Lecture 25 Flashcards
What methods of sex determination are there?
Mammals have a system which uses XY, XX is female, XY is male, a Y chromosome in humans means male, hence XXY would be male. In things like bees fertilised eggs will end up as diploid and hence be female, unfertilised eggs will be haploid and hence male. Birds you a Zw system, ZZ is male and Zw is female.
What is a sex linked trait?
A sex linked trait is one which is found on either of the sex chromosomes.
What does the presence of an X linked trait gene in males mean vs in females? Why? Which parents can pass this on?
In males, who only have one X chromosome (passable on only by mum) they are guaranteed to get the trait regardless of it being recessive or dominant. In females who get one copy from mum and one from dad the trait will only be present if the trait is dominant, or if it is recessive she will need the allele from both mum and dad, this protects females from X linked recessive diseases, but allows them to be unknowing carriers.
If a father has an X linked disease what does this mean about his offspring?
All daughters will receive the allele, no sons will. (Assuming normal mum)
What will a carrier mother for an X linked trait lead to in her offspring?
Half of her sons will have the mutation, half of her daughters will be carriers, the rest will be normal. (Assuming normal dad)
How does the convention for fruit fly genes work?
The first mutant (non wild type) discovered provides the letters, the wild type for that gene is given a + after the symbol. The non wild type will typically be recessive.
What are the parental types and recombinant types?
The parental types are offspring which have the same phenotype as one of the parents, the recombinant types are the ones which have a mix of the parental traits.
What is a linked gene?
Genes which are found on the same chromosome, these do not assort independently.
What would be the expected result of offspring for linked genes? Why is this not the case?
All offspring would be expected to be parental types, this does not occur because crossing over of chromosomes during meiosis recombines the alleles.
How does crossing over occur?
Two chromatids of a tetrad (one from each pair) cross at random points (forming chiasma) during late prophase I and swap genetic material, there can be multiple chiasma formed between two chromatids.
What is the recombination frequency and what is it given by?
The recombination frequency is given by the proportion of recombinant genes. This can be gotten by taking the total number of recombinant offspring and dividing this by the total number of offspring.
What is a genetic map? How did it come about? What is a map unit and what is the maximum value? Why?
A genetic map maps the relative distant between two linked genes by using recombinant frequencies. The relative chance of a chiasmata forming at any point is the same, hence further away genes will have a higher recombinant frequency, these are measured in map units (centimorgan, cM, not an actual distance, only relative) and is given by 100x the recombination frequency. The maximum value is 50 as this correlates with a 50% chance, this value cannot be higher than this as at this point the genes are essentially unlinked and are equally as likely to be seperated as brought back together by another cross over.
What does hemizygous mean? What about heterogametic and homogametic?
Hemizygous means an organism has only one of a chromosome e.g human males have only one X chromosome. Heterogametic means two different sex chromosomes (human males) homogametic means two of the same sex hormone (human females).
