Module 6

Module 6—Mendelian Genetics: The Transmission of Traits to the Next Generation

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Chromosome Theory of Inheritance

Before considering how certain traits are linked to gender, recall two of 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. For example, if the parent is heterozygous for height, Tt, then one gamete could receive the allele T for tall, and the other gamete receives the allele t for short.

Mendel's second law explains how two or more allele pairs segregate independently from one another into gametes. As a result, the inheritance of one characteristic can have little relationship to the inheritance of a different characteristic. For example, the allele for height segregates independently and has no affect on the colour of the seed, or the texture of the seed, or the colour of the flower.

In Module 5, you learned that when diploid organisms go through meiosis, homologous pairs of chromosomes are separated during the formation of gametes. Therefore, for each pair of gametes produced, 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? Walter Sutton noticed these similarities and proposed that Mendel’s factors, now called genes, were found on chromosomes. He proposed a theory called the Chromosomal Theory of Inheritance. Read from “Genes and Chromosomes” on page 595 to “Section 17.1 Summary” on page 597. Choose how you will prepare a brief summary of this information and store your summary in your course folder.

Sex-Linked Inheritance

Like most theories, the chromosomal theory of inheritance was not initially widely accepted. It took the work of Thomas Hunt Morgan to finally add some scientific proof of its validity. Morgan was actually trying to disprove the theory, but when his experimental evidence supported the theory, he changed his position. While working with Drosophila (small flies), Morgan and his team 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. However, the white was not equally distributed among males and females. Instead, all of the white eyed flies were male; none of them were female. This observation led Morgan to propose the hypothesis that the gene for eye colour in Drosophila must occur on the X chromosome. Read the introductory section on Thomas Hunt Morgan on page 599 of you textbook. Then read “Sex-linked Inheritance” and “Barr Bodies” on pages 601 to 604.

Watch and Listen

To understand the history and principles of sex-linked inheritance, you may choose to view the entire video “Sex-Linked Inheritance: It All Began with Drosophila.” However, you may also choose to watch only the following segments of the video. Remember that you can ask your teacher for the username and password for the video, if you need them.

• “Examining Fruit Flies”
• “Bio Review: Inheritance of White Eye Colour”
• “Patterned Sex-linked Inheritance”

Begin the video and continue viewing until you reach the section titled “Bio Review: Inheritance of White Eye Colour.” After watching this section, watch the “Incomplete Dominance” section. Answer the following questions for your own understanding and save your work in your course folder.

TR 1. Why do many geneticists study fruit flies?

TR 2. What tells the students that eye colour in fruit flies is not autosomal, but, rather, is linked to gender?

TR 3. What chromosome actually carries the allele for eye colour in fruit flies?

TR 4. What happens in males with the alleles that occur on X chromosomes?

TR 5. Can males pass on X-linked traits to their sons?

TR 6. What is the term used to describe the genes located on the X chromosome in males?

Module 6: Lesson 6 Assignment—Labs

To apply your understanding that some genes are carried on the X chromosome and are said to be X-linked complete the lab simulation, “Sex-linked Traits,” and all related activities. You will be prompted to complete the Module 6: Lesson 6 Assignment as you complete the lab.