Page 32 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

24 Chapter 2 Mendel’s Principles of Heredity

Figure 2.13 Genotype versus phenotype in homozygotes Figure 2.14 How a testcross reveals genotype. An individual
and heterozygotes. The relationship between genotype and of unknown genotype, but dominant phenotype, is crossed with a
phenotype with a pair of contrasting alleles where one allele (Y) homozygous recessive. If the unknown genotype is homozygous, all
shows complete dominance over the other (y). progeny will exhibit the dominant phenotype (cross A). If the unknown
genotype is heterozygous, half the progeny will exhibit the dominant
Genotype for the Seed Phenotype trait, half the recessive trait (cross B).
Color Gene
Cross A Cross B
YY
Homozygous dominant Yellow
P YY yy P Yy yy

Dominant Recessive
allele allele
Yy Yellow F 1 y F 1 y
Heterozygous
Y Yy Y Yy
yy
Homozygous recessive Green y yy

O spring all yellow O spring 1:1 yellow to green
Testcrosses: A way to establish genotype
Before describing Mendel’s answer, we need to define a way, the testcross establishes the genotype behind a domi-
few more terms. An observable characteristic, such as yel- nant phenotype, resolving any uncertainty.
low or green pea seeds, is a phenotype, while the actual As we mentioned earlier, Mendel deliberately simpli-
pair of alleles present in an individual is its genotype. A YY fied the problem of heredity, focusing on traits that come in
or a yy genotype is called homozygous, because the two only two forms. He was able to replicate his basic monohy-
copies of the gene that determine the particular trait in brid findings with corn, beans, and four-o’clocks (plants
question are the same. In contrast, a genotype with two dif- with tubular, white or bright red flowers). As it turns out,
ferent alleles for a trait is heterozygous; in other words, it his concept of the gene and his law of segregation can be
is a hybrid for that trait (Fig. 2.13). An individual with a generalized to almost all sexually reproducing organisms.
homozygous genotype is a homozygote; one with a hetero-
zygous genotype is a heterozygote.
Note that the phenotype of a heterozygote (that is, of a Dihybrid Crosses Reveal the Law
hybrid) defines which allele is dominant: Because Yy peas of Independent Assortment
are yellow, the yellow allele Y is dominant to the y allele for
green. If you know the genotype and the dominance rela- Having determined from monohybrid crosses that genes
tion of the alleles, you can accurately predict the pheno- are inherited according to the law of segregation, Mendel
type. The reverse is not true, however, because some turned his attention to the simultaneous inheritance of two
phenotypes can derive from more than one genotype. For or more apparently unrelated traits in peas. He asked how
example, the phenotype of yellow peas can result from two pairs of alleles would segregate in a dihybrid individ-
either the YY or the Yy genotype. ual, that is, in a plant that is heterozygous for two genes at
With these distinctions in mind, we can look at the the same time.
method Mendel devised for deciphering the unknown gen- To construct such a dihybrid, Mendel mated true-
otype. We’ll call it Y–, responsible for a dominant pheno- breeding plants grown from yellow round peas (YY RR)
type; the dash represents the unknown second allele, either with true-breeding plants grown from green wrinkled peas
Y or y. This method, called the testcross, is a mating in (yy rr). From this cross he obtained a dihybrid F 1 genera-
which an individual showing the dominant phenotype, for tion (Yy Rr) showing the two dominant phenotypes, yellow
instance, a Y– plant grown from a yellow pea, is crossed and round (Fig. 2.15). He then allowed these F 1 dihybrids
with an individual expressing the recessive phenotype, in to self-fertilize to produce the F 2 generation. Mendel could
this case a yy plant grown from a green pea. As the Punnett not predict the outcome of this mating. Would all the F 2
squares in Fig. 2.14 illustrate, if the dominant phenotype in progeny be parental types that looked like either the orig-
question derives from a homozygous YY genotype, all the inal yellow round parent or the green wrinkled parent? Or
offspring of the testcross will show the dominant yellow would some new combinations of phenotypes occur that
phenotype. But if the dominant parent of unknown geno- were not seen in the parental lines, such as yellow wrinkled
type is a heterozygous hybrid (Yy), half of the progeny are or green round peas? New phenotypic combinations like
expected to be yellow peas, and the other half green. In this these are called recombinant types.