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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: will separate during crossing over =

    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.


  • 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 will separate during crossing over =

AB – 12% CB – 7% AC – 5%

A B

C

12 map units

7 map units

5 map units

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

31 map units

A

C

B

28 map units

3 map units


Problem 3: will separate during crossing over =

Genes X Y Z

X - 10 15

Y 10 - 5

Z 15 5 -

15 map units

Z

X

Y

10 map units

5 map units


Crossover Frequency of Some Genes on Chromosome #6 will separate during crossing over =

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 will separate during crossing over =

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: will separate during crossing over =

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 will separate during crossing over = 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 will separate during crossing over =

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.



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.

XB

Xb

XBXb

XbXb

Xb

XBY

XbY

Y

In the F1 generation:


Phenotypic ratios: female with a recessive male. Calculate the phenotypic & genotypic 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 female with a recessive male. Calculate the phenotypic & genotypic ratios.

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

Xb

Xb

XBXb

XBXb

XB

XbY

XbY

Y

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


Dna the molecule of life
DNA the molecule of life female with a recessive male. Calculate the phenotypic & genotypic ratios.

  • 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.


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 – offspring share structural similarities with those of their parents.

  • 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.


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 – offspring share structural similarities with those of their parents.

  • basic unit is the nucleotide

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


  • 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.


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

Watson

Crick


Structure of DNA 3-dimensional model of the DNA molecule.

  • 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? 3-dimensional model of the DNA molecule.

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


The two purines are 3-dimensional model of the DNA molecule. 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 3-dimensional model of the DNA molecule. 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”…)


Structure of DNA 3-dimensional model of the DNA molecule.


Transposons 3-dimensional model of the DNA molecule.

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.



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