Designer Genes

What is a monohybrid cross?

A genetic cross that involves one trait and one pair of alleles.

Who is the geneticist associated with Punnett squares?

Reginald C. Punnett.

What does a Punnett square illustrate?

The cross between alleles and the genotype of the resulting offspring.

What is the genotypic ratio for a cross between two heterozygous plants (Aa x Aa)?

1 homozygous dominant : 2 heterozygous : 1 homozygous recessive.

What is the phenotypic ratio for a cross between two heterozygous plants (Aa x Aa)?

3 dominant : 1 recessive.

What does homozygous mean?

Having two identical alleles for a trait.

What is the phenotypic ratio for a homozygous dominant (AA) crossed with a homozygous recessive (aa)?

4 dominant : 0 recessive.

What is the definition of genotype?

The genetic makeup of an organism, represented by alleles.

What is the definition of phenotype?

The observable traits or characteristics of an organism.

What is a dihybrid cross?

A genetic cross that involves two traits and two pairs of alleles.

What is the phenotypic ratio for a heterozygous dihybrid cross (RrYy x RrYy)?

9 round/yellow : 3 round/green : 3 wrinkled/yellow : 1 wrinkled/green.

What is the genotypic ratio for a heterozygous dihybrid cross (RrYy x RrYy)?

1 D/D : 2 D/H : 1 D/R : 4 H/H : 4 H/D : 1 R/D : 2 R/H : 1 R/R.

What is incomplete dominance?

A form of inheritance where the heterozygous phenotype is an intermediate between the two homozygous phenotypes.

Provide an example of incomplete dominance.

4 o'clock flowers, which are red (homozygous) and white (homozygous) but produce pink flowers (heterozygous).

What is codominance?

A form of inheritance where both alleles in a heterozygote are fully expressed.

Provide an example of codominance.

Roan cows, which display both red and white patches.

What are the two remaining types of crosses not shown in the note?

AA x AA and aa x aa, which result in all offspring being homozygous for that allele.

What is the significance of memorizing genotypic and phenotypic ratios?

It helps in quickly solving genetic problems on tests.

What does the notation 'D' and 'R' represent in dihybrid crosses?

'D' represents dominant traits, while 'R' represents recessive traits.

What is a trihybrid cross?

A genetic cross that involves three traits and three pairs of alleles.

What is the general rule for the phenotypic ratio of dihybrid crosses?

It is always 9:3:3:1 for two heterozygous parents.

What is the purpose of using Punnett squares?

To predict the genotypes and phenotypes of offspring from genetic crosses.

What does the term 'strict dominant/recessive genes' refer to?

Genes where one allele completely masks the effect of another in the phenotype.

What are sex-linked traits?

Traits associated with genes on sex chromosomes, often found on the X chromosome, like red-green color blindness and hemophilia.

Why do X-linked recessive traits occur more often in males?

Males have only one X chromosome; thus, a single recessive allele on that X will express the trait.

What is a Punnett square?

A diagram used to predict the genetic outcome of a cross between two individuals.

What is the inheritance pattern of Y-linked traits?

Y-linked traits are passed directly from father to son, as they are only found on the Y chromosome.

What are multiple genes?

Traits controlled by more than one set of non-allelic genes, such as height and skin color.

What are multiple alleles?

When more than two alleles exist for a gene, such as the ABO blood type system with IA, IB, and i alleles.

What are lethal alleles?

Genotypes that cause death before reproduction, altering expected Mendelian ratios, like HH in Mexican hairless dogs.

What is epistasis?

A genetic interaction where one gene inhibits the expression of another, such as albinism affecting eye color.

What is a pedigree chart?

A graphical representation showing the inheritance pattern of a trait through generations in a family.

How can you determine if a trait is sex-linked from a pedigree?

If the trait appears more frequently in males and skips generations, it is likely X-linked recessive.

What are the components of DNA?

DNA consists of a phosphate group, deoxyribose sugar, and nitrogenous bases (adenine, guanine, thymine, cytosine).

What distinguishes purines from pyrimidines in DNA?

Purines (adenine and guanine) have two rings, while pyrimidines (cytosine and thymine) have one ring.

What is the difference between nucleotides and nucleosides?

Nucleotides include a phosphate group, while nucleosides consist of only a sugar and a base.

Who were the key figures in discovering the structure of DNA?

James Watson, Francis Crick, and Maurice Wilkins; Rosalind Franklin contributed through X-ray diffraction.

What is the significance of the 5' and 3' carbons in DNA?

The phosphate group attaches to the 5' carbon, and the nitrogenous base attaches to the 1' carbon, forming the DNA backbone.

What is the role of Watson and Crick's model of DNA?

They proposed the double helix structure of DNA in 1953, based on Franklin's X-ray diffraction data.

What is the mnemonic for remembering purines and pyrimidines?

Purines are 'Pure As Gold' (A, G) and pyrimidines are 'CUT pie' (C, U, T).

What happens to phenotypic expression when lethal alleles are present?

They can remove expected progeny classes from genetic crosses, affecting observed ratios.

What is the typical phenotype for individuals homozygous recessive for albinism?

They exhibit reddish eyes regardless of their eye color genotype.

How does X chromosome inactivation affect the expression of X-linked traits in females?

It can complicate the dominant/recessive model, as only one X chromosome is active in each cell.

What is the common inheritance pattern for traits controlled by multiple genes?

These traits often show a continuous range of phenotypes, like height or skin color.

What is the significance of the allele combinations in the ABO blood type system?

IA and IB are codominant, while i is recessive, leading to four possible blood types: A, B, AB, and O.

What is the primary function of a pedigree analysis?

To trace the inheritance pattern of a specific trait and determine genotypes of individuals.

What are the two types of nitrogenous bases in DNA?

Purines (double rings) and pyrimidines (single rings).

What is chromatin?

A form of DNA stored in the nucleus, consisting of loose DNA, histones, and RNA.

What are the functions of chromatin?

Packaging DNA, reinforcing DNA for mitosis, preventing DNA damage, and controlling gene expression.

What are the two varieties of chromatin?

Euchromatin (actively transcribed) and heterochromatin (tightly packed, structural functions).

What happens to chromatin during mitosis?

Chromatin condenses into chromosomes, with sister chromatids attached at the centromere.

What is base pairing in DNA?

Adenine pairs with thymine, and guanine pairs with cytosine.

What is Chargaff's rule?

An organism should have equal percentages of adenine and thymine, and cytosine and guanine.

What is the role of helicase in DNA replication?

It separates the DNA strands by breaking hydrogen bonds between nitrogenous bases.

What is the function of primase in DNA replication?

It lays down an RNA primer that forms hydrogen bonds with existing DNA bases, starting the replication process.

What are Okazaki fragments?

Short sections of DNA replicated on the lagging strand in spurts.

What is the role of ligase in DNA replication?

It connects the backbones of adjacent DNA sections by forming phosphodiester bonds.

How does DNA replication differ in prokaryotes and eukaryotes?

Prokaryotes usually have one origin of replication, while eukaryotes can have multiple origins.

What does semi-conservative replication mean?

Each new DNA strand contains one original strand and one newly synthesized strand.

What is RNA and how does it differ from DNA?

RNA is usually single-stranded and contains ribose sugar, with uracil replacing thymine.

What are the three major types of RNA?

Messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

What is the function of mRNA?

It encodes the sequence of amino acids that become a protein.

What is the role of tRNA?

It transports amino acids to ribosomes during translation.

What is the function of rRNA?

It makes up the ribosome, which translates mRNA into proteins.

What is transcription?

The process of transcribing DNA into pre-mRNA, the first step of gene expression.

What occurs during the initiation stage of transcription?

Activator proteins bind to control elements and RNA polymerase II forms an initiation complex at the promoter.

What is the complementary sequence produced during transcription?

A pairs with U in RNA, G pairs with C, and C pairs with G.

What are non-coding RNAs (ncRNA)?

Any RNAs that do not result in a protein product, including tRNA and rRNA.

What is microRNA (miRNA)?

Short sections of RNA that can silence gene expression by preventing translation.

What is the significance of splicing in eukaryotes?

It removes introns from pre-mRNA, producing different transcripts.

What is pre-mRNA?

Newly transcribed RNA that has not yet undergone post-transcriptional processing.

What is heterogenous nuclear RNA (hnRNA)?

RNAs located in the nucleus, including pre-mRNA.

What role do transcription factors play in transcription?

Transcription factors bind to specific DNA sequences known as promoters to initiate transcription.

What is the function of RNA polymerase during transcription?

RNA polymerase binds to the promoter, unwinds the DNA, and synthesizes RNA from free-floating RNA nucleotides.

What is the direction of RNA synthesis in relation to the DNA template?

RNA polymerase synthesizes RNA in the 5' → 3' direction while reading the DNA template from 3' → 5'.

What is the transcription bubble?

The transcription bubble is formed when RNA polymerase unwinds approximately 14 base pairs of DNA.

What is pre-mRNA and how is it processed?

Pre-mRNA is the initial RNA transcript that undergoes processing, including capping, polyadenylation, and splicing, to become mature mRNA.

What is the purpose of the 5' cap added to pre-mRNA?

The 5' cap protects the RNA from degradation and helps the ribosome attach during translation.

What is the role of the poly-A tail in mRNA?

The poly-A tail stabilizes the mRNA and delays degradation by ribonucleases.

What are introns and exons in the context of mRNA processing?

Introns are non-coding sections removed during splicing, while exons are the coding sections that remain in the mature mRNA.

How does translation begin?

Translation begins when the small ribosomal subunit binds to the mRNA and locates the start codon (AUG).

What are codons and how do they relate to tRNA?

Codons are three-nucleotide sequences on mRNA that correspond to specific amino acids, with tRNA carrying the appropriate amino acid to the ribosome.

What happens during the elongation phase of translation?

Amino acids are added to the growing polypeptide chain as tRNA molecules enter the ribosome and bind to codons.

What triggers the termination of translation?

Translation terminates when a stop codon (UAA, UAG, UGA) reaches the A site of the ribosome.

What is the role of release factors in translation termination?

Release factors bind to the stop codon and facilitate the release of the newly synthesized polypeptide chain.

What is the significance of the N-terminus and C-terminus in polypeptides?

The N-terminus has an amino group, while the C-terminus has a carboxylate group, defining the directionality of polypeptide synthesis.

What is degeneracy in the genetic code?

Degeneracy refers to the phenomenon where multiple codons can code for the same amino acid.

What is wobble in the context of codons?

Wobble is the property that allows the third nucleotide of a codon to vary while still coding for the same amino acid.

What is the primary structure of a protein?

The primary structure of a protein is the linear sequence of amino acids in a polypeptide chain.

How do eukaryotic and prokaryotic transcription differ?

In eukaryotes, transcription occurs in the nucleus and is followed by mRNA processing, while in prokaryotes, transcription and translation can occur simultaneously in the cytoplasm.

What is the typical rate of RNA elongation during transcription?

The typical rate of RNA elongation is 10-100 nucleotides per second.

What is the function of chaperone proteins in protein folding?

Chaperone proteins assist in the correct folding of proteins in the rough endoplasmic reticulum.

What is a polypeptide?

A polypeptide is a chain of amino acids linked by peptide bonds, which can fold into a functional protein.

What is the role of snRNPs in mRNA processing?

snRNPs (small nuclear ribonucleoproteins) are involved in splicing, removing introns from the pre-mRNA.

What is the significance of the AAUAAA sequence in transcription?

The AAUAAA sequence signals for polyadenylation, prompting the addition of adenine nucleotides to the mRNA's 3' end.

What occurs during the proofreading mechanism of RNA polymerase?

RNA polymerase checks and replaces incorrect nucleotides during RNA synthesis to ensure accuracy.

What is the purpose of a codon chart?

To match a nucleotide sequence to an amino acid or vice-versa.

What is the first step in interpreting a DNA sequence into an amino acid sequence?

Transcription, where RNA nucleotides pair with DNA nucleotides.

What is the start codon and its significance?

AUG, which corresponds to methionine and starts translation.

What are the stop codons?

UAA, UAG, and UGA; they do not code for an amino acid.