Patterns of Inheritance

Patterns of inheritance refer to the way certain traits are passed down genetically from parents to offspring. Mendelian patterns of inheritance typically follow four basic patterns and involve only one gene.

First, let’s review the basics of genetics. Each gamete, also known as a sex or germ cell, such as sperm or ova, contains 23 chromosomes, which carry genetic information. When a sperm fertilizes an ovum, these chromosomes combine to form a zygote, or fertilized ovum, that now has 46 chromosomes, arranged into 23 pairs. These pairs of chromosomes are collectively called the genome.

Now, the first 22 pairs of chromosomes are autosomes, whereas the 23rd pair are sex chromosomes. Sex chromosomes typically include an X chromosome from the mother and either an X or Y chromosome from the father, resulting in either a female offspring with XX chromosomes or a male offspring with XY chromosomes.

Every chromosome contains multiple genes, which are regions of DNA that carry information for a specific trait. Different versions of a gene are called alleles. Alleles provide information for a phenotype, or the observable traits of an individual, such as eye color or height. Alleles can be dominant or recessive, with recessive alleles being masked by dominant alleles. Additionally, if an individual inherits two of the same alleles, they’re homozygous for that trait; whereas if the alleles are different, they’re heterozygous for that trait.

For example, the allele for brown eyes is dominant and the allele for blue eyes is recessive. So, if an individual has one allele for brown eyes and one for blue eyes, the dominant allele masks the recessive allele, resulting in an individual with brown eyes, and they’re considered heterozygous for that trait. On the other hand, if an individual has two alleles for blue eyes, they’ll have blue eyes and be considered homozygous for that trait.

Alright, let’s take a closer look at Mendelian patterns of inheritance. They’re classified based on whether the affected gene is located on an autosome or sex chromosome and whether the trait is recessive or dominant. So, Mendelian patterns of inheritance include autosomal recessive and autosomal dominant; and when sex chromosomes are involved, they can be X-linked recessive or X-linked dominant.

Each of these patterns can be visualized using a Punnett square, which is a diagram that helps predict the probability of an inherited genotype. First, imagine a box with four squares. Using uppercase letters to represent dominant alleles and lowercase letters to represent recessive alleles, we can put the genotype of one parent horizontally across the top and the genotype of the other parent down one side vertically. Then, we can fill out the Punnett square by taking one letter from the top and one from the side and filling in the square.

Okay, so let’s take a look at autosomal recessive disorders which require two copies of a mutated allele, one from each parent, to cause disease, making the affected offspring homozygous for that trait. On the other hand, if just one mutated allele is present, the offspring is heterozygous for that trait and is considered a carrier, meaning they typically don’t show signs of disease. Examples of autosomal recessive disorders include sickle cell anemia, cystic fibrosis, and phenylketonuria.

Using sickle cell anemia as an example, let’s draw a Punnett square to show the possible genotypes of offspring from two carrier parents. Now, in autosomal recessive disorders, the normal allele is dominant, so we’ll represent it with an uppercase “S.” The mutated allele is recessive, so it’s represented with a lowercase “s.” Then, we’ll put the alleles of one parent on the horizontal row and the alleles of the other parent on the vertical column, and complete the Punnett square. When we’re finished, we see the possible genotypes for the offspring. Because 1 of the 4 boxes have two copies of the recessive, mutated allele, there’s a 25% chance that an offspring will inherit sickle cell anemia. Likewise, because 2 of the 4 boxes have a dominant and recessive allele, there’s a 50% chance an offspring will be a carrier. Finally, because 1 out of 4 boxes have two dominant alleles, there’s a 25% chance an offspring will be an unaffected, non-carrier.