Genetics and Gene Technology
Senior 3 · Biology
Genetics and Gene Technology
Lesson 1 of 5
25 min
Between 1856 and 1863, in the garden of St Thomas's Abbey in Brno (now in the Czech Republic), the monk Gregor Mendel crossed pure-breeding tall pea plants with pure-breeding dwarf ones. Every plant in the next generation was tall. He then let those tall plants pollinate themselves, and the dwarf form came back. In one count of 1,064 plants, 787 were tall and 277 were dwarf.
The dwarfness had been carried, unseen, through a whole generation of tall plants. Mendel explained this in 1866. Each plant carries two "factors" for height, one from each parent, and one factor can mask the other. A plant passes on only one of its two factors in each sex cell. Biologists now call his factors genes and alleles. This lesson sets out these words and the other key terms, because every cross in this unit is written in them.
Definition
Heredity (inheritance): the passing of characteristics from parents to offspring. Genetics is the study of heredity and variation.
Gene: a length of DNA, at a fixed place on a chromosome, that controls one characteristic by coding for a protein. Example: the gene that controls stem height in peas.
Locus (plural loci): the position of a gene on a chromosome.
Allele: one of the different forms of the same gene. Example: the height gene of peas has two alleles, one for tall (T) and one for dwarf (t).
In Unit 5 you saw that a diploid () cell holds its chromosomes in homologous pairs: one chromosome of each pair came from the father and one from the mother. The two chromosomes of a pair carry the same genes at the same loci. So a diploid cell has two copies of every gene, one on each chromosome of the pair. The two copies may be the same allele or two different alleles.
Figure 1. A homologous pair, drawn simply, with three gene loci marked. The two chromosomes carry the same genes at the same loci, but not always the same alleles. (In a real pea plant these three genes lie on different chromosomes; they are drawn on one pair here to show the idea.)
Definition
Genotype: the pair of alleles an organism has for a gene, written in letters. Example: Tt.
Phenotype: the characteristic that results and can be seen or measured. Example: tall. The phenotype depends on the genotype and on the environment.
Homozygous: having two identical alleles of a gene (TT or tt). A homozygous organism is pure-breeding (true-breeding) for that characteristic.
Heterozygous: having two different alleles of a gene (Tt). A heterozygous organism is sometimes called a hybrid.
Dominant allele: an allele that shows in the phenotype whenever it is present, in one copy or two. Written with a capital letter: T.
Recessive allele: an allele that shows in the phenotype only when no dominant allele is present, that is, in the homozygous form. Written with the small form of the same letter: t.
| Genotype | Name of the genotype | Phenotype |
|---|---|---|
| TT | homozygous dominant | tall |
| Tt | heterozygous | tall (T masks t) |
| tt | homozygous recessive | dwarf |
Two different genotypes, TT and Tt, give the same phenotype. That is why you cannot always read a genotype from what you see. A dwarf plant is the exception: a recessive phenotype tells you the genotype at once, because it can only be tt.
Genes set what is possible, and the environment acts on it. A TT pea grown in dry, poor soil may be shorter than a well-fed Tt pea. You will study this in Unit 14.
Worked example
Worked example 1: reading genotypes. In cattle, the allele for having no horns (polled), H, is dominant to the allele for horns, h.
(a) Give the genotype of a horned cow. (b) Give the possible genotypes of a polled bull. (c) Which of these genotypes are homozygous?
Solution
Meiosis separates the two chromosomes of each homologous pair (Unit 5). So a gamete receives only one chromosome of each pair, and with it only one allele of each gene. At fertilisation, a male and a female gamete fuse and the zygote has two alleles again. This is Mendel's law of segregation: the two alleles of a gene separate when gametes form, and each gamete carries only one of them.
Figure 2. Segregation: a heterozygous parent puts T into half of its gametes and t into the other half.
Worked example
Worked example 2: counting gametes. A heterozygous tall pea plant (Tt) makes 2,000 pollen grains. About how many would you expect to carry the allele t?
Solution
Answer: about 1,000. A TT plant would make only T gametes, and a tt plant only t gametes.
Warning
Choosing letters. Use one letter for each gene: the capital for the dominant allele and the small form of the same letter for the recessive one. "TD" for a heterozygous tall plant is wrong, because T and D look like two different genes. Avoid letters whose capital and small forms look alike in handwriting, such as C and c, O and o, S and s, or W and w. Also, "dominant" does not mean "more common" or "better": it only describes which allele shows in a heterozygote.
Alleles and Genes, from the Amoeba Sisters. Watch for how two chromosomes of a pair carry the same gene but can carry different alleles, and for the terms homozygous and heterozygous.
Tip
G for genes, Ph for physical. The genotype is the letters for the genes; the phenotype is the physical result you see. Homo- means "same" and hetero- means "different", so homozygous means the two alleles are the same.
Practice: the language of genetics
Key points
Sources and credits
Written by Ganzaa: the text, Figures 1 and 2, the worked examples and the practice questions.
