Those who are sicklers or carriers of the sickling trait know the importance of seeking genetic counselling before making a decision about who to settle down with in a marriage. Carriers of the sickling traits know that marrying a fellow carrier makes them liable to produce offspring that will be sicklers. Ordinarily, the chance is 1/4 or 25%. However, being 1/4 does not mean that for every 4 offspring produced, 1 will be a sickler. Each fertilized egg has a 25% percent chance of turning out to be a baby with the sickling trait. Hence, it is possible for consecutive children of a carrier couple to be phenotypically normal just as it is possible for them to be sicklers.
In my last post, I discussed one of the laws guiding the inheritance of characters. Some of the key information in the post that will be useful in understanding my today's post are the following:
- Each heritable trait or character is made up of 2 alternate forms known as alleles.
- Alleles of the same gene could be homozygous dominant or recessive when they are similar. When they are dissimilar (heterozygous), one could be dominant and the other recessive, both could be equally dominant (codominance) or one could exhibit incomplete dominance over the other.
Now, let's look at some of the different forms of inheritance of traits that exist in nature.
Autosomal recessive inheritance
In the case of the sickling trait above, the non-sickling alternate version of the gene is dominant over the sickling allele. Hence, heterozygous individuals appear normal while carrying the sickling trait in recessive form. When they reproduce with their fellow carriers, the recessive allele in each parent can merge together to make a homozygous recessive, sickling progeny. This form of inheritance is referred to as autosomal recessive inheritance.
Generally, autosomal traits are traits whose genes are located on the non-sex-determining chromosomes in the human body. Out of the 23 pairs of chromosomes in the body of a normal human, a pair determines the gender of an individual and is referred to as sex chromosomes while the remaining 22 pairs are called autosomes.
An individual that phenotypically carries an autosomal recessive trait genotypically carries the trait in homozygous recessive form. The two alleles representing the gene are both in recessive forms. This is why most autosomal recessive inheritance tends to skip generations only to resurface sometimes into the future. This is why it is pertinent to seek genetic counseling if one observes that there exists a history of a disorder in one's family before settling down maritally.
Autosomal dominant inheritance
This form of inheritance is similar to autosomal recessive inheritance in that the genes are located on autosomes. However, unlike autosomal recessive inheritance, only one dominant allele is needed for the trait to physically manifest itself in an individual.
For autosomal dominant traits, once either of the parents is heterozygous for the, there is a 50% chance that their progeny will have the trait. The situation of being a carrier does not exist. If a parent is homozygous for the trait, all the progeny will become affected by the trait, culminating in a 100% chance. The same thing happens if the two parents are either heterozygous or homozygous for the trait.
The implication of having an autosomal dominant disorder is that the person is at a 50% risk of producing an affected child with a normal partner while two affected partners are 100% sure to produce affected children all through.
X-linked traits are also referred to as sex-limited traits or inheritance. The genes for these traits are located on the X chromosome of the sex chromosomes only. Recall that the sex chromosome of females is XX while that of males is XY. The implication of this is that males will only need one affected allele in order to be affected phenotypically for a trait irrespective of whether the trait is a dominant or a recessive one.
There are two types of X-linked inheritance:
- Dominant X-linked inheritance: In this case, only one dominant allele is needed for a trait to manifest itself irrespective of the other allele. Both males and females have an equal chance of being affected by this kind of inheritance.
Human pedigree illustrating dominant X-linked inheritance By Madibc68 - Own work, CC BY-SA 4.0
- Recessive X-linked inheritance: In this case, two recessive alleles are needed for the trait to manifest itself in females. Because the trait can only be on the X-chromosome of the sex chromosomes and males have only got one X chromosome, only one recessive allele is needed for such trait to manifest itself in males. If the trait is a disorder, it means male individuals are at a higher risk of being affected than female individuals. Females can be a carrier while the males do not have the luxury.
Human pedigree illustrating a recessive X-linked inheritance By Simon Caulton - Own work, CC BY-SA 3.0
Tl:dr? Here is a quick summary
- Autosomal traits are located on autosomes while X-linked traits are located on sex chromosomes.
- Autosomal recessive traits require two recessive alleles and as such, often skips generations. Autosomal dominant traits require at least, one dominant allele and they hardly skip any generation. Both males and females have an equal chance of being affected by autosomal inheritance.
- X-linked dominant traits require only one dominant allele in order to manifest themselves. Both males and females have an equal chance of being affected. X-linked recessive traits require two recessive alleles for females to be affected (due to XX condition) but one recessive allele for males to be affected (due to XY condition). Hence, males are at a much higher risk of being affected for X-linked disorders than females.
Thank you all for reading.