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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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The chromosomal theory is as follows:
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
The closer genes are to each other, the less likely they will separate during crossing over = linked genes.
eg.) if the crossover frequency of a gene is 5%, then the two genes are 5 map units apart.
crossover % = number of recombinations x 100
total number of offspring
- ordering fragments of DNA
- studying chromosomal alterations
- performing crosses to see how frequently crossing over occurs between fragments.
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
Genes X Y Z
X - 10 15
Y 10 - 5
Z 15 5 -
15 map units
10 map units
5 map units
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
So far, what do you know about sex chromosomes?
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.
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.
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.
a. Hemophilia - lack or deformity of blood clotting factor VII or IX.
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.
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:
1 brown eyed girl: 1 brown eyed boy: 1 blue eyed girl: 1 blue eyed boy
1XBXb: 1XbXb: 1XBY: 1XbY
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 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.
Chromosomes are composed of relatively equal amounts of proteins and nucleic acids.
- basic units are amino acids
- made up of 20 different amino acids which can be arranged to make an almost infinite amount of proteins.
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.
1953 – James Watson & Francis Crick developed a 3-dimensional model of the DNA molecule.
a. The bases form the “rungs” of the ladder
b. The sugar and phosphate form the “uprights”
Human DNA is 3 billion base pairs and about 2 m in length.
Double ring structures
The two pyrimidines are thymine and cytosine
Single ring structures
Cytosine pairs with guanine
Adenine pairs with thymine
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.