SEX CHROMOSOMES AND X-LINKED INHERITANCE
Figure 4.12 shows the chromosomal basis
of sex determination in all mammals, many insects (including Drosophila),
and some (but not all) other animals. Female mammals have two X chromosomes,
and their progeny all get one of these X's (random which one). Male mammals
have an X chromosome and a Y chromosome. During gamete formation, the X and
Y form a bivalent during meiotic prophase I, and thus end up in separate gametes.
Resulting progeny that get the X chromosome become female and those progeny
that get the Y chromosome become male. ( So, all men have received their X chromosome
from their mother, and will transmit it to their daughters. All men have received
their Y chromosome from their father, and will transmit it to their sons).
1. What do we know about the details of the human X and Y chromosomes?
Here is a good photo showing the relative sizes of the human X and Y chromosomes. The detailed sequence analysis of the Y chromosome was published in 2003, and that of the X chromosome in the journal Nature in March of 2005. Here is the link to the summary (abstract) of this article. Print this.
The human Y chromosome is small and mostly heterochromatic, containing only a small number of genes. The human X chromosome is approximately average size among the human chromosomes, and has approximately an average number of genes. So, let's look at some aspects of inheritance of genes that are on the X chromosome.
2. How is normal Mendelian inheritance altered if the gene in question is on the X chromosome ?
Figure 4.15 shows the case for a Drosophila eye color gene (with dominant allele "W+" and recessive allele "w") that is on the X chromosome. The Y chromosome does not have this gene.
For normal autosomal Mendelian inheritance, the outcomes of crosses involving homozygous parents are not dependent on which parent contains which alleles. For any gene on the X chromosome, however, it definitely DOES matter, as the details of Figure 4.15 make clear. Note that the reciprocal parental crosses (labelled as crosses A and B in the figure) give differences in both the F1 and F2 generations.
3. What about X-linked inheritance in humans?
Here is an example. On the long arm of the human X chromosome is the gene called F8C, which codes for the protein called "coagulation factor VIII". The wild-type allele F of this gene codes for the functional protein, while the mutant allele f codes for a non-functional defective version of the protein. Having one F allele is enough to give a normal phenotype, but someone who does not have the F allele has the inherited disease hemophilia A. So, the only way a female has the disease is to be ff, while a male has the disease if he is (simply) f, which occurs at 50% probability if his mother is a heterozygous (Ff) carrier of the f allele.
Figure 4.17 shows a historical pedigree for hemophilia A spanning four generations.
4. What are the effects of "nondisjunction"?
Figure 4.19 shows a somewhat rare event in which the two X chromosomes of a female do not separate properly during meiosis I. Such a failure of chromosomes to separate, called nondisjunction, can involve either sex chromosomes or autosomes. Nondisjunction during meoisis I involves a bivalent remaining intact and going "left" or "right" in meiotic anaphase I.
Nondisjunction can also occur in meiosis II, involving a two-chromatid chromosome remaining intact and going left or right in meiotic anaphase II.