SEX LINKAGE

1/13/92, rvsd 1/26/94, 1/11/95, 1/17/96, 1/13/97, 14 Jan 00, 10 Jan 01, 14 Jan 04, 10 Jan 05, 16 Jan 08

GMSLG 7^th: 38-40, 73-75, GWLC 9^th: 61-65

1905:     Stevens with beetle Tenebrio, saw heteromorphic chromosomes, named X and Y

1906:     Sex linkage discovered by Doncaster and Raynor crossed magpie moths (Abraxas):

Reciprocal crosses:

              light-winged females x dark males                 F₁:         all dark winged

              dark-winged female x light male                    F_1:       50:50 light-winged females, dark-winged males (???)

crisscross inheritance: daughters like fathers, sons like mother

Bateson, crossed chickens, got similar results: barred x uniform colored chickens (SEX - LINKED):

               barred male x non-barred female progeny = all barred

              non-barred male x barred female progeny = sons barred, daughters non-barred

Moths and chickens, sex is determined and males carry recessive genes:

              males = ZZ,         females = ZW =  heteromorphic for shape, hemizygous for genes (♀ here)

Different means of sex determination:           species                 male      female

                                                                        Moths, chickens  ZZ         ZW

                                                                        Mammals, flies    XY        XX

                                                                        Hymenoptera       XO        XX

Thomas Hunt Morgan, 1909, used Drosophila melanogaster for genetic studies:

              12 day life cycle, after emerge from pupa, ready to mate 12-14 hrs

Found a single white-eyed male, crossed times red-eyed female:

              white-eyed male x red-eyed female:                             F₁ = all red.

                                                                                                    F₂ = 1/4 white, but were all male

Henking studied chromosomes in Hemiptera: 11 pairs chromosomes, one unpaired element

              called it an X element:       females = XX

                                                         males = XO

Sex-linked vs autosomal genes

SEX-LINKED EXAMPLES:

In humans. female = XX, male = XY (remember: condition called hemizygous)

X linked human recessive traits (show sample pedigrees):                                                           affected ♂:

              red-green colorblind:                                                                                                                   1/8

              hemophilia A, factor XIII missing:                                                                                              1/2,500

              Duchenne's muscular dystrophy: muscle atrophy by 6 yrs, dead by 20:                                     1/3,500

Sex Chromosome:             signs: (MM, VX, p. 2148-2150)

Turner's XO        pathognomic: lymphedema of dorsal hands&feet, neck                            1/3000

triple X                XXX     little effect, may be sterile, may be retarded                               1/1000

Klinefelter's         XXY     tall, eunuchoid, sterile, a few are retarded                                                1/700

super male           XYY     acne, language dysfunction (1/15 in prisons for violent crimes)               1/1000

How to tell sex-linked for a rare phenotype:

              1)           many more males affected

              2)           None of offspring of affected males affected (usually).

                            All of his daughters are carriers, half of their sons will be affected

              3)           None of sons of affected male will be affected, will not transmit to offspring

Calico cats, Lyonization, Barr bodies, mosaics