Sex determination and sex linkage Flashcards
(21 cards)
Mammalian Sex Determination
Mammals have a chromosome-based mechanism for sex determination
The biological sex of a mammal depends upon which of two sex determining chromosomes are present in the sperm fertilizing an ovum
What does XX mean
If genotype is XX, the sex chromosomes are a homologous pair
The same genes are present at the same loci on each chromosome of a pair
If genotype is XY, the sex chromosomes are non-homologous
XY are
X and Y have different genes
Y chromosome is much smaller and carries few functional genes compared to X
There is a small portion of Y that is homologous to X
Ensures pairing during prophase I and thus segregation during anaphase I
1:1 ratio of X and Y in sperm
Y chromosomes of mammals
Presence of the Y chromosome triggers development of the male phenotype
XO have only one X chromosome and are phenotypically female
XXY have two X chromosomes and are phenotypically male
Absence of a Y chromosome results in the female phenotype
Presence of a Y results in the male phenotype
SRY is
The Y chromosome contains the SRY gene (sex-determining region of Y chromosome)
During early embryological development the gonads are capable of developing into ovaries or testes, based on the hormonal environment
A functioning SRY gene triggers testosterone production and the gonads become testes
Sex determinations in birds
In birds, butterflies, moths, and some reptiles and fish:
Heterogametic ZW – female traits
Homogametic ZZ – male traits
Sex determinations in some insects
In some insects, such as crickets, there is only one type of sex-chromosome
Female traits - XX
Male traits - XO
Ants, wasp and bees sex determination
O Sex is determined by ploidy
O Diploid individuals (2n) have female traits
O Haploid individuals (1n) have male traits
Environment sex-determination is and affects who
some reptiles and fish
Sex ratio of developing crocodile and alligator eggs is a function of temperature
Variations in XY Distributions
Occasionally, the XY chromosomes do not segregate as expected during meiosis
Offspring have variations in the number of sex chromosomes
Monosomy X (XO) is and what do they look like
Individuals are phenotypically female
XO genotype mares:
Often small in size overall
Have a small uterus and small, undifferentiated ovaries
Infertile
Klinefelter Syndrome (XXY) is
Individuals are phenotypically males
Infertile because presence of two X chromosomes blocks spermatogenesis
Recognized most commonly in cats, but occurs in other species too
Sex chromosomes determine and why
Although sex-chromosomes determine the biological sex of an individual, they do not carry all the genes responsible for sexual characteristics
These are scattered throughout the autosomes
The sex-chromosomes also carry genes determining non-sexual characteristics
For genes found on the X chromosome, the recessive phenotype is much more common in XY individuals
Expression of xlinked genes
X-linked genes are located on the X-chromosome
XY individuals cannot be homozygous or heterozygous for alleles of X-linked genes
They can only possess one allele, so are hemizygous for those genes
Features Associated with Sex-linked genes
Heterozygous females transmit their X-linked recessive allele to ~ half of their daughters and half of their sons
Expression only occurs in the sons
Males that inherit an X-linked recessive allele exhibit that trait
Y chromosome has no counterpart allele
Hemizygous recessive
Reciprocal crosses give different results
X-linked Recessive Trait Pedigrees characteristics
Typically have 3 characteristics:
It is more frequent in males than females
It fails to appear in females unless it also appears in the paternal parent
It seldom appears in both father and son
Father does not pass it to the son, the allele must come from the mother
X-linked Dominant Inheritance
Dominant traits are expressed even if there is only a single copy of the gene
Pedigrees for X-linked dominant traits show the following characteristics:
Trait is expressed in each generation
Affected offspring must have an affected parent
Affected males only pass the condition to their daughters
Affected females will produce affected sons and daughters
Inactivation of the X-chromosome
In every XX genotype cell only one of the X chromosomes is functional
X chromosome inactivation
Is a random event
In all subsequent mitosis events from that cell the same X chromosome remains inactivated
Early in development one of the X chromosomes becomes highly condensed
Referred to as a Barr body
Animals that are heterozygous for X-linked genes are called mosaics
Half of their somatic cells will express one allele, the rest express the other allele
Why X-Inactivation?
X-inactivation achieves dosage compensation
Inactivation of one X chromosome in XX individuals reduces the number of expressed copies of X-linked genes from 2 to 1
Expression of X-linked genes in XY and XX individuals is therefore equal
Sex determination in some invertebrates
In some invertebrates a single locus with two alleles determines sex
Mosquitoes
If heterozygous for the gene develop male traits
If homozygous recessive develop female traits
What affects mosquitos sex characteristics
In some invertebrates a single locus with two alleles determines sex
O Mosquitoes
O If heterozygous for the gene develop male traits
O If homozygous recessive develop female traits
