4.3 : Theoretical Genetics
The instructions for making proteins in a cell are carried in the cell's DNA. The sequence of bases in the DNA determines the sequence of amino acids in the protein. A length of DNA which codes for one protein is called a gene. A gene coding for a specific protein may have several forms, or alleles. . Each allele has a different base sequence, and so produces a different protein
By coding for the kinds of proteins made in a cell, genes influence everything which takes place in the cell, and in the organism as a whole.
The study of inherited characteristics, and the way they are passed on from one generation to another, is called genetics. Our knowledge of the subject is expanding rapidly, and this knowledge is dependent on an understanding of the molecule of DNA. Mendelian inheritance looks at the transmission of hereditary characteristics from parent to their children; it underlies much of genetics. They were initially derived from the work of Gregor Mendel published in 1865 and 1866 which was "re-discovered" in 1900, and were initially very controversial. When they were integrated with the chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics.
Causes of variation
The overall appearance of an organism is a result of the characteristics that it has inherited from its parents and the characteristics that result from the effects of the environment. Permanent characteristics that can be inherited are due to the genetic make up of the organism. This genetic make up may be altered due to mutation or as a result of sexual reproduction.
Mutation
A mutation is a change in the type or amount of DNA and can be a result of:
i. Mistakes in the copying of DNA
ii. Damage to the DNA
iii. Uneven distribution of chromosomes during cell division
Sexual Reproduction
Sexual reproduction mixes up genetic material in three ways:
i. Crossing over – exchange of genetic material, between homologous pairs, during meiosis.
ii. Independent assortment – homologous pairs may line up in different ways (random) during meiosis
iii. Any male gamete can fertilise any female gamete during fertilisation.
The combined effects of mutation and sexual reproduction lead to an enormous amount of variation between individuals.
Inheritance of characteristics
Chromosomes carry genetic information as a series of genes ie flower colour. Each chromosome in the nucleus of a diploid organism has a partner that carries the same genes. Such a pair of chromosomes is called a homologous pair. Each chromosome in a pair may carry alternative forms of the same gene. These alternative forms are called alleles. The position of a gene on homologous chromosomes is termed its locus.
For example, the gene for eye colour has alleles that code for blue or brown. If both alleles are found in a particular cell nucleus, then the cell is heterozygous for that characteristic. If the nucleus carries the same allele on both members of the homologous pair, then the cell is homozygous.
If an organism is heterozygous, the characteristic displayed by the organisms will depend on which allele is dominant and which one recessive. The dominant allele has the same effect on the phenotype whether it is present in the homozygous or heterozygous state. The recessive allele only has an effect on the phenotype when present in the homozygous state.
Genotype – the alleles of an organism Phenotype – the characteristics of an organism
A: Bb Brown
B: BB Brown
C: bb Blue
Patterns of inheritance
Scientists called geneticists study the inheritance of characteristics by carrying out breeding experiments. There is a conventional pattern for describing the results of such experiments, a sort of genetic shorthand.
The diagram on the left shows a cross between a purple grained and white grained plant. The chromosomes are drawn with the alleles attached, in this case G and g. Gametes are then shown (notice the reduction in chromosome number), and then the offspring genotype and phenotype is given. In this case all offspring have the genotype Gg and the phenotype purple
Monohybrid crosses
A genetic cross, like the one shown above, in which a single pair of alleles is transmitted to the next generation , is called a monohybrid cross. The first information about inheritance came from the monohybrid crosses carried out by an Austrian Monk called Gregor Mendel (1822 -84). Modern genetics is based on his results.
The pictures below show a cross between a homozygous purple flowered and homozygous white flowered plant. All the F1 generation have purple flowers, thus showing purple to be dominant. If the purple flowers are selfed (crossed with themselves) then 25% of the F2 will be white.
The format used to carry out the cross is called a Punnet square after the British scientist Reginald Crundall Punnett (1875 – 1967)
The Test Cross
It might be important to know whether an organism is heterozgous or homozygous for a trait, particularly in the breeding of domestic animals. To do this, geneticists use a test cross. The principle is outlined on the diagram on the left.
In this case a purple flowered plant of unknown genotype is crossed with a homozygous recessive individual. From the phenotypes of the offspring produced geneticists can work out the genotype of the unknown purple flowered plant.
100% purple means unknown was homozygous.
50% purple, 50% white means the unknown was heterozygous.
Co-dominance
When neither alleles is dominant both alleles will have an effect on the heterozygous phenotype. This sort of inheritance is called codominance and in this case the two alleles can produce three phenotypes.
In human blood groups A is co-dominant to B, but both are dominant of O.
Sex Determination
The photograph below shows a karyotype for a complete set of male and female human chromosomes . The two chromosomes labelled X and Y are the sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).
· It is the presence of absence of the Y chromosome which decides the sex of the individual. Therefore it is the male who determines the sex of the offspring.
· Sex chromosomes carry genes concerning with sexual development.
· Sex chromosomes also carry a few genes that are not concerned with sex. These genes are called sex linked genes and may only show their characteristic in one of the two sexes.
An example of a sex linked gene is the hairy ear gene . This is carried on the Y chromosome, therefore it only occurs in males.
Sex Linked inheritance
One well known sex linked characteristic is colour blindness. This is a disease in which sufferers have difficulty distinguishing between colours, particularly green and blue. The gene for this disease is recessive and is carried on the X chromosome. Because of this it is a disease which occurs much more often in males than females. Test yourself for colour blindness.
Online tutorial and questions
http://www.biology.arizona.edu/mendelian_genetics/problem_sets/monohybrid_cross/01t.html
Pedigree charts
This is a diagram of a family tree over several generations showing the descendants from particular ancestors, their relationships, and the presence or absence of the trait in all the members. In the charts the males are represented by squares, the females as circles. Shading indicates the incidence of the particular phenotype under investigation. Solid shading shows diseased individuals, half shaded shows carriers, unfilled objects show healthy individuals.Each generation is set out along one line of the page, the birth sequence of the family running from left to right. Succeeding generations are shown on following lines.
Analysis of the pedigree chart enables us to detect the difference between a trait that is dominant from one that is recessive. A dominant trait tends to occur in members of every generation. A recessive trait is seen infrequently, often skipping one or more generations.
Albinism is due to a recessive mutation in a single gene causing a block in the body’s biochemical pathway that produce the pigment melanin. An albino has white hair, very light skin and pink eyes. (picture below shows the black albino and reggae star ‘Yellowman Johnson’)
Brachydactyly is due to a dominant mutation, and causes shortness of fingers and toes.
Haemophilia is a sex linked recessive trait. It is an inherited disorder that impairs the body’s ability to control blood clotting. It is found on the X chromosome and therefore occurs more frequently in males. Below is a pedigree chart for the British Royal family