Sexual Development and Traits Inherited on Sex Chromsomes









I.  Sexual Development:

    A.   See Figure 6. 1 for an over view of human sexual development.

     B. Sex Chromosomes:

    Human males and females have equal numbers of autosomes (22), but males have one X chromosome and one Y
    chromosome, and females have two X chromosomes.

    Heterogametric sex:
 

    Homogametric sex:

    X chromosome contains more than a thousand identified genes, while the Y chromosome has 85 genes.
 

    Y chromosome:  It doesn't have a homolog with which to cross over;
                                It has many repeated DNA sequences;
                                It has very few genes.

    Y chromosome genes are divided into three broad groups, based on similarity to genes on the X chromosome:

        First group:  located at both tips of the Y chromosome, known as pseudoautosomal regions, termed
                             PAR1 and PAR2.  Comprise 5 percent of the chromosome, they have counterparts on the X
                             chromosome and can cross over with them.
                              They encode a variety of proteins that function in both sexes, involving in the control of
                              bone growth, signal transduction, the synthesis of hormones and receptors, and energy
                              metabolism.

       Second group:  X-Y homologs genes, they are similar to certain genes on the X chromosome, but are not
                                 identical as are pseudoautosomal genes.  They are expressed in nearly all tissues.

        Third group:   Genes unique to the Y chromosome, including the SRY gene, and other genes occur in
                                multiple copies.  They encode proteins that participate in cell cycle control and in
                                the regulation of gene expression, enzymes, and receptors for
                                immune system biochemicals.

        Question:  1.  Where do you think the Y chromosome comes from?

                           2.  Read Figure 6.4 on page 118, and understand how a chromosomal "he" develops as a
                                phenotypic "she"?
 

        C.  Gender Identity - Is Homosexuality Inherited?

            Homosexuality:  A human behaviour, a person's phenotype and genotype are consistant, but physical attraction is
                                     toward members of the same sex.  It is seen in all cultures and has been observed for
                                     thousands of years.

            Is it genetically inherited?

                a:  homosexual individuals developed the feeling as young children, well before they know of the existance or the
                     meaning of the term.

                b:  Twin studies suggested a genetic influence... different sizes in brain areas, 1991

                c:   Five DNA markers on X chromosome in 40 pairs of homosexual brothers, they were indentical in 33 of the
                      sibling pairs.  Did not identify a causative gene, not always true among siblings, and did not find the X
                      chromosome markers between pairs of lesbian sisters.

                d:  Genetically manipulated male fruit flies.   Lowered brain serotonin is associated with homosexual behavior.  Fig.
                     6.5
 

II. Sex-linked genes have unique patterns of inheritance.
 

In humans, the term sex-linked usually refers to X-linked characters.

Fathers pass X-linked alleles to all their daughters, but not sons.

Mothers can pass sex-linked alleles to both sons and daughters.

Far more males than females have disorders that are inherited as sex-linked recessives.

Female needs to have double dose of recessive, while male only needs a single dose of recessive allele .
 

Sex-linked disorders in humans.  X-linked disorders
Color blind:  a mild disorder.
 

Hemophilia:  sex-linked recessive trait, absence of a certain protein, required  for blood clotting, a serious
                    disorder:

X-linked recessive trait got transmitted from Queen Victoria (X+X-):
 
 

III. Sex-limited Traits

Traits expressed in only one sex.  They may be controlled by sex-linked or autosomal loci (locations of certain alleles on the chromosome). Although, the genes are present in both sexes.
 

Ex.  Sex-linked traits:

 Woman:  breast and ovary formation.
 Man:  facial hair distribution and sperm production.

Female bird:  Not brightly colored plumage patterns.
Male bird:     Brightly colored plumage patterns.

Sheep:    Horns found only in males.
 

IV. Sex-Influenced Traits (Sex-conditioned traits):

Traits that appear more often in one sex than in another.
Traits are not necessarily transmitted by sex chromosome.

Ex.  Baldness in human beings----- sex- influenced trait.
It is controlled by an allele that appears to be dominant in men, but recessive in woman.  Male hormone, testosterone is required for a full expression of the allele.
For woman, it is usually expressed as a thinning of hair rather than a balding.