Page 121 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

4.6 Validation of the Chromosome Theory 113

produce many diploid cells, the primary spermatocytes. We have presented thus far two circumstantial lines of
Unlike primary oocytes, primary spermatocytes undergo a evidence in support of the chromosome theory of inheri-
symmetrical meiosis I, producing two secondary sper- tance. First, the phenotype of sexual morphology is as-
matocytes, each of which undergoes a symmetrical meio- sociated with the inheritance of particular chromosomes.
sis II. At the conclusion of meiosis, each original primary Second, the events of mitosis, meiosis, and gametogene-
spermatocyte thus yields four equivalent haploid spermatids. sis ensure a constant number of chromosomes in the so-
These spermatids then mature by developing a characteristic matic cells of all members of a species over time; one
whiplike tail and by concentrating all their chromosomal ma- would expect the genetic material to exhibit this kind of
terial in a head, thereby becoming functional sperm. A human stability even in organisms with very different modes of
sperm, much smaller than the ovum it will fertilize, contains reproduction. Final acceptance of the chromosome the-
22 autosomes and either an X or a Y sex chromosome. ory depended on researchers going beyond the circum-
The timing of sperm production differs radically from stantial evidence to a rigorous demonstration of two key
that of egg formation. The meiotic divisions allowing con- points: (1) that the inheritance of genes corresponds with
version of primary spermatocytes to spermatids begin only the inheritance of chromosomes in every detail, and (2)
at puberty, but meiosis then continues throughout a man’s that the transmission of particular chromosomes coin-
life. The entire process of spermatogenesis takes about cides with the transmission of specific traits other than
48–60 days: 16–20 for meiosis I, 16–20 for meiosis II, and sex determination.
16–20 for the maturation of spermatids into fully functional
sperm. Within each testis after puberty, millions of sperm
are always in production, and a single ejaculate can contain Mendel’s Laws Correlate with
up to 300 million. Over a lifetime, a man can produce bil- Chromosome Behavior During Meiosis
lions of sperm, almost equally divided between those bear-
ing an X and those bearing a Y chromosome. Walter Sutton first outlined the chromosome theory of in-
heritance in 1902–1903, building on the theoretical ideas
and experimental results of Theodor Boveri in Germany,
essential concepts E. B. Wilson in New York, and others. In a 1902 paper,
Sutton speculated that “the association of paternal and ma-
• Diploid germ cell precursors proliferate by mitosis and
then undergo meiosis to produce haploid gametes. ternal chromosomes in pairs and their subsequent separa-
• Human females are born with oocytes arrested in tion during the reducing division (that is, meiosis I) . . .
may constitute the physical basis of the Mendelian law of
prophase of meiosis I. Meiosis resumes at ovulation
but is not completed until fertilization. Spermatogenesis heredity.” In 1903, he suggested that chromosomes carry
begins at puberty and continues through the lifetimes Mendel’s hereditary units for the following reasons:
of human males. 1. Every cell contains two copies of each kind of chro-
• The two meiotic divisions of oogenesis are asymmetrical, mosome, and two copies of each kind of gene.
so a primary oocyte results in a single egg. The two 2. The chromosome complement, like Mendel’s genes,
meiotic divisions of spermatogenesis are symmetrical, so appears unchanged as it is transmitted from parents to
a primary spermatocyte results in four sperm. offspring through generations.
• All human oocytes contain a single X chromosome; 3. During meiosis, homologous chromosomes pair and
human sperm contain either an X or a Y. then separate to different gametes, just as the alternative
alleles of each gene segregate to different gametes.
4. Maternal and paternal copies of each chromosome
4.6 Validation of the pair move to opposite spindle poles without regard to
Chromosome Theory the assortment of any other homologous chromosome
pair, just as the alternative alleles of unrelated genes
assort independently.
5. At fertilization, an egg’s set of chromosomes unites
learning objectives with a randomly encountered sperm’s set of chromo-

1. Describe the key events of meiosis that explain somes, just as alleles obtained from one parent unite
Mendel’s first and second laws. at random with those from the other parent.
2. Infer from the results of crosses whether or not a trait 6. In all cells derived from the fertilized egg, one-half
is sex-linked. of the chromosomes and one-half of the genes are
3. Predict phenotypes associated with nondisjunction of maternal origin, the other half of paternal origin.
of sex chromosomes. The two parts of Table 4.4 show the intimate relationship
between the chromosome theory of inheritance and