Law of Independent Assortment

Law of Independent Assortment

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Have you ever wondered why some siblings in your family have the same hair color but different eye colors? Well, it's all thanks to the law of independent assortment in genetics. This is the third and final law in Mendelian genetics, which explains that different traits on different genes don't affect each other's ability to be inherited or expressed. In simpler terms, all combinations of alleles at different locations in our DNA are equally likely. This law was first discovered by Mendel using garden peas and is one of the reasons why genetic variety exists among siblings. In this article, we'll delve deeper into the law of independent assortment, including its definition, some real-life examples, and how it is different from the law of segregation.

The law of independent assortment states that...

The law of independent assortment in genetics means that the inheritance of one gene does not affect the inheritance of another gene. This means that alleles, or different versions of genes, are inherited independently of each other. For example, inheriting a particular allele for eye color does not affect the ability to inherit any other allele for hair color. In simpler terms, traits are passed down randomly and independently from each other. This law was discovered by Mendel using garden peas and is a fundamental concept in genetics.

Definitions for understanding the law of independent assortment in biology:

means that the inheritance of one gene does not affect the inheritance of another gene. This is because genes are located on different loci or positions on the chromosome, and each allele at a locus is inherited randomly and independently of the alleles at other loci.

To understand this concept, we need to zoom out and look at the chromosome, which is made up of homologous chromosomes. These chromosomes contain the same genes, but each homologous chromosome can have different alleles. When gametes are formed during meiosis, homologous chromosomes randomly mix and match, breaking off and reassembling in a process called recombination. This ensures that alleles are separated into different gametes.

During this process, the law of independent assortment states that no allele affects the likelihood of another allele being packaged in the same gamete. In other words, the inheritance of one gene does not affect the inheritance of another gene, and different combinations of alleles are equally likely to be passed on to the offspring. This is why siblings can have different combinations of traits, such as eye color and hair color, even though they come from the same parents.

Summarize the law of independent assortment

The genotypes of the parental generation 1 (P1) and parental generation 2 (P2) of the dihybrid cross performed by Mendel are as follows:

P1: YYRR (homozygous dominant for color and shape)

P2: yyrr (homozygous recessive for color and shape)

The F1 generation of this dihybrid cross will have a genotype of YyRr (heterozygous for both color and shape). This is because the gametes produced by P1 and P2 will only have one color allele and one shape allele each, and the combination of these gametes will result in the F1 generation having a genotype of YyRr.

Dihybrid cross: F1 x F1 - an example of the law of independent assortment

Here's where it gets interesting. Mendel took two F1 plants and crossed them to each other. This is called a dihybrid cross, when two dihybrids for identical genes are crossed together.

Mendel saw that the P1 x P2 cross had only led to one phenotype, a yellow round pea (F1), but he had the hypothesis that this F1 x F1 cross would lead to four distinct phenotypes! And if this hypothesis held true, it would support his law of independent assortment. Let's see how.

F1 x F1 = YyRr x YyRr

There are four possible gametes from F1 parents, considering one allele for color and one allele for shape must be present per gamete:

YR, Yr, yR, yr.

We can make from these a massive Punnett square. Because we're examining two different genes, the Punnett square has 16 boxes, instead of the normal 4. We can see the possible genotypic outcome from each cross (Fig. 2).

Dihybrid cross for pea color and shape
Dihybrid cross for pea color and shape

The Punnett square shows us the genotype, and thus the phenotype. Just as Mendel suspected, there were four different phenotypes: 9 yellow and round, 3 green and round, 3 yellow and wrinkled, and 1 green and wrinkled.

The ratio of these phenotypes is 9:3:3:1, which is a classic ratio for a dihybrid cross. 9/16 with dominant phenotype for traits A and B, 3/16 with dominant for trait A and recessive for trait B, 3/16 recessive for trait A and dominant for trait B, and 1/16 recessive for both traits. The genotypes we see from the Punnett square, and the ratio of phenotypes they lead to, are both indicative of Mendel's law of independent assortment, and here's how.

If every trait assorts independently to find the probability of a dihybrid phenotype, we should simply be able to multiple the probabilities of two phenotypes of different traits. To simplify this, let's use an example: The probability of a round, green pea should be the probability of a green pea X the probability of a round pea.

To determine the probability of obtaining a green pea, we can do an imaginary monohybrid cross (Fig. 3): Cross two homozygotes for different colors to see the color and proportion of colors in their offspring, first with P1 x P2 = F1:

YY x yy = Yy.

Then, we can follow this up with an F1 x F1 cross, to see the outcome of the F2 generation:

Monohybrid cross outcomes
Monohybrid cross outcomes

Yy and yY are the same, so we get the following proportions: 1/4 YY, 2/4 Yy (which = 1/2 Yy) and 1/4 yy. This is the monohybrid genotypic cross ratio: 1:2:1

To have a yellow phenotype, we can have the YY genotype OR the Yy genotype. Thus, the probability of yellow phenotype is Pr (YY) + Pr (Yy). This is the sum rule in genetics; whenever you see the word OR, combine these probabilities by addition.

Pr (YY) + Pr (Yy) = 1/4 + 2/4 = 3/4. Probability of a yellow pea is 3/4, and probability of obtaining the only other color, green is 1/4 (1 - 3/4) (Fig. 4).

Monohybrid crosses for pea shape and color
Monohybrid crosses for pea shape and color

To summarize, the law of independent assortment states that alleles of different genes segregate independently during gamete formation. This means that the inheritance of one gene does not affect the inheritance of another gene. The product rule is used to calculate the probability of two or more independent events occurring together.

The law of segregation, on the other hand, explains how alleles of the same gene segregate during gamete formation. It states that each parent contributes one allele to their offspring, and the two alleles segregate during gamete formation.

However, gene linkage is an exception to the law of independent assortment. It occurs when two genes are located close to each other on the same chromosome and tend to be inherited together more often than expected by chance. This is because recombination is less likely to occur between closely located genes during gamete formation.

Law of Independent Assortment - Key takeaways The law of independent assortment explains that alleles assort independently into gametes and are not impacted by other alleles of other genes. During gametogenesis, the law of independent assortment is on display A dihybrid cross can be done to exemplify the law of independent assortment The monohybrid genotypic ratio is 1:2:1 while the dihybrid phenotypic ratio is 9:3:3:1Gene linkage limits recombination of certain alleles, and thus creates potential for exceptions to Mendel's law of independent assortment.

Law of Independent Assortment

what is the law of independent assortment?

this is the 3rd law of mendelian inheritance

what does mendel's law of independent assortment state?

The law of independent assortment states that alleles of different genes are inherited independently of one another. Inheriting a particular allele for one gene doesn't affect the ability to inherit any other allele for another gene.

how does the law of independent assortment relate to meiosis?

during meiosis; breakage, crossing over and recombination of alleles on different chromosomes occur. This is culminated in gametogenesis, which allows for the independent segregation and assortment of alleles on different chromosomes.

Does independent assortment occur in anaphase 1 or 2

It occurs in anaphase one and allows for a new and unique set of chromosomes following meiosis.

What is the law of Independent Assortment and why is it important? 

The law of independent assortment is the third law of mendelian genetics, and it is important because it explains that the allele on one gene impacts that gene, without influencing your ability to inherit any other allele on a different gene.

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