Transcript The Chromosomal Basis of Inheritance

Please do the following activities on the student media (Chapter 15):
- MP3 tutor: Chromosomal Basis of Inheritance
- Activity: Sex-Linked Genes
- Activity: Linked Genes and Crossing Over
CONCEPT 3: APPLYING THE
CHROMOSOMAL BASIS OF
INHERITANCE TO ANALYZE THE EFFECTS
OF SEX CHROMOSOMES, LINKED
GENES, AND RECOMBINED
RECOMBINES GENES
Campbell: Chapter 15
Holtzclaw: pg 108 - 110
Learning Intentions
Goal: to analyze mechanisms of chromosomal
inheritance

You must know:
 How
the chromosome theory of inheritance connects the
physical movement of chromosomes in meiosis to
Mendel’s laws of inheritance
 The unique pattern of inheritance in sex-linked and
linked genes
Also:
 Lab 3
 The
roles of segregation, independent assortment, and
crossing over in generating genetic variation
 How to calculate map distance from experimental data
 Lab
7
 How
to use data to determine the mode of transmission and
genetic make-up of the parents
 How to use a Punnett square to verify your conclusions
Remember...Chromosome Theory of Inheritance

What is the relationship between a pair of alleles
and a homologous pair of chromosomes?
Remember...Chromosome Theory of Inheritance


Alleles are located on chromosomes.
The homologous pair of chromosomes code for the
same genes, each having one allele.
Homologous chromosomes
Remember...Chromosome Theory of Inheritance

Since genes have specific locations on chromosomes
(loci), when chromosomes segregate and assort
independently so do the genes!
Sex-Linked Genes

A sex linked gene is a gene located on the X sex
chromosome.
Would this individual
be a male or a
female? How do you
know?
Read pages
109-110 in
Holtzclaw
Morgan’s Sex-Linked Discovery in 1910!
He got a Nobel Prize...
He’s kind of a big deal.
Thomas Hunt Morgan
1866-1945
The Fly Room!
Conclusion?
The gene for
fly eye colour
is found on
the “X” sex
chromosome!
Sex-Linked Genes – Checking In...



Can Fathers pass sex-linked genes to their sons?
For males, do the terms homozygous and
heterozygous apply?
Do most genes on the “X”
sex chromosomes have
to do with sex?
Sex-Linked Genes



NO. Fathers pass sex-linked genes to their
daughters but not their sons.
NO. For males, the terms homozygous and
heterozygous do not apply for sex-linked genes
since males only have one copy of the gene.
NO. Most genes on the “X”
sex chromosomes have
nothing to do with sex.
Colour Blindness: Sex-Linked Gene

Can you read the number?
Colour Blindness: Sex-Linked Gene

What happens if you cross a normal female with a
colour blind male?
Colour Blindness: Sex-Linked Gene
All females are carriers
All males are normal
Colour Blindness: Sex-Linked Gene

What happens if you cross a carrier female with a
normal male?
Colour Blindness: Sex-Linked Gene
½ females are carriers
½ females are normal
½ males are colour-blind
½ males are normal
Colour Blindness: Sex-Linked Gene

What happens if you cross a carrier female with a
colour-blind male?
Colour Blindness: Sex-Linked Gene
½ females are colour blind
½ females are carriers
½ males are colour blind
½ males are normal
Colour Blindness: Sex-Linked Gene
All females are carriers
½ females are carriers
and ½ males are
colour-blind
½ females are
colour blind, ½
females are carriers.
½ males are
colourblind
Other examples of Sex-Linked Genes:

Duchene Muscular Dystrophy
 Progressive
muscle weakening,
loss of coordination

Hemophilia
 Reduced
ability to clot blood
Try This!

Neither Tom nor Rhonda has muscular distrophy, but
their firstborn son has it.
 What
is the probability that a second child will have
this disease?
 What
if the child was a boy?
 A girl?
Try This!

Neither Tom nor Rhonda has muscular distrophy, but
their firstborn son has it.
 What
is the probability that a second child will have
this disease?
 What
if the child was a boy?
 A girl?
Try This!

Neither Tom nor Rhonda has muscular distrophy, but
their firstborn son has it.
 What
is the probability that a second child will have
this disease? 1/4
 What
if the child was a boy? 1/2
 A girl? 0, but ½ carrier
X-Inactivation

Since females get two copies of the “X” sex
chromosomes, they need to inactivate one of them!
 Done
by adding methyl groups to DNA...
Bunches up the chromosome
 Happens randomly in each cell when the female is an
embryo
 Inactive chromosome is called
a Barr body
Example of X-Inactivation: Calico Cats!
The kitten on the left “CC” is an identical clone of the cat on
the right “Rainbow”. How can you explain the different
colouring?
http://scientificcuriosity.blogspot.com/2006/09/that-cute-kitten-is-copycat-or-cc-first.html
CC (copy cat) was born in 2001. She is the first cloned cat.
Discuss...

During early embryonic development of female
carriers for colour blindness, the normal allele is
inactivated by chance in about half the cells. Why,
then, aren’t half of female’s colour blind?
Discuss...

During early embryonic development of female
carriers for colour blindness, the normal allele is
inactivated by chance in about half the cells. Why,
then, aren’t half of female’s colour blind?
 Eye
cells must come from multiple cells in early embryo
 Having ~1/2 of mature eye cells expressing proteins
for colour detection must be enough for normal colour
vision
Try This!

Handout with questions...
 “Sex
Linked Practice”