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X Marks the Spot!. Sex-linked Inheritance. 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) .
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X Marks the Spot! Sex-linked Inheritance
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-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-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?
