King Henry VIII and his six wives

King 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.

Sex Determination in Humans:

Phenotype: 50% female , 50% male

Genotype: 50% XX , 50% XY

Prince William and Kate Middleton

The firstborn of Prince William and Kate Middleton will be heir to the throne, whether it’s a boy or a girlunder the new successionrules.

Sex-Linked Inheritance• Sex linked traits: Occur in humans, for which genes are on the X

chromosome.

• Each X-chromosome in the female will have one allele for a gene for a total of 2 alleles per gene

• Because Males have only one X-chromosome they will only have one allele for a particular gene.

Female Male

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 for a woman in order for the phenotype to be expressed, whereas a man only needs one recessive allele

Dominant alleleRecessive allele

Sex- Linked Disorders

• There are various disorders that arise from sex-linked inheritance. (Note: the disorder is considered to be the phenotype).

Severe Combined Immunodeficiency Haemophilia

Colour Blindness

The gene that causes colour blindness is on the X chromosome.

Sex-linked Inheritance Problem:

• Color blindness is an sex-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 Xh

XH

Y

X H X H Xh X H

X H Y Xh Y

Phenotype: 2 normal females 1 normal male 1 color blind male (Xh Y)

Genotype: 25% XHXH

25% XH Y 25% Xh XH

25% Xh Y

Defective allele