Heredity (A): genes, alleles and Mendel’s first law
Week 10 · Unit 3 Variation of Traits in Living Organisms, opener; Lesson 1 Heredity and Variation of Traits (A): DNA, genes and chromosomes, alleles, dominant and recessive, genotype and phenotype, Mendel's first law, the Punnett square · 3 lessons
1
Genes, chromosomes and alleles
Science Prep 3 Term 1, Unit 3: pp. 64–70
GoalSay how DNA, chromosomes, genes and alleles fit together and how a child gets two copies of each gene.
New words
gene · a segment of the genetic material that carries the information for one traitallele · one of the different forms of the same genegamete · a sex cell (sperm or ovum) that carries half of the genetic material
Explanation
Genetics studies how information is passed from parents to children. The genetic material is DNA, found in the nucleus of the cell. DNA is very long and coils with protein into thread-like chromosomes. A gene is a segment of this material that carries the information for one trait, for example the colour of a pea flower. The sex cells, called gametes (sperm and ovum), are made in the gonads by a kind of cell division called meiosis. Each gamete carries half of the genetic material, so just one copy of each gene. At fertilisation the two nuclei fuse, and the new individual has two copies of every gene: one from each parent. The different copies of one gene are called alleles, and a gene may have more than two alleles in a population. An individual normally has two alleles for a trait. Careful: each parent gives one allele, not both; and a trait is often controlled by several genes, so real human traits are more complicated than our simple models.
nucleus→chromosomes (DNA coiled with protein)→gene: a segment of DNA for one trait→alleles: forms of the gene
Worked examples
Count the copies. A pea plant makes pollen and ovules. Each gamete has 1 copy of the gene for seed shape. After fertilisation the new seed has 1 + 1 = 2 copies of that gene, one from each parent plant.
A short, imaginary piece of DNA, drawn as a ladder of letters. The letters A, T, G and C are the bases, and the order of the bases carries the information. A real gene is far longer than this picture.
Why can one gene have several alleles? Different versions of a gene can exist, for example a gene for flower colour in a plant with alleles for purple and for white. Each plant carries only two of the available alleles.
Class activity
Model with beads. Use two colours of beads as alleles. Each parent “has” a pair of beads in a cup. Draw one bead from each parent’s cup without looking, and put them together: this is the new individual with two alleles. Repeat 10 times and record the pairs. Draw the DNA, chromosome and gene as a hierarchy: nucleus, chromosome, gene, allele.
Practice
1
Where in the cell is the genetic material found?
In the nucleus, as DNA coiled into chromosomes.
2
What is a gene?
A segment of the genetic material that carries the information for one trait.
3
How many copies of each gene does a gamete carry, and how many does the individual have after fertilisation?
A gamete has one copy; after fertilisation the individual has two.
