Top3-Ch4-P78-97End Flashcards
What is genic balance system?
sex is determined by a balance between genes on the autosomes and genes on the X chromosome.
Explain sex determination of fruit fly and what the system is called? Also give scientific name for fruit fly.
The system is called the genic balance system. Scientific name is drosophila melanogaster.
Sex is determined between genes on X chromosome and balance of genes on autosomes. Despite having XX (females) and XY (males) the Y chromosome does not determine maleness. Instead maleness comes from the autosomes A. To determine fruit fly sex must calculate X:A ratio with 1.0 being female and 0.5 being male.
Examples, XX:AA = 1.0 female, XY:AA=0.5 male, XXX:AA= 1.5 metafemale, XO:AAA0.33 metamale, XXXY:AA= 1.5 metafemale
What is the name of the gene on the Y chromosome that determines maleness?
SRY for Sex determining region Y which was discovered in 1990. The gene is found in XX males or XY females. This happens because the gene can move off the Y chromosome onto another chromosome to still create maleness.
See diagram below of where SRY gene is located.
Explain Turner Syndrome
Only has a single X chromosome referred to as XO. Affects females, seen in 1 in 3000 females, frequently short, relatively broad chested, folds of skin on neck. Intelligence usually normal, usually sterile women from this.
No known cases in which a person misses both X chromosomes indicating that at least one X chromosome is necessary for human development.
Explain Klinefelter syndrome
- Individuals have multiple X chromosomes and one or more Y chromosomes (usually XXY), but can be XXXY, XXXXY or XXYY.
- Phenotype is male
- Reduced facial hair
- Sterile
- Occurs 1 in 1000 male births.
Explain Poly-X females
- Has three X chromosomes which is called triplo-X-Syndrome
- Have normal female phenotype and slightly greater chance on sterility and mental retardation
- Mental retardation occurs when more than three X-chromosomes and severity increases with each added X chromosome.
Explain roles of sex chromosomes (5 points)
- X-chromosome is essential, at least one copy has to be active
- male determining gene is on the Y chromosome
- absence of Y chromosome means female phenotype
- genes affecting fertility on both X and Y chromosomes
- additional copies of the X chromosome may affect normal development
Androgen-insensitivity syndrome
Contain XY, but appear externally as female and have male testes inside. Reason, is SRY creates gonads that produce testosterone to have affect testosterone must bind to an androgen receptor. But the receptor is defective in females with androgen-insensitivity syndrome so their cells are insensitive to testosterone and female characteristics develop.
Sex-Linked Characteristics are?
Determined by genes located on the sex chromosomes, X-linked characteristics and Y-linked characteristics
Who was the first biologist to explain sex-linked inheritance? And what did he show?
Thomas Hunt Morgan and he showed that eye colour was an X-linked trait in Drosophila
Are there homologous alleles on the Y chromosome?
No.
Explain how eye colour for fruit fly work on X and Y chromosomes.
eye colour alleles are only on X chromosome. Y chromosome does not have homologous alleles. So females can be homozygous or heterozygous for eye colour alleles. But for males with just one X chromosome can only carry a single eye color allele on the Y. Males are then said to be hemizygous for X-linked loci.
Look on P82 for Thomas Hunt Morgan’s fruitfly experiment. See attached diagram in answers
non-disjunction is?
when homologous chromosomes fail to separate during meiosis
When Thomas Hunt Morgan crossed his original white eyed males with homozygous red-eyed females all 1237 of the progeny had red eyes except for 3 white eyed males. His student Calvin Bridges did experiments to show what happened. See diagram on next page.
See diagram for x-linked colour blindness in humans.
Xc is allele for red-green colour blindness which is recessive. Allele for blue pigment vision is on chromosome 7. X+ is allele for normal colour vision.
Is there crossing over between X- and Y chromosomes in males?
No. Because of this the Y chromosome reduces in size and becomes more and more mutated
What is the Lyon Hypothesis?
that there is a random inactivation of one of the X chromosomes in each cell during 32-64 cell stage of development. The descendant cells follow the same X is inactive. This explains mosaic patterns as the inactive X can vary throughout the body.
What is a Barr body
it is an inactivated X chromosome.
See below the dark Barr body on the left cell but not on the right cell.
