7 - Sex determination and Sex Linkage

Question 1

What are the 2 sex chromosomes of mammalians

Answer 1

X and Y
XX - female - homogametic
XY - Male - Heterogametic

Question 2

What are some differences between X and Y genes

Answer 2

Y chromosomes are smler w/ less function genes than X
A small portion of Y is homologous w/ X (ensures pairing during prophase I and thus segregation during anaphase I)
1:1 ratio of X and Y in sperm

Question 3

Explain the importance of the Y chromosomes in mammals

Answer 3

Presences of Y chromosomes triggers development of the male phenotype
XO have only one X chr and are phenotypically female
XXY have 2 X chr and are phenotypically male
ABSENCE of Y chr results in fem phemo
PRESENCE of Y chr results in male pheno

Question 4

What is SRY?

Answer 4

Y chr contains the SRY gene or sex-determining region of Y chr
During early mbryological development the gonads are capable of developing into ovaries or testes based on hormonal environment
a functioning SRY gene triggers testerone prod and the gonads become testes

Question 5

What are sex determinants in birds, butterflies, moths and some reptiles and fish?

Answer 5

hetergametic ZW - female
Homogametic - ZZ male

Question 6

What are some sex deterimants in crickets, ants, wasps, and bees?

Answer 6

Crickets only 1 type of sex chr
Fem - XX, Male -XO
Ants, wants, bees lack sex chr
Sex determined by ploidy
diploids (2n) = fem
Haploids (1n) = male

Question 7

What are some sex determants of some invertebrates

Answer 7

a single locus w/ 2 allels determine sex
ex mosquitos
is hetero for gene develop male
if homo for gene develop female

Question 8

How might temperate in reptiles and fish affect sex determinants?

Answer 8

Enviromental sex-determination where sex ratio of crocodile and alligator eggs is dependant on temperature

Question 9

What is monosomy X (XO)

Answer 9

individuals phenotypically female
XO genotype mares: small is size overall, small uterus and undifferentiated ovaries, infertile

Question 10

What is klinefelter syndrome? (XXY)

Answer 10

individuals are phenotypically male
infertile bc of presence of two x chro blocks spermatogenesis
recognized in cats but some species too

Question 11

What genes are responsible for sexual characteristis?

Answer 11

Autosomes

Question 12

What might sex-chromosomes also carry?

Answer 12

Genes determining non-sexual characteristics
for genes found on X chr, the recessive phenotype is more common in XY

Question 13

How might the expression of X-linked genes work?

Answer 13

x-linked genes are on X-chromosomes
XY individuals cannot be homozygous or hetero for alleles of Xlinked genes (can only possess one allele and so are hemizygous for these genes)

Question 14

Give an example of X-linked genes in regards to Hemophilia.

Answer 14

Hemophilia A caused by recessive X-linked allele
Dom XH prod functional clotting factor
Rec Xh prod non-functional clotting protein
Any individual homo or hemi for rec allele has blood clotting issues

Question 15

What happens if the mother is heterozygous (a carrier) for the recessive allele and the father is unaffected (for hemophilia)

Answer 15

any daughters will be unafected but 50% carriers
any sons have 50% change of having hemophilia

Question 16

What are some common features associated with sex-linked genes?

Answer 16

hetero fems trasmit their Xlinked rec allele to half of their daughters and sons (expression only occurs in the sons)
male that inherit the rec allele will exhibit that trait (y chr has no counterpart allele so hemi reccessive)
reciprocal crosses give diff results

Question 17

In terms of xlinked recessive trait, where are 3 common characteristics?

Answer 17

1. more frequent in males than fems
2. fails to appear in fems unless it also appears in paternal parent
3. seldom appears in both father/son (allele must come from mother

Question 18

What should we keep and mind and consider in regards to x-linked genes and how is passes?

Answer 18

non of the children of an affected male show the trait, but it may be seen again in his grandchildren
Only a female can inherit the recessive allele from an affected male
she will be a carrier and could pass the rec allele so any sons that receive it will be affected

Question 19

What is the differences of x-linked genes when there is dominant inheritance?

Answer 19

dominant traits are expressed even if there is only a single copy of the gene
pedigrees for x-linked dom show following characteristics
traits expressed in each generation
affected offspring have affected parent
affected males only pass to daughters
affected fems will prod affected sons/daughters

Question 20

Give an example of x-linked dominant

Answer 20

hypophasphatemia in humans and mice (low blood phpshprous lvls, AKA vitamin D-resistant rickets)
Alport syndrome in dogs - form of neprhitis

Question 21

What is inactivation of X-chromosomes?

Answer 21

in every XX genotype cell only 1 of the X chr is functional (x chromosome inactivation)
A random event
in all subsequence mitosis events from that cell the same X chr remains inactivated
in early development one of the X chr becomes highly condensed and referred to as a barr body

Question 22

What might animals that are heterozygous for x-linked genes be called?

Answer 22

mosaics
half of their somatic cells will express one allele and the rest the other

Question 23

Why X-inactivation?

Answer 23

achieves dosage compensation
inactivation of one X chr in XX individuals reduces the number of expressed copies of X-linked genes from 2-1
expression of Xlinked genes in XY and XX individuals is therefore equal

Question 24

In which individuals do X-inactivation occurs?

Answer 24

XX individuals