Eukaryotic Genetics: Heredity, Linkage, and Recombination Flashcards
What does Mendel’s 1st Law (Law of Segregation) state
Each organism carries 2 alleles that segregate equally into gametes
What does Mendel’s 2nd Law (Law of Independent Assortment) state
Alleles at different loci assort independently during gamete formation
Who proposed that chromosomes carry hereditary factors
Sutton and Boveri (1902) – observed chromosome behaviour matching Mendel’s laws
What problem did the chromosomal theory of inheritance have with Mendel’s 2nd Law
It was hard to reconcile independent assortment with the idea that genes are on specific chromosomes
Who discovered genetic linkage
Thomas Hunt Morgan using Drosophila (fruit flies)
What did Morgan’s experiment with white-eyed flies show
Eye colour gene is linked to the X chromosome, violating Mendel’s 2nd law
What did Morgan mean by linkage
Genes on the same chromosome tend to be inherited together
Who first observed physical crossover structures in meiosis
Frans Janssens (1909)
What did T.H Morgan hypothesise about recombination
Chromosomes physically exchange genetic material via crossing over
What causes strand exchange in single-strand break recombination
Breaks in DNA followed by ligation and branch migration
What structure is formed during recombination involving 4 DNA strands
Holliday Junction, resolved by resolvase enzymes
What are the steps in double-strand break recombination
- Alignment
- Double strand break
- End resection
- Strand invasion + synthesis
- Double Holliday junction formation
Why was the F2 generation crucial in Morgan’s experiment
Because it revealed a sex-specific pattern: only males showed the white-eyed phenotype, indicating the gene was X-linked and recessive
What are Holliday junctions and why are they important
Four-stranded DNA structures formed during recombination, they are intermediates that must be resolved to complete crossing over
What is the main difference between recombination after a single-strand break vs a double-strand break
Single-strand recombination involves simple strand invasion and migration, while double-strand break repair includes end resection, synthesis, and potentially forms double Holliday junctions
What is ‘branch migration’
A process during recombination where the crossover point moves along the DNA, extending the region of hybrid DNA
Why do linked genes not show a 9:3:3:1 or 1:1:1:1 phenotypic ratio
Because their inheritance is biased toward parental combinations due to reduced recombination between them
Why is recombination suppressed in sex chromosomes
To ensure that sex-determining genes and other sex-linked traits are inherited together and maintain functionality
What is the evolutionary benefit of recombination
It increases genetic diversity by allowing new combinations of alleles and reshuffling mutations, which can be acted upon by natural selection
How do chromosomal inversions affect recombination?
They can suppress recombination within the inverted region, as crossing over there produces inviable gametes due to duplications/deletions
What happens if recombination occurs within an inversion
It results in abnormal chromosomes (with duplications or deletions), usually leading to non-viable gametes or inviable offspring
What causes strand exchange in single-strand break recombination
Breaks in DNA followed by ligation and branch migration
What are the steps in double-strand break recombination
- Alignment
- Double strand break
- End resection
- Strand invasion + synthesis
- Double Holliday junction formation
Is linkage ever absolute
No, crossing over can still produce recombinant alleles
