Problem 3 - Campbell Biology Concepts & Connections

Question

Edward was found to be heterozygous (Ss) for sickle-cell trait. The alleles represented by the letters \(S\) and \(s\) are a. linked. b. on homologous chromosomes. c. both present in each of Edward's sperm cells. d. on the same chromosome but far apart.

Step-by-Step Solution

Answer

b. on homologous chromosomes.

Step 1: Understand the genetic terminology

Edward is heterozygous (Ss) for sickle-cell trait. This means he has two different alleles for this trait: one normal allele (S) and one mutant allele (s).

Step 2: Rule out option a

Linked alleles are usually inherited together because they are close to each other on the same chromosome. Being heterozygous does not tell us if the alleles are linked. Therefore, option a is unlikely to be correct for this information.

Step 3: Analyze option b

In a heterozygous individual, the two different alleles (S and s) are located on homologous chromosomes. This is consistent with Edward having (Ss) alleles, meaning 'b' could be correct.

Step 4: Rule out option c

Each sperm cell receives only one allele from a homologous pair. Hence, Edward's sperm cells would either carry the S allele or the s allele, but not both. Therefore, option c is incorrect.

Step 5: Evaluate option d

The alleles being on the same chromosome but far apart does not apply here. Heterozygosity indicates that each allele is on a separate homologous chromosome, not the same chromosome. Thus, option d is incorrect.

Key Concepts

heterozygous

The term 'heterozygous' plays a pivotal role in understanding genetic variation. When an individual is heterozygous for a particular trait, like Edward in the case of the sickle-cell trait, it means they possess two different alleles for that trait. In Edward's instance, these are the normal allele (S) and the mutant allele (s).

Being heterozygous can impact the expression of certain traits, especially if dominant and recessive alleles are involved. In this case, 'S' is the dominant allele while 's' is the recessive one. Consequently, Edward's 'Ss' combination allows him to express the normal trait while still carrying the sickle-cell trait.

alleles

Alleles are different versions of the same gene. They dictate variations in inherited characteristics. For a specific trait, such as sickle-cell disease, there can be multiple alleles.

One allele can be dominant (like S in Edward's case), and the other recessive (s for sickle-cell). An individual inherits one allele from each parent. Thus, Edward, being heterozygous, has inherited the 'S' allele from one parent and the 's' allele from the other. Each parent provided a different version of the gene responsible for the sickle-cell trait.

homologous chromosomes

Homologous chromosomes are pairs of chromosomes that have the same structure and gene sequence but may carry different alleles. Humans have 23 pairs of homologous chromosomes, one set from each parent.

In the context of Edward's heterozygous sickle-cell trait (Ss), one chromosome carries the S allele and its homologous partner contains the s allele. This configuration means Edward's sperm cells will receive either the S or the s allele during meiosis, but not both. This separation ensures genetic diversity in the offspring.

sickle-cell trait

The sickle-cell trait is a genetic condition resulting from an allele mutation in the hemoglobin gene. Being heterogeneous for this trait, as in Edward's case (Ss), doesn’t lead to sickle-cell disease but means he carries one normal allele (S) and one sickle-cell allele (s).

People with only the sickle-cell trait usually don't exhibit symptoms of sickle-cell disease but can pass the mutant allele to their offspring. If a child's genotype includes two sickle-cell alleles (ss), they will develop sickle-cell anemia, a condition where red blood cells become crescent-shaped, causing health complications.