Sex Determination: Inheritance of Sex-Linked Traits
In higher plant and animals, the cells which make up the body cells of the adult
are usually diploid and referred to as somatic cells. The gametes which are produced in
the sex organs are always haploid.
Fill in the following chart and place in your notes.
| Terms |
Description of terms |
| Somatic Cells |
|
| Gametes |
|
| Diploid |
|
| Haploid |
|
Read the pages in you text describing sex determination and sex linkage. Study behavioral objectives 14, 15 and 16. Sex determination in
mammals and some insects, is determined by the presence of a special pair of chromosomes
called sex chromosomes. In males there are 22 homologous chromosome pairs and two
essentially unpaired chromosomes of different sizes carrying different genes. These
unpaired chromosomes (sex chromosomes) may be indicated by
XY. The human male cell contains 44 regular chromosomes (autosomes)
and two sex chromosomes. The human female cell contains 23 homologous chromosome pairs.
The female cells contain 44 autosomes and two sex chromosomes. These pair of chromosomes
are usually indicated by XX. The early embryological differentiation of males and
females is apparently based on the presence or absence of the Y chromosome. The presence
of the Y chromosome results in the production of the combination of phenotypic
characteristics of the male. Absence of the Y chromosome results in the physical
development of a phenotypic female. For the most part the genes on the Y chromosome are
nonfunctional. The Y carries relative few genes other than those determining maleness.
Example: The gene which produces testosterone is located on the Y chromosome. Read the
pages concerning sex linked traits and character and study behavioral
objectives 14, 15 and 16.
An individual with a XX chromosome will develop into a female and an individual
with a XY chromosome pair will develop into a male.
1. How many autosomes are present in a human male's somatic cell?
Answer as follows:
The human male's somatic cell contains 44 autosomes and
two sex chromosomes.
2. How many autosomes are present in a human male's gamete (sperm)?
Answer as follows:
The human male's gamete contains 22 autosomes and two
unpaired chromosomes.
3. Which of the following chromosome combinations would be found in a female's
somatic cell?
a) 22 autosomes and 1 X chromosome
b) 44 autosomes, 1 X and 1 Y chromosome
c) 46 autosomes
d) 44 autosomes and 2 X chromosomes
e) 22 autosomes and 2 X chromosomes
Answer as follows:
The correct answer is "d".
4. Which of the following chromosome combinations would be found in a male's
diploid somatic cell?
a) 22 autosomes and 1 X chromosome
b) 44 autosomes, 1 X and 1 Y chromosome
c) 46 autosomes
d) 44 autosomes and 2 X chromosomes
e) 22 autosomes and 2 X chromosomes
Answer as follows:
The correct answer is "b".
5. Which of the following chromosome combinations would be found in a female's
gamete (egg)?
a) 22 autosomes and 1 X chromosome
b) 44 autosomes, 1 X and 1 Y chromosome
c) 46 autosomes
d) 44 autosomes and 2 X chromosomes
e) 22 autosomes and 2 X chromosomes
Answer as follows:
The correct answer is "a".
6. Which of the following chromosome combinations would be found in a male's
haploid cell (sperm)?
a) 22 autosomes and 1 X chromosome
b) 44 autosomes, 1 X and 1 Y chromosome
c) 46 autosomes
d) 44 autosomes and 2 X chromosomes
e) 22 autosomes and 2 X chromosomes
Answer as follows:
The correct answer is b.
During meiosis in females since the somatic cells all
contain XX chromosomes, the gametes (eggs) will all contain a single X chromosome. During
meiosis in males since the somatic cells all contain XY
chromosomes, the gametes (sperm) will contain a single X chromosome and a single Y
chromosome.
7. Work the problem showing the segregating of the X and Y chromosomes in humans.
XX x XY
Answer as follows:
| //////////// |
X |
Y |
| X |
XX |
XY |
| X |
XX |
XY |
8. Who determines the sex of the child?
Answer as follows:
The Y chromosome contributes to determination of
the sex of the child.
Remember these are entire chromosomes and not genes that controls the sex of an
individual. These chromosomes contain genes which may be passed on to their offspring.
These type of problems are called sex-linked problems. Sex
linked problems are worked like the monohybrid problems you worked earlier. Keep in mind
that when you work six-linked problems the Y chromosome is nonfunctional.
Example: Red/green color blindness is a sex linked trait. Individual who are not red/green
color blind (normal) contain a dominant allele (N) for this trait. Individuals who are
red/green color blind are homozygous recessive for this trait. Since these genes are
located on the X and Y chromosome the genotypes may be written as follows. Note: Alleles
are not placed on the Y chromosome because they are nonfunctional. The combination of X
and Y determines the sex and the superscript allele
represents the gene associated with the sex chromosomes.
XNXN is a female
normal for red/green color blindness (not color blind).
XNXn is a
female normal for red/green color blindness (not color blind);
however, she is a carrier for this allele.
XnXn is
a female who is red/green color blinded.
XNY is a male
normal for red/green color blindness (not color blind).
XnY is
a male who is red/color blinded.
9. Can males be heterozygous for sex linked traits?
Answer as follows:
No. Only the "X" chromosome is active,
therefore; whatever allele is located on the "X" chromosome will be present.
10. In fruit flies (Drosophila) the gene for red or white eyes is located on the
sex chromosome. Red (RR or Rr) is dominant to white (rr). State the possible genotypes of
both parents when a red-eyed male Drosophila is crossed with a red-eyed female and
the resulting offspring include:
19 red-eyed males
11 red-eyed females
10 white-eyed males
Answer as follows:
XX = female, XY = male
19 red eyed males = XRY
The XR had to come from the female parent.
11 red eyed females = XRX-
Since all females have red eyes the male must have a
XR allele.
10 white eyed males = XrY and
XRY Since there are both red eye and white
eye males the mother must be heterozygous.
Parents are: XRXr x
XRY
11. In fruit flies (Drosophila) the gene for red or white eyes is located on the
sex chromosome. Red (RR or Rr) is dominant to white (rr). State the possible genotypes of
both parents when the following F1 offspring were observed:
10 red-eyed males
9 red-eyed females
9 white-eyed males
8 white-eyed females
Answer as follows:
XRXr x
XrY
12. Normal vision (C) is dominant over color blind (c). This gene is located on
the sex chromosome. A normal vision man and woman produce three boys, one of which is
color blind and the other two with normal vision. What are the genotypes of the parents.
Answer as follows:
XCXc x
XCY
Work Punnett square to check.
13. The disease hemophilia is caused by a recessive allele
carried on the X chromosome. What are the possible genotypes of a father when all his sons
are hemophiliacs?
Answer as follows:
Can not be determined. Only the mother passes sex-linked to the
boys.
14. Consider the following parents: XHXh x
XhY
What percent of the male's sperm contain the Xh allele?
Answer as follows:
50% will contain the chromosome with the Xh alleles and
50% will contain the Y chromosome.
15. Consider the following parents: XHXh
x XhY
What percent of the female's eggs contain the dominant allele?
Answer as follows:
| \\\\\\\\\\\\\\\\\\\\\\\ |
XH |
Xh |
| Xh |
XHXh |
XhXh |
| Y |
XHY |
XhY |
50% of the females eggs will contain XH
alleles.
16. Consider the following parents: XHXh
x XhY
What are the chances for these two parents to have a boy with hemophilia?a girl with
hemophilia?
Answer as follows:
| \\\\\\\\\\\\\\\\\\\\\\\ |
XH |
Xh |
| Xh |
XHXh |
XhXh |
| Y |
XHY |
XhY |
The changes of having a boy with hemophilia is 25% and to
having a girl with hemophilia is 25%.
This is the end of the lesson involving sex linkage. In the next lesson you will
learn that many genes may be located on the same chromosome. You work the problems the
same way as demonstrated in the lesson involving monohybrid crosses, however two or more
genes will segregate out on the same chromosome. Go back to the home page and go to
the lesson, Linked Genes and Crossing-over.
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