Honors Biology

Module 8

Mendelian Genetics Part 2

December 12, 2013

Class ChallengeSharing 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 numberIs 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, VI-12] 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.

Homework

Complete Pedigree Review worksheet.

Finish Reading Module 8

Finish OYO for Module 8

Finish Study Guide for Module 8

Class Quiz:

Class Challenge: