Genetics and Molecular Biology: Study Guide for Lab Quiz 4

Genetics: Mendelian Inheritance and Punnett Squares

Single Gene Crosses and Punnett Squares

Understanding how traits are inherited requires knowledge of Mendelian genetics and the use of Punnett squares to predict offspring genotypes and phenotypes.

• Punnett Square: A diagram used to predict the outcome of a genetic cross by showing all possible combinations of alleles from the parents.

• Alleles in Gametes: Each gamete (sperm or egg) carries one allele for each gene. The combination of alleles from both parents determines the genotype of the offspring.

• Genotype: The genetic makeup of an organism (e.g., AA, Aa, aa).

• Phenotype: The observable traits of an organism resulting from its genotype.

• Genotypic Ratio: The ratio of different genotypes produced in a cross (e.g., 1:2:1 for AA:Aa:aa).

• Phenotypic Ratio: The ratio of observable traits (e.g., 3:1 for dominant:recessive phenotype in a monohybrid cross).

• Listing Ratios: Genotypes and phenotypes are typically listed in order of dominance or frequency.

• Example: Crossing two heterozygous individuals (Aa x Aa) yields genotypes AA, Aa, and aa in a 1:2:1 ratio.

Human ABO Blood Types

Blood Type Genetics and Inheritance

The ABO blood group system is determined by multiple alleles and codominance, resulting in four main blood types in humans.

• Blood Types: A, B, AB, and O.

• Genotypes:

  • Type A: IAIA or IAi

  • Type B: IBIB or IBi

  • Type AB: IAIB

  • Type O: ii

• Antibodies Produced:

  • Type A: Anti-B antibodies

  • Type B: Anti-A antibodies

  • Type AB: No antibodies against A or B

  • Type O: Both anti-A and anti-B antibodies

• Universal Donor: Type O (no A or B antigens)

• Universal Recipient: Type AB (no anti-A or anti-B antibodies)

• Punnett Square Application: Used to predict offspring blood types from parental genotypes.

• Example: Crossing IAi (Type A) with IBi (Type B) can produce all four blood types in offspring.

Blood Type	Genotype(s)	Antibodies Produced	Can Receive From	Can Donate To
A	IAIA, IAi	Anti-B	A, O	A, AB
B	IBIB, IBi	Anti-A	B, O	B, AB
AB	IAIB	None	A, B, AB, O	AB
O	ii	Anti-A, Anti-B	O	A, B, AB, O

Molecular Biology: Transcription and Translation

Transcription: DNA to mRNA

Transcription is the process by which a segment of DNA is copied into messenger RNA (mRNA) for protein synthesis.

• DNA Template Strand: The strand of DNA that is used as a template for mRNA synthesis.

• Coding (Nontemplate) Strand: The DNA strand not used for transcription; its sequence matches the mRNA (except T is replaced by U).

• Orientation: mRNA is synthesized in the 5' to 3' direction.

• Enzyme: RNA polymerase catalyzes the synthesis of mRNA from the DNA template.

• Example: If the template DNA sequence is 3'-TACGGA-5', the mRNA sequence will be 5'-AUGCCU-3'.

Translation: mRNA to Protein

Translation is the process by which the sequence of an mRNA molecule is used to build a polypeptide (protein).

• Structure: Ribosome is the cellular machinery that translates mRNA into protein.

• Genetic Code: The set of rules by which information encoded in mRNA is translated into amino acids.

• Amino Acids: There are 20 different amino acids used in protein synthesis.

• Codon: A sequence of three mRNA nucleotides that specifies a particular amino acid.

• Reading the Codon Table: Each codon corresponds to one amino acid; for example, AUG codes for methionine (start codon).

• Start Codon: AUG (methionine) signals the beginning of translation.

• Stop Codons: UAA, UAG, UGA signal the end of translation.

• Translation Process: The ribosome moves along the mRNA, reading codons and adding corresponding amino acids to the growing polypeptide chain.

• Example: mRNA sequence 5'-AUGGCCUAA-3' translates to Met-Ala (stop).

Codon	Amino Acid	Function
AUG	Methionine	Start codon
UAA, UAG, UGA	None	Stop codons
UUU, UUC	Phenylalanine	Standard amino acid
GCU, GCC, GCA, GCG	Alanine	Standard amino acid

Key Equations and Concepts

• Transcription: $\text{DNA (template strand)} \xrightarrow{\text{RNA polymerase}} \text{mRNA}$

• Translation: $\text{mRNA} \xrightarrow{\text{ribosome}} \text{polypeptide (protein)}$

Summary Table: Transcription vs. Translation

Process	Location	Main Enzyme/Structure	Product
Transcription	Nucleus (eukaryotes)	RNA polymerase	mRNA
Translation	Cytoplasm	Ribosome	Polypeptide (protein)

Additional info: Mutations are excluded from this quiz but will be covered for the exam. The codon table includes 64 codons, three of which are stop signals, and the rest code for 20 amino acids.