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Biology: Genetics, Reproduction, and Biotechnology - Genes, Alleles, and the Genetic Code

Grade 8IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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DNA (Deoxyribonucleic Acid) is the molecule that carries genetic information. It is structured as a double helix composed of nucleotides, which consist of a phosphate group, a deoxyribose sugar, and one of four nitrogenous bases: Adenine (AA), Thymine (TT), Cytosine (CC), and Guanine (GG).

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A Gene is a specific segment of DNA located on a chromosome that codes for a particular protein, which in turn determines a specific trait.

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Alleles are alternative forms of a single gene. They are usually represented by letters: capital letters for dominant alleles (e.g., BB) and lowercase letters for recessive alleles (e.g., bb).

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Genotype refers to the genetic makeup of an organism (e.g., BBBB, BbBb, or bbbb), while Phenotype refers to the observable physical characteristics resulting from the genotype.

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Homozygous organisms have two identical alleles for a trait (e.g., BBBB or bbbb). Heterozygous organisms have two different alleles (e.g., BbBb).

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The Genetic Code is read in sets of three nitrogenous bases called codons. Each codon, such as AUGAUG or GCAGCA, corresponds to a specific amino acid in a protein chain.

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Punnett Squares are diagrams used to predict the probability of genotypes and phenotypes in offspring from a genetic cross between two parents.

📐Formulae

[A]=[T] and [C]=[G][A] = [T] \text{ and } [C] = [G]

[A]+[T]+[C]+[G]=100%[A] + [T] + [C] + [G] = 100\%

P(trait)=Number of specific offspring genotypesTotal number of possible genotypes×100%P(\text{trait}) = \frac{\text{Number of specific offspring genotypes}}{\text{Total number of possible genotypes}} \times 100\%

Phenotypic Ratio=Number of Dominant Phenotypes:Number of Recessive Phenotypes\text{Phenotypic Ratio} = \text{Number of Dominant Phenotypes} : \text{Number of Recessive Phenotypes}

💡Examples

Problem 1:

In pea plants, the allele for tall height (TT) is dominant over the allele for short height (tt). If two heterozygous tall plants (TtTt) are crossed, what is the probability that an offspring will be short (tttt)?

Solution:

The Punnett square for Tt×TtTt \times Tt results in: TtTTTTttTttt\begin{array}{c|c|c} & T & t \\ \hline T & TT & Tt \\ \hline t & Tt & tt \end{array} Out of 4 possible combinations, only 1 is tttt. P(tt)=14=25%P(tt) = \frac{1}{4} = 25\%

Explanation:

A short phenotype only occurs when the genotype is homozygous recessive (tttt). In a monohybrid cross of two heterozygotes, the genotypic ratio is 1:2:11:2:1 (TT:Tt:ttTT:Tt:tt), leading to a 25%25\% chance for the recessive trait.

Problem 2:

A sample of DNA is analyzed and found to contain 22%22\% Adenine (AA). Calculate the percentage of Cytosine (CC) present in the sample.

Solution:

According to Chargaff's rule, [A]=[T][A] = [T]. Therefore, if [A]=22%[A] = 22\%, then [T]=22%[T] = 22\%. Total A+T=22%+22%=44%A + T = 22\% + 22\% = 44\%. The remaining percentage for C+GC + G is: 100%−44%=56%100\% - 44\% = 56\% Since [C]=[G][C] = [G], we divide the remaining percentage by 2: [C]=56%2=28%[C] = \frac{56\%}{2} = 28\%

Explanation:

Because DNA bases pair specifically (AA with TT and CC with GG), their amounts must be equal. Subtracting the known pair from 100%100\% gives the total for the other pair, which is then halved.