Source material from UCF Molecular Biology I Difficulty: Introductory to Intermediate Prerequisites: Study Notes Part 1 (History and the Central Dogma). You should already know that DNA carries genetic information and that genes encode proteins.
Tags: genetics, Mendel, law of segregation, independent assortment, genotype, phenotype, alleles, dominant, recessive, homozygous, heterozygous, mitosis, meiosis, crossing over, gene linkage, codominance, incomplete dominance, polygenic inheritance, chromosomal theory, sex-linked traits
This unit covers how genetic information is inherited from one generation to the next. Mendel worked out the rules of inheritance in the 1860s using pea plants, long before anyone knew DNA existed. Sutton and Boveri later connected those rules to chromosomes. The concepts here (segregation, independent assortment, dominance, linkage) are the vocabulary you will use for every genetics problem in the course. Cell division (mitosis and meiosis) is the physical mechanism that makes inheritance work, and crossing over during meiosis is what generates genetic diversity. If you are behind, start with the key terms, then work through Mendel's two laws before tackling the more complex inheritance patterns.
Mendel's Law of Segregation says each gamete gets one allele per gene. His Law of Independent Assortment says genes on different chromosomes sort independently. Real inheritance is often more complex: incomplete dominance, codominance, multiple alleles, and polygenic traits all deviate from simple Mendelian ratios. Mitosis makes identical diploid cells; meiosis makes genetically diverse haploid gametes, with crossing over as the main source of recombination.
Allele
A variant form of a gene at a given locus. Different alleles arise by mutation and may produce different phenotypes. Think of it as one version of a particular instruction. You carry two copies (one from each parent), and they might not match.
Homozygous
Having two identical alleles at a locus (e.g. AA or aa). In simple terms, both copies of the gene say the same thing.
Heterozygous
Having two different alleles at a locus (e.g. Aa). In simple terms, the two copies of the gene differ, and which one shows up depends on dominance relationships.
Dominant allele
An allele whose phenotype is expressed in heterozygotes. Written as an uppercase letter (A). Think of it as the louder voice: if one copy is present, you see its effect.
Recessive allele
An allele whose phenotype is masked in heterozygotes and only expressed when homozygous (aa). In simple terms, you need two copies for it to show.
Genotype
The genetic makeup of an organism at one or more loci (e.g. Aa, AaBb). Think of it as what the DNA says.
Phenotype
The observable characteristics of an organism resulting from the interaction of genotype and environment. Think of it as what you can see or measure.
Diploid (2n)
A cell with two complete sets of chromosomes, one from each parent. Most human somatic cells are diploid (2n = 46). In simple terms, every chromosome has a partner.
Haploid (n)
A cell with one set of chromosomes. Gametes (sperm and egg) are haploid (n = 23 in humans). In simple terms, half the usual chromosome count, ready to merge with another gamete at fertilisation.
Segregation
The separation of the two alleles of a gene into different gametes during meiosis. Think of it as dealing one card from each pair into separate hands.
Independent assortment
The principle that alleles of genes on different chromosomes are distributed to gametes independently of one another. In simple terms, inheriting your mother's version of gene A does not determine which version of gene B you inherit (provided A and B are on different chromosomes).
Crossing over (recombination)
The physical exchange of segments between homologous chromosomes during meiosis I, producing new allele combinations on a single chromosome. Think of it as shuffling sections of two instruction manuals to create new combinations.
Gene linkage
The tendency of genes located close together on the same chromosome to be inherited together, because crossing over between them is rare. In simple terms, nearby genes travel as a package unless recombination breaks them apart.
Autosomal trait
A trait encoded by a gene on one of the non-sex chromosomes (autosomes 1 through 22 in humans).
Sex-linked trait / X-linked trait
A trait encoded by a gene on the X chromosome. Males (XY) are hemizygous for X-linked genes, so a single recessive allele on X will be expressed. In simple terms, males only have one X, so they cannot be carriers of an X-linked recessive condition: they either have it or they do not.
Genetic marker
A specific, known DNA sequence with a known location on a chromosome, used to track inheritance patterns in genetic mapping.
Law of Segregation – During gamete formation (meiosis), the two alleles for each gene separate so that each gamete carries only one allele. At fertilisation, offspring receive one allele from each parent, restoring the diploid state.
Law of Independent Assortment – Genes on different chromosomes assort independently during meiosis. This produces 2^n possible gamete combinations, where n is the number of heterozygous gene pairs. For humans, the theoretical number of genetically distinct gametes from independent assortment alone is 2^23, or roughly 8.4 million.
Sutton's Chromosomal Theory – Walter Sutton and Theodor Boveri independently proposed that chromosomes are the physical carriers of genes. This explained Mendel's laws in terms of chromosome behaviour during meiosis: segregation reflects homologous chromosomes separating in meiosis I, and independent assortment reflects the random orientation of different chromosome pairs at the metaphase plate.
Not all inheritance follows simple dominant/recessive rules:
Incomplete dominance – The heterozygote has an intermediate phenotype between the two homozygotes. Classic example: red (RR) crossed with white (WW) snapdragons produces pink (RW) offspring. Neither allele fully masks the other.
Codominance – Both alleles are fully expressed simultaneously in the heterozygote. Classic example: AB blood type, where both A and B antigens appear on red blood cells.
Multiple alleles – More than two allele variants exist in the population for a single gene (though any individual still carries at most two). The ABO blood group system has three alleles: I^A, I^B, and i.
Polygenic inheritance – Multiple genes contribute to a single trait, producing a continuous distribution of phenotypes rather than discrete categories. Examples: height, skin colour, blood pressure. These traits often follow a bell curve in a population.
Mitosis – One round of division producing two genetically identical diploid daughter cells. Function: growth, repair, and asexual reproduction. No crossing over under normal circumstances, so daughter cells are clones of the parent cell.
Meiosis – Two rounds of division (meiosis I and meiosis II) producing four genetically unique haploid cells (gametes). Genetic diversity arises from three mechanisms:
Crossing over during prophase I
Independent assortment of homologous pairs at metaphase I
Random fertilisation (which gamete meets which)
Genes on the same chromosome do not assort independently; they are linked. The closer two genes are on a chromosome, the less likely crossing over will separate them, and the more frequently they are inherited together.
Recombination frequency between two genes is used to estimate their distance apart. One percent recombination equals approximately one map unit (centimorgan).
X-linked traits follow a distinct inheritance pattern because males are hemizygous. X-linked recessive conditions (e.g. haemophilia, red-green colour blindness) appear far more frequently in males.
Janssen's Hypothesis – Proposed that crossing over involves the physical exchange of segments between non-sister chromatids of homologous chromosomes during meiosis I. Later confirmed cytologically.
Barbara McClintock – Discovered that both crossover and non-crossover events during meiosis influence chromosome behaviour and genetic recombination. Her later work on transposable elements ("jumping genes") further expanded the understanding of genome dynamics.
Mutations introduce new alleles into a population. They are the ultimate source of all genetic variation.
Recombination (crossing over and independent assortment) shuffles existing alleles into new combinations each generation.
Together, mutation and recombination provide the raw material for natural selection and adaptation.
Understanding inheritance patterns is the basis of genetic counselling. A family history of a recessive condition like cystic fibrosis, for instance, allows counsellors to calculate carrier probabilities using Mendelian ratios. Genetic mapping (using linkage analysis and recombination frequencies) was one of the key tools in the Human Genome Project and remains important in identifying disease-associated genes.
Students often assume "dominant" means "more common" or "better." Dominance is simply about which phenotype appears in the heterozygote. A dominant allele can be rare and harmful (e.g. Huntington disease).
Students frequently forget that independent assortment only applies to genes on different chromosomes (or far apart on the same chromosome). Linked genes violate independent assortment, and that is the whole point of linkage.
Mitosis and meiosis are sometimes confused. A useful anchor: mitosis = maintenance (identical copies for growth and repair); meiosis = mixing (diverse gametes for reproduction).
Incomplete dominance and codominance are not the same thing. In incomplete dominance, the heterozygote is a blend (pink). In codominance, both phenotypes appear fully and simultaneously (A and B antigens both present).
⚠️ Be able to solve a basic monohybrid cross and predict phenotypic and genotypic ratios (3:1 phenotypic for a heterozygous cross with complete dominance).
⚠️ Know the difference between incomplete dominance (blended intermediate) and codominance (both fully expressed).
⚠️ Understand why X-linked recessive traits appear more often in males. Be prepared to work through an X-linked Punnett square.
⚠️ Explain how crossing over and independent assortment each contribute to genetic diversity, and why linked genes do not assort independently.
⚠️ Be able to compare mitosis and meiosis: number of divisions, ploidy of products, genetic identity vs. diversity.
True or false: A heterozygous individual (Aa) always shows the dominant phenotype. (True, under complete dominance. False if incomplete dominance or codominance applies, so read the question carefully.)
Fill in the blank: Mendel's Law of ______ states that the two alleles for a gene separate during gamete formation.
True or false: Genes on the same chromosome always travel together during meiosis. (False – crossing over can separate them, especially if they are far apart.)
Fill in the blank: Meiosis produces ______ (number) genetically ______ (identical/unique) haploid cells.
True or false: Codominance produces an intermediate, blended phenotype. (False – that is incomplete dominance. Codominance means both phenotypes are fully expressed.)
Q: In a cross between two heterozygous parents (Aa x Aa), what are the expected genotypic and phenotypic ratios of the offspring under complete dominance?
A: Genotypic ratio: 1 AA : 2 Aa : 1 aa. Phenotypic ratio: 3 dominant : 1 recessive.
Q: Why do X-linked recessive disorders like haemophilia affect males more often than females?
A: Males have only one X chromosome (XY), so a single recessive allele on X is expressed because there is no second X to carry a dominant allele. Females (XX) would need two copies of the recessive allele to be affected; with one copy they are carriers.
Q: Explain how crossing over during meiosis I increases genetic diversity.
A: During prophase I, non-sister chromatids of homologous chromosomes exchange segments. This creates recombinant chromosomes with new combinations of alleles that were not present on either parental chromosome, increasing the variety of gametes produced.
Q: Two genes are located on the same chromosome and show 8% recombination frequency. Are they considered linked? How far apart are they?
A: Yes, they are linked (recombination frequency is well below 50%, which would indicate independent assortment). They are approximately 8 map units (centimorgans) apart.
Q: How does polygenic inheritance differ from a simple Mendelian single-gene trait?
A: In polygenic inheritance, multiple genes each contribute a small additive effect to a single trait, producing a continuous range of phenotypes (a bell curve distribution). A single-gene Mendelian trait produces discrete phenotypic categories (e.g. purple vs. white flowers).
Mendel's "factors" are the genes whose molecular identity was established by the experiments in Part 1 of these notes (Avery, Hershey-Chase). The behaviour of chromosomes during meiosis, covered here, is the physical explanation for Mendel's laws. Clinical correlations (sickle cell anaemia, Huntington disease, cystic fibrosis) covered in the biochemistry notes are direct applications of inheritance patterns: a single mutation altering a protein and causing disease. The biochemistry of DNA replication and transcription connects to how genetic information is copied and expressed from one generation to the next.
Mendelian genetics, law of segregation, law of independent assortment, Sutton chromosomal theory, Boveri, genotype, phenotype, allele, dominant, recessive, homozygous, heterozygous, diploid, haploid, mitosis, meiosis, crossing over, recombination, gene linkage, X-linked, sex-linked traits, autosomal, incomplete dominance, codominance, multiple alleles, polygenic inheritance, genetic mapping, centimorgan, recombination frequency, Punnett square, genetic marker, Janssen, McClintock, transposable elements, UCF molecular biology