Life Sciences Term 2

Sex determination in humans

There are 22 pairs of autosomes and one pair of sex chromosomes (gonosomes) in the human karyotype. Females have XX sex chromosomes while males have XY sex chromosomes.   Each time fertilisation occurs, there is a 50% chance of the zygote being male and a 50% chance of the zygote being female, X + X = XX and X + Y = XY.

An example of a genetic cross to show the inheritance of sex:

P₁ Phenotype: male x female
Genotype: XY x XX
Meiosis
Gametes: X, Y x X, X
Fertilization

	X	X
X	XX	XX
Y	XY	XY

       

 

 

                 

F₁ Genotype: XX XX : XY XY
1      :   1

Phenotype: 50% females : 50% males

Sex-linked alleles
Some characteristics or traits are carried on the sex chromosomes. 

HAEMOPHILIA and COLOUR BLINDNESS are two sex linked disorders that forms part of our curriculum. This is taught, if learners see this they should immediately know that it is sex linked and they should use this method. Any other sex-linked disorder will be specified as a sex-linked disorder in the question.
 
Haemophilia is a sex-linked condition where blood fails to clot properly.  This recessive allele is found only on the X chromosome of the sex chromosomes.  Males have only one X chromosome The Y chromosome has no gene for blood clotting. This means that the condition of haemophilia is seen in males with only one recessive allele present. A female with one recessive allele will be a carrier because the other X chromosome will carry the normal dominant gene. A female will only be haemophilic if she has both homozygous recessive alleles.

EXAMPLES OF HAEMOPHILIA GENETIC CROSSES:

Example 1: For a normal male and female carrier (heterozygous) cross:

P₁  Phenotype: unaffected normal male x female carrier
Genotype: X^HY x X^HX^h 
Meiosis
Gametes: X^H, Y x X^H ,  X^h
Fertilization

	XH	Y
XH	XH  XH	XH Y
Xh	XH Xh	XhY

F₁  Genotype: 1 X^H X^H  :  1 X^H X^h   1 X^H Y  1 X^h Y
normal female : female carrier : unaffected male : haemophilic male
25% : 25%  : 25%  : 25% 
Phenotype : 50% unaffected females : 25% unaffected males : 25% haemophilic males

Example 2: An affected male with an unaffected female

P₁  Phenotype: affected male x unaffected female
Genotype: X^h Y x X^H X^H                                                                           
Meiosis
Gametes: X ^h , Y x X ^H ,  X^H
Fertilization

	Xh	Y
XH	XHXh	XHY
XH	XH Xh	XHY

F₁ Genotype: X^HX^h  X^HX^h  X^HY  X^HY
50% of  F₁  is female carriers : 50%  of  F₁  is normal males
1 : 1 

Phenotype : 100% normal