Module 6 Lesson 7 - 2

Chromosome Theory of Inheritance

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Read pages 596 - 597

Before considering how certain traits are linked to sex, you need to review two ideas that you already know.

First, review Mendel's laws.

• His first law explains how parents have two alleles for a trait and that these alleles separate during the formation of gametes. As a result, gametes have only one allele for each trait.
  
  
• His second law explains how two or more allele pairs segregate independently of one another into gametes. As a result, the inheritance of one characteristic does not affect the inheritance of a different characteristic.

Now, consider the previous module on cellular division and meiosis. When diploid organisms go through meiosis, homologous pairs of chromosomes are separated during the formation of gametes. Therefore, each of the two gametes has only one homologous chromosome. In meiosis I, during metaphase I, each pair of homologous chromosomes lines up on the equator and then separates independently of every other pair.

Can you see the similarities between the description of Mendel's laws and the movement of chromosomes in meiosis? A scientist named Walter Sutton noticed these similarities and proposed a theory called the Chromosomal Theory of Inheritance in which he stated that genes are found on chromosomes.

Read about genes and chromosomes in your textbook on pages 596 and 597.

Sex-Linked Inheritance

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Similar to most theories, the chromosomal theory of inheritance was not widely accepted in the beginning. The work of Thomas Hunt Morgan added some scientific proof of its validity. Morgan was trying to disprove the theory, but when his experimental evidence supported it, he changed his position.

 

Thomas Morgan worked with fruit flies (Drosophila melanogaster) to study patterns of inheritance. The normal (wild type) eye colour of the fruit fly is red. During their research, Morgan and his team discovered male fruit flies with white eyes. They noticed that, when crossing red eye dominant flies with white eye recessive flies, he obtained the expected 3 red : 1 white ratio in the F2 generation. However, the white eye phenotype was distributed unequally among males and females. Instead, all white-eyed flies were male. This observation led him to propose the hypothesis that the gene for eye colour in Drosophila must occur on the X chromosome, or be sex-linked.

Consider Morgan's research using Punnett squares. Let X^R represent the dominant allele for red eyes and X^r represent the recessive allele for white eyes.

P1 cross was between a pure breeding red-eyed female and a white-eyed male: X^RX^R crossed with X^rY.

	Xr	Y
XR	XRXr	XRY
XR	XRXr	XRY

Note about the symbols

For sex-linked genes, we use the XX notation for females and XY notation for males. To indicate the dominant alleles on the X chromosome, we use a superscript: upper case letter for dominant and lower case letter for recessive allele. Because the gene is expressed only on the X chromosome, we place the symbols only on the X chromosome and not on the Y chromosome.

F1 cross was between a red-eyed female and a red-eyed male.

	XR	Y
XR	XRXR	XRY
Xr	XRXr	XrY

F2 generation result was as following:

• 2 red eyed females = X^RX^R and X^RX^r
  
• 1 red eyed male
• 1 white eyed male

Although the phenotype ratio is 3 red eyes : 1 white eyes, white eye is expressed only in male fruit flies.

Read the first section about Thomas Hunt Morgan on page 599 of your textbook. Then, skip the sections on linked genes, chromosome maps, and crossing over, and continue to read about sex-linked inheritance on pages 601 to 603.