In our discussion of inheritance, we have so far focused on organisms other than humans. Now, we will get the chance to think about how the rules of genetics apply to our own families. Normal humans have twenty-three homologous pairs of chromosomes (see figure 10.2). Twenty-two of these pairs are truly homologous, that is each member of the pair has the same genes, but potentially different alleles. The exceptions to this arrangement are the human sex chromosomes. Females have two homologous X-chromosomes, while males have one X and one Y chromosome as their twenty-third pair. Thinking about how these chromosomes move during meiosis and fertilization (figure 10.3) shows how sex is determined. Female gametes (eggs) must all carry an X chromosome, while male gametes (sperm) may carry either an X or a Y. Eggs that are fertilized by X-carrying sperm develop as female, while eggs that are fertilized by Y-carrying sperm develop as male. During development, "uncommitted" internal and external genitalia arise, then develop as either male or female, depending on the presence or absence of the Y chromosome. Note that the X chromosome has a full complement of thousands of genes, while the Y chromosome has only a few genes. Among these genes is SRY, the gene that signals the uncommitted genitalia to develop as male. In the absence of this signal, humans develop as female.

For genes on the X-chromosomes, females have two copies ( are diploid) while males have one copy (are haploid). For this reason, males are more likely to show the phenotypic consequences of recessive alleles of genes located on the X chromosome. These alleles are called X-linked recessives, and they are responsible for a number of genetic disorders, from color-blindness to hemophilia, that are more prevalent in males than in females. For the other twenty-two pairs of chromosomes, males and females are both diploid. Any time two genes are on the same chromosome, they are called linked genes, meaning that they travel together in meiosis, rather than assorting independently.

Now look at figure 10.7, and remember what happened during synapsis in prophase I of meiosis - crossing over. Since crossing-over events occur at random places along the chromosome, the further apart two genes are, the more likely it is that a crossing-over event will occur between them. This fact means that, although linked genes will occasionally seem to assort independently due to crossing over, the closer the genes are on the chromosome, the less likely this is to happen. This is the basis for the generation of chromosome maps.

Humans, though, are not good subjects for genetic experiments. For obvious reasons, we cannot do test matings to observe the phenotype of the offspring of a chosen pair of parents. Instead, we have to analyze the genotypes and phenotypes of families that already exist. The tool for this type of analysis is called the pedigree, a chart that traces the genetic connections between individuals. Figure 10.8 shows some of the basics of pedigree construction. Make sure to learn how to pedigrees, because we will use this type of analysis extensively in solving genetics problems in the lab and on the exam. They are useful for answering questions about the probability of individuals having a certain genotype, a question that is especially interesting to families that know or suspect that they carry alleles that cause genetic disorders. Remember that humans are diploid organisms, and have two copies of each gene. Most humans carry several recessive alleles that could cause genetic disorders in homozygous, but these are masked by the presence of a dominant allele. When two of these recessive alleles combine at fertilization, the result is an affected individual (see figure 10.9). In addition to the autosomal recessive mode of inheritance we have just described, genetic disorders may be inherited as autosomal dominant alleles or X-linked dominant or recessive alleles (see figures 10.10 and 10.11). Because the Y chromosome carries so few genes, Y-linked disorders are very rare.

Changes in chromosome number due to nondisjunctions in either meiotic division can lead to other types of genetic disorder, like the well-known Down syndrome. Changes in the number of sex chromosomes gives rise to Turner, XYY and Kleinfelter syndromes. Advances in understanding of the molecular rules of inheritance have led to the ability to test not just for changes in chromosome number using karyotype analysis, but to test for the presence or absence of specific alleles. (Figure 10.9)