Sex determination and sex linkage

Question 1

Mammalian Sex Determination

Answer 1

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

Question 2

What does XX mean

Answer 2

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

Question 3

XY are

Answer 3

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

Question 4

Y chromosomes of mammals

Answer 4

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

Question 5

SRY is

Answer 5

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

Question 6

Sex determinations in birds

Answer 6

In birds, butterflies, moths, and some reptiles and fish:
Heterogametic ZW – female traits
Homogametic ZZ – male traits

Question 7

Sex determinations in some insects

Answer 7

In some insects, such as crickets, there is only one type of sex-chromosome
Female traits - XX
Male traits - XO

Question 8

Ants, wasp and bees sex determination

Answer 8

O Sex is determined by ploidy
O Diploid individuals (2n) have female traits
O Haploid individuals (1n) have male traits

Question 9

Environment sex-determination is and affects who

Answer 9

some reptiles and fish
Sex ratio of developing crocodile and alligator eggs is a function of temperature

Question 10

Variations in XY Distributions

Answer 10

Occasionally, the XY chromosomes do not segregate as expected during meiosis
Offspring have variations in the number of sex chromosomes

Question 11

Monosomy X (XO) is and what do they look like

Answer 11

Individuals are phenotypically female
XO genotype mares:
Often small in size overall
Have a small uterus and small, undifferentiated ovaries
Infertile

Question 12

Klinefelter Syndrome (XXY) is

Answer 12

Individuals are phenotypically males
Infertile because presence of two X chromosomes blocks spermatogenesis
Recognized most commonly in cats, but occurs in other species too

Question 13

Sex chromosomes determine and why

Answer 13

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

Question 14

Expression of xlinked genes

Answer 14

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

Question 15

Features Associated with Sex-linked genes

Answer 15

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

Question 16

X-linked Recessive Trait Pedigrees characteristics

Answer 16

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

Question 17

X-linked Dominant Inheritance

Answer 17

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

Question 18

Inactivation of the X-chromosome

Answer 18

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

Question 19

Why X-Inactivation?

Answer 19

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

Question 20

Sex determination in some invertebrates

Answer 20

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

Question 21

What affects mosquitos sex characteristics

Answer 21

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