Page 158 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

150 Chapter 5 Linkage, Recombination, and the Mapping of Genes on Chromosomes

FAST FORWARD Sprinters: © Robert Michael/Corbis RF

Mapping the Crossovers that Generate the Chromosomes of Individual Human Sperm
Using DNA analysis technologies that will be described in Next, the researchers determined which SNP alleles
Chapters 9 through 11, scientists can now examine the base were present in individual sperm genomes. Then, by compar-
pair sequences of the whole genomes of single sperm. In one ing the SNP alleles present on each chromosome in an indi-
such study, by comparing the DNA sequences in each of the vidual sperm with the corresponding SNP alleles on each
pairs of homologous chromosomes in a man’s somatic cells with homolog of the man’s somatic cells (prior to crossing over
those of individual sperm the same man produces, researchers during meiosis), the locations of crossovers were revealed
could locate specific recombination events that occurred in the (Fig. B). By analyzing the crossovers in 91 different sperm,
man’s primary spermatocytes. the researchers found that about 1 crossover per chromo-
The homologous chromosomes any person inherits from his some took place in each gamete, and crossover hotspots
or her father or mother differ in about 1 out of every 1000 base were detected.
pairs. The base pair differences in different genomes are called The information obtained from this study and others like
SNPs (pronounced snips; for single nucleotide polymorphisms). it is useful to scientists studying the biochemistry of recombi-
From comparisons of the genome sequences of many individuals, nation. In addition, you will see in later chapters of the book
approximately 50,000,000 locations in the genome have been that the ability to determine the base pair sequence of indi-
identified where SNPs commonly can occur. The different base vidual chromosomes and individual gamete genomes has
pair sequences of SNPs are considered different alleles of the widespread applications in the study of mutation and human
SNP locus (Fig. A). Researchers can zero in on SNP loci and deter- evolution.
mine which alleles of millions of SNPs are present in a genome.
In order to map recombination sites, first the scientists de- Figure B Crossover map of a single sperm’s
veloped new technology to isolate individual chromosomes autosomes. The autosomes (chromosomes 1–22) of a sperm
from diploid somatic cells. Once isolated, the SNP alleles of in- are depicted, where the dark blue and light blue regions
dividual homologs could be determined (Fig. A). correspond to the different homologs in the man’s somatic cells
(see Fig. A). Most chromosomes are the products of a single
Figure A DNA sequences of homologs reveal SNP loci. crossover; in this example, chromosomes 2 and 10 are exceptions.
At a particular SNP locus, maternal (M) and paternal (P) homologs
can have different alleles (for example, an A−T base pair or a G−C 1 2 3 4 5 6 7 8 9 10 11
base pair).
M P
A:T G:C

Homologous
chromosomes 22 21 20 19 18 17 16 15 14 13 12

about 1 million base pairs. In yeast, however, where the female germ line than in males. This fact means that the
frequency of recombination per length of DNA is much same two genes will appear roughly twice as far apart on a
higher than in humans, one map unit is approximately 2500 genetic map generated by measuring RF in female meiosis
base pairs. Thus, although map units are useful for estimat- than they would if crossing-over during male meiosis were
ing relative distances between the genes of an organism, measured instead. The Fast Forward Box Mapping the
1% RF can reflect very different expanses of DNA in dif- Crossovers that Generate the Chromosomes of Individual
ferent organisms. Human Sperm explains how new technology allows
Recombination frequencies sometimes vary even be- analysis of the DNA sequences of individual human sperm
tween the two sexes of a single species. In humans, the genomes. Researchers now can detect crossovers directly
frequency of crossovers is about twofold higher in the in each chromosome of single sperm. The results of these