Honors Biology
Module 8
Mendelian Genetics Part 2
December 12, 2013
Class Challenge
Sharing the Christmas spirit…..What did you
do this past week?
Dec 19 Grades
Next week Semester grades will be given
out.
Today see me and Mrs Barwick to review
and post any missing work.
Thank you!
Collect the Lab books for
• Experiment 8.1 The Earlobe Pedigree
Quiz
Write a least one paragraph on Gregor
Mendel’s contribution to science in the
area of genetics.
Sex – Linked Genetic Traits
Humans have 23 homologous pairs of
chromosomes. The last pair is different,
they determine if the baby will be male or
female.
So we call the first 22 pairs of chromosomes
autosomes and the last pair sex
chromosomes.
If a person’s sex chromosomes are perfectly
homologous, XX pair, the person is a female.
In males, the sex chromosomes are not
homologous, there is a XY pair.
There are fewer genes on the second
chromosome (called the Y chromosome) than
there are on the X.
This causes an interesting effect called sex-linked
genetic traits.
See Punnett Square on Page 247
In meiosis I, the homologous pairs of
chromosomes are split up. Since females have
two X chromosomes, each cell at the end of
meiosis I has an X chromosome.
As a result, all gametes produced by a female
have an X chromosome. At the end of meiosis I
in the male, however, one cell has an X
chromosome and the other has a Y. As a result,
half of the gametes produced by the male will
have an X chromosome and half will have a Y.
50 / 50 chance
Half of the offspring will have XX
chromosomes and be female and
Half will have XY chromosomes and will be
male.
Sex-linked Genetic Traits
Exist because the Y chromosome does not
carry as many alleles as does the X
chromosome.
As a result, there are some traits for which
alleles only exist on the X chromosome
and not the Y chromosome.
What does that mean?
For certain genetic traits that reside on the sex
chromosome, males have only one allele
instead of two. This results in expressing the
recessive phenotype for such a trait is much
more likely for males.
If you have two alleles for a given genetic trait,
both recessive alleles must be present for your
to express the recessive phenotype.
For sex-linked traits, however, the male has only
one allele. The male needs only one recessive
allele to express the recessive phenotype.
Last week we looked at the
Fruit Fly, Genus Drosophila
We looked at a fruit fly under the microscope.
Most have red eyes, however a few have
white eyes and that phenotype is much
more likely among the Male than the
female.
The reason for this is that the eye color allele
for Drosophila exists on the X chromosome
but not on the Y chromosome.
So to Have White eyes…
a female must have two recessive alleles.
A male has only one allele to begin with. If
that allele is recessive, the male’s eyes will
be white.
When a trait is much more prevalent among
males than females, it is often governed
by a sex-linked, recessive trait.
Medical scientists often use this as a means
of trying to understand diseases and other
health problems.
Autism is a neurological disorder that affects
four times as many males as it does
females.
Researches think that autism has a genetic
component that is sex-linked. This has
caused some medical researchers to
concentrate on studying the differences in
X chromosomes between autistic and
nonautistic people, hoping that such
analysis will help them find the genetic
component to autism.
Punnett Square
Analyzing sex-linked characteristics, we can
still use a Punnett Square, but it is just a
little more involved.
Instead of representing an allele with a
capital letter when it is dominant and a
small letter when recessive, we note that it
exists only on the X chromosome.
Scientists use an X with a subscript. X^
Dr Wile’s Explaination 8.7
Chromosomal Genetics
http://youtu.be/rIe7mPXkYhs
Genetic Disorders and Diseases
There are at least 5 means by which genetic
abnormalities occur:
1. Autosomal inheritance
2. Sex-linked inheritance,
3. Allele Mutation
4. Changes in the Chromosome structure
5. Changes in the chromosome number
Autosomal Inheritance
Inheritance of a genetic trait not on a sex
chromosome.
A gene exists on an autosome that causes a
particular problem. Sometimes the genetic
disorder is recessive, and the offspring need to
have both recessive alleles in order to have the
disease. In other cases, the genetic disorder
comes from a dominate allele.
Ie: Lactose Intolerance p. 253
Genetic Disease Carries
A person who is heterozygous in a recessive
genetic disorder.
Ie: Hungington’s Disorder
Sex-Linked Inheritance
Inheritance of a genetic trait located on the
sex chromosome.
Ie: hemophilia
Exp. 8.3
Mutation
Is a radical chemical change in one or more
alleles.
Hutchinson-Gilford progeria Syndrome.
page 254
Change in Chromosome Structure
Is a situation in which a chromosome loses
or gains genes during meiosis.
When a chromosome has too many or too
few genes, the body does not understand
all of the genetic information. Genetic
disorders occur.
Ie: Cat’s-cry disease. Page 254
Change in Chromosome number
Is a situation in which abnormal cellular events in
meiosis lead to either none of a particular
chromosome in the gamete or more than one
chromosome in the gamete.
Down’s syndrome occurs when a person has three
of chromosome number 21. This happens
because either the father’s sperm or the
mother’s egg has two of that chromosome rather
than just one. This produces mental retardation.
(1 in 1000 newborn’s in North America)
page. 255.
Experiment 8.3
Queen Victoria and her extended
family
•
Hemophilia A is an inherited X-linked recessive disease,
characterized by improper clotting of the blood due to a deficiency
in Coagulation Factor VIII. Hemophilia was introduced into the
germ line of European royal families by a
"sporadic" mutation in Queen Victoria [circle & dot, III-2], who
passed the allele on to three of her children. The allele passed to the
Russian royal family through Victoria's granddaughter Tsarina
Alexandra [V-8] and her son (Tsarevitch Alexis [solid square, VI12] had it), as well as the Spanish royal family.
King Juan Carlos [VII-14] did not inherit it, though two of his uncles
did.
Alexis' hemophilia is historically significant. The concern of Tsar
Nicholas II for the Tsarevitch's health distracted him from difficulties
facing the Russian Empire before and during World War I, and
brought his wife under the influence of the monk, Rasputin.
The abdication of the Tsar in 1917 was materially affected by his belief
that the Alexis would be unable to succeed him. The power vacuum
contributed to the onset of the Bolshevik Russian Revolution of
1917. The book and movie "Nicholas and Alexandra" are an
historically accurate account.
Can you take Genetics to far?
Human Animal Hybrids – John Paul Jackson
Homework
Complete Pedigree Review worksheet.
Finish Reading Module 8
Finish OYO for Module 8
Finish Study Guide for Module 8
Class Quiz:
Class Challenge: