SEX-LINKED INHERITANCE

X Marks the Spot!

Sex Chromosomes

• Humans normally possess 23 pairs of chromosomes.– 22 of these pairs are identical in both

genders. They are called autosomes.– 1 of these pairs differs between the

genders; this pair is the sex chromosomes (X and Y).• Females possess a homologous pair of X’s

(XX).• Males have one X and one Y (XY; not

homologous).

Sex Chromosomes cont’d

• Since females are XX, all female gametes carry an X chromosome.

• Male gametes may carry either an X or a Y (depending on chance).– So it is the sperm that

determines the gender of the offspring.

Sex Chromosomes cont’d

Sex-Linked Traits

• Traits controlled by genes on the sex chromosomes are called sex-linked traits.–We usually focus on traits

controlled by the X chromosome• This is because it has many more

genes than the Y chromosome!– (Y does code for some traits, too,

but they aren’t as commonly taught.)

Sex-Linked Traits cont’d

Notation for sex-linked genes:• Genes carried on the X

chromosome:–Use the letter X to indicate that

the gene is on the X chromosome–Use a superscript letter for the

trait itself.– Eg: XB = dominant gene for

normal colour visionXb = recessive gene for colour-blindness

Sex-Linked Traits

Examples:• Red-green colour-blindness (X-

linked)– XB = normal; Xb = colour-blindGenotype Phenotype

XBXB

XBXb

XbXb

XBY

XbY

Genotype Phenotype

XBXB Female; normal vision

XBXb Female; normal vision (carrier)

XbXb Female; colour blind

XBY Male; normal vision

XbY Male; colour blind

Sex-Linked Traits cont’d

Examples cont’d:• Hemophilia (deficiency in blood

clotting factors)• Duchenne muscular dystrophy• Some forms of diabetes insipidus

(another form of diabetes – excessive thirst and urination)

• (These are all recessive conditions)

Sex-Linked Traits cont’d

• Note: Because males only have one X chromosome, they cannot be carriers of sex-linked disorders or conditions.– They either have the gene and

show the condition, or don’t have it and don’t show the condition.

Special Cases in Sex Linkage

• X-inactivation:• Females have two copies of the X

chromosome, but males don’t.• The duplicate genes on the second X

chromosome aren’t all necessary! – The body selectively “deactivates” X

chromosomes in some female cells.• Deactivated X’s become inert Barr bodies.

X-Inactivation in Cats

• Coat colour genes are on the X chromosome

• If the cat is heterozygous, then its coat will change from place to place depending on which X chromosome gets inactivated (Calico cat).

Sex-Linked Inheritance Practice Problems

• Both the mother and the father of a colorblind male appear to be normal. From whom did the son inherit the allele for colorblindness? What are the genotypes of the mother, father, and the son?

• A woman is colorblind. What are the chances that her son will be colorblind? If she is married to a man with normal vision, what are the chances that her daughters will be colorblind? Will be carriers?

• Both the husband and the wife have normal vision. The wife gives birth to a colorblind daughter. Is it more likely the father had normal vision or was colorblind? What does this lead you to deduce about the girl’s parentage?

• What is the genotype of a colorblind male with long fingers is s=long fingers? If all his children have normal vision and short fingers, what is the likely genotype of the mother?