Module 6 Lesson 7 - 3

Example 1

You are given one wild type female fruit fly and one white-eyed male fruit fly. Can you produce a white-eyed female fruit fly? Explain.

Solution

Let X^R represent the dominant allele for red eyes and X^r represent the recessive allele for white eyes.

• P1 Female Genotype = X^RX^R
  
• P1 Male Genotype = X^rY
  

To have white eyes, a female fruit fly must have two copies of the recessive alleles: X^rX^r. In other words, it must receive one recessive allele from the mother and another recessive allele from the father. At this point, the P1 female fruit fly is homozygous dominant. To have white eyes, an offspring must have at least one copy of the recessive allele.

P1 Cross: X^RX^R  x  X^rY

	Xr	Y
XR	XRXr	XRY
XR	XRXr	XRY

The F1 generation consists of heterozygous dominant female fruit flies, X^RX^r. Because this fruit fly has a recessive allele, it can pass the white eye gene to its offspring.

The white-eyed male fruit fly, X^rY, can pass the recessive white eye allele to its female offspring. However, it cannot pass the recessive white eye allele to its male offspring because it passes the Y chromosome.

F1 Cross: X^RX^r  x  X^rY

	Xr	Y
XR	XRXr	XRY
Xr	XrXr	XrY

The F2 generation includes a homozygous recessive female fruit fly with white eyes, X^rX^r, and a recessive white-eyed male, X^rY. This Punnett square demonstrates that females can pass the X-linked traits to their sons and daughters, and males can pass the X-linked traits only to their daughters.

Example 2

The inheritance of colour in domestic pigeons involves several genes. The dominance hierarchy of three colour alleles is ash-red > blue-black > brown. These colours are carried on the Z sex chromosome. A male pigeon has two Z sex chromosomes, and a female pigeon has one Z sex chromosome and one W sex chromosome.

A blue-black female pigeon mated with a brown male pigeon. What is the expected phenotypic ratio of the F1 generation?

Solution

The feather colour inheritance is sex-linked in domestic pigeons. Unlike humans, a male pigeon has ZZ chromosomes and a female pigeons has ZW chromosomes.

Let Z^A represent the ash-red colour, Z^B represent the blue-black colour, and Z^b represent the brown colour: Z^A > Z^B > Z^b .

• Male brown pigeon = Z^bZ^b
• Female blue-black pigeon = Z^BW

	Zb	Zb
ZB	ZBZb	ZBZb
W	ZbW	ZbW

Because the blue-black colour is dominant to the brown colour, all male pigeons from the F1 generation have blue-black feathers. All female pigeons from the F1 generation have brown feathers.

Watch and Listen

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Consider the following video on sex-linked inheritance:

 
Answer the following questions for your own understanding.

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 sex?
   
   
3. What chromosome 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 X-linked traits to their sons?
   
   
6. What is the term used to describe that genes are located on the X chromosome in males?