Fig. 5: Gene segregation and genetic recombination (2023)

Science of genetics

Genetics is one of the major sciences underlying plant breeding. Genetics is the science of inheritance,genes, chromosomes and varieties of biological organisms. The science of genetics is often divided into four main sub-disciplines:

• Transmission genetics (also called classical or Mendelian genetics)
• quantitative genetics
• population genetics i
• Molecular genetics

transmission geneticsdeals with how genes and genetic traits are passed down from generation to generation and how genes recombine. The foundations of modern genetics are believed to have been laid in the mid-nineteenth century when Gregor Mendel analyzed the results of crosses he had made between pea plants. Mendel concluded that inherited traits (now called traits or phenotypes) were determined by factors (now known as genes) that he had observed. He also realized that each organism contains two copies of each "factor" (gene), one inherited from the mother and the other from the father. Mendel discovered the principles of inheritance when he understood how inherited characteristics (e.g., round or wrinkled shape of seeds, yellow or green color of pods, axial or terminal position of a flower, or high or low plant height) are passed from parent to parent. kids. This module focuses on transmission (Mendelian) genetics.

Genetic subdisciplines

quantitative geneticsfocuses on the study of inheritance when phenotypes show continuous variation or distribution. In particular, it takes into account the influence of multiple genes that can simultaneously influence such traits, as well as the relative contributions of the environment and the interactions between the genotype and the environment. The quantitative inheritance module focuses on quantitative genetics.

population geneticsinvolves the study of the heritability in groups of individuals of traits that are generally determined by one or only a few genes. It deals with the distribution of genes and genetic diversity within and between populations and subpopulations. Population genetics involves estimating and predicting the response to selection. It describes the relationships between allele frequencies and genotypes resulting from four major evolutionary forces: natural selection, genetic drift, mutation, and gene flow. Population genetics is the focus of the inbreeding and heterosis module.

Molecular geneticsdeals with the molecular structure and function of genes. It includes the study of DNA structure and replication, and deals with gene expression and regulation.

Genes and chromosomes

To understand heredity, it is essential to understand the structure and function of genes. Let's take a look at the key terminology and rules. For a more in-depth overview, look into books on biology or genetics, e.g.De Genes a Genomas (Hartwell et al. 2011), Genetics: A Conceptual Approach (Pierce 2012), lub iGenetics: A Molecular Approach (Russell 2010).

Genes are encoded by DNA. Most of the DNA in plants is found in the cell nucleus and is organized into groups of genes along multiple chromosomes in a linear fashion. Nuclear DNA is subject to Mendelian inheritance, which will be discussed later in this module. In addition to being found in the chromosomes of the nucleus, DNA is also found in organelles present in the cytoplasm of plant cells.

The molecular basis of the chromosome

chromosome– Each chromosome contains one DNA molecule (Figure 2).

DNA

DNA (deoxyribonucleic acid)It consists of two polynucleotide chains. Polynucleotides are also called nucleic acids and are composed of linear polymers which are macromolecules formed by the chemical linkage of many identical or similar units called nucleotides. Each nucleotide in each strand consists of a nitrogen-containing base, a deoxyribose (sugar) and a phosphate group. The nucleotides in each chain are held together by sugar-phosphate (phosphodiester) bonds (Figure 3).

The nitrogen-containing bases are purines (adenine, A and guanine, G) and pyrimidines (cytosine, C and thymine, T). Pairing occurs between purines and pyrimidines and is specific. Sequences of consecutive nucleotides form genes (Fig. 4).

C always agrees with G

You are always paired with A

DNA replication is semi-conservative.

The process of DNA replication is still not fully understood. Basically there are three steps.

1. The two strands of DNA unwind and separate.
2. Free (unbound) nucleotides bind to complementary bases on the original DNA strand.
3. The newly formed strand and the DNA template strand wrap around to form a double helix.

This process is semi-conservative because each nascent double-stranded DNA molecule consists of a newly synthesized strand and a template strand (Fig. 4). Since one strand of each DNA molecule is the original strand, errors are less likely to occur during replication.

genes

There are many genes on each chromosome. Each particular gene appears at a specific point on the chromosome, the genelocalin each of the twohomologous chromosomes. More than one form of a particular genecried, may occupy the same locus on homologous chromosomes.

cried

Alleles are variants that differ slightly in DNA sequence. Diploid plant species have two sets of chromosomes, each of which can carry a different allele for a particular gene. For example, a gene responsible for seed color may have two alleles,AmiA. . . . criedAcauses the phenotype (e.g. brown seed color) and the alleleAcauses a different phenotype (e.g. white seed color). For this gene, the genotype may beAA,aa, Lubaa.

If one allele at a locus on a homologous chromosome partially or completely masks the expression of another affecting the phenotype, the allele that is expressed is called dominant and the masked allele is called recessive. By convention, the dominant form is capitalized and the recessive form is lower case. In the example above, for seed color the alleleAis the dominant allele. If it isAthe allele is completely dominantAallele, individuals zAALubaagenotypes would have a brown seed phenotype, whileaaindividuals would have white seeds.

the individual is heterozygous (aa) when there are two different alleles at a locus and it is homozygous - in this example, or homozygous dominant (AA) or homozygous recessive (aa) — when the same alleles are present on both chromosomes. The alleles in a locus can interact in different ways that its phenotype reveals whether it isheterozygotes are homozygotes.

DNK, proteins and other genetic products

To better understand the concept of genes, which will be the subject of this and the next lesson on connections, it is necessary to understand the chemical nature of genes.DNA. Let's look at the pathways by which genetic information in DNA is transferred from one DNA molecule to another (a process called DNA replication) and from DNA to DNA.ribonucleic acid (RNA)molecule (called transcription) and then transferred from RNA to protein (called translation) using a code that specifies the protein's amino acid sequence (see Figure 6).

Agenis a stretch of DNA along achromosomewhich consists of stringsnucleotides. Recall that genetic information in DNA is encoded in a sequence of four nucleotides, which are shortened depending on the type of nitrogenous bases they contain - purines A and G and pyrimidines C and T. The genetic information of an individual is passed from cell to cell during development and from generation to generation. generation to generation during reproduction.

The structure of DNA

Explore the following to better understand the chemical structure of the nucleotides that make up the basic building blocks of DNA and the process of DNA replication:

Learn about the chemical structure of DNA and what happens during the DNA replication process. DNA replication occurs during the synthesis phase of the cell cycle.

Four types of chemical bases - A, G, C, T - in gene sequences contain instructions for protein assembly (Fig. 8). The base pairs are connected by hydrogen bonds and form the "rungs of the DNA ladder" (Fig. 8).

Nucleotides

Nucleotidesare the basic building blocks of nucleic acids such as DNA and RNA, which are polymers made of long chains of nucleotides. DNA is double-stranded while RNA is single-stranded (Figure 9). Note that there is a chemical principle in RNAuracyl(vas) replacesmine(T).

Genes usually express their effect through codingpolypeptidechains that are polymers of ten or moreaminoacidconnected bypeptidetitles. One or more polypeptides make up a protein. The DNA sequence of a gene is used as the basis for the production of a specific protein sequence. Proteins are complex molecules responsible for most biological functions in a cell.

Gene expression, RNA, translation and transcription

amino acidsthey are the building blocks of proteins. ANDproteinconsists of one or more long chains of amino acids whose sequence corresponds to the DNA sequence of the gene that encodes it. The process of creating proteins from the genetic code in DNA is calledgenetic expression. The general process of gene expression in eukaryotic cells such as plants involves several steps described below.

Transcriptionis the process by which a sequence of nucleotides from a DNA strand of a gene is copied into the nucleotides of an RNA molecule. The nucleic acid sequence in RNA complements the sequence in the DNA strand from which it is transcribed. However, in the RNA strand, uracil (U), not thymine (T), is the base that complements adenine (A). When an RNA transcript is created, each base in the DNA pairs with a base in the RNA nucleotide, which is gradually added to the RNA strand as it grows. Transcription occurs in the cell nucleus (Fig. 13).

In a process known as RNA splicing, intermediate sequences or introns are removed from the transcript by RNA splicing. Introns are a special type of so-called non-coding DNA sequences that do not code for amino acids, but are found in genes until these sequences are removed during RNA processing. (Note that with the exception of intron sequences, most of the non-coding DNA found in chromosomes is between (not inside) the gene loci along the chromosome.) The regions between introns in fully processed RNA are called exons, protein coding sequences (Figure 10). The endings of the transcript have also been changed. The fully processed RNA is called mRNA (messenger RNA). MRNA is a single-stranded nucleic acid sequence that moves from the cell nucleus to the cytoplasm, where proteins are produced (Fig. 10).

Translationit is the process by which mRNA directs the assembly of amino acids in the correct order for the synthesis of a specific protein. Ribosomes in the cytoplasm of the cell read the mRNA base sequence (Fig. 10).

In the translated portion of the mRNA, each adjacent group of three nucleotides is a coding group, orcode. Each codon specifies an amino acid subunit in the polypeptide chain. adapter particles,tRNA(transfer RNA) are complexed with a specific amino acid corresponding to the base sequence of a given mRNA. The tRNA molecules carry the amino acids of some mRNAs to the ribosomes where they are added to the growing protein chain. When the polypeptide chain is complete, it is released from the mRNA and forms a protein molecule. The sequence of amino acids determines the structure of a protein, which affects its function.

Basic steps of transcription

Here are the basic steps of transcription and translation:

1. During transcription, a region of double-stranded DNA briefly opens, separating the two strands and allowingheavyknown as RNA polymerase to build an mRNA strand that fits into that region of DNA.
2. The tRNA anticodon binds to the mRNA codon. The tRNA has a region called the "anticodon" that is complementary to the mRNA codon sequence (Figure 14).
3. The specific amino acid complexed with the tRNA stays in place, while the tRNA-amino acid complex corresponding to the next codon moves into place. A peptide bond is formed between adjacent amino acids, which builds a protein molecule.

Mechanisms

Heredity is based on the preservation of chromosomes and the genes they carry. Whilemesosisand gametogenesis, homologous chromosomes separate. Each gamete receives a (haploid) set of chromosomes. The particular chromosome of a homologous pair assigned to a given gamete is random. When two gametes fuse during fertilization, the zygote receives a set of chromosomes and alleles that each carries from each parent. The resulting combination of alleles in a zygote determines its genotype.

Since the arrangement of homologous chromosomes in gametes is random, the fusion of gametes in a zygote can produce various genetic combinations. Therefore, differences in some features or forms can be observed in the population. If variation in a particular trait is due to contrasting alleles at one or more loci rather than a response to the environment, the variation is heritable and can be passed from parent to offspring. Breeders select plants that exhibit desired traits, and these plants carry the desired allele of the gene encoding the trait of interest.

Each gene or combination of genes and alleles, under the influence of the environment, determines the phenotype or the observed expression of a particular trait. The allelic composition of an individual at the appropriate loci on homologous chromosomes results in the expression of that gene. The alleles at the appropriate loci interact with each other. One allele may mask the presence of another allele(s).

Alleles at a locus can interact in a variety of ways, including non-dominance (also known as additive gene action), partial dominance, complete dominance, and superdominance.

Genetic action

There are several general types of gene action. The type of gene action and the alleles present for a given gene affect the phenotype. Consider the gene action indicated by the phenotype of the heterozygous diploid individual at a given single locus compared to the phenotype of its parents.

Partial domain (incomplete)

Heterozygous offspring have a higher phenotypic valueaverage value (MPV), but less than in the homozygous dominant parent.

superdominance

The phenotype of heterozygous offspring is greater than that of both parents.

Research questions 1: Comparing the values ​​of parents and offspring

Two diploid plants with different phenotypes for traits A, B and C are crossed. The progeny are bred and their phenotypes are assessed. Suppose both parents are homozygous at each locus. Compare the parent and offspring values ​​for each character. Select gene action at each locus.

One place	genotype	phenotype value
father one	AA	75
Father of two children	aa	40
offspring	aa	75

The H5P interactive element has been removed from this version of the text. It can be seen online here:
https://iastate.pressbooks.pub/cropgenetics/?p=1087#h5p-35

Place B	genotype	phenotype value
father one	bed and breakfast	60
Father of two children	bed and breakfast	20
offspring	Bed and breakfast	55

The H5P interactive element has been removed from this version of the text. It can be seen online here:
https://iastate.pressbooks.pub/cropgenetics/?p=1087#h5p-36

Place C	genotype	phenotype value
father one	CC	28
Father of two children	cc	15
offspring	Copy	28

The H5P interactive element has been removed from this version of the text. It can be seen online here:
https://iastate.pressbooks.pub/cropgenetics/?p=1087#h5p-37

Multiple alleles

With complete dominance of the type in question, there are two different alleles for the trait, but only one of the alleles is visible in the phenotype. But it's important to understand that dominance doesn't affect how genes are inherited. For some traits, there are causes other than inter-allelic dominance at the same locus that explain deviations from the expected phenotypes.

more alleles- instead of just two - can appear in one place. Examples of multiple alleles at one locus includeTHEYblood group system in humans orSalleles controlling self-incompatibility in plants. Multiple alleles at a locus are sometimes called allele series. However, while there may be more than two alleles per gene present in apopulation, note that the genotype anyIndividuala diploid plant has only two alleles in a population.

penetrationis a measure of the proportion of individuals with a given genotype that express the expected phenotype. Incomplete penetrance occurs when the genotype does not always produce the expected phenotype.

clarityis a related term that describes the degree to which a character is expressed.

incomplete penetration

Incomplete penetrance and variable expression result from the influence of other genes or environmental factors that alter the function of a particular gene. For example, the phenotype produced by an enzyme encoded by a particular gene may only be expressed over a narrow range of temperatures. A recessive allele appears in barley that produces albino plants when grown in cooler temperatures. The allele inhibits the production of chlorophyll. But if barley plants homozygous recessive for this allele grow above a critical temperature, the effect is absent and the plants have normal chlorophyll and are green.

flight allelesit may also alter expected phenotypic ratios. Lethal alleles cause death if present, so the offspring of the cross will be missing one or more genotypes. Lethal alleles can be recessive (cause death only in homozygotes) or dominant (both homozygotes and heterozygotes with the allele will die). Dominant lethals rarely persist in populations.

essential genesare genes that, when mutated, can result in a lethal phenotype.

Terminology

The parental generation of the cross is often calledgeneration P. Using symbolism based on what is calledsymbol F, usually called offspring from the union of two parentsF₁or the first filial generation.The next generation formed by self-pollination or cross-breeding of F₁offspring (type of mating, so-calledkinship) it is namedF₂Generation, Lubson's second generation. The offspring resulting from the self-pollination of each successive generation after F₂is called F₃, F₄, F₅, and so on. It is based on a different type of symbolismSymbol S. The symbol S is used to describe the offspring of a cross - especially a cross between two homozygous parents.Symbole F i Swere developed to describe the offspring resulting from hybridization and selfing.

crosses

A cross involving a single trait (e.g. seed color) is called amonohybrydowya cross, and one that includes two characteristics (e.g. seed color and plant height) is called a crosstwo hybridscross. Conventionally, in the equations used to symbolize the cross, the female parent is listed first and the male parent second, as in this single locus example in diploid individuals:

AA x aa ⇒ Aa

Trips performed in both directions are calledmutual crosses. For example, the cross inverse of the above would be:

aa x AA ⇒ Aa

Forecasting segregation rates

If the genetic basis of a trait is known, Mendel's rules can be used to predict the outcome of the cross. There are three common approaches used to analyze segregation results, two of which use a systematic enumeration of all possible genotypes and phenotypes of zygotes and gametes, and the other uses mathematical rules.

• OPunnett square methodit works best in situations involving one or two genes. All possible gametes are recorded in a square and then systematically combined to represent the genotype range of the offspring.
• OBranched or forked line method[See Appendix C for examples] also works well in situations involving one or two genes. It uses a numbering system in the branch diagram.
• OProbability methodis based on two principles of the mathematical theory of probability - naMultiplication rule and addition rule- and manages the frequency of events.

Parent monohybrid cross

F₁monohybrid crossing

Two-hybrid crossing

expected F₂phenotype for each trait

The principle of uniformity

If both parents are homozygous, your F₁is genetically homogeneous.

On the right is a Punnett square illustrating an example of this phenomenon, showing the genotypic and phenotypic proportions and the chromosomes of diploid parents, haploid gametes and F₁Generation.

Segregation principle

In heterozygotes, two different alleles of a gene locus separate from each other during gamete formation. Below are two images (one using a Punnett square and the other using a branch or fork diagram method) that show an example of Mendel's law of segregation. Figures show genotypic and phenotypic ratios and F chromosomes₁heterozigotne, haploidalne gamety i F₂Generation.

The principle of independent selection

Alleles at different gene loci are passed on independently during gamete production. Below are two images (one using the Punnett square and the other using the bifurcation method or branching diagram) that show an example of Mendel's law of independent distribution. The numbers represent the genotypic and phenotypic ratios and the chromosomes of the parents, F₁heterozigotne, haploidalne gamety i F₂Generation.

Drag the correct inheritance onto the appropriate trait graph

The cross was made between a plant homozygous for green seeds (GG) and a plant homozygous for white seeds (gg) — monohybrid crossing. Suppose the species is diploid and normally self-pollinating, aGthe allele is completely dominant. By convention, "X" means cross-pollination, and "The symbol indicates self-pollination.

In each generation, you will determine and complete the missing phenotypic and genotypic proportions. You will drag some of the options below to the corresponding empty field.

Progeny test

Ooffspring testevaluates the genotype of an individual based on the performance of its offspring. Progeny testing can be used to:

1. Distinguish between heritable phenotypes and phenotypes attributed to environmental effects.
2. Determine the genotype or allele composition of the individual.

Stages of examining the offspring

1. Hybridize (pair) two plants, A and B.

   

2. They grow and self-pollinate F₁plants.

   

3. Cultivation and self-fertilization of F₂plants.
   1. Determine the phenotypic ratio of the trait you are interested in.
   2. Collect the seeds from each plant separately.
4. Part of the F plant₃seeds of each phenotype separately.
   1. Determine the phenotypic ratio in each group - The phenotypic ratio reveals which F₂which plants were homozygous and which were heterozygous for the trait(s) of interest.
   2. Based on the phenotypic data, calculate the genotypic coefficient.

In this example, the phenotypic proportions of F₃plants reveal the following genotypic information about each of the F₂Pais:

Both the red and green phenotypes occur in proportions consistent with hereditary characteristics. Thus, these phenotypes have a genetic basis (ie, these phenotypes are not simply the result of environmental conditions).

Steps in Testcross

1. Hybridize (pair) two plants. Parent 1 genotype is unknown, A (?). Parent 2 is homozygous recessive for the trait of interest, aa.

2. Rasti F₁plants and estimate the phenotypic proportion:
   1. If you split 1:1, then you know Parent 1's genotype was heterozygous, Aa.

2. If all plants have the parent 1 phenotype, then you know that parent 1 was homozygous dominant, AA.
   

Oback passthis is a special kind of progeny test. It's an F cross₁any of the original parents. This procedure is widely used in basic genetic research, but is not often used by plant breeders to determine the genotype of plants.

Grow improved varieties

To develop improved varieties, plant breeders often combine favorable traits from one plant or variety with desirable traits from another plant or variety, stockpiling the desired alleles for key traits. To obtain an improved genetic mix, breeders conduct a series of matings, selecting the best offspring to produce the next generation. Breeders rely on several genetic mechanisms to produce new genetic combinations.

1. Segregation- Homologous chromosomes from different parents are separated and randomly distributed into cells during meiosis.
2. recombination— Creating new combinations of genes by mating individuals with different genotypes.

Segregation

Segregation is the result of the independent or random distribution of homologous chromosomes and the genes they carry in gametes. During meiosis, pairs of alleles are separated and distributed to different cells, which then go through gametogenesis.

Genes located on different pairs of chromosomes are sorted independently. This means that the random distribution of a particular chromosome, say one of these green chromosomes, in a cell has no effect on the distribution of the yellow chromosome. Independent diversity facilitates recombination and leads to segregation in successive generations.

Cross two plants, one heterozygous and one homozygous at the G and H loci. Identify all possible gamete types and then all possible genotypes that would result from the offspring of this mating. Let Father 1 be the female mother and Father 2 the male parent on this cross. Check each step and make adjustments if necessary before proceeding to the next step.

Step 1: From the types below, select the possible types of gameteseggsand drag 4 matching types to the boxes below.

step 2: From the types below, select the possible types of gametesspermand drag 4 matching picks to the box below.

phase 3: Fertilization: When the gametes unite, the zygote receives half of its genes from each parent. All possible genotype combinations are listed below. Choose the right combinations and drag them to their place on the table.

Step 4: What is the genotypic relationship of these offspring?

Step 5: What is the phenotypic ratio of these offspring?

Step 6: Identify parental and recombinant types by clicking the appropriate button under each example.

It was a homozygous plant

• visok prinos (Y_ = visok, yy = nizak),
• low protein content (P_ = high, pp = low),
• early ripening (E_ = late, ee = early), i.e
• with white flowers (W_ = purple, ww = white)

was crossed with a homozygous low-yielding, high-protein, early maturing plant with purple flowers.

useful advice

• 3/4 is high efficiency (Y_)
• 3/4 will be high protein (P_)
• everything will ripen early (ee)
• 1/4 will have white flowers (ww)

Let's find out by looking at the possible combinations of genes in gametes. There are eight combinations.

YPeW YPew YpeW Ypew yPeW yPew ypeW ypew

To check all genotypes in F₂, we can create a Punnett square with these eight egg and sperm combinations, creating an 8 x 8 table of 64 combinations in F₂zygotes. Only those F₂with the genotype Y_P_eeww (indicated by X in the table below) will have the phenotype: high yield, high protein content, early maturity and white flowers.

Hybrid features

A breeder cannot improve a trait unless there is some variation in that trait within which selection can be made. Hybridization of plants with different phenotypes (and genotypes) and selection among recombinants gives the breeder a chance to progress in improving crops. However, recombination and segregation may not provide the expected variability for two general reasons.

• population size— A minimum of offspring from the cross must be bred and scored. If the number is too small, the likelihood of the desired recombinant appearing in the population decreases. As the number of independently distributed genes increases, the number of plants that need to be assessed increases exponentially. Therefore, a suitable population is essential for successful progress towards breeding goals. The minimum population size required for all genotypes to be represented in the population can be calculated as follows:
  1. Determine the number of segregating pairs of genes. Set this number as 'n'.
  2. Calculate the minimum population size: Minimum population size = 4^N

• Genetic interaction“Although genes are involvedepistaticmipleiotropicinteractions can vary independently, their interactions often affect phenotypic and genotypic ratios.
• connection“As stated earlier, nearby loci on the same chromosome tend to move together and do not sort independently.

Cross genetic data

When analyzing data from genetic crosses, it is often appropriate to use some type of statistical analysis as the data is often quantitative. A frequently used statistical procedure to test results from triage data is the so-calledhigh square (χ²) attempt.The chi-square test is also known as the "fit" test.

The developers ask whether the data supports or matches a particular hypothesis and thus helps to explain the results. For example, does the range of phenotypes seen in the offspring of the cross correspond to a particular segregation ratio, say 3:1 or 9:3:3:1? The chi-square procedure helps growers understand the meaningdeviation of observed results from expected resultstested hypothesis. ANDNull hypothesisis formed which says there is no real difference between observed and expected data. If the differences are due to chance, the hypothesis can be accepted, otherwise the null hypothesis is rejected and the developer can modify the hypothesis in favor of a better one. The equation used to calculate (χ²) statistics are as follows

\[x^2 = \sum \frac{(\textrm{observed} - \textrm{expected})^2}{expected}\]

The chi-square procedure will be covered in more detail in the Quantitative Methods course.

Characteristic

When multiple genes control a particular trait or set of traits, gene interaction can occur. Generally, such interactions are detected when genetic proportions deviate from normal phenotypic or genotypic proportions.

• Plejotropia— Genes that affect the expression of more than one character
• Epistasisepistasis“Genes at different loci interact to produce the same phenotypic trait. Epistasis occurs when two or more loci are presentinteract to form new phenotypes. Epistasis also occurs when an allele at a locusmaskthe effects of an allele at one or more loci or whether an allele at a locuseditallele effects at one or more loci. There are several types of epistatic interactions.

Epistasis is expressed inphenotypiclevel. It should be noted that genes involved in epistatic interactions may still show independent assortment ingenotypiclevel. The slides below show some examples of epistasis extracted from different plant species.

Double recessive epistasis

Double recessive epistasis(also known ascomplementary action): a ratio of 9:7 observed in the flower color of the progeny of a cross of a pure line pea with purple flowers (genotypeCCPP) with a purebred homozygous recessive white-flowered plant (ccpp). z₁all plants are purple and have a genotypeCcPp, Ali F₂the offspring will have a modified 9:7 ratio because color is only produced when both genes have at least one dominant allele. These genes control flower color by controlling the expression of biochemicals called anthocyanins that provide the flower with pigment. Pigmentation in this case is controlled by a two-step chemical reaction. One of these genes controls the first step and the other controls the second.

Double dominant epistasis

Double dominant epistasis(also known asduplicated action): A 15:1 ratio observed in the offspring of shepherd's pouch hybrids. If either of the two genes are involved in fruit formation (TLubV) occur separately or both together (TV), then all plants will produce triangular fruits. Only the homozygous recessive genotype (ttvv) produces an oval seed capsule.