6.2: the dihybrid cross Flashcards
What’s a test-cross?
In a testcross a recessive parent is crossed with an unknown genotype.
Where does a 9:3:3:1 ratio occur?
A 9:3:3:1 ratio occurs in the F2 generation of a dihybrid cross.
What are the steps of a dihybrid cross?
Gen 1: two pure breeding lines for two separate traits
- E.g. purely round/square + purely green/yellow
F1: dihybrid generation: all heterozygous so only dominant phenotype of each trait is expressed.
F2: each of parents can provide either gene so literally all combos are possible: homo dom, homo rec, het, and green wrinkled, green round, yellow wrinkled, yellow round. These are expressed in a 9:3:3:1 ratio.
How to calculate probabilities of inheriting different traits together without punnet squares?
- Product rule is used to calculate probability of combos of traits occuring. If one or the other, use sum rule.
How did Mendel explain the 9:3:3:1 ratio of F2 progeny resulting from a dihybrid cross?
Mendel explained the 9:3:3:1 ratio resulting from a dihyrbid cross using Mendel’s 2nd Law, the Law of Assortment
What’s the Law of Assortment?
Mendel’s 2nd Law (The Law of Assortment) states that…
During gamete formation, the segregation of the alleles of one gene is independent of the segregation of alleles in another gene
–> Genes are inherited separately
–> Modern science: not always true for all genes bc epistatic interactions.
What’s the product rule and why is it useful?
The product rule says that the probability of independent events occuring together is the PRODUCT of their individual probabilities.
Useful to calculate the probability of inheriting a given genotype for multiple different genes, e.g. the probability that you will produce round seeds that are ALSO yellow and that you ALSO produce purple flowers. To do this, calculate the probability of each of these occuring in isolation (e.g. 3/4, 1/2, and 1/2) and multiply them together (e.g. =3/16).
What’s the sum rule and why is it useful?
The sum rule says that the probability of two mutually exclusive events occuring is the SUM of their individual probabilities.
e.g. odds of rolling 3 and 5 OR 6 and 2 = 1/36 + 1/36
How do we test correlation of theoretical and experimental results?
The Chi Squared test can be used to assess whether results follow a predicted pattern by finding the probability of the null hypothesis being corrrect.
What’s a genetic marker?
A genetic marker is a gene of known map position existing in multiple alleles (exhibits polymorphism) with visible phenotypic effects.
What’s a classical marker?
Classical markers are polymorphisms in genes (with visible phenotypic effects)
What’s a molecular marker?
Molecular markers are polymorphisms in genes OR non-coding DNA which is detected directly at the DNA level
- Variety of types
- May not produce visible phenotypic effects
What’s a dominant marker?
A dominant marker is one where you can’t distinguish between heterozygous and homozygous phenotypes.
–> Mostly classical
What’s a co-dominant marker?
A codominant marker is one where het.zy and homo.zy phenotypes are expressed differently
–> Mostly molecular
