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: X B = dominant gene for normal colour vision X b = recessive gene for colour-blindness

Sex-Linked Traits Examples: Red-green colour-blindness (X-linked) – X B = normal; X b = colour-blind GenotypePhenotype XBXBXBXB XBXbXBXb XbXbXbXb XBYXBY XbYXbY GenotypePhenotype XBXBXBXB Female; normal vision XBXbXBXb Female; normal vision (carrier) XbXbXbXb Female; colour blind XBYXBYMale; normal vision XbYXbYMale; 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?