How is three factor cross different from two factor cross?

In classical genetics, different types of crosses are used to study inheritance patterns and gene linkage. Two of the most commonly used crosses are the two-factor cross and the three-factor cross. To understand how they differ from each other, we need to compare them based on certain defined criteria as mentioned below:

1. Based on Number of Genes Studied

Two-Factor Cross: In this cross, inheritance of only two genes is studied at a time. These genes may or may not be located on the same chromosome.

Three-Factor Cross: In this method, inheritance of three genes is studied together. These three genes are usually located on the same chromosome and are studied to find their relative positions.

2. Based on Purpose of the Cross

Two-Factor Cross: The main purpose is to identify whether the two genes are linked or independently assorted. It also helps in calculating the recombination frequency between the two genes.

Three-Factor Cross: It is mainly used for gene mapping, to find the linear order of the three genes and to determine how far apart they are from each other using crossover data.

3. Based on Gametes and Phenotypic Classes

Two-Factor Cross: This cross generally produces 4 types of gametes depending on the arrangement of alleles. As a result, 4 phenotypic classes are commonly observed in the offspring.

Three-Factor Cross: This type of cross produces 8 types of gametes, because three genes can be assorted in more combinations. So, the progeny usually shows 8 different phenotypic classes.

4. Based on Detection of Double Crossovers

Two-Factor Cross: It is not able to detect double crossover events between genes. So, it may give an incomplete picture of the recombination events.

Three-Factor Cross: This method can clearly identify both single and double crossover events. This is helpful in getting more precise genetic distances.

5. Based on Gene Order Determination

Two-Factor Cross: Since only two genes are involved, we can only study their linkage or recombination frequency, but not their linear order.

Three-Factor Cross: This is specially designed to find the exact sequence or order of the three genes on a chromosome, which is essential in genetic mapping.

6. Based on Accuracy in Mapping

Two-Factor Cross: Gene mapping is possible, but it is limited in accuracy. It gives rough estimation of distance between two genes.

Three-Factor Cross: Provides highly accurate gene mapping by using recombination frequencies from both single and double crossovers, giving a clear picture of gene positions.






Comments

Post a Comment

Popular posts from this blog

What are non-coding genes? Give examples

Non-coding genes are segments of DNA that do not code for any protein but still play very important roles in the cell. These genes are transcribed into functional RNA molecules instead of mRNA. These RNAs do not get translated into proteins but perform regulatory, structural, or catalytic roles directly as RNA. Non-coding genes are a major part of the eukaryotic genome and help in many cellular processes such as gene regulation, RNA processing and maintaining genome stability. Many people think that only protein-coding genes matter, but non-coding genes also play a crucial role in regulating cellular activities. They help control when and how genes are turned on or off, assist in processing other RNAs and participate in building cell machinery like ribosomes and spliceosomes. There are several important types of non-coding genes, which can be grouped based on the RNA they produce and their functions. The main categories are: 1. rRNA Genes (Ribosomal RNA Genes) These genes produce ribos...
Read more

What are the differences between gene enhancers and gene silencers? How do enhancers and silencers regulate eukaryotic gene expression?

Enhancers and silencers are two important types of regulatory DNA sequences that play opposite roles in controlling gene expression in eukaryotic cells. They are non-coding DNA elements which do not produce proteins but control when, where and how much a gene is expressed. They work by interacting with transcription factors and RNA polymerase to either increase or decrease the level of transcription. Differences Between Gene Enhancers and Gene Silencers Gene enhancers and gene silencers are both regulatory DNA elements found in eukaryotic genomes. Their main role is to control the level of gene expression, but they work in opposite directions. The differences between them can be explained based on the following criteria: 1. Based on Function Enhancers   increase  the transcription of a gene. They make the gene more active and allow it to produce more RNA. On the other hand,  silencers   reduce  or  completely block  transcription. They stop or decrease...
Read more

Define and distinguish sex-linked, sex-limited and sex-influenced characters

In genetics, traits can be influenced or expressed differently depending on the sex of the individual. Some traits are linked to sex chromosomes, while others are affected by hormonal or physiological differences between males and females. To describe these traits more precisely, geneticists use three main terms:  sex-linked, sex-limited and sex-influenced traits.  Although these terms may sound similar, they refer to different types of genetic expression related to sex. Understanding the distinction among these three is important for grasping how certain traits are inherited and expressed differently in males and females. 1. Sex-linked characters: These are traits controlled by genes that are located on the  sex chromosomes,  usually on the X chromosome in humans. Because males have only one X chromosome (XY) and females have two (XX), the pattern of inheritance and expression is different in both sexes. Most sex-linked traits are X-linked, and very few are Y-linked...
Read more

What are epigenetic modifications? Give examples

Epigenetic modifications are heritable and reversible changes in gene expression that occur  without altering the DNA sequence  itself. These changes play a major role in how genes are  turned on or off  in different cells and at different times. Epigenetics helps explain how the same DNA sequence can produce different types of cells like skin cells, nerve cells and liver cells in the same organism. These modifications are very important during development, cellular differentiation, X-chromosome inactivation in females, genomic imprinting, aging and in many diseases such as cancer. There are the following three main types of epigenetic modifications: 1. DNA Methylation This is the most studied and well-understood form of epigenetic modification. In this process, a methyl group (–CH₃) is added to the cytosine base in DNA, mainly at CpG dinucleotides. These CpG regions are often found in clusters called CpG islands, which are located near  gene promoters.  DN...
Read more

What are the regulatory sequences of a typical eukaryotic gene? Give examples

In eukaryotic cells, gene expression is highly controlled by specific regulatory DNA sequences. These sequences do not code for proteins, but they decide when, where and how much a gene should be expressed. They mainly control the process of  transcription,  where RNA is made from DNA. A typical eukaryotic gene contains several important regulatory sequences, such as: Promoters Enhancers Silencers Insulators Response elements 1. Promoter The promoter is the main regulatory region of a gene. It lies just before the  transcription start site,  which is the point where RNA starts getting made. This is the site where RNA polymerase and transcription factors attach to begin transcription. One well-known part of the promoter is the  TATA box,  found around 25 to 35 base pairs before the start site. It helps RNA polymerase find the correct place to begin. Another element, the  Initiator (Inr)  sequence, is present near the +1 position and supports proper...
Read more

Thomson's Plum Pudding Model of the Atom

Image
The Plum Pudding Model is one of the earliest models of atomic structure. It was built upon two major scientific contributions. The idea that atoms might contain both positive and negative charges was first proposed by William Thomson (commonly known as Lord Kelvin ) around the late 19th century. He suggested that positive and negative charges must be present together inside the atom to make it neutral. However, this idea remained a speculation until Sir Joseph John Thomson (Commonly known as J.J. Thomson ) made a major breakthrough in 1897 when he discovered a negatively charged particle, later called the electron, through his cathode ray tube experiments. He demonstrated that these rays were made of tiny, negatively charged particles that were much lighter than atoms. This was the first strong experimental evidence that atoms were divisible and had internal structure. After this discovery, in 1904,  J.J. Thomson  proposed a concrete atomic model to explain how these ele...
Read more

Describe what happens when a nonsense mutation is introduced into the gene encoding transposase within a transposon

Transposons,  also known as  jumping genes,  are segments of DNA that can move from one position in the genome to another. This movement depends on a special enzyme called  transposase,  which is produced by a gene located inside the transposon itself. The transposase enzyme performs important functions like cutting the transposon from one place and helping it to insert into another. For the transposon to move, the transposase must be produced correctly and completely. When a  nonsense mutation  occurs in the transposase gene, it replaces a codon that normally codes for an amino acid with a  stop codon.  This causes the ribosome to stop protein synthesis early, resulting in a shortened and usually non-functional transposase enzyme. This leads to several important changes inside the cell. These changes are mainly: 1. Early Termination of Transposase Production The nonsense mutation introduces a premature stop codon in the transposase gene. Thi...
Read more

Describe the components of the promoter region of a eukaryotic gene

In eukaryotic genes, the promoter region is a special stretch of DNA that lies just before the gene. Its main job is to control when and where transcription starts. It does this by providing binding sites for RNA polymerase II and other transcription factors. The promoter region can be divided into two main parts: Core Promoter Proximal Promoter Elements Each of these parts has different types of DNA sequences that help start and regulate transcription. 1. Core Promoter This is the most essential part of the promoter and lies near the transcription start site  (called the +1 position).  It directly helps in assembling the transcription machinery. The core promoter usually includes the following elements: TATA Box: A short DNA sequence (TATAAA) usually located 25–35 base pairs upstream from +1 site. It helps in positioning  RNA polymerase II.  A special protein called TBP (TATA-binding protein) binds here to start the transcription complex. Initiator (Inr) Sequence: F...
Read more

PYQ – MZOE-001: Parasitology (Solved Q&A) | MZOE-001 | MSCZOO | M.Sc.Zoology | IGNOU | December 2024

Image
M.Sc. (Zoology) (MSCZOO) Term-End Examination December, 2024 MZOE-001 : PARASITOLOGY Time : 2 Hours| Maximum Marks : 50 Note: (i)  Attempt any five questions.            (ii) All questions carry equal marks. 1. (a) What are endoparasites? Give any two examples. (2 Marks) Endoparasites are parasites that live inside the body of their host. They usually infect internal organs like the intestine, liver, lungs, or blood. These parasites depend on the host for nutrients and often cause harm by damaging tissues, stealing nutrients, or releasing toxins. They are commonly found in vertebrates, including humans and may be transmitted through contaminated food, water, or direct contact. Examples: 1. Taenia solium (pork tapeworm) – It lives in the human intestine and causes taeniasis. 2. Plasmodium vivax – A blood parasite that causes malaria in humans, transmitted by female Anopheles mosquitoes. (b) What is the difference between mechanical vector and biologic...
Read more

What are transcription factors? Describe the different categories of transcription factors

Transcription factors are special types of proteins that control the process of  transcription  in eukaryotic cells. Transcription is the first step of gene expression where the DNA is copied into RNA. These factors do not make RNA directly but help the enzyme RNA polymerase to start, stop or control the speed of this process. Transcription factors help the cell know when to turn a gene ON, when to turn it OFF and how much product the gene should make. They play a very important role in development, cell cycle, cell differentiation, stress response and hormone signaling. Transcription factors are mainly divided into two broad categories: Based on their function Based on their structure and DNA-binding domain 1. Types Based on Their Function There are the following two types: a) General Transcription Factors These transcription factors are needed for the transcription of almost all protein-coding genes. They help in forming the transcription initiation complex near the promoter...
Read more