Henry VIII and his six wives Henry VIII of England married six times in an attempt to have a legitimate male heir to the English throne. Henry VIII blamed his wives for their inability to produce a son. Was Henry correct in blaming his wives? Explain.

Mother’s Sex Chromosomes Father’s Sex Chromosomes Phenotype: 50% female, 50% male Genotype: 50% XX, 50% XY

Unusual Pairing in Meiosis What are some genetic conditions that arise from unusual combinations of the sex-chromosomes? (i.e. extra or absence of sex chromosomes) Why does nature favour XX and XY complements?

Sex-Linked Inheritance Sex linked genes: Genes that are carried on either the X or Y chromosome. Any traits that are controlled by X chromosome are “X-linked” Any traits controlled by Y chromosome are “Y-linked” Each X-chromosome in the female will have one allele for a particular gene (for a total of 2 alleles per gene).

Sex-Linked Inheritance Males carry one copy of the X-chromosome and one copy of the Y chromosome. Therefore, males have only one allele for a particular gene carried on the X chromosome. Males will also have only one allele for a particular gene carried on the Y chromosome.

Sex-Linked Inheritance Types of inheritance: X-linked Dominant: dominant allele carried on one or both of the X- chromosomes will cause expression of the phenotype X-linked Recessive: recessive alleles must be present on both X- chromosomes in order for the phenotype to be expressed Y-linked: as long as an allele is present on the Y-chromosome, the phenotype will be expressed Dominant geneRecessive gene

Sex- Linked Disorders There are various disorders that arise from sex-linked inheritance. (Note: the disorder is considered to be the phenotype). Most of the disorders are X-linked (either dominant or recessive) Possible gametes for an X-linked disorder: X H - Dominant Allele X h - Recessive Allele

Sex-linked Inheritance Problem: Color blindness is an X-linked recessive disorder. Suppose a heterozygous female (carrier) is crossed with a normal male. What would be the genotype and phenotype proportions of the F1 generation? X H XhXh XHXH Y XhXh YXhXh Y Phenotype: 2 normal females 1 normal male 1 color blind male (X h Y) Genotype: 25% X H X H 25% X H Y 25% X h X H 25% X h Y Defective allele

Consolidation Questions 1) What gender is affected by color blindness in this case? 2) Why is the female not affected by color blindness? 3) If the disorder had been dominant would the phenotype and genotype ratios change?