Download 122 lec 05 recomb sex link pedigree

Announcements
• Colloquium sessions for which you can get
credit posted on web site:
– Feb 20, 27
– Mar 6, 13, 20
– Apr 17, 24
– May 15.
• Review study CD that came with text for lab
this week (especially mitosis and meiosis).
Objectives
• Compare mitosis and meiosis.
• Recognize how chromosome movement during
meiosis results in Mendel's laws of Segregation
and Independent Assortment.
• Define Chromosomal Theory of Inheritance.
• Understand sex-linkage and why it supports the
Chromosomal Theory of Inheritance.
• Learn how to use pedigrees to track Mendelian
inheritance.
Figure 13.6 Overview of meiosis: how meiosis reduces chromosome number
Comparison of mitosis and meiosis
Event
Mitosis
Meiosis
# divisions
One
Two
Homologue pairing
None
Yes, Prophase
# daughter cells
Genetic composition
Two
identical
to parent
Four
differs from
parent
Chromosome number
same
as parent
asexual
reproduction,
etc.
1/2 that of
parent
gamete
formation
Role in life cycle
1
Figure 13.10 The results of alternative arrangements of two homologous
chromosome pairs on the metaphase plate in meiosis I
Recombination
• How does recombination occur?
– Independent orientation of chromosomes
# possibilities = 2n, where n is the haploid
chromosome number
– crossing over adds to possibilities
• Each offspring receives different genetic
material from its parent
Figure 15.1 The chromosomal basis of Mendel’s laws
Meiosis and inheritance (15.1)
• Each locus on a different chromosome
• Rule of Independent Assortment follows
from independent orientation at Metaphase I
• Rule of Segregation follows from separation
of homologues at Anaphase I
2
Meiosis and inheritance (15.1)
Figure 15.1 The chromosomal basis of Mendel’s laws
• Each locus on a different chromosome
• Rule of Independent Assortment follows from
independent orientation at Metaphase I
• Rule of Segregation follows from separation
of homologues at Anaphase I
Chromosomal theory of inheritance
• Traits inherited according to Mendel's laws
are on chromosomes
• Work on sea urchins supported this theory
– Scramble up chromosomes in eggs
– Misshapen sea urchins result
• Discovery of sex linkage, using fruit flies,
confirmed this theory
Drosophila as a model
organism for genetics
• Flies have short generation times (2 weeks)
• Easy to rear large numbers of flies
• Drosophila species have only four large
chromosomes
• Sex is determined genetically (females XX,
males XY)
3
Introduction to sex linkage
• Some traits are linked to genes that
determine sex
• Sex linked traits may occur in either
sex
• This is different than ‘sex-limited’
traits, which are only found in one sex.
The white eye locus in fruit flies
• At the turn of the 20th century, Morgan and
coworkers bred thousands of flies
searching for ones that differed from the
‘wild-type.’
• A white eyed ‘mutant’ male was discovered
and crossed with a red-eyed female.
Figure 15.2 Morgan’s first mutant
Allele naming in flies
• When a mutant is discovered, the locus is
named after the mutant phenotype (e.g.
w, the white eye locus)
• The ‘typical’ phenotype is called wild-type
– mutant allele = w
– wild-type allele = w+
4
Figure 15.3 results of parental cross
P generation
Results of first cross
Females homozygous
for ‘wild-type’ allele
Males have one copy
of ‘mutant’ allele
• F1 generation
– females all red eyes
– males all red eyes
F1 generation
Females heterozygous
• F2 generation
Males have
X chromosome with
‘wild-type’ allele
Y chromosome
– females all red eyes
– 1/2 males white eyes
– 1/2 males red eyes
Males are
‘heterogametic’ and
‘hemizygous’
Figure 15.3 Naming genotypes for sex-linked alleles
P generation
Xw+Xw+
F1 generation XwXw+
Figure 15.3 Naming genotypes for sex-linked alleles
X
XwY
P generation Xw+Xw+
X
XwY
X
Xw+Y
F1 generation XwXw+
X
Xw+Y
Xw Xw+
Xw+Xw+
Xw Y
Xw+Y
5
Reciprocal cross of white eye females
with red eye males
• F1 generation
– females all red eyes
– males all white eyes
• F2 generation
– 1/2 females white eyes
– 1/2 females red eyes
– 1/2 males white eyes
– 1/2 males red eyes
Diagram of reciprocal cross
P generation
Xw Xw
X
Xw+ Y
F1 generation
F2 generation
Xw+Xw
X
XwY
Xw+Xw
Xw Xw
Xw+Y
Xw Y
Conclusions
• Reciprocal crosses yield differing results
• Sex linked traits show criss-cross
inheritance
• The Y chromosome was associated with
males and not found in females
• The gene for eye color was on the X
chromosome
• This constitutes proof of the chromosomal
theory of inheritance
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