Texbook Flashcards
Why were pea plants ideal for Mendel’s experiments?
They have distinct traits, a short generation time, self-pollinate naturally, and can be easily cross-pollinated.
What are the three key principles Mendel discovered?
The Law of Segregation, the Law of Independent Assortment, and the Principle of Dominance.
What is the Law of Segregation?
During gamete formation, the two alleles for each gene separate, so each gamete gets only one allele.
How does meiosis relate to Mendel’s Law of Segregation?
Homologous chromosomes separate during anaphase I of meiosis, ensuring each gamete receives one allele.
What is the Law of Independent Assortment?
Genes for different traits assort independently of one another during gamete formation.
How does genetic linkage affect independent assortment?
Linked genes, located close together on the same chromosome, are inherited together and do not assort independently.
What is a dihybrid cross?
A genetic cross between two individuals heterozygous for two traits.
What phenotypic ratio results from a dihybrid cross of two heterozygous individuals (AaBb × AaBb)?
9:3:3:1.
What is the purpose of a Punnett square?
To predict the genotypic and phenotypic outcomes of a genetic cross.
What is the product rule in genetics?
The probability of two independent events occurring together is the product of their individual probabilities.
What is the sum rule in genetics?
The probability of either of two mutually exclusive events occurring is the sum of their probabilities.
How does a branching diagram simplify multihybrid cross predictions?
It breaks each gene’s inheritance into individual probabilities and combines them at the end.
Why do Punnett squares become impractical for multihybrid crosses?
The number of boxes increases exponentially with more traits, making it cumbersome to analyze.
What inheritance pattern does Huntington’s disease follow?
Dominant inheritance.
What inheritance pattern does cystic fibrosis follow?
Recessive inheritance.
How can a pedigree help determine if a trait is dominant or recessive?
Dominant traits appear in every generation, while recessive traits can skip generations.
Why are recessive alleles often associated with nonfunctional proteins?
They usually result from mutations that cause a loss of function in the encoded protein.
What is epistasis?
When one gene’s expression masks or modifies the effect of another gene.
Provide an example of epistasis in animals.
Coat color in Labrador Retrievers, where one gene determines pigment and another gene determines pigment deposition.
What is the biochemical role of the Sbe1 gene in Mendel’s peas?
It encodes an enzyme that converts unbranched starch to branched starch, affecting seed shape.
What does the Sgr gene control in Mendel’s pea experiments?
It controls the breakdown of chlorophyll, affecting seed color.
How did Mendel use statistical methods to validate his hypotheses?
He analyzed large sample sizes to observe consistent patterns that matched predicted ratios.
Why was Mendel’s approach revolutionary?
He applied quantitative analysis and statistics to biological inheritance, which was novel at the time.
What was the main goal of the Human Genome Project?
To sequence the entire human genome and identify all its genes.
How did the Human Genome Project advance genetics?
It improved understanding of Mendelian and complex traits, aiding in genetic counseling and personalized medicine.
What is genetic linkage?
The tendency of genes close together on a chromosome to be inherited together.
How does crossing over affect genetic linkage?
It creates recombinant offspring by exchanging DNA between homologous chromosomes.
How do deviations from Mendel’s ratios occur?
Through phenomena like linkage, epistasis, or polygenic inheritance.
What are polygenic traits?
Traits controlled by multiple genes, such as height or skin color in humans.
Why must eukaryotic DNA be compacted?
Because each cell contains about 2 meters of DNA, it must be tightly packed to fit into a tiny nucleus (~6 µm).
What is the fundamental unit of chromatin compaction?
The nucleosome, which is DNA wrapped around a core of eight histone proteins (H2A, H2B, H3, H4).
What is the role of histone H1?
Histone H1 helps stabilize the DNA where it enters and exits the nucleosome, further compacting the chromatin.
What forms after nucleosomes coil more tightly?
The 30 nm fiber, an additional level of compaction created by coiling nucleosomes together.
What are condensins?
Protein complexes that use ATP to loop and condense chromosomes during mitosis, making them visible under a microscope.
How does heterochromatin differ from euchromatin?
Heterochromatin is tightly packed and usually transcriptionally inactive, while euchromatin is loosely packed and actively transcribed.
What is Position-Effect Variegation (PEV)?
Variegated gene expression caused when a gene moves near heterochromatin, resulting in its random silencing in some cells.
What is X-chromosome inactivation?
In female mammals, one X chromosome is silenced (forms a Barr body) to balance X-linked gene expression with males.
What role does the Xist gene play in X-chromosome inactivation?
Xist produces a long noncoding RNA that coats the X chromosome, recruiting proteins that silence it.
What are histone modifications?
Chemical tags (like acetylation or methylation) added to histone tails that can loosen or tighten chromatin to regulate gene expression.
What is the function of telomerase?
Telomerase adds repetitive DNA to telomeres, helping certain cells maintain chromosome ends and avoid premature aging.
Why are telomeres important?
They protect the ends of linear chromosomes and prevent them from being recognized as DNA breaks.
What is the centromere?
A specialized region on a chromosome where kinetochores form and spindle fibers attach during cell division.
How do cohesin complexes help in cell division?
They hold sister chromatids together until anaphase, ensuring accurate segregation of chromosomes.
What is FISH (Fluorescence In Situ Hybridization)?
A technique that uses fluorescent DNA probes to locate specific sequences on chromosomes under a fluorescence microscope.
What is G-banding?
A karyotyping technique using Giemsa stain to produce light and dark bands on chromosomes, aiding in their identification.
Why is DNA replication considered semi-conservative?
Each new double helix contains one original (parent) strand and one newly synthesized (daughter) strand.
What is the leading strand in DNA replication?
The strand synthesized continuously in the 5’→3’ direction as the replication fork opens.
What is the lagging strand?
The strand synthesized discontinuously in short Okazaki fragments, also in a 5’→3’ direction but moving away from the replication fork.
What is the role of the shelterin complex?
It binds and protects telomeres at chromosome ends, preventing them from being recognized as DNA breaks.
What are Okazaki fragments?
Short stretches of DNA synthesized on the lagging strand during replication.
Why are centromeres often composed of satellite DNA?
These repetitive sequences, combined with special histones, form the unique structure required for kinetochore assembly and spindle attachment.
What are the four main classes of chromosomal rearrangements?
Deletions, duplications, inversions, and translocations.
What is a deletion in a chromosome?
The loss of a segment of chromosomal DNA, removing genes from that region.
How can deletions affect an organism?
They can cause haploinsufficiency, unmask recessive alleles, or be lethal if essential genes are lost.
What is a duplication?
A repeat of a chromosomal segment, resulting in extra copies of certain genes.
Give an example of a duplication-related phenotype.
The Bar-eye phenotype in Drosophila, caused by tandem duplications on the X chromosome.
What is an inversion?
A segment of a chromosome is flipped 180° within the same chromosome.
How do inversions affect fertility in heterozygotes?
Inversion loops form during meiosis, often creating unbalanced gametes and reducing fertility.
What is a translocation?
A segment of one chromosome is attached to a nonhomologous chromosome.
How do reciprocal and Robertsonian translocations differ?
Reciprocal translocation: segments swap between two nonhomologous chromosomes.
Robertsonian translocation: two acrocentric chromosomes fuse into one.
What is the Philadelphia chromosome?
A reciprocal translocation between chromosomes 9 and 22 that can cause chronic myelogenous leukemia.
Why might rearrangements reduce fertility?
They lead to abnormal pairing and unbalanced gametes during meiosis, causing inviable or missing genes in offspring.
What are some techniques to detect chromosomal rearrangements?
FISH (Fluorescence In Situ Hybridization), chromosome painting, and genomic sequencing methods.
What is haploinsufficiency?
When one functional gene copy is not enough for a normal phenotype, leading to disorders if the other copy is deleted/mutated.
What causes position-effect variegation?
A gene being relocated (often by inversion) near heterochromatin, resulting in variable gene silencing in different cells.
What is aneuploidy?
An abnormal number of chromosomes (e.g., monosomy = 2n−1, trisomy = 2n+1) rather than a full set.
Which aneuploidy leads to Down syndrome in humans?
Trisomy 21 (having three copies of chromosome 21).
Why are sex chromosome aneuploidies generally more tolerable than autosomal aneuploidies?
Because of X-inactivation (in females) and the relative lack of genes on the Y chromosome.
What is polyploidy?
Having more than two complete sets of chromosomes (e.g., 3n, 4n).
What are the two main types of polyploidy?
Autopolyploidy: extra sets of chromosomes from the same species.
Allopolyploidy: chromosome sets from different species.
Why is polyploidy significant in plant evolution?
It can lead to new species formation, increased genetic diversity, and sometimes larger, more vigorous plants.
What is the major difference between aneuploidy and polyploidy?
Aneuploidy involves gains/losses of individual chromosomes, while polyploidy involves extra entire sets of chromosomes.
How does nondisjunction lead to aneuploidy?
Chromosomes or chromatids fail to separate properly during meiosis, creating gametes with extra or missing chromosomes.
What is unequal crossing-over and how does it relate to duplications?
Misalignment of homologous chromosomes during meiosis can result in one chromosome gaining extra segments (duplication) and the other losing them (deletion).
Why can deleting one copy of a gene be more harmful in some cases than losing two copies?
In haploinsufficiency, a single functional gene copy can’t produce enough protein for normal function, causing an abnormal phenotype.
How do rearrangements sometimes drive evolution?
By rearranging genes, creating new gene dosage patterns, or generating novel gene fusions, organisms can adapt or evolve new traits.
