which is only on for part of the time
- dominance arises from expression of the allele, not competition between the two
- what will be the phenotype of B R heterozygotes?
- Answer: brown piglets
- the brown allele is recessive to black but is dominant to red Biology 1001A | 2012
- dominance and reces