Cross-Dressing or Crossing-Over: Sex Testing of Women Athletes

Maureen Knabb, Department of Biology, West Chester University, and Joan Sharp, Biological Sciences, Simon Fraser University

Caster’s Story Caster Semenya won the gold

medal in the 800-meter race at the 2009 World Championships in Berlin, Germany.

Following her gold medal performance, her sex was officially questioned.

Caster won the silver medal in

the 800m at the 2012 Olympic games

18 August 2009: “South African athlete Caster Semenya fails gender test”

• Top South African woman athlete Caster Semenya, who won a gold medal at a recent international championship, has failed a gender test, according to official reports.

• Sports writer KP Mohan said that a team of doctors, including a gynecologist, endocrinologist and psychologist, normally examines athletes and puts them through physical and clinical examinations during a gender test.

• Caster Semenya's test was done soon after Semenya won the women's 800m race on August 18, but it is not clear how or why she failed the test

Gender vs Sex: Gender refers to the psychological/societal aspects of being male or female, sex specifically to the physical aspects. Do not interchange. (American Medical Association Manual of Style)

How do you know if someone is male or female?

• As a class, let’s try to identify at least 5 characteristics to distinguish males from females.

How is Sex Determined in Humans?

• Anatomy: Primary and secondary sexual characteristics - genitalia, body hair, shape of pelvis (e.g. pelvic angle), etc.

• Physiology: Function and interaction of the sex organs including concentrations of sex hormones such as estrogen, progesterone, and testosterone.

• Genetically: How might this work???

CQ1: Imagine you are a member of the committee assigned to determine whether Caster is female. Here are possible results of the initial tests (we don’t know the real results):

Female genitalia? - Yes Breasts and pubic hair? - Yes Regular menstrual cycle? - Never reported a menstrual cycle

From this information, you would conclude that Caster is:

A: Male B: Female

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What are chromosomes?

What are chromosomes?•Human cell = >6 billion nucleotide base pairs (~2m)•Wrapped around protein = chromatin•DNA/protein = chromosome

How Many Chromosomes Do Humans Have?

• The two copies of each chromosome in human cells are homologous

• Humans are diploid (2n)• Two of each chromosome, one from each parent.• n = 23 unique chromosomes (haploid #)• 2(n) = 46 total chromosomes

•Different versions - same genes in same locations but different DNA sequence. •Different versions (alleles) of a gene may

promote different traits (e.g. hair type).

Curly hair allele

Strait hair allele

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CQ#2: The diploid chromosome number in standard laboratory mice (genus Musculus) is 40.

What is n for this organism?

Is there Another Way that Sex Can Be Characterized in Humans?

• Chromosomes: – Females possess two X chromosomes in

each of their cells, whereas males have one X and one Y chromosome.

• How do you visualize chromosomes?– Chromosomes are only visible when

preparing for nuclear division.– Chromosomes must be stained in order to be

visualized with the light microscope.

Karyotype(picture of chromosomes in a cell)

In diploid organisms, chromosomes come in pairs (e.g., 23 pairs for humans).

How do you make a karyotype?

Human Karyotype

What types of information can be learned from a karyotype?

Human Karyotyping Activity

AmniocentesisProcess of removing amniotic fluid from the womb for the purposes of biochemical/genetic analysis

Preparing a Karyotype

Creating a Karyotype

Chromosome spread Karyotype

Sex Determination

• In mammals, males have one X and one Y chromosome.

• In males, half the daughter cells formed by Meiosis I get an X chromosome and half get a Y chromosome.

• Following Meiosis II and sperm differentiation, half the sperm are X-bearing and half are Y-bearing.

How is Sex Determined in Humans?

• Sex is determined by the sex chromosome carried by the sperm.

• What sex chromosome is carried by the egg?

sperm

eggXX XY

XX XY

girls boys

X

X

Y

X

CQ3: Could Caster have an XX karyotype and be male?

A: No, an XX individual is always female.B: Yes, this is common.C: Yes, if a male-determining control gene is

carried on one of her X chromosomes.

CQ4: A karyotype was performed on an athlete. Here are the results:

From these results, the athlete is: A: Male B: Female

CQ5: So if Caster is a normal female, her karyotype would show which combination of sex chromosomes:

A: XX B: XY C: YY D: XXY

Do Now - How Many Chromosomes?Normal human body (i.e. somatic) cells each contain 46 chromosomes. Mitosis produces daughter cells that are virtually identical to the parent cell. Working with a partner at your table, discuss and answer the following questions on your Do Now handout.1. How many chromosomes would a sperm or an egg cell

contain if either cell were produced by mitosis?2.!If a sperm containing 46 chromosomes fused with an egg

containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? Do you think this would create any problems in the developing embryo?

3.!In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?

4. What is meant by “virtually identical to the parent cell?”

The Fate of Genetic Material During Sexual Reproduction

• You contain genetic material from both your Mom and Dad.

• You have 46 chromosomes, and so do (will?) your children.

• How does this happen?

SEXMom 46 chromosomes Dad (2n = diploid)

Egg 23 chromosomes Sperm (1n = haploid) Zygote (fusion) 46 chromosomes again!

CQ6: Assume that this is one of Caster’s chromosomes. This chromosome is composed of two chromatids joined by shared centromeres.

A: These chromatids make up a diploid chromosome.

B: The cell that contains these sister chromatids must be diploid.

C: The sister chromatids were formed by replicating a single chromatid.

D: The sister chromatids were joined by fertilization, bringing together a maternal and paternal chromatid.

Meiosis• Purpose of meiosis: in animals, produce gametes

with just half the parents’ genetic material. Gametes are ______ (haploid/diploid).

• Mechanism: Only one of each pair of homologous chromosomes gets into each gamete.

• The happy ending: Gametes from the two parents get to fuse, restoring the original number of chromosomes in the fertilized egg (zygote).

Stages of Meiosis

• Meiosis (in animals) produces 4 haploid cells from 1 diploid cell.

• At the end of the first division (Meiosis I) the 2 cells are already haploid.

• The second division (Meiosis II) splits the 2 sister (identical, replicated DNA) chromatids to 1 chromatid.

CQ7: What is true after Meiosis I?

A: Four cells have been produced.B: The daughter cells are haploid.C: The DNA will be replicated before

Meiosis II.D: The cells are ready to perform as

gametes.E: Each chromosome consists of a single

strand of DNA.

CQ8: During Meiosis II:

A: Homologous chromosomes separate.B: The DNA is replicated.C: Gametes fuse.D: Sister chromatids separate.E: All of the above.

Sources of Genetic Variability in Meiosis

Independent Assortment

• Each pair of homologous chromosomes lines up at Metaphase I plate.

• There is a 50:50 chance that a daughter cell will get one or the other copy of the chromosome.

CQ9: How many possible combinations of maternal chromosomes are possible in a human ovum due to independent assortment during meiosis?

A: 23 combinations.B: 46 combinations.C: 232 = 529 combinations.D: 223 = ~ 8 million combinations.

Sources of Genetic Variability in MeiosisCrossing Over During Meiosis

• When homologous chromosomes pair during Prophase I, non-sister chromatids exchange homologous portions.

• A chiasma forms between the non-sister chromatids at the point of crossing over.

• Following crossing over, the chromatids are recombinant, combining paternal and maternal genes.

Crossing Over

• Crossing over can occur anywhere along the autosomes.

• In males, the sex chromosomes (X and Y chromosomes) normally cross over only at their tips.

CQ10: During crossover, all of the following happen EXCEPT:A: The homologous chromosomes line up in

pairs.B: The homologous chromosomes swap

pieces with their adjacent partners.C: Crossing over decreases the genetic

variability in the gametes.D: Each chromatid is unique after it has

crossed over.

How is Sex Characterized in Humans?

In addition to anatomy, physiology, and chromosomes, there is a 4th answer:

Genes: Specific genes determine whether an embryo will develop as a male or female.

SRY (Sex- Determining Region of the Y chromosome) Gene

• Early in development, the immature gonads of males and females are indistinguishable.

• Males: In the 7th week of development, the SRY gene on the Y chromosome activates a number of genes, and the gonads develop as testes.

• Females: With no SRY gene, gonads develop as ovaries by default.

SRY gene

X

Y

Can you have an XX male or XY female?

• What sex is XX (SRY+)?• What sex is XY (SRY-)?

Sex reversal occurs in 1 in 20,000 births!

CQ11: If you were a member of the International Olympid committee (IOC) and Caster’s karyotype revealed that she is XY and SRY-, what would you do?

A: She has female genitalia, allow her to keep her medal.

B: She is genetically male, take her medal away.

C: Perform additional tests.

CQ12: What do you think about requiring gender testing for female athletes in international competition?

A: It should be banned because gender determination is so complex.

B: It is necessary to ensure an even playing field.C: It is necessary, but needs to include a large

number of genetic tests to ensure fairness.D: It should be required for all athletes, both male

and female.

What about Caster Semenya?

Caster was stripped of her gold medal. An anonymous official in the know has reported that Caster’s genetic tests revealed more Y chromosomes than are usually present in the genetic make-up of a female. No official statement has been made and we don’t know Caster’s true medical condition.