Premed · Premed · General Biology 1
Lecture 19: Chromosomal Basis of Inheritance
General Biology I — Molecular & Cellular
Learning Objectives
By the end of this lecture, students will be able to:
- Explain the chromosome theory of inheritance and the evidence supporting it
- Describe X-linked inheritance patterns and solve X-linked genetics problems
- Explain X-inactivation and its consequences
- Define gene linkage and explain how recombination frequency relates to gene distance
- Describe common chromosomal abnormalities and their phenotypic consequences
Lecture Content
I. The Chromosome Theory of Inheritance
The chromosome theory of inheritance, proposed independently by Walter Sutton and Theodor Boveri in 1902-1903, unifies Mendelian genetics with cell biology by establishing three key principles: genes are physically located on chromosomes, chromosomes undergo segregation and independent assortment during meiosis, and the behavior of chromosomes during meiosis directly accounts for Mendel's laws. The most compelling experimental support came from Thomas Hunt Morgan in the 1910s, working with the fruit fly Drosophila melanogaster. Morgan discovered the first sex-linked gene--the white-eye allele (w), which is recessive and located on the X chromosome. Reciprocal crosses (swapping which parent carried the white allele) produced different offspring ratios, a result explicable only if the gene resided on the X chromosome. This work established that specific genes reside on specific chromosomes.
II. Sex Determination
In many organisms, biological sex is determined by sex chromosomes. In the XX-XY system used by mammals, most insects, and some plants, females are XX (the homogametic sex) and males are XY (the heterogametic sex). The SRY gene on the Y chromosome encodes a transcription factor that triggers testis development; without it, the default developmental program produces ovaries. Other systems exist: birds and some reptiles use a ZW-ZZ system (males ZZ, females ZW), some insects use an XX-X0 system (females have two X chromosomes, males have only one), and bees and ants employ haplodiploidy (females develop from fertilized diploid eggs, males from unfertilized haploid eggs). In some reptiles, sex is determined not by chromosomes at all but by the temperature of the environment during embryonic development.
III. X-Linked Inheritance
The X chromosome carries approximately 800 protein-coding genes, far more than the roughly 70 on the much smaller Y chromosome. Because males have only one X, they are hemizygous for X-linked genes and will express whatever allele they carry, whether dominant or recessive. Females, with two X chromosomes, can be homozygous dominant, heterozygous (carriers), or homozygous recessive.
X-linked recessive traits produce a distinctive pedigree pattern: affected individuals are predominantly male, carrier females are phenotypically normal, affected males inherit the allele from their carrier mothers, and there is no male-to-male transmission (since a father passes his Y, not his X, to his sons). All daughters of an affected male are obligate carriers. Classic X-linked recessive conditions include hemophilia A (Factor VIII deficiency causing excessive bleeding), Duchenne muscular dystrophy (progressive muscle wasting due to absence of dystrophin), red-green color blindness (defective opsin genes), and G6PD deficiency (hemolytic anemia triggered by certain drugs or foods). X-linked dominant conditions are rare; in these disorders, both sexes can be affected, but an affected father passes the trait to all daughters and no sons.
<image>A pedigree diagram showing X-linked recessive inheritance of hemophilia across three generations. The carrier female (XHXh) in generation I is shown with a half-filled circle. Her mating with a normal male (XHY) produces: carrier daughters (XHXh, half-filled circles), normal daughters (XHXH, open circles), affected sons (XhY, filled squares), and normal sons (XHY, open squares). Genotypes are labeled below each individual. A Punnett square to the side shows the cross XHXh x XHY, with the four possible offspring types and their expected ratios (1/4 each). An annotation indicates "no male-to-male transmission" with the X chromosome inheritance pattern highlighted.</image>
IV. X-Inactivation
In female mammals, one X chromosome in each cell is randomly inactivated during early embryonic development, condensing into a dense, transcriptionally silent structure visible at the nuclear periphery called a Barr body. This mechanism, described by Mary Lyon in 1961 (the Lyon hypothesis), ensures dosage compensation--equalizing X-linked gene expression between XX females and XY males.
Inactivation is random with respect to which X (maternal or paternal) is silenced in any given cell, but once the decision is made, all descendant cells maintain the same inactive X. The result is that females are genetic mosaics--a patchwork of cell populations expressing one or the other X chromosome. Calico cats beautifully illustrate this: the gene for orange versus black coat color is X-linked, so heterozygous females display a mosaic of orange and black patches. The molecular mediator of inactivation is the XIST gene, which produces a long non-coding RNA that coats the chromosome from which it is expressed, recruiting proteins that add repressive histone modifications and DNA methylation to silence it. Not all X-linked genes are inactivated--some genes in the pseudoautosomal regions escape silencing, which is why individuals with abnormal numbers of X chromosomes (such as those with Turner or Klinefelter syndrome) show phenotypic effects despite X-inactivation.
V. Linked Genes and Genetic Recombination
Linked genes are genes located on the same chromosome. Because they are physically connected, they tend to be inherited together, violating the law of independent assortment. Linked genes can be in coupling (cis) configuration--both dominant alleles on one chromosome and both recessives on the other (AB/ab)--or in repulsion (trans) configuration (Ab/aB).
Recombination during meiosis can separate linked genes through crossing over. The frequency of recombinant offspring depends on the distance between the two genes: genes that are close together recombine rarely (tight linkage), while genes far apart recombine more frequently. When the recombination frequency approaches 50%, the genes assort independently for all practical purposes, even though they reside on the same chromosome.
VI. Genetic Mapping
The relationship between recombination frequency and physical distance provides the basis for constructing genetic (linkage) maps. By convention, 1% recombination frequency equals 1 centimorgan (cM) or 1 map unit. A three-point testcross--crossing a triple heterozygote with a triple homozygous recessive--allows simultaneous determination of gene order and distances between three genes. The parental classes are the most frequent, single-crossover classes are intermediate, and double-crossover classes are the rarest. The gene in the middle is identified by comparing double-crossover offspring to the parental types. Interference describes the phenomenon whereby one crossover event reduces the probability of a second crossover nearby, quantified by the coefficient of coincidence (observed double crossovers divided by expected) and interference (1 minus the coefficient of coincidence).
<image>A genetic mapping diagram. Top: A testcross between AaBbCc (cis configuration: ABC/abc) and aabbcc. Middle: A table listing the eight offspring classes with their phenotypes and frequencies, sorted from most frequent (parental types: ABC and abc) to least frequent (double crossover types). Bottom: A linear genetic map showing the three genes (A, B, C) with distances in centiMorgans between them based on recombination frequencies. Arrows indicate which crossover events produced each recombinant class.</image>
VII. Chromosomal Abnormalities
A. Alterations in Chromosome Number
Aneuploidy involves an abnormal number of individual chromosomes. Trisomies (2n+1) include Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). Turner syndrome (45,X) is the only viable human monosomy (2n-1). Several sex chromosome aneuploidies are compatible with life: Klinefelter syndrome (47,XXY) produces tall, typically infertile males; triple X syndrome (47,XXX) usually causes mild or no symptoms; and XYY syndrome (47,XYY) is generally without significant clinical effects. Polyploidy--additional complete chromosome sets--is lethal in most animals but common and often beneficial in plants, where it contributes to larger cell size and has played a major role in crop domestication.
B. Alterations in Chromosome Structure
Deletions remove chromosomal segments and can cause serious developmental consequences--cri-du-chat syndrome, caused by a deletion on chromosome 5p, produces intellectual disability and a distinctive cat-like cry. Duplications repeat a segment and can lead to gene dosage effects. Inversions reverse a segment's orientation; they may not affect the individual carrying them but can produce unbalanced gametes and thus problems in offspring. Translocations move a segment to a non-homologous chromosome. Reciprocal translocations exchange segments between two chromosomes, while Robertsonian translocations fuse two acrocentric chromosomes at their centromeres. Approximately 4% of Down syndrome cases result from a Robertsonian translocation involving chromosome 21 rather than from standard trisomy.
VIII. Genomic Imprinting (Brief Introduction)
Some genes are expressed differently depending on whether they were inherited from the mother or the father. In genomic imprinting, one parental allele is selectively silenced by DNA methylation established during gametogenesis. The same chromosomal deletion in region 15q11-13 produces two entirely different syndromes depending on the parent of origin: Prader-Willi syndrome when the deletion is on the paternal chromosome (the maternal copy of the relevant genes is imprinted and silent), and Angelman syndrome when the deletion is on the maternal chromosome (the paternal copy is imprinted). These disorders powerfully demonstrate that for certain genes, not just the sequence but the parental origin of each allele matters.
