Sex Determination & Sex Linked Characteristics

An Intro to Sex

• Sexual reproduction involves Alternation of generations:

• Sex: the sexual phenotype of an individual

– Ultimately, what gametes does an individual produce?

• Hermaphrodites: both sexes are present in same individual

Mechanisms of Sex Determination

• Sex determination: is the mechanism by which sex is established.

– Sex chromosomes:

• The chromosomes that …

• Genes on these chromosomes are responsible for determining sex

• Heterogametic:

• Homogametic:

– Autosomes:

• XX – XY sex determination

• XX – XO sex determination

– Females have two X chromosomes (XX)

– Grasshoppers & other insects

• ZZ – ZW sex determination

– Birds, amphibians, moths, and some fishes

• Haplodiploidy

– Bees, wasps & ants …

– Sex is based on the number of chromosome sets

• Males are haploid – develop from unfertilized eggs - Produce sperm by mitosis

• Females are diploid.

• Genic Sex Determination: no sex chromosomes.

– Sex is determined by genes on chromosomes that look the same in both the male & the female

• Environmental Sex Determination:

– Sequential hermaphroditism: each individual can be both male & female (at different times)

– Temperature during development determines sex

• Turtles crocodiles, alligators & some fish.

• Sex Determination in Drosophilia (fruit flies):

– Normally males are XY and females are XX

• BUT it is not the Y that determines sex

– Genic Balance system: Ratio of the # of X chromosomes to the number of haploid sets of autosomal chromosomes (X:A ratio)

Changes In Number of Sex Chromosomes

Nondisjunction causes changes in chromosome number

chromosomes do not separate during anaphase of either meiosis I or meiosis II

• Aneuploidy - One extra or one less chromosome

– Trisomy

– Monosomy

• Turner Syndrome X0 (one X, no Y)

– 1/3,000 female births

– Infertile females, small stature, reduced secondary sex characteristics, low hairline, broad shoulders

– Usually normal intelligence

– 75% of cases have to do w/ non-disjunction in sperm

– 98% spontaneously abort

• Klinefelter’s Syndrome: multiple X chromosomes with one or more Y chromosomes

– 1/1,000 male births

– Tall males, sterile w/ small testes, some breast development, reduced facial & pubic hair

– Usually normal intelligence

– Many show few symptoms

– Primarily due to nondisjunction in egg (67%)

• Poly X females (triplo-X syndrome)

– 1/1,000 female births

– Tendency to be tall & thin

– Some are infertile, most have normal intelligence

– More than 3 X’s lead to mental retardation & physical difficulties

• Super Males (XYY)

• In humans sex is ultimately determined by the presence of a Y chromosome

The Human Y Chromosome

• The Y chromosome is much smaller than the X, but it has some very important male stuff on it.

• The SRY gene (sex determining region on the Y chromosome)

– When expressed, …

– If not expressed, ….

Human Embryo and Sex Organ Development

If SRY is expressed at 8-weeks, then male sex organs develop

Other Genes Influence Sexual Development in Humans

• Androgen insensitivity (testicular feminization syndrome) leads to XY “females”

– Individuals have testes & produce testosterone

– Defective testosterone receptors

• Guevedoces: XY “female then male”

– external genitalia appear at birth as female; clitoral-like penis, undescended tests so scrotum (bifid) appears like labia

– at puberty, penis enlarges, testes descend, male characters appear,

– reduced enzymatic activity involving conversion of testosterone to DHT

Gene Linkage

• There are many thousands of genes in the human genome and only 23 pairs of chromosomes.

– This means there must be many genes on each chromosome

– Linkage groups – groups of genes that are linked together

• X-linked genes: those on X chromosome

• Y-linked genes: those on Y chromosome

Morgan’s X-linked Crosses

• To explain the inheritance of the white eyed characteristic in fruit flies

• Proposed that the gene for eye color must be on the X chromosome

• Note the new notations + & w…

X-Linked Colorblindness in Humans

• Red green colorblindness is an X-linked recessive trait

– recessive allele can be masked in females, but not in males

– x-linked conditions shows up much more frequently in men

– Males are hemizygous

• Hemophilia – inability of the blood to clot – is another X-linked recessive trait.

Dosage Compensation

• We have seen that too much gene product can cause problems (XXX, XXY etc.)

• Why then can males have only one X, while females can handle two X’s?

– Dosage compensation:

– Lyon Hypothesis: all but one of the X chromosomes in each cell becomes inactive (random)

– Barr Body: