Topic 11 pt 2: Sex Linked Inheritance Flashcards
What did Thomas Hunt Morgan see in the F2 generation when crossing drosophila with red or white eyes
3:1 ratio (expected) BUT 1/4 white was all male
Thomas Hunt Morgan drosophila eye colour experiment
Crossed an X+X+ (red eyed) female with an XwY (white) male, got all red eyed offspring (dominant) in F1
Crossed the red eyed offspring (male x female) and got a 3:1 red:white ratio in F2, but all white eyed were males
Why did THM’s female flies always have red eyes
Because the gene controlling this trait is on the X chromosome, female has two X’s so is more likely to get at least one WT X
What is a reciprocal cross
Switching what sex has what trait
How can a reciprocal cross be used to determine if a trait is X linked
Switch what sex has what trait, if the ratios are not the same (one of the crosses has non-mendelian F1 and F2) then it is X linked
Where are the colour sensing cone-cells in our retinas located
Blue = pigment gene on chr 7
Green + red = pigment gene located together on the X chr
Red/green colourblindness allele has…
Both genes function (red and green) are knocked out
What allele is dominant in colour blindness
Wildtype, fully penetrant and haplosufficient
Things to note regarding colour blind crosses
- Male affected= normal mendelian ratios, female affected= 1:1 ratio
- Mother passes the mutant phenotype to their sons but not daughters (when crossed with normal male)
- fathers pass on mutant trait to the grandsons through carrier daughters
- female must have mutant allele from both parents to be affected
- male must receive mutant allele from mother
Y-linked traits characteristics
Very diff than X
Only in males
All sons of affected males are affected
What is dosage compensation
Mechanism used to balance or equalize amount of protein produced by sex chromsomes (since Y does not have same genes)
Why is dosage compensation required
Number of copies of a gene effects the amount of protein produced: autosomal to sex chromosome product will be imbalanced between sexes
What are Barr bodies? What is the lyon hypothesis
Barr bodies = highly condensed inactive X chr
Lyon hypothesis = Mary Lyon said one x-chr is deactivated in females, and that barr bodies are the deactivated chr
Which X chromosome is deactivated in females
Random, different in diff cells
50% of cells express one allele while 50% express the other
When does X chr deactivation occur
Early in development
Once inactivated remains inactive in that cell and its descendants
