Cell Division, Chromosomes, and Inheritance: Study Notes for General Biology

5.1 Cell Division Provides for Reproduction, Growth, and Repair

Cell Theory

Cell division is a fundamental biological process that enables organisms to grow, repair damage, and reproduce. All living things are composed of cells, and all cells arise from preexisting cells.

• Cell division allows organisms to grow, repair damage, and reproduce.

Sexual Reproduction

• Involves two parents (male and female).

• Gametes (sperm and egg) are produced in the gonads through cell division.

• Fertilization joins two gametes to form a zygote containing genetic material from each parent.

• Growth requires many rounds of cell division to form an embryo, then a mature adult.

• Cell and repair continue throughout life as new cells replace old or damaged ones.

• Offspring are genetically unique (mix of both parents' DNA).

Asexual Reproduction

• Involves one parent only—no sperm or egg.

• Offspring are genetically identical to the parent.

• Common in unicellular organisms, some plants, and animals.

Examples of Asexual Reproduction

• Binary Fission: A single-celled organism divides into two identical offspring.

• Plant Asexual Reproduction:

  • Sprouting: e.g., potatoes growing new plants from "eyes".

  • Runners: e.g., strawberry plants sending out horizontal stems to give new plants.

• Regeneration: Some animals can regrow body parts or even regenerate an entire organism from a part.

5.2 Chromosomes are Associations of DNA and Protein

DNA and Chromosomes

Every eukaryotic cell contains chromosomes in the nucleus. Chromosomes are long pieces of DNA associated with proteins that help organize and compact the information.

• All life uses DNA to store information and pass it to the next generation.

• Chromosomes contain genes, which are units of hereditary information.

Chromosome Number

• All eukaryotic cells have a nucleus where chromosomes are stored.

• Each species has a specific, consistent number of chromosomes in its cells.

• Example: Human body cells contain 46 chromosomes (23 pairs).

• Members of the same species have nearly identical chromosomes.

• The number of chromosomes does not indicate an organism’s size or complexity.

• The DNA of two humans of the same sex is about 99.5% identical; only about 0.5% accounts for unique traits.

Chromosome Structure

• Each chromosome is a very long piece of DNA, combined with proteins that keep it tightly organized.

• The combination of DNA and protein is called chromatin.

• Before cell division, the sister chromatids are joined at a centromere.

5.3 Cells Have Regular Cycles of Growth and Division

The Cell Cycle

The cell cycle is an ordered sequence of events that leads from the creation of a cell to its division. It consists of two broad phases: interphase and the mitotic phase.

• Interphase: The cell grows, performs its normal functions, and duplicates its chromosomes.

• Mitotic Phase: The cell divides its nucleus and cytoplasm, forming two genetically identical cells.

Interphase

• Makes up about 90% of the cell cycle.

• The cell performs its normal life functions (e.g., energy capture, protein synthesis).

• The cell grows in size, builds cytoplasm and organelles, and keeps chromosomes duplicated.

• As the cell prepares to divide, it duplicates its chromosomes within the nucleus.

Chromosome Duplication

• Occurs near the end of interphase.

• Each chromosome is copied, forming two sister chromatids joined at the centromere.

5.4 During Mitosis, the Nucleus of the Cell Divides

Mitotic Phase: Mitosis and Cytokinesis

The mitotic phase is when the cell actually divides. It includes two overlapping processes:

• Mitosis: The nucleus divides, and the duplicated chromosomes are distributed evenly into two nuclei.

• Cytokinesis: The cytoplasm divides, separating the cell into two distinct offspring cells.

Result: Two genetically identical cells, each with a complete set of chromosomes, begin their own cell cycles.

Before Mitosis: Interphase

• During interphase, the chromosomes are duplicated in preparation for division.

• Each chromosome is copied to form two sister chromatids, joined at the centromere.

• The duplicated chromosomes remain uncondensed and not yet visible under a microscope.

Mitosis: Division of the Nucleus

• Mitosis is the process by which the duplicated chromosomes are organized, aligned, and separated into two sets.

• Each set of chromosomes is enclosed within its own new nucleus.

• As a result, the two new nuclei contain identical genetic information.

5.5 During Cytokinesis, the Cell is Split into Two

Cytokinesis: The Final Step in Cell Division

Cytokinesis is the division of the cytoplasm, the last step in the cell cycle. In animal cells, cytokinesis proceeds via the formation of a cleavage furrow. In plant cells, it proceeds via the formation of a cell plate.

Cytokinesis in Animal Cells: Cleavage Furrow

• The first sign of cleavage is the formation of a cleavage furrow, an indentation around the equator of the cell.

• The cleavage furrow contains a ring of protein filaments that contract and pinch the cell in two.

• This process separates the cytoplasm, forming two separate, independent cells.

Cytokinesis in Plant Cells: Cell Plate Formation

• Plant cells have a rigid cell wall, so they cannot divide by pinching like animal cells.

• Instead, plant cells form a cell plate along the center line of the cell.

• The cell plate is made of membrane and cell wall materials that collect in the middle of the dividing cell.

• The cell plate grows outward and eventually fuses with the plasma membrane, forming a new cell wall that separates the two offspring cells.

5.6 Nuclear Transfer Can Be Used to Produce Clones

Cloning: Artificial Manipulation of Cell Division

Cloning is the production of genetically identical offspring from a single parent. Nuclear transplantation produces a cloned embryo, which may be used to produce a new individual (reproductive cloning) or stem cells (therapeutic cloning).

Nuclear Transplantation (Animal Cloning)

• The nucleus is removed from an adult donor cell.

• This nucleus is injected into an egg cell that has had its own nucleus removed.

• The egg is stimulated to divide and grow into an embryo.

• The embryo may develop into a complete organism, which is a clone of the animal that donated the nucleus.

• The cloned animal will have identical DNA to the donor that provided the nucleus.

Plant Cloning

• Artificial cloning is widely used in agriculture and horticulture.

• Small pieces of plant tissue or even single cells are placed in a growth medium.

• These cells divide and grow into new plants that are genetically identical to the parent plant.

• This process allows farmers to quickly and consistently reproduce desirable plant traits.

Reproductive Cloning

• In reproductive cloning, the cloned embryo created by nuclear transplantation develops into a living individual.

• This method was used in 1997 to create Dolly the sheep, the first cloned adult mammal.

• Since then, scientists have cloned many other mammals, including dogs, cats, cows, mice, and even endangered species such as the mouflon, gray wolf, and gaur.

Therapeutic Cloning and Stem Cells

• Therapeutic cloning uses embryos (natural or cloned) to collect stem cells, which are undifferentiated cells capable of becoming various cell types.

• Embryonic stem cells can develop into any cell type in the body (muscle, bone, skin, nerve, etc.).

• Scientists hope to use these cells to treat diseases and injuries, such as nerve damage or heart attacks.

• Other sources of stem cells include adult bone marrow and umbilical cord blood.

5.7 Gametes Have Half as Many Chromosomes as Body Cells

Sexual Reproduction and Genetic Variety

The human life cycle involves fertilization, the fusion of haploid gametes to produce a diploid zygote, growth and development via cell division, and the production of gametes through meiosis. Each human body cell has 46 chromosomes; gametes have 23 chromosomes each.

The Human Life Cycle

1. Adults: Every somatic (body) cell is diploid (2n)—it contains 46 chromosomes total. One set of 23 chromosomes comes from your mother, and the other set comes from your father.

2. Gamete Formation: Gametes are formed in the gonads:

   • Testes in males produce sperm.

   • Ovaries in females produce eggs.

   • A special type of cell division (meiosis) creates haploid (n) gametes, each carrying 23 chromosomes.

3. Fertilization: When a sperm and egg fuse during fertilization, they create a zygote. Each gamete contributes one haploid set (23 chromosomes), forming a diploid zygote (46 chromosomes total). This zygote contains one set from the father and one set from the mother, forming a unique combination of DNA.

4. Zygote and Development: The zygote (fertilized egg) is the original cell from which all your other cells descend. Through repeated cell divisions, the zygote develops into:

   • A diploid embryo

   • Then a diploid baby

   • And eventually a diploid adult

   Every one of your body’s millions of cells can be traced back to this single original zygote.

An Inventory of Chromosomes (Karyotype)

• A karyotype is a photographic inventory of all the chromosomes in a cell.

• Each chromosome appears as two sister chromatids joined at a centromere (this shows the duplicated form before cell division).

• Homologous chromosomes: Each pair carries genes for the same traits in the same location (for example, gene for freckles). The alleles (forms of a gene) may be identical or different.

5.8 Meiosis Produces Gametes

Overview of Meiosis

Meiosis is a type of cell division that produces haploid gametes from diploid body cells. One round of chromosome duplication is followed by two rounds of division. The result is four offspring cells, each with half the number of chromosomes as the starting cell.

Sexual Reproduction and Meiosis

• Sexual reproduction produces tremendous genetic variety among offspring because each offspring inherits a unique combination of genes from two parents.

• This process depends on meiosis, a special type of cell division that occurs in the gonads (testes in males and ovaries in females) to produce gametes—sperm and egg cells.

Haploid vs. Diploid Cells

• Diploid (2n) cells:

  • Contain two sets of chromosomes—one from each parent.

  • Examples: liver, skin, and brain cells, each with 46 chromosomes.

• Haploid (n) cells:

  • Contain only one set of chromosomes, half the number found in diploid cells.

  • Examples: gametes (sperm and egg), each with 23 chromosomes.

Overview of Meiosis

• Purpose: To produce haploid gametes for sexual reproduction.

• Similarity to mitosis: Chromosomes are duplicated once before division begins.

• Key difference: Mitosis—one cell division → 2 identical diploid cells; Meiosis—two rounds of cell division → 4 unique haploid cells.

• In short: Chromosomes duplicate once, then are divided in half twice, resulting in gametes that contain half the number of chromosomes as the original diploid cell.

5.9 Mitosis and Meiosis Have Important Similarities and Differences

Sexual Reproduction and Meiosis

• Sexual reproduction creates genetic variety among offspring.

• Each offspring inherits a unique combination of genes from two parents.

• Meiosis occurs in the gonads:

  • Testes in males—produce sperm.

  • Ovaries in females—produce eggs.

• Purpose: To form gametes (sperm and egg) used in fertilization.

Haploid vs. Diploid Cells

• Diploid (2n) cells:

  • Contain two sets of chromosomes (one from each parent).

  • Found in body (somatic) cells, such as:

    • Liver cells

    • Skin cells

    • Brain cells

  • Humans: 46 chromosomes per diploid cell.

• Haploid (n) cells:

  • Contain one set of chromosomes, half the number found in diploid cells.

  • Found in gametes (sperm and egg).

  • Humans: 23 chromosomes per haploid cell.

Fertilization

• When two haploid gametes fuse, they form a diploid zygote (46 chromosomes total).

Overview of Meiosis

• Purpose: To produce haploid gametes for sexual reproduction.

• Similarity to mitosis: Chromosomes are duplicated once before cell division begins.

• In short:

  • Chromosomes duplicate once, then are divided twice.

  • Produces four genetically unique haploid gametes.

  • Ensures offspring have half the chromosomes from each parent.

5.10 Several Processes Produce Genetic Variation Among Sexually Reproducing Organisms

Core Idea

Genetic variation is created during sexual reproduction through independent assortment of chromosomes, random fertilization, and crossing over. These processes shuffle chromosomes and create new combinations of genes.

1. Independent Assortment

• During Meiosis I, homologous chromosomes (one from each parent) line up in pairs.

• The orientation of each pair is random—either parent’s chromosome can go to either side.

• This random lineup means each gamete receives a different mix of maternal and paternal chromosomes.

• Humans have 23 homologous pairs, with 2 possible arrangements → $2^{23} = 8,388,608$ possible chromosome combinations in gametes!

2. Random Fertilization

• Random fertilization multiplies genetic variety because any sperm can fertilize any egg.

• The total number of possible chromosome combinations in a zygote = $8$ million × $8$ million = over $64$ trillion possibilities!

3. Crossing Over (Recombination)

• During Meiosis I, homologous chromosomes exchange segments of genetic material.

• This process, called crossing over, occurs when the chromosomes physically swap pieces.

• The result is hybrid chromosomes that contain genes from both parents.

• This creates new combinations of genes within each chromosome—increasing genetic diversity even further.

Summary Table: Sources of Genetic Variation

Source of Variation	When It Occurs	How It Increases Diversity
Independent Assortment	Meiosis I	Random alignment of chromosomes
Crossing Over	Meiosis I	Exchange of DNA between homologous chromosomes
Random Fertilization	After Meiosis (Fertilization)	Combines chromosomes from two parents

5.11 Mistakes During Meiosis Can Produce Gametes with Abnormal Numbers of Chromosomes

Nondisjunction and Chromosome Abnormalities

Nondisjunction is an error during meiosis that produces gametes with abnormal numbers of chromosomes. If such a gamete produces a zygote, it will have an abnormal number of chromosomes. This is usually fatal but can occasionally produce an adult with a characteristic syndrome.

What is Nondisjunction?

• Nondisjunction happens when chromosomes fail to separate properly during gamete formation.

• This can occur in the ovaries or testes during gamete formation.

• The result is abnormal gametes:

  • Some with one extra chromosome (n + 1)

  • Some with one missing chromosome (n – 1)

• When these gametes are fertilized:

  • The zygote may have 47 (2n + 1) or 45 (2n – 1) chromosomes instead of 46.

• Most embryos with this error do not survive, making nondisjunction a major cause of miscarriage.

• If they survive, they often show physical or developmental abnormalities.

Example: Trisomy 21 (Down Syndrome)

• Trisomy 21 occurs when a person has three copies of chromosome 21 instead of two.

• Total chromosome count: 47.

• This condition causes Down syndrome, the most common human chromosomal abnormality.

• Characteristics may include:

  • Short stature

  • Heart defects

  • Flattened facial features and almond-shaped eyes

  • Developmental delays

  • Higher risk of certain diseases

Sex Chromosome Abnormalities

• Nondisjunction can also affect sex chromosomes (X and Y).

• An embryo that lacks an X chromosome entirely cannot survive.

• Other abnormalities involving extra or missing sex chromosomes are usually not fatal, though they can cause certain physical or developmental effects.

• Examples include:

  • Turner syndrome (XO)—only one X chromosome

  • Klinefelter syndrome (XXY)—an extra X chromosome in males

5.12 Mendel Deduced the Basic Principles of Genetics by Breeding Pea Plants

Heredity and Genetics Study Guide

• Heredity: Transmission of traits from one generation to the next.

• Genetics: The scientific study of heredity.

• Founded by Gregor Mendel (1822–1884) through experiments with pea plants.

• Mendel discovered that "heritable factors" (now called genes) are passed from parents to offspring and retain their identity through generations.

Characters and Traits

• Character: An inherited feature that varies among individuals (e.g., flower color in pea plants, eye color in humans).

• Trait: A variation of a character (e.g., purple or white flowers, brown, blue, or green eyes).

Alleles

• Gene: Units of inherited DNA located on chromosomes.

• Most organisms (including humans and pea plants) are diploid (2n)—they have two sets of chromosomes (one from each parent).

• Alleles: Alternative forms of a gene.

  • Homozygous: Two identical alleles (AA or aa).

  • Heterozygous: Two different alleles (Aa).

Dominant vs. Recessive Alleles

• In heterozygous individuals:

  • The dominant allele (uppercase letter) determines the organism’s appearance.

  • A recessive allele (lowercase letter) has no visible effect but can be passed on.

Genotype vs. Phenotype

• Genotype: Organism’s genetic makeup (the alleles it carries).

• Phenotype: Observable traits (physical expression of the genotype).

• Relationship: Genotype + environment = phenotype.

Genotype Table Example

Genotype Type	Allele	Phenotype Example
Homozygous (Dominant)	FF	Freckled face
Homozygous (Recessive)	bb	Chocolate Labrador
Heterozygous	Pp	Purple pea flower

5.13 A Punnett Square Can Be Used to Predict the Results of a Genetic Cross

Genetic Cross Basics

• Genetic cross: An experiment where two parents (P generation) are bred to produce offspring (F generation).

• Used to predict how traits will appear in offspring.

• Represented using a Punnett square, which shows possible combinations of sperm and egg alleles.

Monohybrid Cross

• A monohybrid cross involves parents that are heterozygous for one trait (example: coat color).

• Homozygous: Two identical alleles (BB or bb).

• Heterozygous: Two different alleles (Bb).

• Dominant allele (B): black coat.

• Recessive allele (b): chocolate coat.

Example

• Cross: Bb × Bb

• Possible offspring genotypes:

  • BB → black

  • Bb → black

  • bb → chocolate

• Phenotype ratio: 3 black : 1 chocolate

Key Point

• The Punnett square helps predict both genotypes and phenotypes of offspring.

Law of Segregation

• During meiosis, paired alleles for a trait separate (segregate) into different gametes.

• Each gamete gets only one allele from each pair.

• Since allele distribution is random, all possible sperm and egg combinations must be considered.

5.14 Mendel’s Law of Independent Assortment Accounts for the Inheritance of Multiple Traits

Core Idea

A dihybrid cross involves individuals who are heterozygous for two characters. When forming gametes, the law of independent assortment states that alleles for each character will segregate (assort) independently of the alleles for other characters.

Dihybrid Cross Overview

• A dihybrid cross involves two traits instead of one.

• Both parents are heterozygous for two genes (example: seed color and seed shape in Mendel’s peas).

• Helps predict how two different traits are inherited together.

Example

• Traits:

  • Seed color—yellow (Y) dominant, green (y) recessive

  • Seed shape—smooth (S) dominant, wrinkled (s) recessive

• Cross: YySs × YySs

Law of Independent Assortment

• States that the inheritance of one trait does not affect the inheritance of another.

• Each pair of alleles separates independently during meiosis.

• Applies to all sexually reproducing organisms (peas, dogs, humans, etc.).

Key Concept

• A gamete’s alleles for one gene (like coat color) have no influence on which allele it gets for another gene (like hearing).

• This produces multiple combinations of gametes.

Example: Labrador Retriever

• Traits:

  • Coat color—B (black, dominant) / b (chocolate, recessive)

  • Hearing—D (hearing, dominant) / d (deaf, recessive)

• Parent genotype: BbDd

• Possible gametes (4 types):

  • BD

  • Bd

  • bD

  • bd

• Each gamete is equally likely, leading to four combinations of traits in the offspring.

Dihybrid Cross Results

• The Punnett square for a dihybrid cross has 16 boxes (4 gametes × 4 gametes).

• Phenotypic ratio for a typical dihybrid cross = 9 : 3 : 3 : 1

  • 9 show both dominant traits (e.g., black & hearing)

  • 3 show one dominant trait (e.g., black & deaf)

  • 3 show the other combination (e.g., chocolate & hearing)

  • 1 shows both recessive traits (e.g., chocolate & deaf)

5.15 Pedigrees Can Be Used to Trace Traits in Human Families

Overview

• Mendelian genetics applies to many human traits.

• However, humans cannot be bred for experiments, so Punnett squares alone aren’t practical.

• Instead, scientists use observational tools like pedigrees to study inheritance.

Human Genetic Characters

• Many human traits are not caused by a single gene.

• These traits follow simple dominant/recessive inheritance patterns.

• Some traits are rare, but the most common = simply marks the allele whether both alleles are present.

Key Terms

• Wild-type trait: most common form (e.g., normal pigmentation).

• Mutant trait: less common form (e.g., albinism, freckles).

Examples Table

Character	Wild-Type (Most Common)	Mutant Trait (Less Common)	Dominance
Freckles	Non-freckled	Freckled	Dominant
Pigmentation	Normal	Albinism	Recessive
Height	Normal	Achondroplasia (dwarfism)	Dominant
Lungs	Healthy	Cystic Fibrosis	Recessive

Carriers

• Most genetic disorders are recessive.

• A person must inherit two copies of the recessive allele to have the disease.

• A carrier is heterozygous—only one normal allele and one disease allele.

• Carriers do not show symptoms but can pass the allele to offspring.

Punnett Square for Two Carriers (Aa × Aa)

Parents	Possible Genotype	Phenotype
A (normal), a (disease)	AA (normal)	Normal
	Aa (carrier)	Carriers (no symptoms)
	aa (disease)	Afflicted

Genotype Ratio: 1:2:1 Phenotype Ratio: 3 normal : 1 afflicted

Pedigrees

• A pedigree is a family genetic history chart that shows how traits are passed across generations.

• Used to track inheritance and determine genotypes of family members.

• Commonly used in:

  • Medical genetics (e.g., tracking inherited diseases)

  • Animal breeding (purebred lineages)

Symbols

• □ Square = male

• ○ Circle = female

• ■ or ● Shaded = individual expresses the trait

• □ or ○ Unshaded = individual does not express the trait

• Half-shaded = carrier

5.16 The Inheritance of Many Traits is More Complex Than Mendel’s Laws

Core Idea

Classical Mendelian genetics describes the inheritance of many traits, but real traits require extensions to the simple rules. Examples of complications include incomplete dominance, multiple alleles, pleiotropy, polygenic inheritance, and environmental influences.

1. Incomplete Dominance

• The heterozygous genotype produces an intermediate phenotype, not just dominant or recessive.

• Example:

  • Red (RR) × White (rr) → All Pink (Rr) offspring.

  • When two pinks (Rr × Rr) cross → 1 Red : 2 Pink : 1 White.

  • Seen in snapdragon flower color and other traits where blending occurs.

2. Multiple Alleles & Codominance

• Some genes have more than two alleles in a population, though an individual carries only two.

• Example: Human Blood Type

  • Alleles: IA/IA, IA/IB, IB/IB, and ii.

  • Possible Genotypes & Phenotypes:

  Genotype	Blood Type
  IA/IA or IA/i	A
  IB/IB or IB/i	B
  IA/IB	AB
  ii	O

• Codominance: IA/IB and IB/IB are both expressed equally (Type AB shows both antigens).

3. Pleiotropy

• One gene affects multiple traits.

• Example: Sickle-cell disease

  • A single mutation changes the hemoglobin protein.

  • Leads to sickle-shaped red blood cells, causing:

    • Blocked blood flow

    • Organ damage

    • Pain and anemia

  • Shows how one gene = many effects.

4. Polygenic Inheritance

• Many genes influence one trait, leading to a continuous range of phenotypes.

• Examples:

  • Human height

  • Skin color

• Traits controlled by polygenic inheritance show a bell-shaped distribution (most people have intermediate forms).

5. Environment vs. Genetics

• Some traits are mostly genetic (e.g., blood type).

• Others result from both genes and environment (e.g., skin color).

• Some are purely environmental (e.g., tattoos, piercings).

• Only genetic traits are heritable—environmental influences are not passed to offspring.

5.17 Linked Genes May Not Obey the Law of Independent Assortment

Core Idea

Genes are located on chromosomes, so the behavior of chromosomes accounts for the inheritance of genes. Genes located near each other on the same chromosome (called linked genes) are often inherited as a set, but crossing over of homologous chromosomes during meiosis can produce recombinant chromosomes.

1. Linked Genes

• Mendel’s law of independent assortment states that each gene is inherited independently.

• Exception: Genes located close together on the same chromosome are linked and tend to be inherited together.

• Effect: Offspring ratios differ from Mendelian expectations.

• Example: Cross of homozygotes for two linked genes (AABB × aabb) may yield:

  • 3 double dominant : 1 double recessive

  • No mixed dominant/recessive offspring

2. Genetic Recombination

• During meiosis, homologous chromosomes pair up and can exchange segments in crossing over.

• Recombinant chromosomes result from this exchange, containing parts of both original chromosomes.

• Cross-over frequency gauges the distance between genes:

  • Genes far apart—more likely to be separated by crossover—appear to assort independently.

  • Genes very close together—rarely separated—remain linked.

5.18 Sex-Linked Genes Display Unusual Inheritance Patterns

Core Idea

Human cells contain 44 autosomes and 2 sex chromosomes. A person with two X sex chromosomes is genetically female; males have one X and one Y. Because they are located on the X chromosome, sex-linked genes display unusual inheritance patterns.

Human Chromosomes

• Humans have 46 chromosomes total:

  • 44 autosomes (common to all body cells)

  • 2 sex chromosomes (determine genetic sex)

• Sex determination:

  • Male: XY

  • Female: XX

• Gamete contribution:

  • Male sperm: 22 autosomes + X or Y

  • Female egg: 22 autosomes + X

  • Offspring sex depends on whether sperm contributes X or Y.

Sex-Linked Genes

• Genes located on the X chromosome (but not the Y) are sex-linked.

• Males have one X—a single allele can determine the trait.

• Females have two Xs—two alleles influence expression.

• Examples of X-linked traits:

  • Color blindness

  • Hemophilia (recessive disorder common in royal families)

• Key Point: X-linked recessive traits are more common in males because a single defective X expresses the trait.

Unique Patterns of Inheritance

• Females (XX): Can be carriers if heterozygous (one normal, one mutant allele).

• Males (XY): Cannot be carriers; the single X chromosome determines phenotype.

• Y chromosome: Very small; mostly carries genes for maleness.

Key Takeaways

• Sex is genetically determined by X and Y chromosomes.

• Sex-linked traits follow unique inheritance patterns due to differences in male and female chromosomes.

• Recessive X-linked disorders appear more frequently in males than in females.