SCN3230-ABED_1: Module 6 Lesson 7 - 4 (Lab)

Lesson 7 — Chromosomal Theory and Sex-linked Inheritance

Lab: Sex-linked Traits

Genes are carried on chromosomes, and some of those genes are carried on the X chromosome. Those on the X chromosome are said to be X-linked.

This explains why males have a higher incidence of certain traits than females have. Females have two X chromosomes; therefore, they receive two alleles for all traits carried on the X chromosomes. Males have only one X chromosome, and they can inherit only one allele for each trait carried on the X chromosome. Remember that the Y chromosome holds very few genes. (Genetics studied in high school considers Y to be empty.)

As a result, males always express whatever alleles they have on their X chromosome, and females follow the rules for dominant and recessive traits. This can enable a "normal" gene on one X chromosome to mask or cover a problematic one.

In this simulation, you will explore sex-linked traits. With Drosophila, you will conduct various breeding cycles involving eye colour, and you will analyze the results.

Purpose

The purpose of this Investigation is to explore the sex-linked gene for eye colour in Drosophila.

Materials

For this simulation, you require access to the Internet and a word-processing program or a blank piece of paper to record your results. You will also record your results in Assignment 6D.

Procedure

Click the following link to access the virtual lab: Drosophila Eye Colour Lab
Click on the Procedures tab on the right side of your screen.
Follow the procedures for Exercise 1A, Exercise 1B, Exercise 2A and Exercise 2B.
Record your results (see question 15 in Assignment 6D).

Results

Record your results in your observation table in Assignment 6D (question 15)

Analysis

Complete questions 16 and 17 in Assignment 6D

In a mating between a red-eyed male fruit fly and a red-eyed heterozygous female, what percentage of the female offspring is expected to be carriers?

In a mating between a red-eyed male fruit fly and a white-eyed female fruit fly, what percentage of the male offspring is expected to have white eyes?

Hemophilia is a blood disorder in humans that results from a sex-linked recessive allele. A daughter from a mother without the allele and a father with the allele marries a man with hemophilia. What is the probability that the daughter's children will be affected by hemophilia?

Colour blindness results from a sex-linked recessive allele. What is the genotype of the offspring that result from a cross between a colour blind male and a homozygous female with normal vision?

Check Your Answers.

	X^R	Y
X^R	X ^RX^R	X ^RY
X^r	X ^RX^r	X ^rY

50% of the female offspring is expected to be carriers of the recessive allele.

	X^R	Y
X^r	X ^RX^r	X^rY
X^r	X ^rX^r	X ^rY

All males resulting from this cross will have white eyes. If males are carriers of the recessive allele, they will be affected because they have only one copy of the X chromosome.

	X^h	Y
X^H	X ^HX^h	X ^HY
X^H	X ^HX^h	X ^HY

A daughter from homozygous dominant mother and affected father will be a carrier (heterozygous).

	X^h	Y
X^H	X ^HX^h	X ^HY
X^h	X ^hX^h	X ^hY

There is a probability of 0.50 that her children will be affected by hemophilia whether boys or girls.

	X^c	Y
X^C	X ^CX^c	X ^CY
X^C	X ^CX^c	X ^CY

When an affected male and homozygous normal female mate, none of the children will be affected by colour blindness. All female children will be carriers of the recessive alleles.