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Sex Chromosomes
So far, what do you know about sex
chromosomes?
In addition to their role in determining sex, the sex
chromosomes, especially X chromosomes, have genes
for many characters unrelated to sex.
We call these sex-linked alleles.
Female cells can differ from male cells in two ways:
1. Female cells show dark spots of chromatin
(called Barr Bodies) during interphase, male
cells do not.
2. Female cells contain 2 X chromosomes and
males contain only one X chromosome.
The Y chromosome carries few genes.
There are very few genes on the Y
chromosome that are common on the X
chromosome, and because of that, little
crossing over may occur between an X
and a Y.
eg.) Calico cats
Male cats tend to be black (XBY) or orange
(X0Y). Female cats can be black (XBXB),
orange (X0X0) or calico (XBX0) – a mixture
between black and orange.
Very few male cats can be calico, why?
Those who do, carry a hidden X
chromosome, and are likely sterile.
• A male embryo does not differ from a female fetus until
the 6th/7th week of pregnancy.
• At this point, the “testes determining factor” (TDF)
gene on the Y chromosome is activated.
• The TDF gene initiates the production of a protein that
stimulates the testes to begin secreting male hormones.
Examples of sex linked traits.
a. Hemophilia - lack or deformity of
blood clotting factor VII or IX.
b. Red Green colorblindness
c. Pattern baldness - sex influenced not sexlinked.
i. Humans carry two alleles for
baldness.
ii. In females the allele for baldness is
recessive but in males, due to
testosterone, it is dominant.
We can also perform monohybrid crosses between
sex chromosomes.
For example:
Brown eye color (B) is dominant to blue (b).
Eye color is carried on the X chromosome.
Homozygous dominant femaleXBXB (brown)
Heterozygous female
XBXb (brown)
Homozygous recessive female XbXb (blue)
Dominant male
XBY (brown)
Recessive male
XbY (blue)
Draw a Punnett square for a cross between a heterozygous
female with a recessive male. Calculate the phenotypic &
genotypic ratios.
XB
Xb
Xb
XBXb
XbXb
Y
XBY
In the F1 generation:
XbY
Phenotypic ratios:
1 brown eyed girl: 1 brown eyed boy: 1 blue
eyed girl: 1 blue eyed boy
Genotypic ratios:
1XBXb: 1XbXb: 1XBY: 1XbY
Example #2
Is it possible to get a blue eyed female from crossing a blue
eyed female with a brown eyed male? Explain.
Xb
Xb
XB
XBXb
XBXb
Y
XbY
XbY
No it is not possible, all females would be browned eyed
Chromosomal Theory
The chromosomal theory is as follows:
i) Chromosomes carry genes, the units of hereditary
ii) Paired chromosomes segregate during meiosis. Each
sex cell or gamete has half the number of
chromosomes found in a somatic cell
iii) Chromosomes sort independently during meiosis.
Each gamete receives one of the pairs and that one
chromosome has no influence on the movement of a
member of another pair
iv) Each chromosome contains many different genes
Chromosome Mapping and Gene Linkage
• A single chromosome contains many genes
linked together and so does the other
chromosome in the homologous pair.
• The sequence of genes on each chromosome
pair should match each other exactly.
• Gene linkage reduces the chance for genetic
recombination and variety among the offspring.
• Parts of a chromosome holding many genes,
may separate and switch places with the
matching part of the other chromosome =
crossing over.
• The closer genes are to each other, the less
likely they will separate during crossing over =
linked genes.
• Scientists use crossover frequencies on genes to
determine their positions on chromosomes
eg.) if the crossover frequency of a gene is
5%, then the two genes are 5 map units
apart.
• Crossover frequency is determined by the
following formula:
crossover % = number of recombinations x 100
total number of offspring
• Gene markers are usually recessive genes that
are easily observed in offspring and can be used
to identify other genes found on the same
chromosome.
• By using crossover frequencies, we can
determine gene maps.
• Gene maps show the relative positions of
genes on a chromosome (loci).
• Gene maps are constructed by:
- ordering fragments of DNA
- studying chromosomal alterations
- performing crosses to see how frequently
crossing over occurs between fragments.
Problem 1: 3 genes A, B, C
AB – 12%
CB – 7%
A
AC – 5%
12 mapB units
C
5 map units
7 map units
Problem 2: AB - 3%
BC - 28% AC - 31%
31 map units
A
B
28 map units
3 map units
C
Problem 3:
Genes X Y Z
X 10
15
Y 10 5
Z 15 5
-
X
15 map units
10 map units
Z
Y
5 map units
Crossover Frequency of Some Genes on
Chromosome #6
Genes
Cross-over Frequency
Diabetes(1) and Ovarian cancer (2)
21%
Diabetes (1) and RH blood group(3)
12%
Ragweed allergy (4) and RH blood group(3)
RH blood group(3) and ovarian cancer (2)
Ragweed allergy (4) and ovarian cancer (2)
Hint: Start here
10.5
9%
19.5
Transposons (see handout)
Gene Therapy: when defective genes are replaced
with normal genes in order to cure genetic diseases
Human Genome Project: to determine the complete
sequence of the 3 billion DNA subunits (bases),
identify all human genes, and make them accessible
for further biological study.