The Chromosomal Basis of Inheritance

Core Concept

Mendel's abstract "factors" are physically located on chromosomes. The chromosome theory of inheritance states that genes reside at specific positions (loci) on chromosomes, and that the behavior of chromosomes in meiosis — segregation of homologs and independent assortment — is the physical basis of Mendel's laws. Studying chromosomes also explains sex-linked traits, linked genes, and chromosomal disorders caused by errors in chromosome separation.

Key Concepts

The chromosome theory of inheritance

Early in the 1900s, Thomas Hunt Morgan's work on the fruit fly Drosophila showed that a gene for eye color resides on the X chromosome, providing the first concrete link between a specific gene and a specific chromosome. The theory holds that chromosomes carry genes, homologous chromosomes carry alleles at corresponding loci, and meiosis's chromosome movements account for segregation and independent assortment.

Sex chromosomes and X-linked inheritance

Humans have 22 pairs of autosomes plus one pair of sex chromosomes (XX in females, XY in males). The X chromosome carries many genes unrelated to sex; the Y chromosome is small and gene-poor (with the SRY gene that triggers male development). X-linked (sex-linked) traits are carried on the X chromosome. Because males have only one X, they express X-linked recessive alleles with a single copy — this is why red-green color blindness and hemophilia are far more common in males, who inherit the allele from their (carrier) mother and have no second X to mask it. Fathers cannot pass X-linked traits to their sons (sons get the Y from dad) but do pass them to all daughters.

Linked genes and recombination frequency

Genes on the same chromosome tend to be inherited together and are said to be linked — they do not assort independently. However, crossing over in prophase I can exchange segments between homologs, producing recombinant gametes. The recombination frequency (percent of offspring that are recombinants) is lower when genes are close together and higher when they are far apart. Morgan's student Alfred Sturtevant used this to build the first linkage maps: a 1% recombination frequency defines roughly one map unit (centimorgan). Genes on different chromosomes (or far apart on the same one) assort independently (≈50% recombination, the maximum).

Nondisjunction and aneuploidy

Nondisjunction is the failure of homologous chromosomes (meiosis I) or sister chromatids (meiosis II) to separate properly. The resulting gamete has an extra or missing chromosome, and after fertilization the zygote is aneuploid — carrying an abnormal chromosome number. Trisomy = three copies of a chromosome; monosomy = one copy. Down syndrome is trisomy 21 (three copies of chromosome 21) and is the most common viable human aneuploidy; its frequency rises with maternal age. Sex-chromosome aneuploidies (XXY/Klinefelter, XO/Turner) also occur. Most other autosomal aneuploidies are lethal early in development.

How It Works

In meiosis, each homolog carries the same genes at the same loci. Segregation of homologs (meiosis I) is the physical basis of Mendel's Law of Segregation; random orientation of different chromosome pairs at metaphase I is the basis of independent assortment. If two genes sit on the same chromosome, they travel together unless crossing over swaps them — the closer they are, the less often they recombine. When chromosomes fail to separate (nondisjunction), gametes gain or lose whole chromosomes, producing aneuploid embryos such as trisomy 21.

How it works

In meiosis, each homolog carries the same genes at the same loci. Segregation of homologs (meiosis I) is the physical basis of Mendel's Law of Segregation; random orientation of different chromosome pairs at metaphase I is the basis of independent assortment. If two genes sit on the same chromosome, they travel together unless crossing over swaps them — the closer they are, the less often they recombine. When chromosomes fail to separate (nondisjunction), gametes gain or lose whole chromosomes, producing aneuploid embryos such as trisomy 21.

Common confusions

• "X-linked recessive means only males can have it." Wrong — females can be affected (if homozygous) or carriers; males are just affected more often because they have one X.
• "Linked genes always stay together." Wrong — crossing over can separate them; the farther apart, the more recombination.
• "50% recombination means the genes are on the same chromosome but far apart." Ambiguous — 50% is indistinguishable from independent assortment, so you can't tell far-apart linked genes from genes on different chromosomes without more data.
• "Down syndrome is inherited." Usually wrong — it is most often caused by a spontaneous nondisjunction event in gamete formation, not inherited from a parent.
• "Aneuploidy means any mutation." Wrong — aneuploidy specifically means an abnormal number of chromosomes, not a change within a gene.

Quick review

• Genes are on chromosomes; meiosis underlies Mendel's laws.
• X-linked traits: more common in males; dads → daughters, moms → carrier sons.
• Linkage = same chromosome; crossing over creates recombinants.
• Recombination frequency measures map distance (1% = 1 cM).
• Nondisjunction → trisomy/monosomy (aneuploidy).
• Down syndrome = trisomy 21.