Genetics, revised edition. Karen Vipond
Chromosomes are the body’s genetic material as they possess all four characteristics.
Figure 1.24 Expression of stored information
| ACTIVITIES 1.7 AND 1.8 |
1.7. Explain and contrast a chromosome and a gene.
1.8. Identify the role of the following cellular components in the storage, expression and transmission of genetic information:
• chromatin;
• nucleus;
• ribosome;
• mitochondrion;
• centromere.
| SUMMARY |
• Cells are made up of organelles and chromosomes.
• Chromosomes are composed of DNA that encodes for proteins.
• There are 23 pairs of chromosomes in the nucleus of every cell (46 in total). There are 44 autosomes (22 pairs) and 2 sex chromosomes (X and Y).
• 23 individual chromosomes are inherited from each parent.
• Cells replicate to produce identical cells by mitosis. To halve the chromosomal number in germ cells, the cells replicate by meiosis.
• There are four different bases included in the DNA structure. A sequence of three bases (a codon) code for one amino acid. Amino acids link together to form protein.
• RNA is needed to copy and carry the genetic code out of the nucleus and to assemble the amino acid chain within the cytoplasm.
• Proteins are assembled following transcription and translation of the genetic code.
• Mitochondria have their own genome, although most mitochondrial proteins are coded for by the nuclear genome.
FURTHER READING
There are many good physiology texts that have a whole chapter dedicated to the biology of the cell.
Marieb, E. and Hoehn, K. (2006) Human anatomy and physiology. Harlow: Pearson International
Martini, F.H. and Nath, J.L. (2008) Fundamentals of anatomy and physiology. Harlow: Pearson International
Stanfield, C.L. and Germann, W.J. (2007) Principles of human physiology. Harlow: Pearson International
There are also some more in-depth texts on cellular biology.
Cooper, G.M. and Hausman, R.E. (2009) The cell: A molecular approach. Basingstoke: Palgrave Macmillan
Alberts, B., Bray, D., Hopkin, K., Johnson, A., Lewis, J., Raff, M., Roberts, K. and Walter, P. (2009) Essential cell biology. Oxford: Garland Science
For interactive web pages on cellular activities, the following websites provide some good animations in cellular activities.
www.cellsalive.com
www.biology.arizona.edu/cell_bio/cell_bio.html
www.johnkyrk.com/index.html
| LEARNING OUTCOMES |
The following topics are covered in this chapter:
• The Mendelian principles of transmission:
♦ unit inheritance: genes and alleles;
♦ dominance: allelic relationships;
♦ segregation – single gene inheritance patterns, Punnet squares;
♦ independent assortment – inheriting two or more genes.
• Exceptions to the rules:
♦ mitochondrial inheritance;
♦ penetrance;
♦ genomic imprinting;
♦ sex-related effects;
♦ mutations;
♦ genetic linkage;
♦ polygenic and multifactorial inheritance;
♦ epistasis;
♦ pleiotropy.
INTRODUCTION
The fact that biological traits can be inherited has long been established. The first significant discoveries regarding the mechanisms of inheritance resulted from the work of Gregor Mendel in the late nineteenth century.
Mendel studied the patterns of inheritance within pea plants while he was working as a monk. His work went largely unnoticed until after the start of the twentieth century. Scientists who were studying the function of chromosomes rediscovered Mendel’s publications and realised that Mendel had discovered the way in which biological traits were inherited. Mendel became known as the Father of Genetics, and the branch of genetics involved with simple inheritance is known as Mendelian genetics.
Although Mendel’s work involved plants, his findings are relevant to human genetics. From his work, he derived certain laws that have become the principles of transmission genetics. Mendel proposed four principles of inheritance: unit inheritance, dominance, segregation and independent assortment. It is these four principles that form the basis of inheritance today.
1. The Principle of Unit Inheritance
Biological traits are determined by genes. Genes are the basic units of heredity. Strands of DNA that encode for one protein form a gene. As chromosomes occur in pairs after fertilisation, genes can be found on both the paired chromosomes. The individual ‘genes’ on each chromosome are termed alleles.1 An allele is a version of a gene that has a paired version of the same gene in the same location on the opposite chromosome (see Figure 2.1).
Figure 2.1 A gene is usually made of two alleles, one on each of the paired chromosomes.
2. The Principle of Dominance
Alleles can present as different versions of the same gene. If two alleles carried a different sequence of DNA, the effect of one allele might be masked by its partner allele. A dominant allele will be expressed regardless of any instructions carried by the other allele.
In humans, the allele that codes for freckles is dominant over the allele for no freckles. Therefore, an individual who carries two different alleles for this gene – an allele for freckles and an allele for no freckles – will have freckles on their skin. This is because the freckles gene is dominant and will be expressed in that individual. An individual who has two different types of alleles for a single trait (like freckles) is said to be heterozygous for that trait.
An allele that is not expressed, due to the presence of a dominant partner allele, is termed recessive. Recessive alleles are only expressed when both alleles are in a recessive form. Individuals who have either two recessive alleles