Unformatted text preview: Lecture 18
June 7, 2011
Chapter 4 Sex Determination and SexLinked Characteristics
1. Sex determination and Human sex
chromosomes An introduction
2. X linked gene inheritance
3. Dosage compensation Sex and Sex Reproduction
Sex refers to sexual phenotype.
Most organisms have only two sexual phenotypes: male
Sexual reproduction involves the combination of haploid
gametes (sperm and egg) from two sexes to form a diploid
embryo (zygote) Sex Determination
Sex determination refers to the mechanism or genetic
basis for sex establishment
1. Chromosomal sex determination (focus of our
2. Genic sex determination (by genes on
3. Environmental sex determination (e.g. marine mollusk)
Warm temp. =
Cool temp. =
Warm temp. =
Cool temp. =
females slipper limpet Human Sex Chromosomes
Humans have 23 pairs of chromosomes (discovered in 1956)
22 autosomal chromosomes (same in male and female) 2 sex chromosomes: X and Y
Y is 1/3 the size of the X and
has only 1/100th as many genes
Though differ in shape and size,
X and Y can still pair in meiosis
X and Y contain at least 20
common genes near the ends of
Allow for pair and recombination
between the X and Y . Human Y Chromosome
The Y chromosome is an acrocentric chromosome (NOT Yshaped!)
95% of Y = nonrecombining region of the Y (NRY) or malespecific region of the Y (MSY)
Y chromosome has the SRY gene, which determines the
male and encodes a protein called the testis-determining
XY female miss
functional SRY gene
XX male has SRY
Absence of Y
chromosome results in
(turner syndrome) Human Sex Inheritance
Female: homogametic XX
Produce two same gametes in
terms of sex chromosome
Male: heterogametic XY
Produce two different gametes
in terms of sex chromosome
Inheritance of human sex results
in equal numbers of male and
http://www.youtube.com/watch?v=8YMhkweABfQ&feature=related Inheritance of Sex-linked Genes
The Y, although behaving like a homolog of X, has much
If a locus controlling a trait is located on X not Y
chromosome, the frequency at which the trait is observed
will be different in males and females
In Female XX: a gene that is on the X will be two copies
In male XY: a gene on the X will be one copy
E.g. Color blindness, hemophilia (reduced blood
clotting) = x linked recessive abnormality.
Different ratio from the Mendelian principles
A major extension of Mendelian inheritance principles X-linkage
Firstly discovered by Thomas H. Morgan in 1910.
He found genes controlling eye color, body color and
other traits in Drosophila are located on X chromosomes.
The linked genes are inherited together (linkage)
according to sex. http://bcs.whfreeman.com/WebPub/Biology/pierce4e/Animations%20%20Podcasts/ch04/0401_X_linked_inheritance.html Wild-type eye color in Drosophila = red
Due to the production of two pigments.
Morgan found a mutant male with white eyes.
Two pigments are missing due to a recessive allele.
Crosses between the white-eyed male mutant to the wildtype (red) females results in all (F1) progeny flies with red
red eye is dominant to white eye.
Male and female F1 progeny were then crossed with each
Females: 100% red eyes.
Males: ½ red eyes: ½ white eyes (1:1). Explanation
The gene that controls eye color is located on the X
The Y chromosome does not carry any alleles associated
with the eye color gene.
Males; therefore, have only one copy of the gene.
Inherited from their mother along with their only X
In Meiosis Examples of X-linked Inheritance in
Males need to inherit only one recessive allele to show an
X-linked trait, but females need to inherit two, one from the
mother and one from the father.
Male has more chance to exhibit the recessive
The genes for visual pigments that are responsible for
the perception of red and green are found on the X
X-linked recessive trait in human F i g : 4-14, K & G Color
Inheritance Sex-linked Gene Inheritance Practice
Betty (wife): normal vision, but her mom is
color blind. Bill (husband) is color blind
50% Dosage Compensation
Human female XX: 2 copies of X-linked genes
Human male XY: 1 copy of X-linked genes
So, X-linked gene in females would produce twice as much
products as in males?
No, because there is a genetic mechanism in females that
compensates for X dosage.
Inactivation of one X-chromosome in female Dosage Compensation
In female somatic cells, one can see an inactivated X
chromosome = Barr body (darkly staining chromatin) during
Barr body = inactivated X-chromosome Areas (genes) on
one of the X chromosomes visible in nucleus of a cell in
a female mammals
Female cell bar body Male cell, no bar body Number of bar body in human cells with
different complements of sex chromosomes
Turner Syndrome (1/2000 female births)
45, X: no Barr body
person is female, abnormal ovaries
Klinefelter Syndrome (1/500 male births)
47,XXY: one Barr body
male, but sterile and slight feminization
- 47, XXX: two Barr bodies (1/1200 female births)
many times they are normal females
- 47, XYY: one Barr body
normal male (> 6 feet)
- 46, XX (normal): one Barr body Dosage Compensation
Which X gets inactivated? Is it the same X in every cell?
Mary Lyon (1961): Lyon Hypothesis
X inactivation is random
Happens early in development (within first few weeks)
In heterozygous female, 50% cell express one allele and
50% cell express the other allele.
Once an X is inactivated, is remains inactivated in all
somatic cells that descend from the cell.
Neighboring cells have the same X chromosome
Produce a patchy or mosaic pattern for the expression
of an X-linked character in heterozygous females. Patchy distribution caused by X
E.g. Calico cat or tortoiseshell
gene (of orange or black allele) for coat color on X
orange/black patches in female
So, females are mosaics for all heterozygous X-linked alleles Mechanism of X-Inactivation
X inactivation yielding a Barr body = lyonization
Xist (X-inactivation-specific-transcription) gene
On the X chromosome destined to become inactivated, the
Xist gene produce an RNA molecule that costs the X
chromosome and inactivate the genes on it by altering the
On the active X chromosome, the Xist gene is repressed. ...
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- Summer '08
- Genetics, X chromosome, Sex-determination system