Download sex determination and sex linked traits

SEX DETERMINATION
AND
SEX LINKED TRAITS
Mrs. Sprenkle
Biology
TODAY’S OBJECTIVES
Demonstrate how sex is determined in
humans and other organisms using a
Punnett Square
Use knowledge of sex determination to
create a Punnett Square showing sex
linked traits on the X chromosome
Explain why sex linked traits appear in
males more often than females
I. SEX CHROMOSOMES
A. Initial organism studied:
Drosophila melanogaster
Fruit fly
** This would make a
great bonus question
B. WHY?
1.
Have only 4 chromosomes
2.
Reproduce in 10-15 days
3.
Small

1 pair of chromosomes in male is different
than the female = sex chromosomes
C. Human Sex Chromosomes
1.
FEMALE = X X
2.
MALE = X Y
3.
Who determines sex of offspring?
Male can donate X or Y. Female
can only donate X.
D. PUNNETT SQUARE
1. P cross
2.
3. XY
4. male
XX
female
Geno. Ratio:
XX : XY
2 : 2
Pheno Ratio:
Female:male
2 : 2
II. SEX LINKED
A. Sex Linked- genes carried on the X
chromosome
1.
2.
3.
Any recessive allele present on the male’s X
chromosome WILL BE EXPRESSED
WHY?
a) Because there is no gene on the Y to be
dominant over the recessive
WHY ARE RECESSIVE SEX LINKED
TRAITS LESS COMMON IN FEMALES?
a) Females have another chance to receive a
dominant gene from the second X chromosome
SEX LINKED
4. Fruit fly eye color is a SEX LINKED
TRAIT
RED EYES = Dominant
white eyes = recessive
5. EXAMPLE

white male x RED female = all F1 offspring
are RED eyes
HOWEVER…

F1 cross shows only males have white eyes
SEX LINKED
1. P cross
2.
3. XrY
4. white,male
XRXR
red,female
Geno. Ratio
XRXr:XRY
2:2
Pheno Ratio:
Red/carrier/female 2
red/male 2
SEX LINKED
1. F1 cross 2.
3. XRY
4. red,male
XRXr
red, carrier, female
Geno. Ratio
XRXR:XRY:XRXr:XrY
1:1:1:1
Pheno Ratio:
Red/female 1
Red/male 1
Red CARRIER
female 1
White/male 1
III. OTHER SEX LINKED
TRAITS
X-linked genes are responsible for
abnormal conditions such as:
1.
2.
3.
4.
5.
hemophilia
red-green color blindness
congenital night blindness
high blood pressure
duchene muscular dystrophy
CONCLUSION
A. In humans and many other organisms
sex is determined by X and Y
chromosomes.
B. Traits carried only on the X chromosome
are referred to as “SEX LINKED”
1.
Males tend to “show” a sex linked trait more
than females because the Y chromosome
does not carry the trait. Therefore, if the trait
given to the male on the X chromosome is
recessive, the male will show that trait.