Genes, Chromosomes, Inheritance & Epigenetics – Unit 2 AOS 1

A comprehensive set of Q&A flashcards covering DNA structure, genes, chromosomes, karyotypes, meiosis, inheritance patterns, genotype vs phenotype, as well as environmental and epigenetic influences on gene expression.

What is the primary function of DNA in an organism?

To act as the genetic blueprint that codes for and determines an organism’s physical, behavioural and physiological characteristics.

Describe the basic structure of a DNA molecule.

Double-stranded nucleic acid composed of repeating nucleotides arranged in a double helix.

Name the three components of a nucleotide.

A phosphate group, a deoxyribose sugar and a nitrogen-containing base.

Which nitrogen bases pair together in DNA?

Adenine with thymine, and guanine with cytosine.

Define gene.

A sequence of DNA nucleotides that codes for a specific protein and contributes to a particular trait.

Define allele.

An alternative form of the same gene with small differences in nucleotide sequence.

How many alleles for each gene does a diploid organism inherit?

Two—one from the mother and one from the father.

Define genome.

The sum total of all genes in an organism, measured by the number of base pairs in a haploid set.

Approximately how many base pairs make up the human genome?

About 3 billion base pairs.

What is a chromosome?

A linear structure of tightly coiled DNA wrapped around histone proteins (chromatin).

Name four key parts of a duplicated chromosome.

Telomere, centromere, chromatid, and locus.

Differentiate unduplicated vs duplicated chromosome.

Unduplicated: one chromosome, one chromatid; Duplicated: one chromosome, two sister chromatids.

Define haploid (n).

A cell containing one copy of each chromosome; found only in gametes.

Define diploid (2n).

A cell containing two copies of every chromosome; found in somatic cells.

How many chromosomes are in human haploid and diploid cells?

n = 23; 2n = 46.

What happens to telomeres after each mitotic division?

They shorten, contributing to cellular ageing and eventual cell death.

What enzyme maintains telomere length in gametes?

Telomerase.

What are homologous chromosomes?

A pair of chromosomes of similar length, centromere position, and gene loci—one maternal, one paternal.

Define karyotype.

A visual display of all the chromosomes in a cell, arranged in decreasing size.

Distinguish autosomes from sex chromosomes.

Autosomes (chromosomes 1–22) carry most body-function genes; sex chromosomes (X or Y) determine biological sex and related traits.

Define aneuploidy.

Addition or loss of a single chromosome within the genome.

Define polyploidy.

Presence of three or more complete sets of chromosomes in the genome.

What chromosomal change causes Turner syndrome?

Monosomy of the sex chromosome (XO).

What chromosomal change causes Down syndrome?

Trisomy of chromosome 21.

What chromosomal change causes Klinefelter syndrome?

An extra X chromosome (XXY).

What is tetrasomy X?

Presence of two extra X chromosomes (XXXX).

Why is polyploidy usually lethal in humans but beneficial in some plants?

Human development cannot tolerate extra sets; plants often gain size and hardiness from them.

State the purpose of meiosis.

To produce haploid gametes for sexual reproduction, ensuring chromosome number is halved.

How many daughter cells result from meiosis and are they identical?

Four genetically different haploid daughter cells.

List two events in meiosis that create genetic diversity.

Crossing over (prophase I) and independent/random assortment (metaphase I).

What is crossing over?

Exchange of DNA segments between non-sister chromatids of homologous chromosomes at chiasmata during prophase I.

Define recombinant chromatid.

A chromatid that contains a new combination of alleles after crossing over.

What is independent (random) assortment?

Random orientation of homologous chromosome pairs on the metaphase plate in metaphase I.

Approximately how many chromosome combinations can independent assortment create in humans?

About 8 million.

Compare mitosis and meiosis in terms of daughter cells produced.

Mitosis: two identical diploid cells; Meiosis: four non-identical haploid cells.

What is nondisjunction?

Failure of chromosomes to separate properly during anaphase, leading to gametes with abnormal chromosome numbers.

Define genotype.

The combination of alleles an organism carries for a particular gene, usually written as two letters.

Define phenotype.

The observable physical, physiological or behavioural traits of an organism.

What is complete dominance?

A pattern where a dominant allele fully masks a recessive allele in the heterozygote phenotype.

What is codominance?

Both alleles are equally and simultaneously expressed in the heterozygote phenotype.

What is incomplete dominance?

The heterozygote phenotype is a blend of the two alleles.

Give the three zygosity terms and their meanings.

Homozygous dominant (AA), heterozygous (Aa), homozygous recessive (aa).

Which human blood group alleles are codominant?

IA (A) and IB (B).

Which blood group allele is recessive?

i (O).

What genotype produces blood type AB?

IAIB.

What is a sex-linked gene?

A gene located on a sex chromosome (X-linked or Y-linked).

Why are X-linked recessive traits more common in males?

Males have only one X chromosome, so any allele present is expressed.

Explain heterozygote advantage with the sickle cell example.

Ss genotype confers malaria resistance without causing full sickle cell anaemia, giving a survival benefit in malaria regions.

State the formula for phenotype influence.

Phenotype = genes + environment (+ epigenetics).

Define proportionate heritability.

The ratio that quantifies how much of a trait is due to genetic versus environmental factors.

Provide the approximate proportionate heritability for human height.

70–80% genes, 20–30% environment.

Give an environmental factor that alters phenotype and an example.

Temperature—Himalayan rabbit fur turns darker in cool body regions.

Define epigenetics.

Study of heritable changes in gene expression that do not alter the DNA sequence.

Name three major epigenetic mechanisms.

DNA methylation, histone methylation, histone acetylation.

How does DNA methylation affect transcription?

Adds methyl groups to DNA, decreasing transcription and silencing genes.

What effect does histone methylation have on DNA packaging?

Tightens DNA around histones, decreasing transcription.

What effect does histone acetylation have on gene expression?

Loosens DNA wound around histones, increasing transcription.

What is somatic epigenetic inheritance?

Transmission of epigenetic marks from parent cell to daughter somatic cells during mitosis.

Define transgenerational epigenetic inheritance.

Epigenetic changes that escape reprogramming in gametes and are passed to offspring.

What are endocrine-disrupting chemicals (EDCs)?

Compounds that interfere with hormone signalling and can cause epigenetic changes.

List two common EDCs and one epigenetic effect of each.

BPA—early puberty & obesity risk; Phthalates—low sperm count & developmental issues.

Which historical drug (DES) caused transgenerational epigenetic effects?

Diethylstilbestrol, linked to cancers and reproductive issues in descendants.

Why do identical twins become more different with age?

Accumulated unique environmental exposures lead to divergent epigenetic modifications.

Name two lifestyle choices that slow telomere shortening.

Mediterranean diet rich in antioxidants and regular physical activity.

What cellular change occurs when tomato DNA methylation decreases?

Ripening genes are activated, leading to fruit ripening.

How can hypermethylation contribute to cancer?

It can silence tumour-suppressor genes, allowing uncontrolled cell growth.

What is the charge relationship between DNA and histones?

DNA is negatively charged; histones are positively charged, enabling binding.

During which meiotic phase do homologous chromosomes separate?

Anaphase I.

During which meiotic phase do sister chromatids separate?

Anaphase II.

Which two meiotic stages immediately follow prophase I and II respectively?

Metaphase I follows prophase I; metaphase II follows prophase II.

What arrests female oocytes during fetal development?

They stop in prophase I until puberty.

Which meiotic division completes just before ovulation in females?

Completion of meiosis I.

What is the centromere’s role?

Holds sister chromatids together and provides attachment for spindle fibres.

Define telomere.

A protective cap at chromosome ends that prevents fusion with other chromosomes.

What is a locus?

The specific physical location of a gene on a chromosome.

Explain why chromosome number does not equal genetic complexity.

Gene count varies; e.g., tomatoes (24 chromosomes, 32 000 genes) vs chickens (78 chromosomes, 17 000 genes).

What is chromatin?

DNA wrapped around histone proteins, forming the material of chromosomes.

Which phase of the cell cycle duplicates chromosomes before meiosis?

Interphase (S phase).

What is a chiasma?

The physical point where crossing over occurs between non-sister chromatids.

How many different paternal/maternal combinations can meiosis create before considering crossing over?

About 8 million due to random assortment alone.

What symbol set represents normal colour vision vs colour blindness in X-linked notation?

XR = normal; Xr = red-green colour blindness.

Which genotype is homozygous dominant for widow’s peak?

HH.

Give an example of incomplete dominance in flowers.

Red (CRCR) × white (CWCW) producing pink (CRCW) offspring.

Which genotype yields type O blood?

ii.

What percentage of normal human somatic cells typically divide before telomere-driven senescence?

Approximately 45–50 divisions.

Why must telomerase activity be tightly regulated?

Excessive activation is linked to cancer cell immortality.

What is the molecular basis of the heterozygote advantage against malaria?

Presence of some sickle-shaped red cells hinders Plasmodium infection.

Which epigenetic change generally increases gene expression?

Histone acetylation.

What is the consequence of adding methyl groups to histones?

Reduced transcription due to tighter DNA winding.

How can environment lead to epigenetic changes?

Factors like diet, toxins or stress modify DNA/histone marks, altering gene activity.