Sex-Linked Crosses – are crosses that involve the X chromosome. Traits are passed are
determined by the mother. (The father “donates” the Y chromosome in males – XY, and only
“donates” one X for females – XX)
a. Representations of the trait:
Sex linked traits – traits that are located on the X chromosome
Females XX Males XY
A dominant allele is represented by X
The recessive allele is represented by Xi
Males only have one opportunity to get the dominant allele
A normal male that does not show the trait will be XY (dominant X)
A male that shows the trait will be XiY (recessive Xi)
Females have two X’s. So if they have a dominant and a recessive, the dominant trait will still be shown
and they will not have the condition. They are known as carriers.
A normal female that does not show the trait will be XX (2 dominant X’s, homozygous dominant)
A carrier female does not show the trait but can pass it on to the offspring will be XiX (One dominant, one
recessive – heterozygous)
A female that shows the trait will be XiXi (2 recessives, homozygous recessive)
In some populations, multiple alleles (3 or more) may determine a trait such as in ABO Blood type
a. Alleles: There are three alleles:
___________________________________________
Alleles A & B are dominant, while O is recessive
Blood Type – traits that are controlled by Multiple (more than one) Alleles
For blood type the dominant allele is represented by I - the recessive allele by i.
There are two types of dominant alleles: Dominant Type A = IA and Dominant
Type B = IB
Genotype Phenotype
ii
Type O
IAi
Type A
A A
I I
Type A
IBi
Type B
IBIB
Type B
A B
Type AB
I I
Type A Blood could be (IA IA ) two dominant type A’s (homozygous) or (IAi) one dominant, one
recessive (heterozygous)
Type B Blood could be (IB IB ) two dominant type B’s (homozygous) or (IBi) one dominant, one
recessive (heterozygous)
Type AB Blood is (IA IB ) one dominant type A and one dominant type B (example of codominance as
both traits show up)
Type O blood is (ii) two recessive alleles
Genetics: X Linked Genes
In fruit flies, eye color is a sex linked trait. Red is dominant to white.
1. What are the sexes and eye colors of flies with the following genotypes:
XIXi
XIXI
_________________
_________________
XIY
XiY
_________________
_________________
2. What are the genotypes of these flies:
white eyed, male ____________
white eyed, female ___________
red eyed female (heterozygous) ________
red eyed, male ___________
3. Show the cross of a white eyed female X i
X i with a red-eyed male X I Y
.
4. Show a cross between a pure red eyed female and a white eyed male.
What are the genotypes of the parents:
___________& _______________
How many are:
white eyed, male___
white eyed, female ___
red eyed, male ____
red eyed, female ____
5. Show the cross of a red eyed female (heterozygous) and a red eyed male. What are the genotypes
of the parents?
___________ & ________________
How many are:
white eyed, male___
white eyed, female ___
red eyed, male ____
red eyed, female ____
Math: What if in the above cross, 100
males were produced and 200 females.
How many total red-eyed flies would there
be?
____________
Genetics: Multiple Allele Traits
Blood Type is controlled by 3 alleles: A, B, O. A & B are codominant, O is recessive.
1. a)
b)
c)
d)
What are the two genotypes possible for a person who as A blood? ______________
What genotype does a person with AB blood have? _______________
What genotype does a person with O blood have? _____________
What are the two genotypes possible for a person who as B blood? ______________
2. A man with type AB blood is married to a woman also with
type AB blood. What blood types will their children have and
in what proportion?
3. A man has type B blood (genotype BB) is married to a
woman with type O blood. What blood type will all their
children have? ________ What is the genotype of the
children? ______
4. A woman with type A blood (genotype AO) is married to a
type B person (genotype BO). What proportion of their
children will have: A blood? _______ B blood? _______ O
blood _____
5. A woman with type A blood is claiming that a man with
type AB blood is the father of her child who is also type AB.
Could this man be the father of the child? _______________
Show the possible crosses; remember that the woman can
have AA or AO genotypes.
6. A man with type AB blood is married to a woman with type
O blood. They have two natural children and one adopted
child. Jane has type A blood, Bobby has type B blood, and
Grace has type O blood. Which child was adopted?
____________________