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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

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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 ofChromosome 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
the chi square test
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 test1
The Chi Square Test
  • The general formula is

(O – E)2

c2 =S


  • 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


Sex-Linked Transmission

(CHAPTER 3 & 5- Brooker Text)

BIO 184

Dr. Tom Peavy

sex determination
Sex Determination
  • In many animal species, chromosomes play a role in sex determination
  • Homogametic vs Heterogametic
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
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
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

Necessary for proper male development

Follows a pseudoautosomal pattern of inheritance