Download Brooker Chapter 2

Linked Genes
& Probability
(CHAPTER 2- Brooker Text)
Nov 13 & 18, 2008
BIO 184
Dr. Tom Peavy
• Scientific basis for chromosome theory of inheritance
– 1. Analysis of the transmission of traits from parent
to offspring
– 2. Inquiry into the material basis of heredity
– 3. Microscopic examination of the processes of
mitosis, meiosis and fertilization
Principles of Chromosome Theory of Inheritance
– 1. Chromosomes contain the genetic material
– 2. Chromosomes are replicated and passed along from
parent to offspring
– 3. The nuclei of most eukaryotic cells contain chromosomes
that are found in homologous pairs
– 4. During the formation of gametes, different types of
(nonhomologous) chromosomes segregate independently
– 5. Each parent contributes one set of chromosomes to its
offspring
PROBABILITY
• Probability calculations are used in genetic problems
to predict the outcome of crosses
• To compute probability, we can use three
mathematical operations
– Product rule
– Sum rule
– Binomial expansion equation
The Chi Square Test
• A statistical method used to determine
goodness of fit
– Goodness of fit refers to how close the observed
data are to those predicted from a hypothesis
• Note:
– The chi square test does not prove that a
hypothesis is correct
• It evaluates whether or not the data and the hypothesis
have a good fit
The Chi Square Test
• The general formula is
c2 = S
(O – E)2
E
• where
O = observed data in each category
E = observed data in each category based on the
experimenter’s hypothesis
S = Sum of the calculations for each category
2-75
Sex-Linked Transmission
(CHAPTER 3 & 5- Brooker Text)
BIO 184
Dr. Tom Peavy
Sex Determination
• In many animal species, chromosomes play
a role in sex determination
• Homogametic vs Heterogametic
• The Y chromosome determines
maleness
• The sex chromosomes are designated Z and W to
distinguish them from the X and Y chromosomes of
mammals
• W chromosome determines femaleness
• In some insects,
– Males are XO and females are XX
• In other insects (fruit fly, for example)
– Males are XY and females are XX
• The Y chromosome does not determines maleness
• Rather, it is the ratio between the X chromosomes
and the number of sets of autosomes (X/A)
– If X/A = 0.5, the fly becomes a male
– If X/A = 1.0, the fly becomes a female
• Males are known as the drones
– They are haploid
– Produced from unfertilized eggs
• Females include the worker bees and queen bees
– They are diploid
– Produced from fertilized eggs
Transmission of Genes Located
on Human Sex Chromosomes
• Genes that are found on one of the two
types of sex chromosomes but not on both
are termed sex-linked
– Indeed, sex-linked and X-linked tend to be used
synonymously, but there are Y-linked genes
• Males have only one copy of the X
chromosome
– They are said to be hemizygous for their Xlinked genes
Reciprocal crosses
• Crosses between different strains in which
the sexes are reversed
• These crosses reveal whether a trait is
carried on a sex chromosome or an
autosome
– X-linked traits do not behave identically in
reciprocal crosses
In poultry, sex is determined by the ZW rather than the XY system.
A dominant sex-linked gene (B) produces barred feathers, and the
Recessive allele (b), when homozygous produces nonbarred feathers.
Suppose a nonbarred male is crossed with a barred female.
What will be the appearance of the F1 birds?
• The X and Y chromosomes also contain
short regions of homology at one end
– These promote the necessary pairing of the two
chromosomes in meiosis I of spermatogenesis
• The few genes found in this homologous
region follow a pseudoautosomal pattern of
inheritance
– Their inheritance pattern is the same as that of a
gene found on an autosome
Contains many
X-linked genes
Involved in
antibody
production
Y-linked gene
Follows a pseudoautosomal
pattern of inheritance
Necessary for proper
male development