Transcript Sex Linked Genes - s3.amazonaws.com

Module 10
Autosomal Traits and Sex
Linked Traits
Objectives
 Understand
the concept of sex linked
genes
 Distinguish between sex-linked and
autosomal genes
 Complete a monohybrid cross using a
gene located on the X chromosome
Allele- alternate forms of a given gene. (green or blue eyes)
Independent assortment- random distribution of unlinked
genes into gametes.
Linked Genes- genes located on the same chromosome
tend to be inherited together. The alleles do not assort
independently.
Sex linked genes- genes located on a sex chromosome.
Crossing over- reciprocal exchange of genetic material
between non-sister chromatids during meiosis I.
CHROMOSOMES
Female
Male
XX /XY
Sex
 Sex
II
III
IV
-----------Autosomal-----------
-chromosomes responsible for
determining sex of an individual
 Autosomal- chromosome that is not directly
involved in determining sex.
Example Phenotypes
W.T.
Ebony
Body
Vestigial
Wings
White
Eyes
Review of Nomenclature
when referring to Autosomal Traits
Autosomal Genes (use + for wt allele)
Genotype
+/+
vg / +
vg / vg
Phenotype
wt (homozygous)
wt (heterozygous)
mutant
Nomenclature
Autosomal Genes (use + for wt allele)
Genotype
+/+
vg / +
vg / vg
+_
Phenotype
wt (homozygous)
wt (heterozygous)
mutant
wt (homo. or het.)
The Law of Independent
assortment
“Each pair of alleles segregates
gametes independently” (Campbell)
For examples:
AaBb will segregate and form 4 different gametes:
AB, Ab, aB, and ab.
into
Autosomal Genes vs. Sex
Linked Genes
 Genes
located on Autosomal
chromosomes are Autosomal Genes.
 Genes that are Sex-linked are located on
the X or Y chromosome.
 Results from a cross using a sex-linked
gene will be much different from a cross
using an Autosomal Gene.
Sex-Linked Genes
 When
a gene is located on the X or the Y
chromosome, it is considered Sex-linked
 Genes located on the X chromosome are
present in both male and females
 Genes located on the Y chromosomes are
only present in males
Example Monohybrid CrossSex Linkage

The gene for eye color is located on the X
chromosome.
 “W” shows a white phenotype (red is wt)


Cross 1: X+Y
X
wild-type male

Cross 2: XWY
white eye male
XWXW
white eye female
X
X+X+
wild-type female
Notice that the Nomenclature for Sex Linked Traits
is different than for Autosomal Traits
Sex Linked Genes- superscript letter denoting genotype
must be attached to respective sex chromosome.
Example below is for white eye color.
Genotype
Phenotype
+
XX
+
wild-type female
+
W
wild-type female
XX
XW XW
White eye female
+
XY
wild-type male
W
White eye male
X Y
Sex linked genes
Example Cross A:
Genotype P:
X+Y
(male WT)
gametes:
X+,Y
Genotype F1:
XWY
Phenotype F1:
Male
white eyes
x
XWXW
(female white eyes)
XW
X+ XW
Female
wild-type eyes
F1 Cross with Progeny from
Sex-linked Parental Cross A
F1xF1:
XWY
X+XW
X
XW
Y
X+
X+XW
X+Y
XW
XWXW
XWY
gametes:
XW ,
F2 genotypes:
X+XW
X+ , XW
Y
XWXW
X+Y
XW Y
•
What is the Phenotypic ratio for the F2
Progeny for the Sex-linked Cross A?
EXPECTED RESULT =
female red eyes (wt)
female white eyes
male red eyes (wt)
male white eyes
1:1:1:1
Summary of the Sex Linked Cross A
P1:
F1:
Phenotype: wt male
+
Genotype: X Y
+
Gametes: X , Y
Genotype
X+XW
XWY
F1 X F1: white eyes male
Genotype: XWY
Gametes:
XW, Y
white eyes female
XW XW
XW
Phenotype
wt female (carrier)
white eyes male
wild-type female
X+XW
X+, XW
Continued: Sex Linked for Cross A
Results of F1 X F1:
F2:
Female
X+XW
XW XW
Male
X+Y
XWY
Phenotype ratio: 1:1:1:1
25% female wt (red eyes)
25% female mutant (white eyes)
25% male wt (red eyes)
25% male mutant (red eyes)
Sex-linked Crosses

First, you must perform the F1 mating for the two sex-linked
crosses described above. See handout on how to perform the
crosses.

For your genetics problem set 2, YOU will follow the same
steps to predict the genotypic and phenotypic ratios for Cross
B.
 Cross B:
XWY
X
X+X+
white eye male
wild-type female

Perform Assignment V in Fly Lab On-line,

A complete assignment for genetics problem set B will be your
handwritten predictions for Sex-linked Crosses 1 and 2
performed in lab and your data generated from fly lab.