Module 6 Mendelian Genetics

3.6.6 page 2

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Chromosome Theory of Inheritance

Before considering how certain traits are linked to gender, we need to put together two ideas that you already know of. First let’s 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 can have little relationship to the inheritance of a different characteristic.

Now, try to think back to 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, 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? A fellow named 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 about genes and chromosomes in your textbook on pages 596–597.

Sex-Linked Inheritance

Like most theories, the chromosomal theory of inheritance was not widely accepted in the beginning. 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 it, he changed his position. While working with Drosophila, 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 him to propose the hypothesis that the gene for eye colour in Drosophila must occur on the X chromosome. Read the first section on Thomas Hunt Morgan on page 599 of you text. Then, skip the sections on linked genes, chromosome maps, and crossing over, and continue reading about Sex-linked inheritance on pages 601–603.

Watch and Listen

Consider the following video on Studying Sex-Linked Inheritance in fruit flies

Please watch the following sections:

• Examining Fruit Flies

•  Inheritance of White Eye Colour

• Patterned Sex-Linked Inheritance

1. Why do many geneticists study fruit flies?
2. What tells the students that eye colour in fruit flies is not autosomal, but rather linked to gender?
3. What chromosome actually carries the allele for eye colour in fruit flies?
4. What happens in males with the alleles that occur on X chromosomes?
5. Can males pass on X-linked traits to their sons?
6. What is the term used to describe the genes located on the X chromosome in males