Page 29 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

2.2 Genetic Analysis According to Mendel 21

perfect ratio of 3 yellow : 1 green. F 1 plants derived from from each parent? Mendel drew on his background in plant
the reciprocal of the original cross produced a similar ratio physiology and answered these questions in terms of the
of yellow to green F 2 progeny. two biological mechanisms behind reproduction: gamete
formation and the random union of gametes at fertilization.
Gametes are the specialized cells—eggs within the
Reappearance of the recessive trait ovules of the female parent and sperm cells within the pol-
The presence of green peas in the F 2 generation was irrefut- len grains—that carry genes between generations. Mendel
able evidence that blending had not occurred. If it had, the imagined that during the formation of eggs and sperm, the
information necessary to make green peas would have been two copies of each gene in the parent separate (or segre-
lost irretrievably in the F 1 hybrids. Instead, the information gate) so that each gamete receives only one allele for each
remained intact and was able to direct the formation of trait (Fig. 2.10a). Thus, each egg and each sperm receives
2001 green peas actually harvested from the second filial only one allele for pea color (either yellow or green).
generation. These green peas were indistinguishable from At fertilization, a sperm with one or the other allele
their green grandparents. unites at random with an egg carrying one or the other al-
Mendel concluded that two types of yellow peas must lele, restoring the two copies of the gene for each trait in the
exist: those that breed true like the yellow peas of the P fertilized egg, or zygote (Fig. 2.10b). If the sperm carries
generation, and those that can yield some green offspring yellow and the egg green, the result will be a hybrid yellow
like the yellow F 1 hybrids. This second type somehow con- pea like the F 1 monohybrids that resulted when pure-breeding
tains latent information for green peas. He called the trait parents of opposite types mated. If the yellow-carrying
that appeared in all the F 1 hybrids—in this case, yellow sperm unites with a yellow-carrying egg, the result will be
seeds—dominant (see Fig. 2.8) and the antagonistic a yellow pea that grows into a pure-breeding plant like
green-pea trait that remained hidden in the F 1 hybrids but those of the P generation that produced only yellow peas.
reappeared in the F 2 generation recessive. But how did he
explain the 3:1 ratio of yellow to green F 2 peas?
Figure 2.10 The law of segregation. (a) The two identical
alleles of pure-breeding plants separate (segregate) during gamete
Genes: Discrete units of inheritance formation. As a result, each sperm or egg carries only one of each
To account for his observations, Mendel proposed that for pair of parental alleles. (b) Cross-pollination and fertilization
between pure-breeding parents with antagonistic traits result in F 1
each trait, every plant carries two copies of a unit of in- hybrid zygotes with two different alleles. For the seed color gene,
heritance, receiving one from its maternal parent and the a Yy hybrid zygote will develop into a yellow pea.
other from the paternal parent. Today, we call these units of
inheritance genes. Each unit determines the appearance of (a) The two alleles for each trait separate during gamete
formation.
a specific characteristic. The pea plants in Mendel’s collec-
Gametes
tion had two copies of a gene for seed color, two copies of (sperm or eggs)
another for seed shape, two copies of a third for stem Y
length, and so forth. Grows into plant Gamete
Mendel further proposed that each gene comes in alter- formation
native forms, and combinations of these alternative forms YY yellow pea Y
from a pure-breeding
determine the contrasting characteristics he was studying. stock
Today we call the alternative forms of a single gene alleles. Grows into plant Gamete y
The gene for pea color, for example, has yellow and green formation
alleles; the gene for pea shape has round and wrinkled al- yy green pea y
from a pure-breeding
leles. In Mendel’s monohybrid crosses, one allele of each stock
gene was dominant, the other recessive. In the P generation,
one parent carried two dominant alleles for the trait under (b) Two gametes, one from each parent, unite at random
consideration; the other parent, two recessive alleles. The F 1 at fertilization.
generation hybrids carried one dominant and one recessive Gametes Zygote F Hybrid
1
allele for the trait. Individuals having two different alleles (one sperm, one egg)
for a single trait are monohybrids.
Y Seed
Fertilization development
Yy
The law of segregation y Yy = yellow pea
showing
If a plant has two copies of every gene, how does it pass dominant trait
only one copy of each to its progeny? And how do the off- Y = yellow-determining allele of pea color gene
spring then end up with two copies of these same genes, one y = green-determining allele of pea color gene