Sex linkage is the phenotypic expression of an allele related to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of inheritance. Since humans have many more genes on the X than the Y, there are many more X-linked traits than Y-linked traits.
X-linked dominant inheritance
A male or female child of a mother affected with an X-Linked dominant trait has a 50% chance of inheriting the mutation and thus being affected with the disorder. All female children of an affected father will be affected (daughters possess their fathers’ X-chromosome). No male children of an affected father will be affected (sons do not inherit their fathers’ X-chromosome).
- Alport’s syndrome
- Aarsog’s syndrome
- Coffin-Lowry syndrome (CLS)
- idiopathic hypoparathyroidism
- incontinentia pigmenti
- Ornithine carbamoyltransferase deficiency
- Rett syndrome (RS)
- vitamin D resistant rickets
- fragile X syndrome
X-linked recessive inheritance
Females possessing one X-linked recessive mutation are considered carriers and will generally not manifest clinical symptoms of the disorder. All males possessing an X-linked recessive mutation will be affected. (Males have a single X-chromosome and therefore have only one copy of X-linked genes). All offspring of a carrier female have a 1 in 2 chance of inheriting the mutation. All female children of an affected father will be carriers. (Daughters possess their father’s X-chromosome). No male children of an affected father will be affected. (Sons do not inherit their father’s X-chromosome).
- Lesch-Nyhan syndrome
- Duchenne muscular dystrophy
- Hunter syndrome
- Menkes disease (kinky hair syndrome)
- Glucose-6-phosphate dehydrogenase deficiency
- Hemophilia A and B
- Fabry’s disease
- Wiskott-Aldrich syndrome
- Bruton’s agammaglobulinemia
- Color blindness
- Complete androgen insensitivity syndrome
- Congenital aqueductal stenosis (hydrocephalus)
- Inherited nephrogenic diabetes insipidus
- Various failures in the SRY genes
Sex-linked traits in other animals
- Fur color in domestic cats: the gene that causes orange pigment is on the X chromosome; thus a Calico or tortoiseshell cat, with both black (or gray) and orange pigment, is nearly always female.
- White eyes in Drosophila melanogaster flies—the first sex-linked gene ever discovered
If there are 4 or more possible phenotypes for a particular trait, then more than 2 alleles for that trait must exist in the population. We call this “MULTIPLE ALLELES”.
Let me stress something. There may be multiple alleles within the population, but individuals have only two of those alleles.
Because individuals have only two biological parents. We inherit half of our genes (alleles) from ma, & the other half from pa, so we end up with two alleles for every trait in our phenotype.
An excellent example of multiple allele inheritance is human blood type. Blood type exists as four possible phenotypes: A, B, AB, & O.
There are 3 alleles for the gene that determines blood type. (Remember: You have just 2 of the 3 in your genotype — 1 from mom & 1 from dad).
The alleles are as follows:
- IA: Type “A” Blood
- IB: Type “B” Blood
- i: Type “0” Blood
Notice that, according to the symbols used in the list above, that the allele for “O” (i) is recessive to the alleles for “A” & “B”. With three alleles we have a higher number of possible combinations in creating a genotype.
A type blood
B type blood
AB type blood
0 type blood
- As you can count, there are 6 different genotypes & 4 different phenotypes for blood type.
- Note that there are two genotypes for both “A” & “B” blood — either homozygous (IAIAor IBIB) or heterozygous with one recessive allele for “O” (IAi or IBi).
- Note too that the only genotype for “O” blood is homozygous recessive (ii).
- And lastly, what’s the deal with “AB” blood? What is this an example of? The “A” trait & the “B” trait appear together in the phenotype.
The term “polygenic inheritance” is used to refer to the inheritance of quantitative traits, traits which are influenced by multiple genes, not just one. In addition to involving multiple genes, polygenic inheritance also looks at the role of environment in someone’s development.
Because many traits are spread out across a continuum, rather than being divided into black and white differences, polygenic inheritance helps to explain the way in which these traits are inherited and focused. A related concept is pleiotropy, an instance where one gene influences multiple traits.
One easily understood example of polygenic inheritance is height. People are not just short or tall; they have a variety of heights which run along a spectrum. Furthermore, height is also influenced by environment; someone born with tall genes could become short due to malnutrition or illness, for example, while someone born with short genes could become tall through genetic therapy. Basic genetics obviously wouldn’t be enough to explain the wide diversity of human heights, but polygenic inheritance shows how multiple genes in combination with a person’s environment can influence someone’s phenotype, or physical appearance.
Skin color is another example of polygenic inheritance, as are many congenital diseases. Because polygenic inheritance is so complex, it can be a very absorbing and frustrating field of study. Researchers may struggle to identify all of the genes which play a role in a particular phenotype, and to identify places where such genes can go wrong. However, once researchers do learn more about the circumstances which lead to the expression of particular traits, it can be a very rewarding experience.