Lecture #25 - Linked genes Flashcards
Sex determination
- What determine in humans?
- What about bees?
Sex chromosomes (x, y) determine the sex of the individual. Other chromosomes are called autosomes
In bees, fertilised eggs develop into diploid females; unfertilised eggs develop into haploid males
Heterogametic and homogametic?
Homo = two same
Hetero = two different
Birds: Females are heterogametic and males are homogametic (carry two identical sex chromosomes (ZZ) and femailes have two different ones (ZW)) Oppostie to humans.
Drosophila females are homogametic - Drosophila females have two X chromosomes and may also have a Y chromosome (like human females). Males have an X chromosome and if they’re fertile, a Y chromosome.
Heterogametic sex (digametic sex) refers to the sex of a species in which the sex chromosomes are not the same. For example, in humans, males, with an X and a Y sex chromosome, would be referred to as the heterogametic sex, and females having two X sex chromosomes would be referred to as the homogametic sex.
Eye colour is X-linked in Drosophilia
Genes are on the X
hemiygous = hemi = half
Sons can’t get white because dad has white on x chromosome which isn’t passed onto son
Appreciate
The daughters are carriers
Red-green colour blindness
It’s an X linked trait in humans
Males have nothing to fall back on if they have the colour blindness alelle whereas females have another X
The sons won’t get colour blindess but may be carriers.
Appreciate this
Appreciate
X^a = recessive
X^A = dominant
Q. How many genes are there in humans?
Q. How many chromosomes are there in humans?
A. around 20,000
A. 23 pairs
That’s about 1000 genes per chromosome
What’s the relationship between the distance between the genes and chiasma forming
Distant have a recombination frequency of what?
Close genes have recombination frequencies of between?
The smaller the distance between two genes - the less likely a chiasma will be formed between them
Distant (unlinked) genes have a recombination frequency of 50% (half recombinant and half parental types).
Close genes have recombination frequencies of between 0-50%.
There is a near linear relationship between distance and recombination frequency and this led A. Sturtevant and T. H. Morgan to an interesting idea
When we cross two BbVv with bbvv - we expect a 1:1:1:1 ratio but that doesn’t happen - what’s this expected from?
The results weren’t that though - what’s the conclusion?
Mendel’s 2nd law (independent assortment of alleles) says it should be 1:1:1:1 ratio because a heterozygote is being mated with a homozygote recessive (test cross).
Conclusion: these two genes do not sort independently. They are linked on the same chromosome.
The parental types must be much larger in number than recombinant types bc parental on same chromosome.
Theoratically, meisosis should what?
Theoretically meiosis should cause all gametes to have parental genotypes
What explains recombination?
‘Crossing over’ during meiosis explains recombination
Recombinant frequency eg “17% of gametes contain a chromosome that has formed a chiasmata between the two genes”
Appreciate
