Question

Three loci, mitochondrial malate dehydrogenase that forms \(a\) and \(b(M D H a, M D H b),\) glucouronidase that forms 1 and 2 \((G U S 1, G U S 2),\) and a histone gene that forms \(+\) and \(-\left(H^{+},\right.\) \(\left.H^{-}\right),\) are located on chromosome \(\\# 7\) in humans. Assume that the \(M D H\) locus is at position \(35, G U S\) at position \(45,\) and \(H\) at position \(75 .\) A female whose mother was homozygous for \(M D H a, G U S 2,\) and \(H^{+}\) and whose father was homozygous for \(M D H b, G U S 1,\) and \(H^{-}\) produces a sample of 1000 egg cells. Give the genotypes and expected numbers of the various types of cells she would produce. Assume no chromosomal interference.

Short answer

Answer: The expected number of cells in a sample of 1000 egg cells are: - 30 cells with MDHa GUS1 H+ genotype - 70 cells with MDHa GUS1 H- genotype - 270 cells with MDHa GUS2 H+ genotype - 630 cells with MDHa GUS2 H- genotype

Step-by-step solution

Calculate recombination probabilities

To calculate recombination probabilities, find the difference between the given positions of the loci and divide by 100. Recombination probability between MDH and GUS is (45 - 35) / 100 = 0.1, and between GUS and H is (75 - 45) / 100 = 0.3.

Determine parental gametes

The mother has genotype MDHa MDHa GUS2 GUS2 H+ H+ and the father has genotype MDHb MDHb GUS1 GUS1 H- H-. The combination of these parental genotypes could produce the following four gametes: 1. MDHa GUS1 H+ 2. MDHa GUS1 H- 3. MDHa GUS2 H+ 4. MDHa GUS2 H-

Calculate expected number of cells for each type

Using the probabilities from Step 1 and the four possible gametes from Step 2, we can calculate the expected number of cells for each type in a sample of 1000 egg cells: 1. MDHa GUS1 H+: 0.1 * 0.3 * 1000 = 30 cells 2. MDHa GUS1 H-: 0.1 * 0.7 * 1000 = 70 cells 3. MDHa GUS2 H+: 0.9 * 0.3 * 1000 = 270 cells 4. MDHa GUS2 H-: 0.9 * 0.7 * 1000 = 630 cells So, in the sample of 1000 egg cells, the expected number of cells with each genotype are: - 30 cells with MDHa GUS1 H+ genotype - 70 cells with MDHa GUS1 H- genotype - 270 cells with MDHa GUS2 H+ genotype - 630 cells with MDHa GUS2 H- genotype

Key concepts

Chromosome Mapping

Chromosome mapping is a vital method used in genetic research to determine the order and spacing of genes on a chromosome. It helps researchers understand how genes relate to one another and their physical distances on the chromosome. In this context, each point on a chromosome, known as a locus, represents a specific gene or marker position.
The loci for the three genes in our example, MDH, GUS, and H, are located at positions 35, 45, and 75, respectively, along chromosome #7. The distances between these loci are significant because they influence the recombination probabilities.
Chromosome mapping serves several purposes:

• It helps in calculating recombination probabilities, which is crucial in determining the likelihood of genes being inherited together.
• It provides insights into the genetic makeup and diversity through understanding the mix and match of parental genes.
• By knowing the positions of genes, researchers can also identify and study genetic disorders linked to specific loci.

Locus

The term 'locus' (plural: loci) refers to a distinct location on a chromosome where a specific gene is situated. Each locus serves as a fixed position, similar to an address or coordinate, that guides geneticists in identifying the exact spot of a gene within the vast chromosomal structure.
In our exercise, we have three loci involved: MDH at 35, GUS at 45, and H at 75. These numerical positions highlight the specific points along chromosome #7 where each respective gene is located. Understanding the loci helps us calculate genetic recombination and predict offspring variations.
Key aspects about locus:

• Loci are crucial for mapping genetic traits and understanding inheritance patterns.
• They provide a physical reference for genes, which is essential for studying gene interactions and mutations.
• The distance between loci on a chromosome can affect how often genes are shuffled during recombination.

Gene Recombination Probability

Gene recombination probability is a core concept in genetics that predicts the likelihood of genes being exchanged during meiosis, the process that forms gametes (sperm and egg cells). This probability is influenced by the distance between genes on a chromosome, and it plays a critical role in genetic diversity.
In our example, to calculate the recombination probability, you subtract the positions of two loci and divide by 100:

• Between MDH (35) and GUS (45), the recombination probability is \(\frac{45-35}{100} = 0.1\).
• Between GUS (45) and H (75), it is \(\frac{75-45}{100} = 0.3\).

Lower probability means the genes are less likely to be separated during gamete formation. This influences the combinations of traits in offspring. Understanding these probabilities:

• Helps in predicting the possible genotypes and expected number of each cell type in progeny.
• Is crucial for geneticists to examine and anticipate the inheritance patterns in populations.
• Aids in identifying potential genetic variations that could lead to different traits or diseases.