What is a dihybrid cross?
A dihybrid cross determines the allele combinations of offspring for two particular genes that are unlinked (not on the same chromosome)
To work out gamete combinations remember FOIL:
• First (AaBb = AB)
• Outside (AaBb = Ab)
• Inside (AaBb = aB)
• Last (AaBb = ab)
Assuming that the flowers' dominant alleles are phenotypes for tall and purple. If there are two heterozygous tall purple flowers crossed, what is the probability of getting offspring with the same phenotype?
What is used to determine whether the difference between an observed and expected frequency distribution is statistically significant?
Chi squared/ X2
Degrees of freedom are determined by the number of types of offsprings, minus one.
Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in the exchange of alleles
- crossing over/chiasmata formed during prophase I of meiosis;
- pairing of homologous chromosomes/synapsis; chromatids break (at same point); (do not accept chromatids overlap)
- non-sister chromatids join up/swap/exchange alleles/parts;
- X-shaped structure formed / chiasmata are X-shaped structures;
- chiasma formed at position where crossing over occurred;
- chiasmata become visible when homologous chromosomes unpair;
- chiasma holds homologous chromosomes together (until anaphase
Draw a diagram of chiasma formed by crossing over
Recombinants formed by crossing over
Explain the relationship between Mendel's Law of Independent Assortment and Meiosis
Meiosis divides the chromosomes into groups of four. Each allele for one trait has a 50% chance of being passed on. But the two alleles combined make four different options, which makes the chances for these being passed on 25%.
State Mendel's Law of Independent Assortment
The alleles of two genes pass into gametes without influencing each other.
What is an autosomal linkage group?
When two genes are located on the same autosomal chromosome
What is polygenic inheritance?
When many genes control the same phenotype (or characteristics). A good example is skin colour.
What do phenotypes of polygenetic characteristic tend to show?
The phenotypes of polygenic characteristics tend to show continuous variation
Explain an example of a cross between two linked genes.
Unlinked genes assort independently in a dihybrid cross between heterozygotes, producing a 9:3:3:1 phenotypic ratio
Linked genes, without crossing over, do not follow the expected 9:3:3:1 phenotypic ratio for a dihybrid cross between heterozygotes
- Instead the result will follow the 3:1 phenotypic ratio of a monohybrid cross, as the two linked genes are inherited together
- This means that all the offspring will produce parental phenotypes
Linked genes with crossing over produce recombinant phenotypes in numbers that reflect the distance between the linked genes
- distant genes will cross over more frequently, producing a higher percentage of recombinants
- genes that are close together will cross over less frequently, producing a lower percentage of recombinants
A test cross involving two linked genes, A and C: AaCc x aacc
- 80% of offspring are parental, either AaCc or aacc
- 20% of offspring are recombinant, Aacc or aaCc
- thus, the two genes are linked and are 20 cM apart on the same chromosome