Brooker Chapter 4 - xdocs.net

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Genetics: Analysis and Principles

Robert J. Brooker

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CHAPTER 4

EXTENSIONS OF

MENDELIAN INHERITANCE

Symbols for Alleles

Dominant alleles are usually indicated either by an italic uppercase letter (D) or by a an italic letter or group of letters followed by a superscript + (Wr+).

Recessive alleles are usually indicated either by an italic lowercase letter (d) or by an italic letter or group of letters (Wr) without the +.

If no dominance exists, italic uppercase letters and superscripts are used to denote alternative alleles (R1, R2; CW, CR).

Morgan’s Experiment The chromosome theory of inheritance was

confirmed through studies carried out by Thomas Hunt Morgan

Morgan tried to induce mutations into the fruit fly Drosophila melanogaster

Treatments included

Rearing in the dark

X-rays

Radium

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After 2 years, Morgan finally obtained an interesting result

A male fruit fly with white eyes rather than the

normal red eyes

Morgan reasoned that this white eyed male must have

arisen from a new mutation that converted a red-eyed

allele into a white-eyed allele

Morgan followed Mendel’s approach in studying the inheritance of this white-eyed trait

He made crosses then analyzed their outcome

quantitatively

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Morgan’s first mutant

The Hypothesis

A quantitative analysis of genetic crosses may

reveal the inheritance pattern for the white eye

allele

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Testing the Hypothesis

Refer to Figure 3.19

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Experimental level

F1 generation

From

F1 generation

F2 generation

x

x

x

1. Cross the white-eyed male to red-eyedfemales.

2. Record the results of the F1generation. This involvesnoting the eye color andsexes of several thousandflies.

4. In a separate experiment, perform a testcross between a white-eyed male anda red-eyed female from the F1generation. Record the results.

Conceptual levelXWY x XW+X+

XW+Y male offspring and XW+XW

female offspring, both with red eyes

1 XW+Y : 1 XWY : 1 XW+XW+ : 1 XW+XW

1 red-eyed male : 1 white-eyed male :2 red-eyed females

XWY x XW+XW

1 XW+Y : 1 XWY : 1 XW+XW+ : 1 XW+XW

1 red-eyed male : 1 white-eyed male :1 red-eyed females : 1 white-eyed female

3. Cross F1 offspring with each other to obtain F2 offspring. Also record the eyecolor and sex of the F2 offspring.

XW+Y x XW+XW

The Data

Cross Results

Original white eyed-male to red-eyed females

F1 generation: All red-eyed flies

F1 male to F1 female F2 generation: 2,459 red-eyed females

1,011 red-eyed males

0 white eyed-females

782 white-eyed males

Test Cross Results

White-eyed male to F1 female

129 red-eyed females

132 red-eyed males

88 white eyed-females

86 white-eyed males

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Interpreting the Data

3-87

The first cross yielded NO white-eyed females

in the F2 generation

These results indicate that the eye color alleles

are located on the X chromosome

Genes that are physically located on the X chromosome are called X-linked genes or X-linked alleles

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3-88

A Punnett square predicts the absence of white-eyed females in the F2 generation

F1 male is Xw+Y

F1 female is Xw+Xw

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+