What is a Punnett lattice? How to work with the Pennet lattice Pennet lattice and Mendelian inheritance

Individuals in the F 1 generation are the result of crossing two parental organisms: male and female. Each of them can form a certain number of types of gametes. Each gamete of one organism with the same probability can meet with any gamete of another organism during fertilization. Therefore, the total number of possible zygotes can be calculated by multiplying all types of gametes in both organisms.

monohybrid cross

Example 7.1. Write down the genotype of individuals of the first generation when two individuals are crossed: homozygous for the dominant gene and homozygous for the recessive gene.

Let's write the letter designation of the genotypes of the parental pairs and the gametes they form.

R AA x aa

Gametes A a

In this case, each of the organisms forms gametes of the same type, therefore, when gametes merge, individuals with the Aa genotype will always be formed. Hybrid individuals developed from such gametes will be uniform not only in genotype, but also in phenotype: all individuals will carry a dominant trait (according to Mendel's first law of uniformity of the first generation).

To facilitate the recording of the genotypes of the offspring, it is customary to indicate the meeting of gametes with an arrow or a straight line connecting the gametes of the male and female organisms.

Example 7.2. Determine and write down the genotypes of individuals of the first generation when crossing two heterozygous individuals analyzed for one trait.

R Aa x Aa

Gametes A; a A; a

F 1 AA; Aa aa; aa

Each parent produces two types of gametes. The arrows show that any of the two female gametes can meet with any of the two male gametes. Therefore, four variants of gametes are possible and individuals with the following genotypes are formed in the offspring: AA, Aa, Aa, aa.

Example 7.3. Hair can be light or dark. The dark color gene is dominant. A heterozygous woman and a homozygous man with dark hair entered into marriage. What genotypes should be expected in children of the 1st generation?

trait: gene

dark color: A

light color: a

R Aa x AA

dark dark

Gametes A; a A

dark dark

Dihybrid cross

The number and types of zygotes in a dihybrid cross depend on how the non-allelic genes are located.

If non-allelic genes responsible for different traits are located in the same pair of homologous chromosomes, then the number of gamete types in a diheterozygous organism with the Aa Bb genotype will be equal to two: AB and av. When two such organisms are crossed, fertilization will result in the formation of four zygotes. Recording the results of such a cross will look like this:

R AVav x Avav

Gametes AB; av AB; av

F 1 ABAB; ABav; ABav; awav

Diheterozygous organisms containing non-allelic genes in non-homologous chromosomes have the AaBv genotype and form four types of gametes.

When two such individuals are crossed, combinations of their gametes will give 4x4 = 16 genotype variants. The genotype of the resulting individuals can be recorded sequentially one after another, as we did with monohybrid crossing. However, such a line-by-line record will be too cumbersome and difficult for further analysis. The English geneticist Pennet proposed to record the result of crossing in the form of a table, which is named after the scientist - the Punnet lattice.

First, the genotypes of the parent pairs and their gamete types are recorded as usual, then a grid is drawn in which the number of vertical and horizontal columns corresponds to the number of gamete types of the parent individuals. The female gametes are written horizontally at the top, and the male gametes are written vertically to the left. At the intersection of imaginary vertical and horizontal lines coming from the gametes of the parents, the genotypes of the offspring are recorded.

Reginald Pannett (1875-1967) as a tool that is a graphical notation for determining the compatibility of alleles from parental genotypes. Along one side of the square are female gametes, along the other - male. This makes it easier and more visual to represent the genotypes obtained by crossing parental gametes.

monohybrid cross

In this example, both organisms have the Bb genotype. They can produce gametes containing either the B or b allele (the former meaning dominance, the latter recessive). The probability of a descendant with the BB genotype is 25%, Bb - 50%, bb - 25%.

		maternal
		B	b
paternal	B	BB	bb
	b	bb	bb

Phenotypes are obtained in a combination of 3:1. A classic example is the color of a rat's coat: for example, B is black wool, b is white. In such a case, 75% of the offspring will have black coats (BB or Bb), while only 25% will have white coats (bb).

Dihybrid cross

The following example illustrates a dihybrid cross between heterozygous pea plants. A represents the dominant allele for shape (round peas), a the recessive allele (wrinkled peas). B represents the dominant allele for color (yellow peas), b represents the recessive allele (green). If each plant has the AaBb genotype, then, since the alleles for shape and color are independent, there can be four types of gametes in all possible combinations: AB, Ab, aB and ab.

	AB	Ab	aB	ab
AB	AABB	AABb	AaBB	AaBb
Ab	AABb	AAbb	AaBb	Aabb
aB	AaBB	AaBb	aaBB	aaBb
ab	AaBb	Aabb	aaBb	aabb

It turns out 9 round yellow peas, 3 round green, 3 wrinkled yellow, 1 wrinkled green pea. Phenotypes in a dihybrid cross are combined in a ratio of 9:3:3:1.

The Punnett lattice helps to establish how a particular gene can be transmitted during sexual reproduction of two living organisms. The completed Punnett lattice contains all possible inheritance variants of a particular gene and allows you to calculate the probability of each option. Building a Punnett lattice will help you better understand the basic concepts of genetics.

Steps

Part 1

Construction of the Punnett lattice

Draw a 2 x 2 table. Draw a square and divide it into four equal squares. Leave free space above and to the left of the square - you will need it for further notes.

Label the alleles in question. Each cell of the Punnett lattice describes a specific gene variant (combination of alleles) that can be obtained in a descendant during sexual reproduction of two organisms. Choose letters to represent alleles. Use an uppercase letter for the dominant allele and a lowercase letter for the recessive allele. Any letter can be used.

• For example, let's denote the dominant allele that causes black coat color with the Latin letter "F", and the recessive allele for yellow color with the letter "f".
• If you don't know which gene is dominant, use different letters for the two alleles.

1. Check the genotypes of the parents. Now you should find out the genotype of each parent for the trait you are interested in. For this or that trait, each parent, like all sexually reproducing organisms, contains two alleles (sometimes they are the same), so their genotype will consist of two letters. Sometimes the genotype of the parents is known in advance, but in other cases it has to be obtained based on other information:

   Label the rows with the genotype of one of the parents. Choose one parent. Usually this is a female (mother), although you can take a male. Place the first allele near the top line of the table, and the second allele of the selected parent near the bottom line.

   • Suppose a female bear is heterozygous for coat color (Ff). Accordingly, write F to the left of the top line and f to the left of the bottom line.

2. Sign the columns of the table with the genotype of the second parent. Write the second genotype for the same trait over the grid. Typically, the columns are for the genes of the male, that is, the father.

   • Suppose a male bear is homozygous recessive (ff). Write an f above each column.

3. Write in the grid cells the corresponding letters from the rows and columns. The cells of the Punnett lattice are filled simply. Start in the top left cell. Take a look at what letters are to the left of it and above it. Write these letters in a cell. Repeat the same procedure for the other three cells. If both types of alleles are present, then it is customary to write the dominant allele in the first place (that is, Ff, not fF).

   • In our example, the F allele from the mother and the f allele from the father are in the upper left cell, resulting in Ff.
   • The upper right cell inherits F from the mother and f from the father, that is, in this cell we write Ff.
   • The bottom left cell contains fs from both parents, resulting in ff.
   • In the lower right cell, there are f alleles from both parents, we get ff.

4. Interpret your results. The Punnett lattice shows the probability of the offspring inheriting certain alleles. There are four possible combinations of parental alleles, and they are all equally likely. This means that the probability of each combination is 25%. If the same combination occurs in more than one cell, then to find its probability, add the corresponding 25% probabilities.

   • In our example, we got two cells with a combination of Ff (heterozygote). Since 25% + 25% = 50%, each offspring can inherit the combination of Ff alleles with a 50% chance.
   • In the other two cells we have ff (recessive homozygote). Thus, each offspring can inherit the ff genes with a probability of 50%.

5. Describe the phenotype. Often it is not the genes of the offspring that are of interest, but its characteristic features. They are fairly easy to determine in most of the simple cases for which the Punnett lattice is commonly used. To determine the probability that an offspring will have a particular trait, add the probabilities of all cells with one or more dominant alleles that match that trait. To find the probability that an offspring will inherit a recessive trait, add the probabilities of cells with two recessive alleles.

   Part 2

   Basic concepts

   1. Learn about genes, alleles, and traits. A genome is a fragment of the "genetic code" that determines one or another characteristic feature of a living organism, such as eye color. In this case, the eyes can be blue, brown or have a different color. Different variants of the same gene are called alleles.

The Punnett grid is a visual tool that helps geneticists identify possible combinations of genes in fertilization. The Punnett grid is a simple table of 2x2 (or more) cells. With the help of this table and knowledge of the genotypes of both parents, scientists can predict what combinations of genes are possible in offspring, and even determine the likelihood of inheriting certain traits.

Steps

Basic information and definitions

To skip this section and go directly to the description of the Punnett lattice, .

Learn more about the concept of genes. Before you begin to master and use the Punnett grid, you should familiarize yourself with some basic principles and concepts. The first such principle is that all living creatures (from tiny microbes to giant blue whales) have genes. Genes are incredibly complex microscopic instruction sets that are built into virtually every cell in a living organism. In fact, to one degree or another, genes are responsible for every aspect of an organism's life, including how it looks, how it behaves, and much, much more.

Learn more about the concept of sexual reproduction. Most (but not all) living organisms known to you produce offspring through sexual reproduction. This means that the female and male contribute their genes, and their offspring inherit about half of the genes from each parent. The Punnett lattice is used to visualize the various combinations of parental genes.

• Sexual reproduction is not the only way for living organisms to reproduce. Some organisms (for example, many types of bacteria) reproduce themselves by asexual reproduction when offspring are created by one parent. In asexual reproduction, all genes are inherited from one parent, and the offspring is almost an exact copy of it.

1. Learn about the concept of alleles. As noted above, the genes of a living organism are a set of instructions that tell each cell what to do. In fact, just like regular instructions, which are divided into separate chapters, paragraphs, and subparagraphs, different parts of the genes indicate how different things should be done. If two organisms have different "subdivisions", they will look or behave differently - for example, genetic differences can cause one person to have dark hair and another to have light hair. These different types of the same gene are called alleles.

   • Since the child receives two sets of genes - one from each parent - he will have two copies of each allele.

2. Learn about the concept of dominant and recessive alleles. Alleles do not always have the same genetic "strength". Some alleles that are called dominant, necessarily manifest in the appearance of the child and his behavior. Others, the so-called recessive alleles appear only if they do not match with dominant alleles that "suppress" them. The Punnett lattice is often used to determine how likely a child is to receive a dominant or recessive allele.

   Representation of a monohybrid cross (single gene)

   1. Draw a 2x2 square grid. The simplest version of the Punnett lattice is very easy to make. Draw a large enough square and divide it into four equal squares. Thus, you will have a table with two rows and two columns.

      Label the parental alleles in each row and column. In the Punnett lattice, columns are reserved for maternal alleles, and rows are reserved for paternal alleles, or vice versa. In each row and column, write down the letters that represent the mother's and father's alleles. In this case, use capital letters for dominant alleles and lowercase letters for recessive ones.

      • This is easy to understand from an example. Suppose you want to determine the probability that a given couple will have a child who can roll their tongue. You can denote this property in Latin letters R and r- the capital letter corresponds to the dominant allele, and the lower case corresponds to the recessive allele. If both parents are heterozygous (have one copy of each allele), then write one "R" and one "r" above the bars and one "R" and one "r" to the left of the hash.

   2. Write the corresponding letters in each cell. You can easily complete the Punnett grid once you know which alleles will come in from each parent. Write in each cell a two-letter combination of genes that represent alleles from mother and father. In other words, take the letters in the corresponding row and column and write them into the given cell.

      Determine the possible genotypes of the offspring. Each cell of the completed Punnett lattice contains a set of genes that is possible in a child of these parents. Each cell (that is, each set of alleles) has the same probability - in other words, in a 2x2 lattice, each of the four possible choices has a probability of 1/4. The various combinations of alleles represented in the Punnett lattice are called genotypes. Although genotypes represent genetic differences, this does not necessarily mean that each variant will produce different offspring (see below).

      • In our example of the Punnett grid, a given pair of parents could have the following genotypes:
      • Two dominant alleles(cell with two R's)
      • (cell with one R and one r)
      • One dominant and one recessive allele(cell with R and r) - note that this genotype is represented by two cells
      • Two recessive alleles(cell with two r's)

   3. Determine the possible phenotypes of the offspring. Phenotype An organism represents actual physical traits that are based on its genotype. An example of a phenotype is eye color, hair color, sickle cell anemia, and so on - although all these physical traits determined genes, none of them is determined by its particular combination of genes. The possible phenotype of the offspring is determined by the characteristics of the genes. Different genes manifest themselves differently in the phenotype.

      • Suppose in our example that the gene responsible for the ability to fold the tongue is dominant. This means that even those descendants whose genotype includes only one dominant allele will be able to roll their tongues. In this case, the following possible phenotypes are obtained:
      • Top left cell: can fold tongue (two R's)
      • Top right cell:
      • Bottom left cell: can fold tongue (one R)
      • Bottom right cell: cannot fold tongue (no capital R)

   4. Determine the probability of different phenotypes by the number of cells. One of the most common uses of the Punnett grid is to find the probability of a given phenotype occurring in offspring. Since each cell corresponds to a certain genotype and the probability of occurrence of each genotype is the same, it is enough to find the probability of the phenotype divide the number of cells with a given phenotype by the total number of cells.

      • In our example, the Punnett lattice tells us that four kinds of gene combinations are possible for given parents. Three of them correspond to a descendant that is able to roll its tongue, and one corresponds to the absence of such an ability. Thus, the probabilities of the two possible phenotypes are:
      • Descendant can fold tongue: 3/4 = 0,75 = 75%
      • Child cannot fold tongue: 1/4 = 0,25 = 25%

   Representing a dihybrid cross (two genes)

   1. Divide each cell of the 2x2 grid into four more squares. Not all combinations of genes are as simple as the monohybrid (monogenic) cross described above. Some phenotypes are determined by more than one gene. In such cases, all possible combinations should be taken into account, which will require b about lshey table.

      • The basic rule for applying the Punnett lattice when there is more than one gene is as follows: for each additional gene, the number of cells should be doubled. In other words, a 2x2 grid is used for one gene, a 4x4 grid is appropriate for two genes, an 8x8 grid is appropriate for three genes, and so on.
      • To make it easier to understand this principle, consider an example for two genes. To do this, we have to draw a lattice 4x4. The method outlined in this section is also suitable for three or more genes - you just need b about Bigger grid and more work.

   2. Determine the genes of the parents. The next step is to find the genes of the parents that are responsible for the property you are interested in. Since you are dealing with multiple genes, one more letter must be added to the genotype of each parent—in other words, four letters must be used for two genes, six letters for three genes, and so on. As a reminder, it is helpful to write the mother's genotype above the bars and the father's genotype to the left of it (or vice versa).

   3. Write different combinations of genes along the top and left edges of the grid. Now we can write above the grid and to the left of it the various alleles that can be passed on to the offspring from each parent. As with a single gene, each allele is equally likely to be passed on. However, since we are considering multiple genes, each row or column will have multiple letters: two letters for two genes, three letters for three genes, and so on.

      • In our case, we should write out various combinations of genes that each parent is able to pass on from his genotype. If the mother's genotype is SsYy on top, and the father's genotype is SsYY on the left, then for each gene we get the following alleles:
      • Along the top edge: sy, sy, sy, sy
      • Along the left edge: SY, SY, SY, SY

   4. Fill in the boxes with the appropriate combinations of alleles. Write letters in each cell of the grid in the same way as you did for one gene. However, in this case, for each additional gene, two additional letters will appear in the cells: in total, each cell will have four letters for two genes, six letters for four genes, and so on. As a general rule, the number of letters in each cell corresponds to the number of letters in the genotype of one of the parents.

      • In our example, the cells will be filled in as follows:
      • Top row: SSYY, SSYY, SSYY, SSYY
      • Second row: SSYY, SSYY, SSYY, SSYY
      • Third row: SsYY, SsYy, ssYY, ssYy
      • Bottom row: SsYY, SsYy, ssYY, ssYy

   5. Find the phenotypes for each possible offspring. In the case of several genes, each cell in the Punnett lattice also corresponds to a separate genotype of possible offspring, there are simply more of these genotypes than with one gene. And in this case, the phenotypes for a particular cell are determined by which genes we are considering. There is a general rule that the presence of at least one dominant allele is sufficient for the manifestation of dominant traits, while for recessive traits it is necessary that all the corresponding alleles were recessive.

      • Since grain smoothness and yellowness are dominant for peas, in our example, any cell with at least one capital letter S corresponds to a plant with smooth peas, and any cell with at least one capital letter Y corresponds to a plant with a yellow grain phenotype. Plants with wrinkled peas will be represented by cells with two lowercase s alleles, and in order for the grains to be green, only lowercase y is required. Thus, we get possible options for the shape and color of peas:
      • Top row:
      • Second row: smooth/yellow, smooth/yellow, smooth/yellow, smooth/yellow
      • Third row:
      • Bottom row: smooth/yellow, smooth/yellow, wrinkled/yellow, wrinkled/yellow

   6. Determine by cells the probability of each phenotype. To find the probability of different phenotypes in the offspring of given parents, use the same method as in the case of a single gene. In other words, the probability of a particular phenotype is equal to the number of cells corresponding to it divided by the total number of cells.

      • In our example, the probability of each phenotype is:
      • Offspring with smooth and yellow peas: 12/16 = 3/4 = 0,75 = 75%
      • Offspring with wrinkled and yellow peas: 4/16 = 1/4 = 0,25 = 25%
      • Offspring with smooth and green peas: 0/16 = 0%
      • Offspring with wrinkled and green peas: 0/16 = 0%
      • Note that the inability to inherit two recessive y alleles resulted in no plants with green seeds among the possible offspring.
   • Remember that each new parental gene causes the number of cells in the Punnett lattice to double. For example, with one gene from each parent you get a 2x2 grid, for two genes a 4x4 grid, and so on. In the case of five genes, the size of the table will be 32x32!

(1875-1967) as a tool that is a graphical record for determining the compatibility of alleles from parental genotypes. Along one side of the square are female gametes, along the other - male. This makes it easier and more visual to represent the genotypes obtained by crossing parental gametes.

Phenotypes are obtained in a combination of 3:1. A classic example is the color of a rat's coat: for example, B is black wool, b is white. In this case, 75% of the offspring will have black coats (BB or Bb), while only 25% will have white coats (bb).

Dihybrid cross

The following example illustrates a dihybrid cross between heterozygous pea plants. A represents the dominant allele for shape (round peas), a the recessive allele (wrinkled peas). B represents the dominant allele for color (yellow peas), b represents the recessive allele (green). If each plant has the AaBb genotype, then, since the alleles for shape and color are independent, there can be four types of gametes in all possible combinations: AB, Ab, aB and ab.

	AB	Ab	aB	ab
AB	AABB	AABb	AaBB	AaBb
Ab	AABb	AAbb	AaBb	Aabb
aB	AaBB	AaBb	aaBB	aaBb
ab	AaBb	Aabb	aaBb	aabb

It turns out 9 round yellow peas, 3 round green, 3 wrinkled yellow, 1 wrinkled green pea. Phenotypes in a dihybrid cross are combined in a ratio of 9:3:3:1.

tree method

There is also an alternative, tree-like method, but it does not display gamete genotypes correctly:

It is advantageous to use it when crossing homozygous organisms: