Session 1: Double Gene Inheritance Flashcards
What are dihybrid crosses?
two unlinked genes - the genes are located on different chromosome pairs, and undergo independent assortment during meiosis. (E.g. two unlinked genes form four allele combinations, whereas one normal gene has two allele combinations.
What is Mendel’s First principle (law)?
Principle of segregation - when any individual produces gametes, the copies of a gene (alleles) separate, so that each gamete receives only one copy.
What is Mendel’s second principle (law)?
Principle of independent assortment - alleles of different genes assort independently of one another during gamete formation.
(This is due to the random orientation of homologous pairs during metaphase I of meiosis)
What are the steps to completing a dihybrid cross?
Step 1: Designate characters to represent the alleles. Capital letter for dominant allele, lower case letter for recessive allele. Step 2: Write down the genotype and phenotype of the parents (P generation). Always pair alleles from the same gene and always write capitals first (e.g. AaBb, not ABab)
Step 3: Write down all potential gamete combinations for both parents. Use the FOIL method to identify all possible combinations. Step 4: Use a Punnett square to work out potential genotypes of offspring (F1 generation). Only include the different gamete combinations for each parent (e.g. AaBB has two combinations = AB and aB). Step 5: Write out the phenotype ratios of potential offspring Phenotypic ratios reflect mathematical probabilities only and may not necessarily reflect actual offspring ratios
What are linked genes (linkage)? and What is recombination?
Linkage is when genes are linked and will be passed on together. Recombination is the opposite to linkage, it’s when crossing over occurs to make new combinations.
How can we tell that genes are linked?
if the offspring phenotypes of a dihybrid cross are different to predicted ratios based on independent assortment. (e.g. if the predicted results are largely different from the actual results)
Recombination can cause…
Recombination can cause alleles previously on the same chromosome to be separated and end up in different daughter cells. The farther the two alleles are apart, the greater the chance that a cross-over event may occur between
them, and the greater the chance that the alleles are separated.
What does distance have to do with recombination? (and what is a centimorgan?)
Thomas Morgan demonstrated that genes that were further apart on a chromosome were more likely to recombine. This is because there are more potential sites for crossing over to occur between distant genes. A centimorgan is the unit used.
Describe centimorgans.
A centimorgan is a unit of measure used to approximate the distance between genes. A centimorgan is not a true measure of distance and will differ in equivalence between species. In humans, 1 cM corresponds to approximately 1 million bp on average. While centimorgans may still be used to quantitate recombination frequencies, they are no longer used to measure distance. Genome mapping has allowed scientists to determine specific distances between genes in kilo bases (kb).
What is the map distance formula?
Map distance = (number of recombinant offspring x 100) / total number of offspring.
What is Chi Squared used for?
to compare observed data with data we would expect to obtain according to a specific hypothesis (we use it to test the null hypothesis - that there is no difference between expected and actual)
