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
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
Chromosome Mapping and Gene Linkage
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.
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
15 map units
10 map units
5 map units
Crossover Frequency of Some Genes on Chromosome #6
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?
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.
Examples of sex linked traits.
a.Hemophilia - lack or deformity of blood clotting factor VII or IX.
b.Red Green colorblindness
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.
Brown eye color (B) is dominant to blue (b).
Eye color is carried on the X chromosome.
Homozygous dominant femaleXBXB (brown)
Heterozygous femaleXBXb (brown)
Homozygous recessive femaleXbXb (blue)
Dominant maleXBY (brown)
Recessive maleXbY (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
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.
- basic units are amino acids
- made up of 20 different amino acids which can be arranged to make an almost infinite amount of proteins.
Structure of DNA
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”…)
Structure of DNA
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.