(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