Lec 27 Flashcards
(45 cards)
Thomas H Morgan( embryologist ) demonstrated that :
Certain genes present on sex chromosome ( X ) can regulate somatic characteristics { eye color }
What is X - linked inheritance?
Inheritance of somatic characteristics from parental X chromosome
What did Thomas Morgan use to demonstrate X-linked inheritance?
Fruit fly / drosophila melanegaster
Why did Morgan use fruit fly / drosophila ?
1- small in size
2- easy to raise in lab
3- life span ~30 days
4- both males & females have autosomes ( 3 pairs ) + sex chromosomes ( 1 pair )
5- chromosomes = 8
How did Dr Morgan discover that genes on X chromosome of drosophila regulate eyecolour?
He noticed that in the lab
most flies have ( red eyes , tan body , large wings ) —> wild type
some flies have ( white eyes , tan body , large wings ) —> mutant most were males
Why are most mutant flies males?
Based on different crosses he proposed that:
The gene/allele that regulates eye colour is not located on Y chromosome but on X
How can a drosophila female have white eyes?
She should carry white eye mutant alleles in both X chromosomes
For a male drosophila to have white eyes he should have
1 X chromosome carrying white eye mutant
What is the dominant eyecolour and the mutant eyecolour? 
Red
White
The affected father —— pass his white eyes to —- son
never
F1
What are double mutant flies?
Black body + short wings
Short wings + white eyes
What are single mutant flies?
Short wings
black body
Based on ——— Dr. Morgan proposed that genes regulating ( eye & body colours + wing size ) must be located on chromosome X close to each other.
Double mutant
Molecular event responsible for flies inheriting single or double mutants from X chromosome ? when does it happen?
crossover
In meiosis cell division during oogenisis
Dr Morgan proposed that the generation of single or mutant flies depends on ?
1- exchange of alleles during cross over
2- generation of novel X gametes
3- fertilisation by type of male gametes
Genes located closer to each other that can be inherited together
Linked genes / linkage
Dr. Morgan discovery was against one of medallion laws which is ?
Law of complete dominance
How did Dr Morgan identify the relative positions / distances between the three genes?
By identifying recombination frequencies between 2 genes by 3 crosses
What are the three crosses that Dr Morgan used to identify the relative position / distance between the three genes ?
1- male ( wild type ) X female ( white eyes + short wings ) TOTAL = 2302
Offspring: red eye normal wings [ 45.5% ] , red eye short wings [ 4.5% ] , white eye normal wings [ 4.5% ] , white eye short wings [ 45.5% ]
2- male ( wild type ) x female ( black body + short wings ) TOTAL = 2300
Offspring: black body short wings [ 41% ] , black body normal wings [ 8% ] , tan body short wings [ 9% ] , tan body normal wings [ 42% ]
3- male ( wild type ) x female ( white eyes + black body ) TOTAL = 1800
Offspring: white eye black body [ 46% ] , White eye tan body [ 4% ] , red eye black body [ 4% ] , red eye tan body [ 46% ]
What is the equation of recombination frequencies?
RF = no.recombinants/total offspring x 100
No . Recombinants ( أقل نسبتين أجمعهم )
Based on recombination frequencies Dr Morgan
1- identified the relative position of three genes on X
2- constructed a linkage map
Doctor Morgan placed ——— recombination frequencies at extreme ends
Higher { wing size & body colour }
He placed the gene controlling eyecolour in ——
The middle
To honour the discovery of genes located on X chromosome that controls somatic characteristics and X-linked recessive inheritance ,Dr Morgan got ?
1- Nobel prize
2- the distance between genes is measured in CENTMORGAN
