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, as sex or germ cells 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 1st 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.

Each 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.

All right, 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 punnet square, which is a diagram that helps predict the probability of an inherited genotype.

First imagine a box with four squares using upper case letters to represent dominant alleles and lower case letters to represent recessive ones.

We can put the genotype of the parent horizontally across the top and the genotype of the other parent down one side vertically, then we can fill out the punnet square by taking one letter from the top and one from the side and filling in the square.

Ok. 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 fetal ketonuria. Using sickle cell anemia.

As an example, let's draw a punnet 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 upper case s the mutated allele is recessive. So it's represented with a lower case 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 punnet square.

When we're finished, we see the possible genotypes for the offspring because one of the four boxes have two copies of the recessive mutated allele.

There is a 25% chance that an offspring will inherit sickle cell anemia. Likewise, because two of the four boxes have a dominant and recessive allele, there's a 50% chance an offspring will be a carrier.

Finally, because one out of four boxes have two dominant alleles. There's a 25% chance an offspring will be an unaffected non carrier.

Next, autosomal dominant disorders require at least one copy of a mutated allele to cause disease. So the affected individual could be either heterozygous or homozygous.

For that gene, examples include familial hypercholesterolaemia, huntington disease, and adult polycystic kidney disease using familial hypercholesterolaemia.

As an example, let's draw a punnet square to show the possible genotypes for offspring from one affected heterozygous parent and one unaffected parent.

So in this case, we'll set up the punnet square with an upper case f for the dominant mutated allele and a lower case f for the normal recessive allele like before we'll put the alleles of one parent on the horizontal row and the alleles of the other parent on the vertical column.

When we're finished with the punit square, we see there's a 50% chance of an offspring with familial hypercholesterolemia because two out of four boxes have at least one dominant mutated allele.

Likewise, there's a 50% chance an offspring won't inherit familial hypercholesterolaemia because two of the four boxes have two normal recessive alleles.

Ok. Now X linked recessive disorders occur when there's a mutation in a gene on the X chromosome and requires two copies of the mutation to cause disease in females but only one copy to cause disease for males.

This is because with the male xy chromosome, there's only one X chromosome. So any mutation on the X chromosome is sufficient to cause disease.

On the other hand, with female XX chromosomes, both chromosomes need to have the mutation to cause disease. Examples of excellent recessive disorders include color blindness, Duchenne, muscular dystrophy and hemophilia.

A. Using color blindness as an example, let's draw a punnet square to show the possible genotypes for offspring from a carrier mother and an affected father in X linked recessive disorders.

The normal allele is dominant. So we'll use a subscripted uppercase B next to an X.

Since the allele is located on the X chromosome, the mutated allele is recessive. So we'll use a subscripted lowercase B next to the X note that the Y chromosome has no allele markings.

Since the mutation is only located on the X chromosome, like before we'll put the alleles of one parent on the horizontal row and the alleles of the other parent on the vertical column.

When we're finished with the punnet square, we see there's a 50% chance that a female offspring will be a carrier because one out of two boxes with XX chromosomes have a mutated allele and a 50% chance a female offspring will have color blindness because one out of two boxes have XX chromosomes and two mutated alleles.

Likewise, there's a 50% chance a male offspring will have color blindness and a 50% chance a male offspring will be unaffected.

Lastly, X linked, dominant disorders can also occur when there's a mutation in a gene on the X chromosome, but require just one mutated allele to cause disease regardless of sex.

An example of an X linked dominant disorder is Fragile X syndrome. So using Fragile X Syndrome as an example, let's draw a punnett square to show the possible genotypes for offspring from an unaffected mother and an affected father in X linked dominant disorders, the mutated allele is dominant.

So we'll use a subscripted uppercase letter F next to an X. Since the mutation is located within the X chromosome, the normal allele is recessive.

So we'll use a subscripted, lower case letter F next to the X. Like before we'll put the alleles of one parent on the horizontal row and the alleles of the other parent on the vertical column.

When we're finished with the punnet square, we'll see that there is a 100% chance all female offspring will have the disease and there's a 100% chance that all male offspring will be unaffected.

All right, is a quick recap patterns of inheritance. Refer to the way certain traits are passed down genetically from parents to offspring.

Mendelian patterns are a type of inheritance that typically follow four basic patterns involve only one gene and include autosomal recessive, autosomal dominant, excellent recessive and excellent dominant.

Using the parents genotypes. A punnet square can be used to predict the probability a certain gene will be inherited