Problem 8 - Campbell Biology

Question

Two genes of a flower, one controlling blue \((B)\) versus white (b) petals and the other controlling round \((R)\) versus oval \((r)\) stamens, are linked and are 10 map units apart. You cross a homozygous blue/oval plant with a homozygous white/ round plant. The resulting \(\mathrm{F}_{1}\) progeny are crossed with homozygous white/oval plants, and 1,000 offspring plants are obtained. How many \(\mathrm{F}_{2}\) plants of each of the four phenotypes do you expect?

Step-by-Step Solution

Answer

Expect 450 blue/oval, 450 white/round, 50 blue/round, and 50 white/oval plants.

Step 1 - Determine Parent Genotypes

The first parent genotype is homozygous blue/oval (BBrr). The second parent genotype is homozygous white/round (bbRR).

Step 2 - Determine F1 Genotype

Cross these two parents to get the F1 progeny. The resulting F1 plants will all have the genotype BbRr.

Step 3 – Set Up the F1 Cross

The F1 plants (BbRr) are crossed with homozygous white/oval plants (bbrr). Use a test cross to determine all possible gametes.

Step 4 - Calculate Recombination Frequency

Since the genes are linked and 10 map units apart, the recombination frequency is 10%. This means 10% of the offspring will have recombinant genotypes, and 90% will have parental genotypes.

Step 5 - Determine Expected Phenotypic Ratios

Expect 90% parental types: 45% blue/oval (Bbrr) and 45% white/round (bbRr). Expect 10% recombinant types: 5% blue/round (BbRr) and 5% white/oval (bbrr).

Step 6 - Calculate Expected Number of Each Phenotype

Multiply the total number of offspring (1,000) by the expected phenotypic ratios to get the expected numbers: blue/oval = 450, white/round = 450, blue/round = 50, white/oval = 50.

Key Concepts

genetic linkage

Genetic linkage occurs when genes are located close to each other on the same chromosome. This means they tend to be inherited together during meiosis because they are less likely to be separated by recombination.
In our example, the genes controlling petal color and stamen shape are linked, meaning they do not assort independently.
This linkage affects the expected ratios of phenotypes in the offspring.
Understanding genetic linkage is crucial for predicting genetic outcomes in crosses.

recombination frequency

Recombination frequency indicates how often crossing over occurs between two genes during meiosis.
It is expressed as a percentage and helps measure the genetic distance between the genes. For instance, if the recombination frequency is 10%, this indicates that crossing over happens 10% of the time.
In our flower example, the recombination frequency of 10% means that 10% of the offspring will exhibit recombinant phenotypes, while 90% will have parental phenotypes.
This concept helps in predicting and calculating the distribution of phenotypes in the progeny.

phenotypic ratios

Phenotypic ratios describe the proportion of different observable traits (phenotypes) among offspring.
These ratios depend on the genetic combinations resulting from a cross.
In our example, due to the genetic linkage and recombination frequency, we calculate that 90% of the offspring will show parental phenotypes (45% blue/oval and 45% white/round), and 10% will show recombinant phenotypes (5% blue/round and 5% white/oval).
Understanding phenotypic ratios aids in predicting the appearance of traits in future generations.

test cross

A test cross involves breeding an individual showing a dominant phenotype (but with unknown genotype) with a homozygous recessive individual.
This helps uncover the genotype of the dominant-phenotype parent based on the offspring's phenotypes. In our flower example, the F1 plants (BbRr) were crossed with homozygous white/oval plants (bbrr).
The resulting phenotypes and their ratios give insight into the genetic linkage and recombination frequency, crucial for understanding the genetic structure of the plants.

map units

Map units (or centiMorgans) measure the distance between genes on a chromosome.
One map unit equals a 1% recombination frequency, showing how often crossing over occurs between two genes.
In our flower example, the genes are 10 map units apart, meaning there's a 10% chance of recombination between them.
By using map units, geneticists can construct linkage maps, helping to visualize gene positions and distances, critical for genetic analysis and breeding strategies.