Linkage and Meiosis - Genetics 3 Flashcards
Meiosis
type of cell division that occurs in sexually reproducing organisms to create gametes (haploid)
Haploid
Cells with one set of chromosomes (23)
Diploid
Cells with 2 sets of chromosomes that are copies of eachother
Alleles in a zygote
2 alleles of each gene (one maternal and one paternal)
How does meiosis produce 2 haploid daughter cells?
One round of DNA replication and two rounds of chromosome segregation
Chromosome Segregation
Distribution of chromosomes/chromatids into daughter cells.
First round of chromosome segregation in meiosis
Anaphase 1 - homologous chromosomes separate (pairs of chromosomes)
Second round of chromosome segregation in meiosis
Anaphase II - sister chromatids separate (one chromosome into its 2 arms) by the centromere
Haploid Daughter Cells
Cells with half the chromosome complement.
Independent Assortment
Random distribution of chromosomes during meiosis depending on how the line up during metaphase I and II
Meiosis I
First meiotic division, specialized chromosome segregation.
Difference between segregation in mitosis and meiosis 1
In mitosis kinetochores (proteins) pull apart sister chromotids by the centromere with spindle fibres whereas in meiosis one homologous pairs ar epulled apart
Key steps for meiosis I to occur
- Pairing of homologous chromosomes
- Synapsis
- Crossing over
- Mono-orientation of kinetochore
What needs to happen in meiosis I for the homologous chromosomes to be separated?
The cell needs to be able to recognise homologous chromosomes in order for pairing to occur. Once they pairs find each other they must stay together before division
Synapsis
Pairing of homologous chromosomes during prophase I of meiosis.
Crossing Over
Exchange of genetic material between homologous chromosomes.
Synaptonemal Complex
Structure facilitating synapsis of homologous chromosomes. Made of highly conserved proteins that link together and is well orchestrated.
The crossing of colours (homologous chromosomes) shows strand break and repair from another chromosome = CROSSING OVER
What holds sister chromatids together?
Chohesin. Most need to be lost in order for the chromatids to later be separated
Separase
Enzyme that cleaves a part of the chromosome arms, so chohesin is no longer keeping them together. Some remains in the central part, keeping the sister chromatids together at the centrophere and next to the kinetochore.
Crossover Recombination
Creation of novel allele combinations in gametes during meiosis one (recombinanat chromatids)
Recombination of genes close to each other on the same chromosome is rare
Meiosis products are parental types
A result of no crossing over between the genes (recombination). Results are in expected ratios
Meiosis products are recombinant (and parental)
Crossing over breaks linkage of genes on chromosomes, creating recombinant genes where some are from paternal and some maternal.
Recombination and linkage tells us that…
Genes are on the same chromosome. It doesn’t tell us how close they are
Genetic Linkage
Tendency of genes on the same chromosome to be inherited together. Distorts mandelian ratios
Phenotypes meet expected ratio
We can make the assertion that we are looking at unlinked genes (on different chromosomes)
Recombinant Frequency
Percentage of recombinant offspring from a cross.
Calculating recombination freqency
Add up amount of recombinant individuals and divide by the total number of individuals
CentiMorgan (cM)
A distance. Map unit for measuring genetic linkage (distance between genes on a chromosome. It = recombination frequency as a %
What would happen to recombination frequency and genetic map distance if there were 2 crossover events between the 2 genes?
A more diverse recombination frequency. Lots of different outcomes of phenotypes.
Why is there a max recombination frequency?
More than 50 = the genes are on different chromosomes. Around 49 suggests the two genes are at either end of the chromosome. The genes are unlinked.
What would happen to recombination frequency and genetic map distance if the 2 genes were closer together on the same chromosome?
Recombination frequency would decrease as crossing over separating the genes is less likely to occur (the genes are close together on the chromosome)
Unlinked genes
Genes located on different chromosomes or very far apart on the same chromosome
Mendelian Ratios
Expected ratios of offspring phenotypes from genetic crosses.
Two Crossover Events
Can maintain parental genotype despite recombination.
Linkage-Based Diagnostic Test
Test determining parental origin of chromosome regions for several embryones to know if offspring is affected, a carrier or unaffected.
Monogenic Disease Screening
Testing for single-gene disorders using linkage.
Gametes
Haploid cells involved in fertilization.
Alleles
Different versions of a gene.
Heterozygous
Having two different alleles for a gene.
Homozygous
Having identical alleles for a gene.
