Understanding Mendel’s Laws and Crossing Examples!

Understanding Mendel’s Law – Parents who have straight hair generally have offspring with straight hair too. The cause: inherited traits or genes. The laws of inheritance are usually considered using Mendel’s Laws. Heredity or the inheritance of traits is a branch of science that examines genetics. Mendel’s law was first discovered by a monk from Austria, Gregor Johann Mendel.

According to the Encyclopedia Britannica, Mendel first conducted crossbreeding experiments on peas in 1854-1856. He chose peanuts as his research media because they have varieties. Inheritance of traits is a process of inheriting traits from parents to their offspring. Chromosomes and genes play a role in determining the inheritance of traits which are then studied in Mendel’s Laws I & II.

Mendel’s law just started to be discovered in 1966 by a scientist from Austria using the same name, namely, GJ. Mendel. In fact, since thousands of years ago, humans have inherited traits according to the parent in their offspring. But only Mendel was someone who succeeded in conducting research on inheritance. Mendel then began his research through crosses in peas.

Now, from the brief explanation above regarding Mendel’s law, it is clear that in the following discussion we will still provide related information regarding the meaning of Mendel’s law and how to apply it properly.

You can see a further discussion of Mendel’s law below!

GJ Mendel Short Biography

Before discussing further about the meaning of Mendel’s laws, it’s a good idea to also listen to the following biography of the originator of the theory:

Gregor Johann Mendel (Czech: Řehoř Jan Mendel; 20 July 1822 – 6 January 1884) was an Austrian botanist and monk who developed the basic concepts of genetics. Investigation of heredity in genetics was carried out by Mendel using peas.

Mendel discovered that the biological inheritance of certain genes of traits in pea plants followed certain patterns, now claimed to be Mendel’s Laws. The deep meaning of Mendel’s work was not recognized until the turn of the 20th century, when the rediscovery of Mendel’s rules pioneered the modern science of genetics.

Gregor Mendel was born into an ethnic German family in Heinzendorf bei Odrau, Austria Silesia, Austrian Empire (now Hynčice, Czech Republic). He is the son of Anton & Rosine (Schwirtlich) Mendel, & has one older sister (Veronica) & one younger sister (Theresia). They live and work on a farm that has been owned by the Mendel family for at least 130 years.

During his childhood, Mendel worked as a gardener, studied beekeeping, and as a young man attended the gymnasium at Opava. From 1840-1843, he studied simple and theoretical philosophy as well as mathematics at the Olomouc University Faculty of Philosophy, taking a year off due to illness.

Definition of Mendel’s Laws


Mendel’s law of inheritance is a rule regarding the inheritance of traits in organisms that was described by Gregor Johann Mendel in his work on Plant Crossing Experiments. This law consists of 2 parts:

  • Mendel’s law of segregation, also known as Mendel’s First Law, &
  • Mendel’s law of independent assortment, also known as Mendel’s Second Law.

Furthermore, below is a more detailed explanation of Mendel’s laws:

Mendel’s Law I

Mendel’s Law I has another name, namely the Law of Segregation. In the Law of Segregation it states that “In the formation of gametes (sex cells) in the 2 genes that are partners, will be separated in 2 daughter cells’. Well, Mendel’s Law I or the Law of Segregation applies to monohybrid crosses, aka crosses using one different trait.

Broadly speaking, Mendel’s Law I will relate to the existence of three points, namely:

Genes have other forms of regulation of variation in inherited traits. This is what makes the concept of 2 types of alleles, namely a) recessive alleles (not always visible from the outside, expressed using lowercase letters, for example w in the picture); & b) alleles are more dominant (shown on the outside, expressed using a capital alphabet, for example R)

Each individual carries a pair of genes, one based on the male parent (eg ww) & one based on the female parent (eg RR). If this pair of genes is 2 incompatible alleles, the more dominant allele will always be expressed (visible visually from the outside). Recessive alleles that are not always expressed, will always be inherited in the gametes (sex cells) that are made in their offspring.

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Look at the cross model between red roses which are more dominant and white roses which are recessive, here it is

Well, Mendel’s First Law also states that the 2 alleles (gene variants) that govern exclusive traits will separate in 2 gametes (sex cells) that are not aligned. Mendel’s first law includes several things, namely:

  • Alleles (gene variations) for inherited trait variations. Example: the colors of 2 different flowers, named by alleles, will occupy a synchronous locus with a homologous pair.
  • Two alleles for a character will separate when gametes (sex cells) are produced. Example:
  • output of a cross containing one parental color allele (purple or white)
  • Each character in each organism will inherit 2 alleles, each of which originates from the parent. Example:
  • the output of a cross that will likely form 1 allele of white color & 1 allele of purple color.

If there are still 2 alleles that are not aligned, then one error can be more dominant, while the other will be recessive. Example:

there is still a marriage of purple flowers with white flowers, so they will form purple offspring.

Broadly speaking, this law covers three points:

Genes have alternative forms that regulate variations in the inherited characters. This is the concept of two kinds of alleles; recessive allele (not always visible from the outside, expressed by a lowercase letter, for example w in the picture on the right), and dominant allele (visible from the outside, expressed by a capital letter, for example R).

Each individual carries a pair of genes, one from the male parent (eg ww in the image below) and one from the female parent (eg RR in the image below).

If this pair of genes are two different alleles (Sb and sB in figure 2), the dominant allele (S or B) will always be expressed (visually visible from the outside). Recessive alleles (s or b) which are not always expressed, will still be inherited in the gametes formed in their offspring.

The cross consists of 2 namely:

1. Dominant cross


A black (dominant) rabbit is crossed with a white (recessive) rabbit. If the first phenotype (F1) has 100 black hairs, then determine the ratio of the second phenotype (F2)!


HH (black) v hh (white)


hh (black)


Hh (black) v Hh (black)


HH(black), Hh(black), Hh(black), hh(white)

So comparison F2 is black:

white = 3:1.

2. Intermediate crosses (semidominant)


A black (dominant) rabbit is crossed with a white (recessive) rabbit. If the first phenotype (F1) is 100r brown hair then determine the ratio of the second phenotype (F2)!

P1: HH (black) v hh (white)

F1: Hh (brown)

P2: Hh (brown) v Hh (brown)

F2: HH (black), Hh (brown), Hh (brown), hh (white)

Mendel’s Law II

In Mendel II’s Law or what is also known as the Law of Independent Assortment or the Law of Independently Grouping of Genes, states that ‘if 2 individuals are not compatible with each other in 2 pairs of traits or more, then the traits of the pair will be inherited without depending on the nature of the partner. other’. The existence of Mendel II’s Law applies to dihybrid crosses (using 2 incompatible traits).

In a dihybrid cross, for example, there is still an individual using the AaBb genotype, then A & a and B & b will separate and then the pair will join independently. Through this, it is possible that the gametes (sex cells) that are formed will have AB, Ab, aB, & ab properties.

In short, Mendel’s Law II states that alleles (gene variations) with genes that are not in harmony do not affect each other. This also shows that the genes that determine plant height, plant color, will not affect each other.


The Difference Between Mendel’s Laws I & II

The difference between Mendel’s I & II Laws is most clearly seen in the crossed properties. In Mendel’s Law I states that the formation of gametes (sex cells) in the 2 parental genes that pair up with alleles, will separate or segregate. This results in each gamete getting one gene according to its parent.

Meanwhile, Mendel II’s Law states that if there are individuals who are not the same as each other in 2 or more pairs of traits, then they will inherit a pair of traits that are independent of other traits.

In conclusion, in Mendel’s Law I will experience a process of segregation or separation of cells freely. While in Mendel II’s Law will experience independent gene grouping.

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Examples of Crosses in Mendel’s Laws

Mendel’s Law I

1. Monohybrid Cross

At that time, Mendel created an experiment by crossing 2 individuals based on peas that had different characteristics, namely between peas with high stems and peas with low stems. While the nature of ‘high’ is generally dominated by the trait of ‘low’, as a result it will make:

if you look again at the theory in Mendel’s Law I which states that in the formation of gametes (sex cells) the allele pairs will separate freely. Well, the incident of separation will be seen when the formation of individual gametes that have a heterozygous genotype, as a result, each gamete (sex cell) will contain one of the alleles wrong.

2. Backcross & Testcross

Backcross is the process of crossing or mating hybrid output individuals (F1) using one parent error. The goal is to be able to know the genotype based on the parent (parental). Consider the following model using relying on the ‘high’ property in peas.

Meanwhile, testcross is the process of crossing F1 individuals using one homozygous recessive parent. The goal is to find out whether the F1 individual is homozygous or heterozygous.

Mendel’s Law II

1. Dihybrid Cross

Through this Dihybrid crossing experiment, Mendel tried to involve 2 traits at once & concluded that in the process of forming gametes (sex cells), each pair of alleles at one locus will segregate independently using other locus allele pairs, & will combine independently using alleles based on the locus. other.

In short, monohybrid is a hybrid using 1 different trait, while dihybrid is a hybrid using two different traits.

At that time, Mendel used pea plants as the object of his observations, using the reasons:

  • Having a pair of different traits that are striking or contrasting.
  • Carrying out self-pollination (autogamy), as a result of which hereditary traits tend to remain.
  • Practically cross-pollinated.
  • Hurry up to make offspring.
  • Can have offspring in large numbers.

The following are the properties possessed by the pea plant, as a result it was used as an object of observation for this dihybrid cross.

The process of dihybrid crosses has special characteristics in the form of:

  • Crossing is done by paying attention to 2 different traits.
  • The number of gametes (sex cells) formed in each individual is 4 (2n)
  • The individual phenotype will be influenced by two kinds of genetic traits.
  • Approximately 16 genotype variations will be found in F2.

Pseudo Deviation of Mendel’s Laws

In Mendel’s Law both I and II there will still be an apparent deflection, which is a form of crossing by making phenotypic ratios asynchronous on a dihybrid basis. Even though it looks out of sync, in fact the phenotypic ratio was a modified form based on the sum of the phenotypic ratios based on all of Mendel’s Laws.

For example, in a marriage between two individuals using two different traits, it turns out that the F2 phenotype ratio is not always 9 : 3 : 3 : 1. However, it is not uncommon to find comparisons that are not synchronous, but are combinations of Mendelian comparison numbers written 9: 3:3:1, namely:

9 : 7 = 9 : ( 3 + 3 + 1 )

12 : 3 : 1 = ( 9 + 3 ) : 3 : 1

15 : 1 = ( 9 + 3 + 3 ) : 1

9 : 3 : 4 = 9 : 3 : ( 3 + 1 )

If it is based on Mendel’s Law II, then one allele will not mutually influence the segregation of other allele pairs in the determination of asynchronous properties. These genes will be independently paired & give rise to exclusive traits in individuals. Well, that’s what is called the ‘Pseudo Deviation of Mendel’s Law’. It is called “pseudo” because the principle of independent segregation is always valid, and is caused by the genes that carry the traits in determining the specific characteristics. The following are the characteristics of ‘Pseudo Deviations of Mendel’s Laws’:

The phenotypic ratios obtained are not in sync with Mendel’s Law.

The existence of exclusive traits in the genes that result in output disparities in filial two is the relationship between genes.


So a brief discussion of the definition of Mendel’s laws. The discussion this time does not only discuss the definition of Mendel’s laws but also discusses further the biography of the originator of the theory and then explains in detail about Mendel’s laws 1 and 2 and finds examples of pseudo intersections of Mendel’s laws.

Understanding the meaning of Mendel’s law gives us additional knowledge about various birth processes and knowing about the cells that make up our bodies are influenced by our parents’ genes which affect our growth and development as children.