Download Grade 11 Biology

Grade 11 Biology
Genetic Continuity
Sex Influenced Traits
Sex-linked Traits
- traits that are dominant in one sex and recessive in another
o e.g. pattern baldness, hemophilia, colour blindness
- this is not the same as sex-limited traits (e.g. breasts)
Chromosomes:
- female:
22 autosome pairs + XX (46 chromosomes)
- male:
22 autosome pairs + XY (46 chromosomes)
o traits inherited on the X & Y chromosomes are sex-linked
o traits inherited on autosome chromosomes follow inheritance as learned earlier
e.g. Hemophilia (blood-clotting disorder of X chromosome):
H: normal
h: hemophiliac
males:
always express trait of their 1 X chromosome
1. dominant allele  dominant trait
XH Y
2. recessive allele  recessive trait
XhY
females:
may be:
1. homozygous dominant  dominant trait XHXH
2. heterozygous  dominant trait but are carriers since they carry recessive trait
3. homozygous recessive  recessive trait
XhXh
XH Xh
Question:
A normal woman marries a man who is normal but their son is a hemophiliac. Show the possible
crosses and state the genotypes of the family. Could the daughter be a hemophiliac?
Hemophilia * X linked:
Father:
XH Y
H: normal
h: hemophila
Mother:
XHXH or XHXh
If mother is homozygous dominant…
XH
Y
H
H H
X
X X
XH Y
XH XH XH XH Y
… there is no possibility any of their children has hemophilia!
If mother is heterozygous…
XH
Y
H
H H
X
X X
XH Y
Xh
XHXh XhY
… 50% of their male offspring will have hemophilia,
and 0% of their female offspring will have hemophilia, but 50% of female offspring will be a
carrier for this disorder.