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Chromosomal Theory The chromosomal theory is as follows: Chromosomes carry genes, the units of hereditary Paired chromosomes segregate during meiosis. Each sex cell or gamete has half the number of chromosomes found in a somatic cell

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Chromosomal Theory

The chromosomal theory is as follows:

  • Chromosomes carry genes, the units of hereditary
  • Paired chromosomes segregate during meiosis. Each sex cell or gamete has half the number of chromosomes found in a somatic cell

iii) Chromosomes sort independently during meiosis. Each gamete receives one of the pairs and that one chromosome has no influence on the movement of a member of another pair

iv) Each chromosome contains many different genes


Chromosome Mapping and Gene Linkage

  • A single chromosome contains many genes linked together and so does the other chromosome in the homologous pair.
  • The sequence of genes on each chromosome pair should match each other exactly.
  • Gene linkage reduces the chance for genetic recombination and variety among the offspring.
  • Parts of a chromosome holding many genes, may separate and switch places with the matching part of the other chromosome = crossing over.

The closer genes are to each other, the less likely they will separate during crossing over = linked genes.

  • Scientists use crossover frequencies on genes to determine their positions on chromosomes

eg.) if the crossover frequency of a gene is 5%, then the two genes are 5 map units apart.


Crossover frequency is determined by the following formula:

crossover % = number of recombinations x 100

total number of offspring

  • Gene markers are usually recessive genes that are easily observed in offspring and can be used to identify other genes found on the same chromosome.

By using crossover frequencies, we can determine gene maps.

  • Gene maps show the relative positions of genes on a chromosome (loci).
  • Gene maps are constructed by:

- ordering fragments of DNA

- studying chromosomal alterations

- performing crosses to see how frequently crossing over occurs between fragments.


Problem 1: 3 genes A, B, C

AB – 12% CB – 7% AC – 5%



12 map units

7 map units

5 map units

Problem 2: AB - 3% BC - 28% AC - 31%

31 map units




28 map units

3 map units


Problem 3:

Genes X Y Z

X - 10 15

Y 10 - 5

Z 15 5 -

15 map units




10 map units

5 map units


Crossover Frequency of Some Genes on Chromosome #6

Genes Cross-over Frequency

Diabetes(1) and Ovarian cancer (2) 21%

Diabetes (1) and RH blood group(3) 12%

Ragweed allergy (4) and RH blood group(3) 10.5

RH blood group(3) and ovarian cancer (2) 9%

Ragweed allergy (4) and ovarian cancer (2) 19.5

Hint: Start here


Sex Chromosomes

So far, what do you know about sex chromosomes?

  • In addition to their role in determining sex, the sex chromosomes, especially X chromosomes, have genes for many characters unrelated to sex.
  • We call these sex-linked alleles.

Female cells can differ from male cells in two ways:

1. Female cells show dark spots of chromatin (called Barr Bodies) during interphase, male cells do not.

2. Female cells contain 2 X chromosomes and males contain only one X chromosome.


The Y chromosome carries few genes.

There are very few genes on the Y chromosome that are common on the X chromosome, and because of that, little crossing over may occur between an X and a Y.


eg.) Calico cats

Male cats tend to be black (XBY) or orange (X0Y). Female cats can be black (XBXB), orange (X0X0) or calico (XBX0) – a mixture between black and orange.

Very few male cats can be calico, why?

Those who do, carry a hidden X chromosome, and are likely sterile.


A male embryo does not differ from a female fetus until the 6th/7th week of pregnancy.

  • At this point, the “testes determining factor” (TDF) gene on the Y chromosome is activated.
  • The TDF gene initiates the production of a protein that stimulates the testes to begin secreting male hormones.

Examples of sex linked traits.

a. Hemophilia - lack or deformity of blood clotting factor VII or IX.


c. Pattern baldness - sex influenced not sex-linked.

i. Humans carry two alleles for baldness.

ii. In females the allele for baldness is recessive but in males, due to testosterone, it is dominant.


We can also perform monohybrid crosses between sex chromosomes.

For example:

Brown eye color (B) is dominant to blue (b).

Eye color is carried on the X chromosome.

Homozygous dominant female XBXB (brown)

Heterozygous female XBXb (brown)

Homozygous recessive female XbXb (blue)

Dominant male XBY (brown)

Recessive male XbY (blue)


Draw a Punnett square for a cross between a heterozygous female with a recessive male. Calculate the phenotypic & genotypic ratios.









In the F1 generation:


Phenotypic ratios:

1 brown eyed girl: 1 brown eyed boy: 1 blue eyed girl: 1 blue eyed boy

Genotypic ratios:

1XBXb: 1XbXb: 1XBY: 1XbY


Example #2

Is it possible to get a blue eyed female from crossing a blue eyed female with a brown eyed male? Explain.









No it is not possible, all females would be browned eyed

dna the molecule of life
DNA the molecule of life
  • The nucleus of every cell of your body contains DNA deoxyribonucleic acid.
  • DNA is the only molecule known that is capable of replicating itself, thereby permitting cell division.
  • DNA provides the directions that guide the repair of worn cell parts and the construction of new ones.

DNA contains instructions that ensure continuity of life – offspring share structural similarities with those of their parents.

However, not all offspring are identical to their parents. Why?

New combinations of genes and mutations – a change in the DNA sequence, affect the uniqueness of descendants.

searching for the chemical of heredity
Searching for the Chemical of Heredity
  • Early 1940’s: biologists began to accept hypothesis that genetic material was found within chromosomes - long threads of genetic material found in nucleus of cells.

Chromosomes are composed of relatively equal amounts of proteins and nucleic acids.

Proteins (histones)

- basic units are amino acids

- made up of 20 different amino acids which can be arranged to make an almost infinite amount of proteins.


Nucleic Acids

  • basic unit is the nucleotide
  • - Nucleotides are made up of phosphates, sugar molecules and one of four different nitrogen bases: adenine, guanine, cytosine & thymine.

At this point in time it was thought that the key to the genetic code lied in the proteins. This hypothesis was logical, but incorrect.

  • 1950 – A chemist named Rosalind Franklin developed a way of using X-rays to take pictures of the DNA molecule.
  • Her pictures showed that DNA was shaped like a spiral, or helix.

1953 – James Watson & Francis Crick developed a 3-dimensional model of the DNA molecule.

  • This model is known as the double helix model and it resembles a twisted ladder.




Structure of DNA

  • The uprights of the DNA “ladder” are composed of phosphates & sugars.

a. The bases form the “rungs” of the ladder

b. The sugar and phosphate form the “uprights”


Did you know?

Human DNA is 3 billion base pairs and about 2 m in length.


The two purines are adenine and guanine

Double ring structures

The two pyrimidines are thymine and cytosine

Single ring structures

Cytosine pairs with guanine

Adenine pairs with thymine


Nucleotides are complementary.

a. A pyrimidine pairs with a purine

Thymine with Adenine

Cytosine with Guanine

b. Bases are held together with relatively weak hydrogen bonds (They can “unzip”…)



Gene Therapy: when defective genes are replaced with normal genes in order to cure genetic diseases

Human Genome Project: to determine the complete sequence of the 3 billion DNA subunits (bases), identify all human genes, and make them accessible for further biological study.