•   X linked

I.      Sex determination

 

A.    Ploidy in the example of bees and wasps, males are haploid and female

              are diploid

       B.    Allelic mechanisms

C.       Environment in the example of reptiles, low temperature (25° C) of incubation yields all

              one sex, high temperature (32° C) yields the other sex

       D.  Substrate to which individuals attach

       E.  Chromosomal: sex with two identical chromosomes is homogametic

               1.    X Y system: XX usually female; XY usually males

        2.    X O system. Insects: XO = male; XX = female; males still produce

                    two kinds of gametes

               3.    ZW in birds: like X-Y, except females are ZW, males ZZ

               4.    Compound, some nematodes: multiple copies of chromosomes; for example, 26A + 8X + Y = male and 26A + 16X = female

       F.  Genetic balance  in Drosophila

               1.       Genetic balance theory

       a.       X:A ratio of at least 1 produces normal female

                            b.    X:A less than or equal to 0.5 produces male

               2.       Sex-lethal (Sxl): when “on” = female; when ``off'' = male

                      a.    X chromosome genes regulating Sxl called numerator elements;

                                4 identified

                      b.   Autosomal genes regulating Sxl called denominator elements

                      c.   Sxl counts gene products of numerator genes; if quantity is high,

                               Sxl is on

     G.    Y chromosome few genes; (Fig. 5.6) pseudoautosomal region on tip of Y

                   allow X and Y to pair

 

II.     Sex determination in humans

      

       A.    H-Y antigen: protein on male cells; gene located near centromere of Y

                    chromosome

B.        ZFY helps sperm development

C.       SRY seems to determine maleness by preventing conversion of testosterone to estradiol

and by stimulating regeneration of female reproductive ducts

 

III.    Sex determination in flowering plants

      

       A.    Most plants hermaphroditic: both male and female parts in one flower

       B.    Unisexual: some with single locus for sex, some with multiple loci, some

                     with an X-Y system

               1.       Monoecious: male and female flowers on one plant (walnut)

               2.       Dioecious: only male or female flower on one plant (date palm)

 

IV.   Dosage compensation may involve X-inactivation in mammals

      

       A.  Females, not males have Barr body highly condensed inactive X chromosome

       B.    Lyon hypothesis all Xs > one are inactivated

  1.   Heterozygous females show allele from only one X chromosome

                mosaicism

                 2.  Tortoise-shell cats: patches of yellow and black

  3.    X inactivation center starts inactivation; XIST makes RNA for

              inactivation

       C.  Drosophila

1.   More activity from X chromosome in male

2.      At least five autosomal genes regulate transcription of X chromosome in males

3.      Proteins plus RNA form compensasome

 

V.    Sex linkage: males and females show different patterns of phenotypes

      

       A.    Sex-linkage usually = X linked; holandric = Y linked

       B.    Males get X chromosome from mother

       C.    Pseudodominance only one allele in males necessary for expression

       D.    Sex-limited expressed in only one sex; bright color feathers in males; milk in females

       E.    Sex-influenced occur more frequently in one sex; baldness, possibly

 

VI.   Pedigree analysis useful for human traits; often more than one possibility (Fig. 5.18)

      

       A.  Penetrance not all individuals with a genotype show trait

       B.  Expressivity different levels of expressing leading to gradation of phenotype

       C.  Proband usually first individual recognized with trait

       D.  Consanguineous between relatives; often causes recessive trait to appear

       E.  Autosomal dominant (Fig. 5.19, 5.20)

               1.       Appears in each generation

               2.       Affected X normal yields 1/2 affected progeny

               3.       Equal distribution among sexes

       F.  Autosomal recessive (Fig. 5.21)

               1.       May skip generations

               2.       Equal distribution among sexes

               3.       Often appear in consanguineous marriages

               4.       Two normal parents produce affected child

       G.    Sex-linked recessive (Fig 5.22)

               1.       Appears more frequently in males

               2.       Affected females have all affected sons

               3.       Affected females must have affected father and at least a carrier mother

               4.       Never passed from father to son

       H.    Sex-linked dominant (Fig. 5.23)

               1.       Appears in each generation

               2.       Affected males yield all affected daughters

               3.       Affected males come from affective mothers