Problem 5 - Biology: Concepts and Applications

Question

\(\mathrm{F}_{1}\) offspring of the cross \(A A \times\) aa are ______. a. all \(A A\) b. all \(a a\) c. all \(A a\) d. \(1 / 2 A A\) and \(1 / 2\) aa

Step-by-Step Solution

Answer

All F1 offspring are 'Aa', which is answer c.

Step 1: Understanding Genotype Symbols

The symbol 'AA' represents a homozygous dominant genotype, meaning the individual has two dominant alleles. The symbol 'aa' represents a homozygous recessive genotype, meaning the individual has two recessive alleles. The question involves a cross between these two types.

Step 2: Setting Up the Punnett Square for the Cross

To determine the offspring genotypes, set up a Punnett square. Place 'AA' on one side of the square and 'aa' on the other side. Each square represents a possible genotype of the offspring.

Step 3: Filling in the Punnett Square

Fill in the Punnett square by combining alleles from both parents. Every combination will result in an 'Aa' genotype because each offspring will receive one 'A' allele from the 'AA' parent and one 'a' allele from the 'aa' parent.

Step 4: Analyzing the Results

The Punnett square shows that all offspring have the genotype 'Aa'. This means that all first-generation (F1) offspring are heterozygous, having one dominant and one recessive allele.

Key Concepts

Genotype

Genotype is the genetic makeup of an organism, specifically the combination of alleles inherited from the parents. It determines many of the physical traits and characteristics of the organism. Genotypes are typically represented using letter symbols, with each letter portraying an allele.

The two letters in a genotype come from each parent's alleles: one from the mother and one from the father. These letters can be either uppercase or lowercase, symbolizing dominant and recessive alleles, respectively.

• A dominant allele (denoted by an uppercase letter like 'A') has the ability to mask the expression of a recessive allele
• A recessive allele (denoted by a lowercase letter like 'a') will only show its effect if the organism has two copies of this allele (homozygous recessive)

Understanding genotypes is essential in predicting an organism's traits and can help in determining the likelihood of certain traits appearing in offspring.

Homozygous

When an organism has two identical alleles for a specific trait, it is termed homozygous. These alleles can be both dominant or both recessive, and they play a crucial role in determining the trait's outcome.

If both alleles are dominant, we call this state 'homozygous dominant' (e.g., 'AA'). In this situation, the dominant trait is visibly expressed because there is no recessive allele to mask it.

On the other hand, if the alleles are both recessive, it is referred to as 'homozygous recessive' (e.g., 'aa'). Here, the recessive trait is shown, as there are no dominant alleles to overshadow it.

• Homozygous dominant: Both alleles are dominant (AA)
• Homozygous recessive: Both alleles are recessive (aa)

In genetics, understanding whether an organism is homozygous can assist in predicting the likelihood of certain traits passing to the next generation.

Heterozygous

Heterozygous refers to having two different alleles for a particular trait. In this configuration, one allele is typically dominant, and the other is recessive. The dominant allele will express the trait, while the recessive allele is masked.

For instance, if an individual has the genotype 'Aa', they are heterozygous. Here, 'A' is the dominant allele and it will determine the trait that is expressed, while 'a' is the recessive allele which is not expressed.

• Trait is determined by the dominant allele
• Recessive allele remains unexpressed unless paired with another recessive allele

Heterozygosity introduces genetic variation and is vital for the diversity we see in biology, as it allows for multiple possible trait combinations in offspring.

Alleles

Alleles are different forms of a gene found at the same place (locus) on a chromosome. Each organism inherits two alleles for each trait, one from each parent. These alleles can either be identical or different, leading to homozygous or heterozygous genotypes, respectively.

Alleles can be either dominant or recessive:

• Dominant alleles will show their effect even if there is only one copy present
• Recessive alleles require two copies to exhibit their associated trait

The combination of alleles that an organism possesses is crucial in determining its phenotype, which is the observable expression of a trait. Alleles therefore are central to understanding inheritance patterns and predicting genetic outcomes in offspring.