07-Linkage

Non-Mendelian genetics explores how alleles can assort differently than expected under Mendelian inheritance due to linkage and recombination.
Key concepts include the following:
- Independent Assortment vs. Linkage: Mendelian inheritance suggests alleles segregate independently, but linked genes do not assort independently.
- Crossing Over and Recombination: This is the process where homologous chromosomes exchange segments during meiosis, leading to new allele combinations.
- Test Crosses with Linked Loci: This gene mapping technique assesses how closely genes are linked on a chromosome.
- Coupling and Repulsion: Refers to arrangement of dominant and recessive alleles in gametes; alleles can be in coupling (cis) or repulsion (trans) configurations.
- Two-point Mapping: A method used to determine the genetic distance between two loci based on recombinant frequencies.
- Three-loci Linkage Maps: Similar mapping for three loci, assessing the distances and relationships among them.

When dealing with two heterozygous gene pairs located on different chromosomes (e.g., AaBb), crossovers in metaphase I lead to independent assortment producing four types of gametes: 1/4 AA, 1/4 Aa, 1/4 Bb, and 1/4 ab.
This results in different allele combinations that are consistent with Mendel's laws.

In cases where gene pairs (e.g., AaBb) are linked (on the same chromosome), only two types of gametes will form based on linkage patterns, indicating alleles are inherited together.

Crossing over occurs during prophase I of meiosis and involves breaking and reunion of non-sister chromatids. This process can yield both nonrecombinant (parental types) and recombinant types of gametes, where the latter are produced at a lower frequency.

Gametes produced are divided into:
- Nonrecombinant (parental, P): Gametes that do not show crossover and maintain original allele combinations.
- Recombinant (nonparental, NP): Result from crossing over, creating new allele combinations.

Presence of Nonrecombinant Gametes: When crossing over occurs but is absent (single crossover event), only two types of gametes could arise leading to a high frequency of them, with no recombination of alleles.
In contrast, if crossing occurs between two loci, both nonrecombinant and recombinant gametes will be produced with a broader variety of types.

A testcross involves crossing a heterozygous individual (MmDd) with a homozygous recessive individual (mmdd) to understand allele frequencies in offspring. This provides insight on how closely genes are linked.
The resulting proportions help in determining the type of gamete and whether they are nonrecombinant or recombinant.

The arrangement of alleles in coupling or repulsion forms influences the gametes formed, affecting phenotypic ratios in offspring.
Coupling Configuration (Cis): Both dominant alleles are on one homologue and recessive on the other (e.g., p+b+ produces more nonrecombinant offspring).
Repulsion Configuration (Trans): Dominant and recessive alleles are distributed across homologues leading to different ratios in progeny.

Requires a cross between heterozygous (for both loci) and homozygous recessive individuals; the frequency of recombinant progeny indicates linkage.
This method allows the mapping of genetic distances between loci, with greater distances correlating to higher recombination probabilities.

Similar to two-point mapping but involves assessing three loci within the same organism. Measurements denote the percentage of recombination between pairs of loci, furthering the assessment of gene position on chromosomes.

Recombination percentages are computed to generate linkage maps, which aid in understanding gene order and distances on chromosomes. These maps are essential for illustrating genetic relationships and distances, as they don’t necessarily correspond to physical distance.