9Linkage and Crossing over

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LINKAGE
▢ Definition: Linkage is the tendency of genes located on the same chromosome to be inherited together.
▢ Cause: Genes present on the same chromosome are physically associated and therefore do not assort independently in all cases.
▢ Linkage Group:
❖ Definition: All genes present on one chromosome constitute one linkage group.
❖ Important Point: Number of linkage groups generally equals the haploid chromosome number of an organism.
▢ Types of Linkage:

Table 1: Types of linkage

Type
Meaning
Complete linkage
  1. Linked genes are inherited together without crossing over between them.
  2. Genes located very close together show strong linkage. CEE 2025
Incomplete linkage
  1. Linked genes are usually inherited together, but crossing over produces some recombinant combinations.
  2. Genes located farther apart show weaker linkage because crossing over is more likely to occur between them.
  3. Greater distance between two genes → greater recombination frequency.
▢ Crossing Over:
❖ Effect: Crossing over can break linkage and produce recombinant offspring.
❖ Site: Occurs between non-sister chromatids of homologous chromosomes during pachytene of prophase I of meiosis.
▢ Important Points:
  • •
    Linked genes are present on the same chromosome.
  • •
    Linked genes tend to move together during inheritance.
  • •
    Linkage reduces the production of recombinant combinations.
  • •
    Recombination frequency is used to estimate the distance between genes.
▢ MCQ Points:
Q1.
Linkage refers to:
Q2.
The number of linkage groups in an organism is generally equal to:
Q3.
Strength of linkage between two genes is greatest when they are:
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SEX LINKED INHERITANCE
Image 1
▢ Definition: Inheritance of traits present on sex chromosomes
▢ Discovery:

Table 1: Sex-linked inheritance facts

Point
Data
First X-linked trait observed by
T. H. Morgan
Organism
Drosophila
Observation
White-eyed fly among red-eyed flies
Inheritance pattern
Criss-cross inheritance
Transmission
Father → daughter → grandson
▢ Key Rule:
  • •
    X-linked gene transmitted from father to grandson through daughter
  • •
    Pattern → criss-cross inheritance
▢ Human Sex-linked Inheritance:
❖ X-linked Inheritance:
◉ Colour Blindness / Red-Green Blindness / Daltonism:
▢ Cause: Defective recessive gene on X-chromosome
▢ Defect: Absence of colour-sensitive cells in retina
▢ Feature: Unable to distinguish red + green colour; appear grey
▢ Transmission: Colour-blind man → daughter → half of next generation
◉ Haemophilia / Bleeder's Disease / Royal Disease:
▢ Cause: Recessive gene on X-chromosome
▢ Defect: Lack of clotting factor in blood; Factor VIII
▢ Clotting Time:

Table 1: Blood clotting time

Condition
Clotting time
Normal
\(2-8\) minutes
Haemophilia
30 minutes–24 hours
▢ Special Points:
  • •
    Common in royal families of England + Russia
  • •
    Inheritance pattern described by Nasse → Nasse’s law
  • •
    Haemophilic son → non-viable / does not attain sexual maturity
  • •
    Males → sufferers
  • •
    Females → carriers
◉ Muscular Dystrophy: Listed only
📚
CROSSING OVER
▢ Definition: Exchange of genetic material between non-sister chromatids of homologous chromosomes → new combination
▢ Key Points:
  • •
    Crossing over ∝ distance between linked genes
  • •
    Single/double/multiple crossing over → depends on number of chiasmata
  • •
    Reported in Drosophila by T. H. Morgan
  • •
    Most accepted theory → precocity/strain theory by Darlington
  • •
    More in females than males
  • •
    Increases at high + low temperature
  • •
    Occurs at tetrad stage of pachytene
  • •
    Well demonstrated in Neurospora
▢ Types:

Table 1: Types of crossing over

Type
Site
Occurrence / Examples
Germinal crossing over
Germinal cells
During gamete formation
Somatic crossing over
Somatic cells
Maize, Aspergillus, Drosophila / MAD
▢ Significance:
  • •
    Basis of linkage maps / genetic maps
  • •
    Produces recombination
  • •
    Strong proof for linear arrangement of genes on chromosome
▢ Recombination Frequency: \(\text{Recombination frequency} = \frac{\begin{subarray}{c} \text{No. of individuals} \\ \text{showing recombination} \end{subarray}}{\text{Total number of offsprings}}\)
Q1.
When two genes are very close to each other and tend to inherit together, it is called
📅CEE MBBS 2025
Q2.
A man whose father was colour blind and mother is normal marries a woman who had a colour blind mother and normal father. What percentage of male children of this couple will be colour blind?
📅CEE MBBS 2024B
Q3.
Which of the following doesn't cause variation?
📅CEE MBBS 2024B
Q4.
Crossing over takes place in:
📅CEE MBBS 2024B
Q5.
A colorblind man marries a normal woman whose father was colorblind. The probability of their son being colorblind is:
📅CEE MBBS 2024a
Q6.
If the number of chromosomes in Pisum sativum is 2n = 14, then the number of linkage groups is:
📅CEE MBBS 2024a
Q7.
Haemophilia is caused by:
📅CEE MBBS 2022
Q8.
A haemophilic woman marries a normal man. The resulting progeny will have:
📅CEE MBBS 2022
Q9.
Unit of crossing over is:
📅CEE MBBS 2022
Q10.
The inheritance of characters from father to grandson through daughter is called:
📅CEE MBBS 2021
Q11.
In human male, the number of linkage groups is equal to:
📅CEE MBBS 2021
Q12.
If hemophilic man is married to heterozygous normal woman, then probability of having hemophilic son is:
📅CEE Nursing 2024
Q13.
If a dihybrid cross fails to give 9:3:3:1 phenotypic ratio and shows 1:1:1:1 phenotypic ratio, then it shows:
📅CEE Nursing 2024
Q14.
In the process of crossing over, the separated chromatid segments are united by: [BPKIHS 1994]
📅BPKIHS 1994
Q15.
Genetic recombination is due to [BPKIHS - 2010]
📅BPKIHS - 2010
Q16.
An exception of Mendel's law is [MOE 2060]
📅MOE 2060