• Describe the contributions of genetic scientists that led to the chromosome theory of inheritance and understanding sex-linked inheritance.
  • Solve sex-linked inheritance problems using Punnett squares.
  • Develop an understanding of gene expression and polygenic inheritance.


The chromosome theory of inheritance states that genes are located on chromosomes and chromosomes provide the basis for segregation and independent assortment of genes.

Sex-linked InheritanceEdit

Sex-linked inheritance is the transmission of genes that are located on one of the two human sex chromosomes. A gene located on the X chromosome is called X-linked, and on the Y chromosome it is Y-linked. Most known sex-linked traits are on the X chromosome. Some examples of X-linked traits are colour blindness and hemophilia.

Morgan's DiscoveryEdit

Thomas Morgan produced a white-eyed male fruit fly by crossing two fruit-flies with red eyes. However when Morgan crossed the white-eyed male with a red-eyed female he could not produce a female with white eyes. Morgan hypothesized that the gene coding for eye colour in fruit flies was located on the X chromosome. He reasoned that the white-eyed trait was recessive so the only way to get a female white-eyed fruit fly was if both parents donated an allele that coded for white eyes.

Sex-linked Traits and Punnett SquaresEdit

Punnet squares can be used to predict the outcome of crosses involving genes that are located on the X chromosome. When creating a Punnet square to determine the outcome of a cross involving a sex-linked trait it is assumed the trait is X-linked unless it it known to be Y-linked. The allele symbol is written on the X chromosome if the trait is X-linked with no allele written on the Y chromosome.

Chromosomes and Gene ExpressionEdit

Males and females produce the same amounts of proteins coded by genes on the X chromosome even though females have two X chromosomes. This is because one of the X chromosomes in each female cell is deactivated at random. The inactivated X chromosome is called a Barr body. Two X chromosomes with different alleles will produce varied effects in expression. For example, some cats have a random distribution of differently coloured patches of fur with each colour being coded by different alleles in the same gene on different X chromosomes.

Polygenic InheritanceEdit

Many traits are controlled by more than one gene, which is called polygenic inheritance. The protein products of different genes work together and produce a range of variation called continuous variation. Continuous variation is the variation among individuals in a population in which there is a gradient of phenotypes for one trait.

Modifier GenesEdit

Some genes, called modifier genes, work with other genes to control the expression of a trait. The expression of eye colour in humans may involve modifier genes. For example, brown eye colour is the result of the presence of the pigment melanin that is coded for by the dominant allele for eye colour and blue eye colour is the result of the absence of melanin coded for by the recessive allele. Modifier genes produce other eye colours.