• The Chapter 12 Homework is due on Sunday, January 11 at 11:59 pm

• The Chapter 12 Test will be on Monday, January 12.

• The Chi-Square Quiz has been postponed until Wednesday.

• Homework for tonight: • Scientific Skills Exercise: Using the Chi-Square Test (This is part of the

Chapter 12 Homework - it will help you prepare for the lab tomorrow and the quiz on Wednesday.)

• “Notes: Chapter 12.1 Mendelian inheritance has its physical basis in the behavior of chromosomes” (I’m not going over this in class because it is just a review of information that was presented in previous chapters.)

Chapter 12Sex chromosomes and sex-linked

genes

You Must Know

• The unique pattern of inheritance in sex-linked genes.

Drosophila melanogaster and Thomas Hunt Morgan

wild-type: normal, common phenotype Mutant

Figure 12.4a

Results

F2

Generation

All offspringhad red eyes.F1

Generation

Experiment

PGeneration

Figure 12.4

Xw Y

Xw+

Xw+ Xw Xw+ Y

Xw+ Xw+ Xw Xw+ Y

Figure 12.4a

All offspringhad red eyes.F1

Generation

Experiment

PGeneration

XwYXw+Xw+

F2

Xw+ Y

Xw+

Xw+ Xw+ Xw+ Y

Xw Xw+Xw Xw Y

Human sex chromosomes

X

Y

SRY gene on the Y chromosome is required for the developments of testes

• A gene that is located on either sex chromosome is called a sex-linked gene.

• Genes on the Y chromosome are called Y-linked genes; there are few of these.

• Genes on the X chromosome are called X-linked genes.

• X-linked genes follow specific patterns of inheritance.

• For a recessive X-linked trait to be expressed– A female needs two copies of the allele (homozygous)– A male needs only one copy of the allele (hemizygous)

• X-linked recessive disorders are much more common in males than in females

• Some disorders caused by recessive alleles on the X chromosome in humans.– Color blindness (mostly X-linked)– Duchenne muscular dystrophy– Hemophilia