Page 58 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

50 Chapter 3 Extensions to Mendel’s Laws

marbled-2, or of marbled-1 with spotted or dotted or clear, fourth (dotted and clear). The fact that all tested pairings of
produce the marbled-1 phenotype in the F 1 generation and a lentil seed coat pattern alleles yielded a 3:1 ratio in the F 2
ratio of three marbled-1 to one of any of the other phenotypes generation (except for spotted × dotted, which yielded the
in the F 2 . These results indicate that the marbled-1 allele is 1:2:1 phenotypic ratio reflective of codominance) indicates
completely dominant to each of the other four alleles. that these lentil seed coat patterns are determined by differ-
Analogous crosses with the remaining four phenotypes ent alleles of the same gene.
reveal the dominance series shown in Fig. 3.6. Recall that
dominance relations are meaningful only when comparing Histocompatibility in humans
two alleles: An allele, such as marbled-2, can be recessive
to a second allele (marbled-1) but dominant to a third and In some multiple allelic series, each allele is codominant
with every other allele, and every distinct genotype there-
fore produces a distinct phenotype. This happens particu-
Figure 3.6 How to establish the dominance relations larly with traits defined at the molecular level. An extreme
between multiple alleles. Pure-breeding lentils with different example is the group of three major genes that encode a
seed coat patterns are crossed in pairs, and the F 1 progeny are family of related cell surface molecules in humans and
self-fertilized to produce an F 2 generation. The 3:1 or 1:2:1 F 2 other mammals known as histocompatibility antigens. Car-
monohybrid ratios from all of these crosses indicate that different
alleles of a single gene determine all the traits. The phenotypes ried by all of the body’s cells except the red blood cells and
of the F 1 hybrids establish the dominance relationships (bottom). sperm, histocompatibility antigens play a crucial role in
Spotted and dotted alleles are codominant, but each is recessive facilitating a proper immune response that destroys intrud-
to the marbled alleles and is dominant to clear. ers (viral or bacterial, for example) while leaving the body’s
Parental Generation F Generation F Generation own tissues intact. Because each of the three major histo-
1
2
Parental seed coat F phenotype Total F 2 Apparent compatibility genes (called HLA-A, HLA-B, and HLA-C in
1
pattern in cross frequencies pheno- humans) has between 400 and 1200 alleles, the number of
Parent 1 Parent 2 and typic
phenotypes ratio possible allelic combinations in an individual creates a
powerful potential for the phenotypic variation of cell sur-
face molecules. Other than identical (that is, monozygotic)
twins, no two people are likely to carry the same array of
marbled-1 clear marbled-1 798 296 3 :1
histocompatibility antigens on the surfaces of their cells.
The extreme variation in these proteins has important
medical consequences, because people can make antibodies
marbled-2 clear marbled-2 123 46 3 :1
to non-self histocompatibility antigens different from their
own. These antibodies can lead to rejection of transplanted
organs. Doctors thus attempt to match as closely as possible
spotted clear spotted 283 107 3 :1 the histocompatibility antigen types of transplant donors
and recipients. Family members usually make the best or-
gan donors, as the closer the genetic relationship between
dotted clear dotted 1706 522 3 :1 two people, the more likely they are to share HLA alleles.

marbled-1 marbled-2 marbled-1 272 72 3 :1 Mutations Are the Source of New Alleles
How do the multiple alleles of an allelic series arise? The
answer is that chance alterations of the genetic material,
marbled-1 spotted marbled-1 499 147 3 :1
known as mutations, arise spontaneously in nature. Once
they occur in gamete-producing cells, they are inherited
faithfully. Mutations that have phenotypic consequences
marbled-1 dotted marbled-1 1597 549 3 :1
can be counted, and such counting reveals that they occur
at low frequency. The frequency of gametes carrying a new
mutation in a particular gene varies anywhere from 1 in
marbled-2 dotted marbled-2 182 70 3 :1
10,000 to 1 in 1,000,000. This range exists because differ-
ent genes have different mutation rates.
Mutations make it possible to follow gene transmission.
spotted dotted spotted/dotted 168 339 157 1 : 2 : 1
If, for example, a mutation specifies an alteration in an
enzyme that normally produces yellow so that it now makes
green, the new phenotype (green) will make it possible to
Dominance series: marbled-1 > marbled-2 > spotted = dotted > clear recognize the new mutant allele. In fact, it takes at least two