You should see a 2 You should see

You should see a 2

You should see a 5

Everyone should have seen a 16

You should see a 42

You should see a 7

You should see a 29

You should see a 6

You should see a 57

You should see a 10

Normal colour vision will see a 5. An individual with Red/Green (the most common) colour blindness will see a 2.

What are our objectives? passed on to offspring differently how from sex-linked parents Investigate are genes

Objectives GR. HS. S 2. GLE 7. EO. d �Investigate how sex-linked genes are passed on from parents to offspring differently.

Sex linked inheritance �In mammals �The X and Y chromosomes determine gender �Females are XX �Males are XY 13

�A group of scientists experimented on fruit flies.

�They crossed a male with white color eyes and a female with red color eyes. The F 1 generation was all red color. �Conclusion till now?

�Then they crossed F 1 x F 1 and they got 75% of F 2 are fruit flies with red color and 25% are fruit flies with white color. �Normal till now. �But they noticed that all the white color fruit flies of F 2 are males! There were no female with white color! �That’s weird!

�When Labelling alleles that are located on the X chromosome you use a capital X and a superscript letter to represent the dominant or recessive allele. �Example: XR represents Red Eyes.

X – Linked Inheritance Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. P generation Xr Y Xr P gametes XR XR Y XR F 1 generation XR Y XR Xr eggs F 1 gametes XR Xr F 2 generation sperm XR XR Xr XR Y Xr Y Y Offspring Allele Key R X = red eyes Xr = white eyes Phenotypic Ratio females: all red-eyed males: 1 red-eyed white-eyed 1 18

X – Linked Inheritance

Extending the Range of Mendelian Genetics � Several X-linked recessive disorders occur in humans: � Color blindness � The allele for the blue-sensitive protein is autosomal � The alleles for the red- and green-sensitive pigments are on the X chromosome. � Muscular dystrophy � Wasting away of the muscle � Caused by the absence of the muscle protein dystrophin � Hemophilia � Absence or minimal presence of clotting factor VIII or clotting factor IX � Affected person’s blood either does not clot or clots very slowly. 20