X-Linked Traits

1. Why do we have two sexes?
   • In humans (and most animals) sex determination is done through sex chromosomes.
   • In humans, females are 2N = 46,XX (meaning their diploid karyotype or chromosome number is organized as a total of 46 chromosomes composed of 44 autosomes and two sex chromosomes, which in the female is XX).
   • In humans, males are 2N = 46,XY (meaning their diploid karyotype or chromosome number is organized as a total of 46 chromosomes composed of 44 autosomes and two sex chromosomes, which in the male is X and Y).
   • In mammals the Y chromosome carries a gene that converts the neutral embryonic gonad into testes. It is a sex-determining gene. It is called SRY (sex regulating region of the Y chromosome).
   • In the absence of the SRY gene (or when it is mutated or prevented from expressing its product) the embryo becomes a female at birth.
   • The advantages of two parents with different sexual functions include: males make lots of small mobile sperm (in humans about 100 million per ejaculate); females make a very large protected egg filled with nutrient (in humans one such egg every 28 days). Both parents produce an immense amount of genetic variation in the kinds of gametes they produce. Sexual reproduction assures genetic diversity for the species.

2. What are consequences of XX females and XY males for traits on their sex chromosomes?
   • Note that unlike autosomes which come in homologous pairs during meiosis, the X and Y are not the same in size or the genes they contain.
   • None of the genes on the Y are essential for life. Females do quite well with two X chromosomes and no Y chromosome (in fact, they live longer than males).
   • There are only some 16 genes on the Y and most are associated with sperm production and testes formation.
   • The X is a typical chromosome. It contains several thousand genes. Most of those genes have nothing to do with sex determination or sexual traits.
   • In the XY male, genes on the X are expressed whether they are dominant or recessive. The male is thus called hemizygous for the X because the Y acts as if it doesn't exist with respect to the X.
   • In the XX female, the genotype is like an autosome: it can be for any allele either homozygous normal, homozygous recessive, or heterozygous.

3. What are the consequences of hemizygous inheritance in the male?
   • In humans certain traits are seen in males but not at all (or very rarely) in females.
   • In all these cases that are called X-linked inheritance, the source of the male's genetic disorder or trait is from the mother. The father only supplies a Y chromosome to his son and that Y chromosome lacks the genes for these traits. The mother is usually heterozygous for the trait and does not express the recessive disorder or trait. The mother supplies the only X chromosome that her sons have.
   • In X-linked traits, the parents are almost always: P1 RY x Rr where the father has the Y and the mother is heterozygous. In this case R = normal color vision and r = deficiency in perceiving red or green color [i.e., red-green color blindness]. Thus the father RY has normal color vision as does the heterozygous mother.
   • For their children, the RY male produces two type of sperm in equal amount. Half will contain the Y and produce sons; half will contain the X and lead to normal daughters because the X contains the allele R. Note that the Y does not contain either R or r.
   • For their children the mother will produce two types of eggs in equal amount. Half will contain the allele R and lead to sons or daughters with normal color vision. The other half will contain the allele r and lead to heterozygous daughters (because the father's X carries the allele R) and to hemizygous rY sons if the r-bearing X in the egg is fertilized by a Y-bearing sperm.
   • If we add up the possibilities we get, from the P1 RY x Rr parents, four possible zygotes which are Rr and RR females and RY and rY males. Thus we get a modified mendelian ratio of 3 color normal children and one color blind individual who happens to be a male. Note that the 3 color normal children have a sex ratio of two females and one male. This is typical X-linked inheritance. We can express this another way: the children from such a cross yield all normal daughters but half the sons will have the mutant condition.
   • Among better known and studied X-linked traits are hemophilia (factor VIII deficiency for clotting blood), Duchenne muscular dystrophy, X-linked mental retardation, Hunter syndrome (a disorder of the "glue" that holds connective tissues together resulting in a gargoyle-like appearance of the children), and Fabry syndrome (a disorder of cellular lysosomes that leads to plugged capillaries and blood blisters, strokes, and kidney failure).

4. Can females be homozygous for X-linked traits?
   • If the trait is mild (like color deficiency) a rare marriage might occur between a colorblind male and heterozygous female (the odds of that are 8% x 8% or 0.64 of 1% of all marriages or matings). Of those rare parents, half the sons and half the daughters will be affected. In the US population that comes out to about 1 color blind female in 600 female births.
   • If you're talking about serious conditions like Duchenne muscular dystrophy it is almost impossible because the disease usually kills before reproductive maturity so all fathers are carrying the normal allele on the X. It would be very rare (about 1 in a million) for conditions like hemophilia because only 1 male in 20,000 might have that disease and reach reproductive maturity and find a spouse (who then has only a 1 in 20,000 chance of being heterozygous unless she is a relative).
   • Note that the Y chromosome makes elimination of X-linked mutations much more rapid than for autosomes where they can repose in heterozygous form for dozens of generations. X-linked mutations usually are of recent origin unless they involve mild traits such as red-green color deficiency.

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