Mendel's Law of Segregation

Mendel's Law of Segregation is part of the trio of laws that make up Mendelian genetics. The first is the Law of Dominance, the second is the Law of Segregation, and the last is the Law of Independent Assortment. The Law of Segregation essentially tells us that gene pairs separate to form individually packaged gametes, and these are the gametes that will fuse to make offspring. Think of Mendel's Law of Segregation as an explanation of two important processes in biology - gametogenesis, and genetics.

Explaining Mendel's Law of Segregation

Mendel's Law of Segregation is the second law of Mendelian inheritance.

Mendel's Law of Segregation

Mendel's Law of Segregation states that when a diploid organism makes its gametes during meiosis, it makes them so that each allele is packaged individually.

Under the principles of this law, each allele pair for a gene is segregated. They are not packaged as a duo into gametes but as a single allele.

This packaging is perfect for future offspring because when two gametes (a maternal gamete and a paternal gamete) fuse, they get one set of alleles from their mom and one set of alleles from their dad. This fusion creates an organism with the standard pair of alleles but with more genetic diversity.

Definitions in Mendel's Law of Segregation

We must understand some terms in reproduction, gametogenesis, and genetics to properly understand Mendel's Law of Segregation.

• What is gametogenesis? It is the process of gamete formation. This process requires meiosis.

• What is a diploid organism? A diploid organism has two sets of chromosomes.
  • What are some examples of diploid organisms? Humans. Almost all mammals and most other animals. Most plants have both diploid and haploid life states that they switch between.
  • What are some examples of exclusively haploid organisms with just one set of chromosomes? Male bees, male ants, and male wasps!

• What is a chromosome? A chromosome is a long strand of DNA that contains all the genes and genetic information of an organism.
  • Thus, because diploid organisms have two sets of chromosomes, they also have two pairs of each gene. Usually, during reproduction and gamete fusion, we get one set of chromosomes from our mother and one from our father.

• What is meiosis? This process is needed in sexual reproduction (in asexual reproduction, only mitosis is needed) to make gametes. The process begins with a somatic, diploid cell. For more details, head to this article on Meiosis!

Example of Mendel's Law of Segregation

Mendel's Law of Segregation can be used to understand and explain many of the phenomena we see in reproduction, genetics, and the inheritance of traits.

Most genetic diseases, thankfully, are inherited in a recessive pattern. That means that the allele that causes the disease has no effect if it is paired next to a normal allele that codes for a normal phenotype. There need to be two copies of a recessive allele and, in the case of a genetic disorder, a mutated allele for that disorder to appear in offspring. Two genes must be "abnormal" to have a diseased phenotype, so most offspring will be normal. But how many will be abnormal, how can we determine this, and how is Mendel's Law of Segregation involved?

Let's use the disorder hemochromatosis to study this. Hemochromatosis is a disorder of iron storage where too much iron is stored in places like the pancreas, the skin, the liver, the heart, and the joints (Fig. 1).

This excessive iron storage causes a syndrome called "Bronzed Diabetes", which is what it sounds like - a person has diabetes and has very tan, bronze-colored skin. They also have a really big heart (cardiomegaly), a big liver (hepatomegaly), painful joints, and sometimes neurological or brain problems.

Hemochromatosis has an autosomal recessive pattern of inheritance. Autosomal chromosomes are the non-sex chromosomes. In humans, the sex chromosomes are X and Y (Fig. 2).

Figure 2: The X and Y chromosomes of mammals, literally look like an X and a Y. Genetic Literacy Project.

Let's take a family where two parents are carriers of the hemochromatosis gene. Hemochromatosis is an autosomal recessive trait that follows all the principles of Mendelian inheritance. If the normal allele is R, and the hemochromatosis allele is r, for this couple where both mother and father have one copy of each (genotype: Rr), let's examine how Mendel's Laws affect genotypes, phenotypes, and inheritance:

1. The Law of Dominance- one allele is completely dominant over the other. The normal allele is dominant over the hemochromatosis allele.
   1. Both the mom and the dad are silent carriers, but neither have the actual hemochromatosis disease. They both have normal, healthy phenotypes.
2. The Law of Segregation- when both parents make gametes for reproduction, they package their hemochromatosis and normal alleles individually and equally. Hence, their offspring have an equal chance of getting either allele.
   1. The mom and the dad will produce an equal number of gametes with the normal allele and gametes with the hemochromatosis allele.
3. The Law of Independent Assortment - inheriting the hemochromatosis allele or the normal allele won't affect their offspring's ability to inherit other alleles at different genes.
   1. So, if the mom also has the allele for dimples, inheriting the normal allele or the hemochromatosis allele won't make the offspring's ability to inherit the dimple allele more or less likely.

Knowing all three of these principles, let's perform a Punnett square of this cross to see the possible outcomes for offspring (Fig. 3)

Figure 3: Rr x Rr cross. Brainly.

We can see that 50% of offspring will have Rr genotype like both their parents, 25% will have RR genotype, and the last 25% will have rr genotype (Fig. 4).

Figure 4: Rr Carrier Mother x Rr Carrier Father and Offspring. BioLibreTexts.

What about phenotype? We know that this hemochromatosis is an autosomal recessive trait, so only those with the rr genotype will have the disorder. So 25% or 1/4th of offspring of this kind of cross will have hemochromatosis. The other 75% or 3/4 of offspring will be completely normal and healthy.

We can see how the Law of Segregation determines that the mother and father's Rr gene pairs split into individual gametes (seen on the Punnett square) with R and r alleles packaged individually. An equal number of R and r alleles was available to and inherited by descendants: within the squares, we can count four capital R's and four lower case r's. Only 25% of offspring have hemochromatosis disease because of Mendel's other law, the Law of Dominance.

Errors In, and Exceptions To, Mendel's Law of Segregation

Sometimes, alleles do not segregate properly, and they are not packaged individually into gametes. Most often, in humans, this leads to genetic disorders. When it happens at the level of a whole chromosome, this leads to chromosomal disorders.

Often, these errors in gene or chromosome segregation happen during meiosis. They can lead to cells with an improper ploidy or number of chromosomes. Let's examine some instances of this.

Cancer Cells:

Some of the ways that cells become cancerous are from developing genetic and chromosomal changes. One of the changes that can occur is the loss of a whole chromosome. Sometimes, cancer cells can even gain a whole chromosome (or two!). These abnormal ploidies can cause them to live longer, multiply abnormally, or metabolize nutrients abnormally, contributing to their carcinogenic potential.

Down Syndrome:

Down syndrome is a genetic condition due to aneuploidy at chromosome number 21. Instead of having a pair of chromosome 21's, individuals with Down Syndrome have three chromosome 21's. Hence, Down Syndrome is also called Trisomy 21 (Fig. 5).

Figure 5: Down Syndrome Karyotype. Futura Sciences.

Polyploid Plants:

Many cultivated plants are polyploid. We bred them to be this way; this is not a diseased state for them. Polyploid plants often have bigger, more bountiful yields (Fig. 6). Some examples of this include species of wheat, peanuts, strawberries, and coffee!

Fig. 6: Polyploid Strawberry. Texas Gateway.

Double Y Males:

We all know the genotype for the female sex is XX, and that of the male sex is XY. But the term double Y males refers to people with the genotype XYY. This genotype is often due to errors in meiosis and leads to this state of aneuploidy. The symptoms of this disorder are not usually drastic, but often these individuals are tall!