Post on 01-Jan-2016
(CHAPTER 2- Brooker Text)
Linked Genes
& Probability
Nov 13 & 18, 2008BIO 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 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
PROBABILITY
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
(O – E)2
E
• where O = observed data in each category
E = observed data in each category based on the experimenter’s hypothesis
= Sum of the calculations for each category
2-75
(CHAPTER 3 & 5- Brooker Text)
Sex-Linked Transmission
BIO 184Dr. 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 X-
linked 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
Y-linked gene
Involved in antibody production
Contains many X-linked genes
Necessary for proper male developmentFollows a pseudoautosomal
pattern of inheritance