Chap 4

Sex Determination

The mechanism by which biological sex is established

hermaphroditism

organisms that display both female and male characteristics

monoecious

species in which an individual organism has both female and male reproductive structures

dioecious

Individual has either male or female reproductive structures

autosome

nonsex chromosomes

heterogametic sex

males produce two different types of gametes with respect to the sex chromosomes

homogametic sex

produce gametes that are all the same with respect to the sex chromosomes

pseudoautosomal region

small region of the X and Y chromosomes that contains homologous gene sequences

genic sex determination

sexual phenotype is specified by genotypes at one of more loci, with no obvious differences between male and female chromosomes

sequential hermaphroditism

sex of an individual changes in the course of its lifetime

turner syndrome

• female in appearance

• XO

• no Y chromosome

• usually sterile

1 of 3000 female births

frequency of turner syndrome

mosaicism

a condition in which cells within the same person have a different genetic makeup

Klinefelter syndrome

• cells contain one or more Y chromosomes and multiple X chromosomes

• commonly XXY, could also be XXXXY or XXYY

• male in appearance with some female characteristics and reduced testes

1 in 1000 male births

frequency of Klinefelter syndrome

triple x syndrome

• contain three X chromosomes

• female phenotype

• some are sterile, most are fertile

the number of chromosomes increases beyond 3

severity of intellectual disability increases as

1 in 1000 female births

frequency of triple X syndrome

XYY Supermales

• no distinctive physical characteristics

• may be taller

• learning disabilities

• 1 in 1000 male births

X chromosome

genetic information essential for both sexes and human development

Y chromosome

single copy of this chromosome produces the male phenotype

2 copies of the X chromosome

genes affecting fertility are found on

only a gene on the Y chromosome is responsible for male sex determination rather than the entire chromosome

XX males revealed that:

SRY Gene

• locate on the Y chromosome

• male determining gene

• discovered 1990

• XX males and XY females

Undifferentiated gonads

Early in development, all humans possess:

a gene on the Y chromosome becomes active causing the gonads to develop into testes

happens six weeks after fertilization when the male determining gene is present

testes secrete testosterone and anti-mullerian hormones

Happens after the testes are developed

Anti-Mullerian hormone

causes the degeneration of female reproductive ducts

Androgen-Insensitivity Syndrome

• female presenting with XY chromosomes

• SRY gene active but lack of androgen receptor

• female characteristics develop

• X linked

X linked

characteristics determined by genes located on the X chromosome

Y linked

characteristics determined by genes located on the Y chromosome

sex linked

Characteristics determined by a sex chromosome

hemizygous

possession of a single allele at a locus

ie: human males with XY chromosomes

Nondisjunction

Failure of homologous chromosomes or sister chromatids to separate properly during cell division, leading to an abnormal number of chromosomes in daughter cells.

dosage compensation

A mechanism to equalize the amount of protein produced by X-linked genes and autosomal genes

fruit fly dosage compensation

achieved by doubling the activity of the genes on the X chromosome of males but not females

Barr body

inactivated X chromosome that appears as a condensed, darkly staining body in the nuclei of most cells of female placental mammals

Lyon hypothesis

• 1961

• one X chromosome in each female cell becomes inactivated

• which X chromosome is random from cell to cell