🩺 Chapter 25- genetics

Language of Science (p. 696)

Say these out loud as you study—these are the vocab keys the chapter will use:

• allele – alternate form of a gene.

• autosome – non–sex chromosome.

• carrier – person with one copy of a recessive disease allele; usually no symptoms.

• codominance – two alleles expressed together.

• crossing‑over – exchange of gene segments between homologous chromosomes during meiosis.

• dominant allele – masks the effect of a recessive allele.

• epigenetics – changes in gene expression (not DNA sequence) often via regulatory switches.

• gamete – sex cell (sperm or ovum).

• gene – DNA segment that codes for a protein.

• gene linkage – genes close together on the same chromosome tend to travel together.

• genetic mutation – change in DNA sequence.

• genetics – study of heredity.

• genome – all the genes/DNA in a cell.

Chapter Objectives (p. 696)

When you finish the chapter you should be able to: explain how genes cause disease; compare dominant vs recessive; describe sex‑linked inheritance and mutations; outline mechanisms of genetic disease; explain nondisjunction → trisomy/monosomy and examples; list tools used in genetic counseling and how they help; and describe treatment approaches. (We’ll hit these as you send more pages.)

Intro: Why genetics matters to disease (p. 696)

• Genetics = scientific study of inheritance.

• Media buzz is about gene therapy and mapping/regulating the human genetic code.

• To understand how misinformation in genes causes disease, we need the basics: what genes are and how they work.

Genetics and Human Disease (p. 696)

• People knew traits ran in families for ages, but scientific genetics began in the 1860s with Gregor Mendel (monk in what is now Czechia).

• Mendel’s key idea: independent units (now called genes) are passed from parents to offspring.

• Genes (often via enzymes) regulate body structure and function.

• Some diseases are inherited directly (ex: hemophilia—a blood‑clotting disorder—passed from a parent with the defective clotting gene).

• Other conditions arise from mutations during development (not passed through egg/sperm). Example they mention from earlier chapters: progeria is a non‑inherited, rare genetic condition.

• Some diseases are partly genetic + partly environment (genetic risk factors). Example: certain skin cancers have a genetic base but only develop with heavy UV exposure.

Chromosomes and Genes — Mechanisms of Gene Function (p. 697)

• Mendel proposed genes are passed in discrete units; modern biology pins those units to DNA (Chapter 3 review).

• A gene is a sequence of nucleotides in DNA; each gene carries a code that is transcribed to mRNA, which moves to a ribosome to be translated into a specific protein.

• Many gene products are enzymes → they drive biochemical reactions, so genes ultimately control body structure and regulatory pathways via the proteins they encode.

• Genes are segments of DNA. When a gene is active, that DNA sits in extended chromatin; during cell division, chromatin coils into chromosomes (Fig. 25‑1).

  • Chromatid vs chromosome: a DNA molecule can be called either a chromatin strand or a chromosome depending on its state. Genes transcribe in chromatin form, not the condensed chromosome form.

• Terminology tip: “chromosome” often used for DNA regardless of state; “gene” = distinct encoding segment within the DNA molecule.

Human Genome (p. 697)

• Genome = total genetic content of each typical human cell (46 nuclear chromosomes + 1 mitochondrial chromosome).

• Human Genome Project (HGP) completed (public collaboration). Takeaways:

  • Humans have ~20,000–25,000 genes (far fewer than originally guessed).

  • <2% of DNA actually codes for proteins; the rest is noncoding DNA (historically nicknamed “junk DNA”).

  • Pseudogenes = broken bits of former genes (usually not functional), though some may still matter in disease.

  • ENCODE suggests ~80% of noncoding DNA has regulatory roles (turning genes on/off).

• Two expanding fields:

  • Genomics – analyzing the genome’s code (where genes are, mutations, organization).

  • Proteomics – analyzing the proteome (all proteins encoded by the genome) to understand function and disease mechanisms.

• Chromosome ideogram (Fig. 25‑1): a simple map showing p‑arm (short) and q‑arm (long), banding patterns, centromere position—useful landmarks for locating genes.

• A gene sequence is written with A, C, G, T letters (nucleotide bases).

Distribution of Chromosomes to Offspring (pp. 698–699)

Fig. 25‑1 (bottom text) bridges into meiosis and inheritance:

• Human cells have 46 chromosomes arranged in 23 pairs.

• Sex chromosomes (one pair) may not match (X and Y).

• The other 22 pairs are autosomes.

• Meiosis (Fig. 25‑2) forms gametes (sperm/ova) with 23 chromosomes (haploid).

• At fertilization, sperm (23) + ovum (23) → zygote with 46.

• Each offspring inherits half from mother, half from father → a unique genetic combo.

Independent assortment & variation (p. 699):

• During Meiosis I, homologous chromosome pairs line up randomly at the equator; different pairs line up independently → >8 million possible chromosome combinations (2²³) per gamete.

• As sperm form, maternal and paternal chromosomes separate and redistribute independently of other pairs → each sperm/ovum is likely genetically different from the previous one.

• Crossing‑over (Fig. 25‑3): during meiosis, matching segments of homologous chromosomes swap genes; sometimes whole groups stay together = gene linkage; crossing‑over adds even more variation among siblings from the same parents.

Gene Expression—Hereditary Traits (start on p. 699)

Gene Pairs

• Each inherited trait is controlled by two sets of similar genes—one from each parent.

• On autosomes, the two genes sit at the same locus on the matching pair.

• Different versions of a gene = alleles (example: both alleles relate to hair color, but not necessarily the same version).

Dominance and Recessiveness (starts here; continues on the next page)

• Mendel also found some alleles are dominant (mask others) and some recessive. We’ll finish details once you send the next page(s).

QUICK CHECK (p. 699) — Answers

1. Founder of scientific genetics? Gregor Mendel.

2. How do genes produce biological traits? A gene’s DNA is transcribed to mRNA, then translated into a specific protein (often an enzyme) that regulates the body’s structure and function—this protein expression creates the trait.

3. Autosome vs sex chromosome? Autosomes are the 22 non‑sex chromosome pairs; the sex chromosomes are the one pair (X and Y) that determine sex and may not match.

4. Mechanisms that increase genetic variation in humans?

   • Independent assortment of homologous chromosomes during meiosis.

   • Crossing‑over between homologous chromosomes.

   • Random fertilization mixing distinct maternal and paternal gametes (each already a unique 2²³ combo).

Gene Expression — Hereditary Traits

Gene Pairs

• Mendel’s pea plant experiments showed that each inherited trait is controlled by two sets of similar genes — one from each parent.

• On autosomes, the two genes sit in the same location (locus) on each chromosome of the pair.

• Variations or different forms of a gene are called alleles.

• Both alleles in a pair can code for the same or different expressions of a trait (e.g., both code for hair color but may code for different shades).

Dominance and Recessiveness

• Dominant allele = masks the effect of a recessive allele for the same trait.

• Written convention: Dominant = uppercase letter, Recessive = lowercase letter.

• Example: Albinism

  • Albinism = lack of melanin pigment in skin and eyes.

  • Gene for normal pigmentation is dominant (A); gene for albinism is recessive (a).

  • AA → typical pigmentation.

  • Aa → typical pigmentation (carrier).

  • aa → albinism (no melanin production).

• Carriers (Aa) can pass the recessive allele to offspring even without showing the trait themselves.

• Inheritance pattern (Fig. 25-4) shows how two carriers (Aa x Aa) can produce:

  • 25% AA (normal)

  • 50% Aa (carriers)

  • 25% aa (affected).

Codominance

• Codominance occurs when two different dominant alleles are both expressed equally.

• Example: one allele for light skin (A¹) and one for dark skin (A²) → A¹A² results in an intermediate skin tone showing both traits.

• ABO blood type and sickle cell trait are other examples of codominance.

Sex Determination (Fig. 25-5)

• Sex chromosomes determine sex:

  • XX → genetic female

  • XY → genetic male

• Y chromosome presence determines male sex; without Y, an individual is genetically female.

• Offspring sex is determined by whether the sperm carries an X or Y chromosome.

Sex-Linked Traits (pp. 701–702)

• Humans have 22 pairs of autosomes + 1 pair of sex chromosomes (X and Y).

• X chromosome = larger; carries many genes not related to sex determination.

• Y chromosome = smaller; contains genes that determine male sexual characteristics.

• X-linked traits = traits determined by genes on the X chromosome.

• Recessive X-linked traits:

  • Females need two copies of the recessive allele to express the trait.

  • Males need only one copy (from their single X chromosome) to express it.

  • More common in males because they lack a second X chromosome to mask recessive traits.

• Example: Red-green color vision deficiency (CVD) (Fig. 25-6)

  • A recessive X-linked condition where photopigments in the retina are defective.

  • A carrier mother can pass CVD to sons; a father with CVD cannot pass it to sons but can make daughters carriers.

Genetic Mutations

• Mutation = change/variation in the genetic code.

• Can occur spontaneously or from mutagens (chemicals, radiation, viruses).

• Types:

  • Change in a single gene (nucleotide sequence).

  • Change to part of a chromosome.

  • Change to an entire chromosome.

• Beneficial mutations → help adaptation/survival; can spread in a population.

• Harmful mutations → reduce survival or reproduction; may disappear from population unless mildly harmful, in which case they can persist for generations.

Mitochondrial Inheritance

• Mitochondria = organelles with their own circular DNA (mtDNA).

• mtDNA is inherited only from the mother because sperm mitochondria don’t survive after fertilization.

• Mutations in mtDNA can cause disorders like:

  • Leber hereditary optic neuropathy (vision loss by age 30).

  • Parkinson disease, Alzheimer disease, myopathy, deafness, cardiomyopathy.

• Experimental IVF protocols aim to prevent transmission by using donor eggs with healthy mitochondria.

Quick Check (p. 702) — Answers

1. Carrier of a genetic trait → person who has one recessive allele but does not express the trait; can pass it to offspring.

2. Codominance → both alleles expressed equally in the phenotype (e.g., A¹A² skin tone, AB blood type).

3. X-linked inheritance → inheritance of traits determined by genes on the X chromosome; males more likely to express recessive X-linked traits.

4. Mutant allele benefit → can aid adaptation/survival, increasing reproductive success and spreading in the population.

Genetic Conditions — Mechanisms of Genetic Disease

Role of Genes in Disease

• Genes aren’t “there to cause disease.”

• Disease happens when a gene or chromosome is altered so it fails to perform its usual function.

• A “disease gene” is really just a gene that, when changed, disrupts a normal process.

• Example: breaking your arm doesn’t mean your arm’s purpose is to break—its purpose is to function normally; disease is the failure of function.

Single-Gene Mechanisms

• Some diseases come from a mutation in one gene — these are single-gene diseases.

• Mutant genes can:

  • Produce an abnormal protein.

  • Fail to produce a needed protein.

• Fig. 25-7 shows examples and chromosome locations:

  • Cystic fibrosis (chromosome 7)

  • Sickle cell disease (chromosome 11)

  • Tay-Sachs disease (chromosome 15)

  • Huntington disease (chromosome 4)

  • …and many others.

Multiple-Gene Mechanisms

• Many diseases involve more than one gene (polygenic).

• Example conditions: hypertension, coronary heart disease, type 2 diabetes.

• Having one “disease-related” gene may not cause illness unless combined with other genes or environmental triggers.

Epigenetics

• Epigenetics studies how environment & behavior influence gene expression without altering the DNA code.

• Genes can be turned on or off by:

  • Chemical changes to DNA/chromosomes.

  • Environmental inputs (e.g., diet of parents or grandparents).

• Explains why identical twins can differ in traits/disease risk.

• Such conditions involve genetic predisposition — not purely genetic.

Chromosomal Mechanisms

• Some diseases are from chromosome structure changes rather than gene mutations.

• Nondisjunction → failure of chromosome pairs to separate during meiosis.

  • Can result in trisomy (extra chromosome) or monosomy (missing chromosome) (Fig. 25-8).

  • Example: Trisomy 21 (Down syndrome).

• These disorders are usually not passed on—affected individuals are often sterile or die early.

Table 25-1: Examples of Single-Gene Conditions

(Dominance type in parentheses)

• Hemophilia (X-linked recessive) → clotting factor deficiency.

• Albinism (recessive) → lack of melanin pigment.

• Sickle cell anemia/trait (codominant) → abnormal hemoglobin, RBCs sickle.

• Red-green color vision deficiency (X-linked recessive) → color blindness.

• Cystic fibrosis (recessive) → thick mucus in lungs/digestive tract.

• PKU (recessive) → can’t break down phenylalanine → brain damage risk.

• Tay-Sachs (recessive) → lipid buildup in brain, fatal by age 4.

• Osteogenesis imperfecta (dominant) → brittle bones.

• Multiple neurofibromatosis (dominant) → tumors in nerve tissue.

• Duchenne muscular dystrophy (X-linked recessive) → progressive muscle wasting.

• Hypercholesterolemia (dominant) → high cholesterol, heart risk.

• Huntington disease (dominant) → progressive neurodegeneration, death by ~55.

• SCID (recessive) → immune deficiency.

Single-Gene Conditions — Details from Text

Cystic Fibrosis

• Recessive mutation in CFTR gene (chromosome 7).

• Affects chloride ion channels → thick mucus in lungs & GI tract → blockage & infection risk.

• Carriers may have resistance to cholera.

• Modern treatments have improved life expectancy.

Phenylketonuria (PKU)

• Recessive mutation in phenylalanine hydroxylase → can’t convert phenylalanine to tyrosine.

• Phenylalanine buildup damages brain tissue → intellectual disability & death risk.

• Diet control from birth prevents symptoms.

• Warning labels on foods with aspartame alert PKU patients.

Tay-Sachs Disease

• Recessive mutation → lack of hexosaminidase enzyme → lipid buildup in brain.

• Symptoms: severe neurodegeneration, death by age 4.

• Common in Ashkenazi Jewish population.

Quick Check (p. 705) — Answers

1. Single-gene vs chromosomal mechanisms → Single-gene: mutation in one gene; Chromosomal: large-scale change in chromosome number/structure.

2. Epigenetics → Environmental/behavioral factors changing gene expression without altering DNA sequence.

3. Nondisjunction → Failure of chromosomes to separate during meiosis; can lead to trisomy (extra chromosome).

Epigenetic Conditions (p. 706)

• Epigenetics: how environment & behavior influence gene activity without changing the DNA sequence.

• Often involves chemical tags that turn genes on/off:

  • Methyl groups (–CH₃): reduction linked to some cancers.

  • Too much methylation: may suppress cancer-preventing genes.

  • Acetyl groups (–COCH₃) or ubiquitin proteins can also regulate proteins/RNA production.

• Some epigenetic diseases:

  • Fragile X syndrome (FXS) — caused by overmethylation of a CGG repeat on the X chromosome → turns off production of needed protein.

    • More common in males (only 1 X chromosome).

    • Females may have milder symptoms if other X chromosome produces enough protein.

  • Possible epigenetic links to type 2 diabetes, cardiovascular disease, and certain cancers.

Chromosomal Conditions (p. 706–707)

Result from nondisjunction during gamete formation → trisomy or monosomy.

Trisomy 21 — Down Syndrome (Fig. 25-9)

• Extra chromosome 21 (three copies instead of two).

• Frequency: ~1 in 600 births; risk increases sharply after maternal age 35.

• Features: intellectual disability, distinctive face (folds near eyes, flat nasal bridge, round face, small hands/feet), possible congenital heart defects, leukemia risk.

• Life expectancy shorter but survival into adulthood possible.

XXY — Klinefelter Syndrome (Fig. 25-10)

• Male with at least two X chromosomes and a Y (XXY most common).

• Symptoms: small testes, breast development, sparse body hair, long limbs, sterility, learning difficulties.

XO — Turner Syndrome (Fig. 25-11)

• Female with only one X chromosome.

• Symptoms: short stature, webbed neck, sexual immaturity (no puberty), sterility, heart problems.

• Some issues improved with estrogen/growth hormone therapy.

Prevention & Treatment of Genetic Diseases (p. 708–709)

Genetic Counseling

• Helps families assess risk of passing on genetic conditions, make informed reproductive decisions, and manage diagnosed conditions.

Pedigree (Fig. 25-12)

• Chart showing family genetic relationships across generations.

• Squares = males, circles = females.

• Shaded = affected, half-shaded = carrier.

• Used to predict inheritance patterns.

Punnett Square (Fig. 25-13)

• Grid predicting probability of inheriting traits.

• Example: Two PKU carriers → 25% affected, 50% carrier, 25% unaffected.

Clinical Application — DNA Analysis

• Electrophoresis separates DNA fragments by size to read genetic code → forms basis of DNA fingerprinting.

• Unique DNA sequences identify individuals and can detect mutations linked to disease.

• Used for both medical genetics and forensic science.

Karyotype

• Photographic map of chromosomes used to detect:

  • Trisomy (extra chromosome)

  • Monosomy (missing chromosome)

  • Structural chromosome defects.

• Cells obtained via cheek swab or blood sample.

Quick Check (p. 707) — Answers

1. PKU diet → avoids phenylalanine to prevent its toxic buildup, protecting the brain.

2. Trisomy 21 → Down syndrome, extra chromosome 21.

3. XO → Turner syndrome.

Genetic Testing Methods

• Amniocentesis (Fig. 25-14): Syringe withdraws amniotic fluid (guided by ultrasound) to collect fetal cells → chemically tested or karyotyped.

• Chorionic villus sampling (CVS): Collects cells from placental chorionic villi through cervix.

• Cell culture: Fetal cells grown, stained in metaphase, photographed, cut out, arranged into a karyotype to check for abnormalities.

• Noninvasive prenatal testing (NIPT): Uses fetal DNA from maternal blood; low risk but may need confirmation by amniocentesis or CVS.

Quick Check (p. 710) — Answers

1. Genetic counseling → professional guidance for families on genetic risk, prevention, and management.

2. Pedigree → chart showing inheritance patterns over generations.

3. Punnett square → grid showing probabilities of inheriting specific traits.

4. Karyotype → arranged photo of chromosomes; used to detect abnormalities.

Treating Genetic Conditions

• Treating symptoms:

  • PKU → diet limits phenylalanine.

  • Klinefelter/Turner → hormone therapy.

  • Many conditions: surgery/therapy to manage complications.

• Gene Therapy: Treats cause rather than symptoms.

Gene Replacement

• Inserts functioning genes into cells to replace defective/missing ones (often via viral vectors).

Gene Augmentation

• Adds genes to boost production of needed protein.

• Methods:

  • Virus-altered cells injected/implanted under skin.

  • Human engineered chromosome (HEC) → extra chromosome carrying therapeutic genes.

RNA Interference (RNAi)

• Silences specific genes to stop harmful protein production.

Gene Therapy in Practice (Fig. 25-15)

• Example: Adenosine deaminase (ADA) deficiency → SCID.

  • WBCs collected → infected with virus carrying ADA gene → cultured → re-infused.

• Also used with plasmids for inhalation therapy in cystic fibrosis.

Quick Check (p. 711) — Answers

1. Most genetic conditions treated by managing symptoms (diet, hormones, surgery).

2. Gene replacement inserts functioning genes to replace defective ones.

3. HEC = human engineered chromosome; acts like an extra chromosome with therapeutic genes.

Science Applications — Genetics and Genomics (p. 712)

• Gregor Mendel: founder of genetics; pea plant experiments showed inheritance patterns.

• Introduced mathematical analysis to biology; pioneered use of statistics in genetics.

• Today:

  • Genetic counselors → guide families on risks.

  • Agricultural scientists → refine crops/livestock.

  • Genetic engineers → manipulate genes for therapies.

  • Genomics scientists → map & analyze genes for better disease treatment.