Transcript Sex Linkage and Genetic Disorders - OISE-IS-BIOLOGY-2011-2012

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.
Father’s Sex Chromosomes
Mother’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 Xchromosomes will cause expression of the phenotype
X-linked Recessive: recessive alleles must be present on both Xchromosomes 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
Recessive gene
Dominant 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:
XH - Dominant Allele
Xh - 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?
Defective allele
X
H
Xh
XH
X HX H
Xh X H
Y
XHY
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
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?