Difficulty: Introductory-Intermediate | Prerequisites: Mendelian genetics, meiosis
Mendel's principles make more sense once you know that genes physically sit on chromosomes. This topic explains why segregation and independent assortment work (chromosomes separate during meiosis), and where they break down (linked genes on the same chromosome). It also covers sex-linked inheritance, which produces different patterns in males and females. You should be comfortable with Mendel's laws and the mechanics of meiosis before starting here.
Genes live on chromosomes, and chromosomes come in pairs. Segregation happens because homologous chromosomes split at meiosis I. Genes on different chromosomes assort independently, but genes on the same chromosome tend to travel together (linkage) unless crossing over separates them. Genes on sex chromosomes produce inheritance patterns that differ between males and females.
Autosome
Any chromosome that is not a sex chromosome. In humans, chromosomes 1 through 22 are autosomes.
Autosomal inheritance
The inheritance pattern of genes located on autosomes. Both sexes are affected equally.
Sex-linked gene
A gene located on either sex chromosome (X or Y). In simple terms, a gene whose inheritance pattern differs between males and females because the sex chromosomes are not a matched pair.
X-linked gene
A gene located on the X chromosome. Males have only one copy (hemizygous), so a single recessive allele will be expressed.
Y-linked gene
A gene located on the Y chromosome. Only passed from father to son.
Linkage
The tendency of genes on the same chromosome to be inherited together. Think of linked genes as passengers on the same bus: they travel together unless something (crossing over) separates them.
Crossing over (recombination)
The physical exchange of segments between homologous chromosomes during meiosis I. This breaks up linkage and produces new allele combinations.
Genetic recombination
The production of offspring with combinations of alleles not found in either parent, typically resulting from crossing over.
Genetic map
A diagram showing the relative positions (loci) of genes along a chromosome, based on recombination frequencies. The farther apart two genes are, the more often crossing over occurs between them.
Genes are located on chromosomes, and chromosomes come in homologous pairs.
The behaviour of chromosomes during meiosis explains Mendel's principles: segregation occurs because homologous chromosomes separate at meiosis I, and independent assortment occurs because different chromosome pairs line up randomly at the metaphase plate.
Non-sex chromosomes are called autosomes. In humans there are 22 pairs of autosomes.
Sex chromosomes (X and Y in mammals) determine biological sex and carry additional genes unrelated to sex determination.
Genes on autosomes follow standard Mendelian patterns. Genes on sex chromosomes follow sex-linked inheritance patterns.
X-linked genes: males (XY) carry only one copy, so a single recessive allele on the X is expressed. Females (XX) need two copies of a recessive allele to show the recessive phenotype.
Y-linked genes: passed exclusively from father to son. Relatively few genes are Y-linked.
Reciprocal crosses involving sex-linked genes produce different results, which is how you identify sex linkage.
Genes on the same chromosome tend to be inherited together, violating Mendel's principle of independent assortment.
Complete linkage (no crossing over) produces only parental (non-recombinant) phenotypes.
Independent assortment predicts a 1:1:1:1 ratio in a test cross. Deviation from this ratio suggests linkage.
During prophase I of meiosis, homologous chromosomes can exchange segments. This is crossing over.
Crossing over between linked genes produces recombinant offspring with new allele combinations.
Crossing over is rare between genes that are physically close together on a chromosome, and more frequent between genes that are far apart.
Recombination frequency (the percentage of recombinant offspring) is used to estimate the distance between genes.
A genetic map plots genes along a chromosome based on recombination frequencies.
One map unit (1 centimorgan) corresponds to a 1% recombination frequency.
These maps show relative positions, not absolute physical distances. They were the first tool for understanding genome organisation, long before DNA sequencing.
Students often confuse "sex-linked" with "sex-limited." Sex-linked means the gene is on a sex chromosome. Sex-limited means the trait is expressed in only one sex, regardless of chromosome location.
Linked genes do not assort independently, but they are not permanently glued together. Crossing over can and does separate them, just at a lower frequency than independent assortment would predict.
A recombination frequency of 50% does not mean the genes are on different chromosomes. Genes very far apart on the same chromosome can also show 50% recombination because multiple crossovers scramble the signal.
"X-linked recessive" does not mean only males are affected. Females can express X-linked recessive traits if they are homozygous (two copies of the recessive allele).
⚠️ Be able to distinguish autosomal from sex-linked inheritance by looking at pedigree patterns and reciprocal cross results.
⚠️ Know the difference between linked and independently assorting genes, and how to recognise linkage from test-cross ratios.
⚠️ Understand why crossing over frequency correlates with physical distance between genes.
⚠️ Be able to use recombination frequencies to construct a simple genetic map (gene order and distances).
Sex-linked inheritance explains why colour blindness and haemophilia are far more common in males: these are X-linked recessive conditions, and males have only one X. Genetic mapping laid the groundwork for the Human Genome Project: early gene maps based on recombination data guided the physical mapping of chromosomes.
True or false: genes on the same chromosome always travel together during meiosis.
False. Crossing over can separate them.
Fill in the blank: a gene on the X chromosome is called an ______ gene.
X-linked.
True or false: a recombination frequency of 10% means the two genes are closer together than a pair with a recombination frequency of 25%.
True. Lower recombination frequency means the genes are closer and crossing over between them is less frequent.
Fill in the blank: non-sex chromosomes are called ______.
Autosomes.
True or false: independent assortment predicts a 1:1:1:1 ratio in a dihybrid test cross.
True.
Q: A colour-blind man (X-linked recessive) and a carrier woman have children. What proportion of their sons will be colour-blind?
A: 1/2 (50%). The mother is a carrier (X^C X^c), so half her X chromosomes carry the recessive allele. Sons get their single X from the mother.
Q: Two genes are located on the same chromosome. In a test cross you observe 85% parental phenotypes and 15% recombinant phenotypes. What is the map distance between the genes?
A: 15 centiMorgans (map units). Recombination frequency equals map distance.
Q: How can you tell whether a gene showing a dominant phenotype is autosomal or X-linked?
A: Perform reciprocal crosses. If results are the same regardless of which parent carries the dominant trait, the gene is autosomal. If results differ, it is likely X-linked.
Q: Complete linkage between two genes would produce what ratio in a dihybrid test cross?
A: Only two phenotypic classes (the parental types), in a 1:1 ratio, rather than the 1:1:1:1 ratio predicted by independent assortment.
This material builds directly on Mendelian genetics: linkage is the main exception to independent assortment, and understanding why it happens requires knowing that genes sit on chromosomes. It connects forward to molecular genetics (what crossing over looks like at the DNA level) and to population genetics (recombination generates the genetic variation that selection and drift act on).
Chromosome theory of inheritance, autosome, sex chromosome, X-linked, Y-linked, sex-linked inheritance, linkage, genetic linkage, crossing over, recombination, recombination frequency, genetic map, centiMorgan, map unit, Morgan, hemizygous, carrier, parental type, recombinant type, non-recombinant