Problem 4 - Campbell Biology

Question

A planet is inhabited by creatures that reproduce with the same hereditary patterns seen in humans. Three phenotypic characters are height \((T=\) tall, \(t=\) dwarf \(),\) head appendages \((A=\) antennae, \(a=\) no antennae \(),\) and nose shape \((S=\) upturned snout, \(s=\) downturned snout). Since the creatures are not "intelligent," Earth scientists are able to do some controlled breeding experiments using various heterozygotes in testcrosses. For tall heterozygotes with antennae, the offspring are tall/antennae, \(46 ;\) dwarf/antennae, \(7 ;\) dwarf \(/\) no antennae, 42; tall/no antennae, \(5 .\) For heterozygotes with antennae and an upturned snout, the offspring are antennae/upturned snout, 47; antennae/downturned snout, \(2 ;\) no antennae/downturned snout, \(48 ;\) no antennae/upturned snout, 3 . Calculate the recombination frequencies for both experiments.

Step-by-Step Solution

Answer

12% and 5%

Step 1: Understanding the Phenotypic Classes

Identify the different phenotypic classes based on offspring counts for both breeding experiments: First for height and head appendages, then for head appendages and nose shape.

Step 2: Determine Total Number of Offspring

Add the number of individuals in each phenotypic class to get the total number of offspring for each experiment.

Step 3: Identify Parental and Recombinant Types

Determine which classes correspond to parental and recombinant offspring, based on highest and lowest counts respectively.

Step 4: Calculate Recombination Frequency for Height and Head Appendages

Use the formula for recombination frequency Recombination Frequency = \frac{Number of Recombinant Offspring}{Total Number of Offspring} \times 100%\(\frac{7 + 5}{46 + 7 + 42 + 5} \times 100 = 12% \).

Step 5: Calculate Recombination Frequency for Head Appendages and Nose Shape

Use the formula for recombination frequency Recombination Frequency = \frac{Number of Recombinant Offspring}{Total Number of Offspring} \times 100%\(\frac{2 + 3}{47 + 2 + 48 + 3} \times 100 = 5%\).

Key Concepts

hereditary patterns

Hereditary patterns describe how genes and traits are passed down from parents to offspring. In these creatures, scientists observe that their hereditary patterns are similar to those in humans. This means that certain traits or phenotypic characters like height, head appendages, and nose shape follow Mendelian inheritance rules.

Mendelian inheritance relies on dominant and recessive alleles. For example:

• For height, the tall allele \(T\) is dominant, and the dwarf allele \(t\) is recessive.
• For head appendages, the presence of antennae \(A\) is dominant over the absence of antennae \(a\).
• For nose shape, the upturned snout \(S\) is dominant, while the downturned snout \(s\) is recessive.

The controlled breeding experiments help scientists understand these patterns by observing offspring from heterozygous parents, which carry both dominant and recessive alleles. By analyzing the offspring, the dominance and segregation of these traits become clear.

phenotypic characters

Phenotypic characters are observable traits an organism exhibits, influenced by its genetic makeup and sometimes its environment. In the creatures from the exercise, three phenotypic characters are given: height, head appendages, and nose shape.

Each of these traits can be described as follows:

• Height: Creatures can be tall (T) or dwarf (t).
• Head Appendages: Creatures may have antennae (A) or no antennae (a).
• Nose Shape: Creatures might have an upturned snout (S) or a downturned snout (s).

Understanding these characters helps in calculating recombination frequencies, because we can distinguish between different trait combinations in offspring. If a particular combination, like tall with antennae, appears more frequently, it suggests certain alleles (gene versions) are inherited together often. Conversely, less frequent combinations indicate recombination, where alleles get shuffled and inherited independently.

recombination frequency

Recombination frequency measures how often two specific gene loci on a chromosome exchange genetic material during meiosis. It helps understand genetic linkage and chromosome mapping.

To calculate recombination frequency in the given exercise, you use this formula:

• Recombination Frequency = \[ \frac{Number \, of \, Recombinant \, Offspring}{Total \, Number \, of \, Offspring} \times 100 \, \% \]

For example, for the height and head appendages characters, we've determined that there are 12% recombination frequency. Here's a breakdown:

• Recombinant offspring (dwarf/antennae and tall/no antennae): 7 + 5 = 12.
• Total offspring: 46 + 7 + 42 + 5 = 100.

So, \[ Recombination \, Frequency \, = \, \frac{12}{100} \times 100 \, = \, 12 \, \% \]

Similarly, for head appendages and nose shape, with a recombination frequency of 5%, calculated as:

• Recombinant offspring (antennae/downturned snout and no antennae upturned snout): 2 + 3 = 5.
• Total offspring: 47 + 2 + 48 + 3 = 100.

Therefore, \[ Recombination \, Frequency \, = \, \frac{5}{100} \times 100 \, = \, 5 \, \% \]
High recombination frequency indicates genes are far apart, while low frequency suggests close linkage on the chromosome.