+ Inheritance

+ Single genes—monohybrid crosses To illustrate the fi rst of Mendel’s Principles of Inheritance we will consider the simplest case—observing the inheritance pattern of a single gene with two alleles coding for a particular trait. This involves conducting a monohybrid cross (‘mono’ meaning single); that is, a cross between organisms that are heterozygous at a single genetic locus, for example, eye colour in blowflies and fl ower colour in snapdragons.

+ The type of experiment that Mendel carried out, investigating just a single characteristic, is called a monohybrid cross There are two alleles controlling pea shape. This means there are three possible genotypes that the F2 generation of plants could inherit, leading to two possible phenotypes. Genotype heterozygous homozygous dominant homozygous recessive Phenotype SS ww Sw smooth wrinkly smooth The likelihood of a trait being produced during a monohybrid cross can be mapped out using a Punnett Square.

+ After his research, Mendel proposed two laws of inheritance. Mendel’s first law: the law of segregation Alternate versions of genes (alleles) cause variation in inherited characteristics. An organism inherits two alleles for each characteristic – one from each parent. Dominant alleles will always mask recessive alleles. The two alleles for each characteristic separate during gamete production. Mendel’s second law: the law of independent assortment Genes for different characteristics are sorted independently during gamete production.

+ A chance event About 1 in 2500 people suffer from a genetic disorder called cystic fibrosis. What is the chance of particular person getting it and why is it important to know? Since the identification of the defective gene in 1989, the DNA of parents can be analysed to find out if they are one of the one in 20 that carry the defective gene. For parents who are carriers for CF, the chance of each child suffering from the disorder is at least one in 4. Analysing pegigree charts and punnet squares can help in identifying the chances of a parent passing on such defective genes.

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+ White eyes—autosomal dominant inheritance Consider the consequences of meiosis for the inheritance of white eye colour in the Australian sheep blow fl y. The production of gametes for each of the genotypes shown below.

+ Note that homozygous genotypes produce only one type of gamete: WW individuals produce only W gametes and ww individuals produce only w gametes. Therefore, in a cross between WW (red-eye) individuals, all of the progeny would be WW (red eye). As long as WW individuals were crossed together, it would be a pure-breeding strain. Being homozygous in every generation, it would produce individuals that were genetically (WW) and phenotypically (red eye) identical. Similarly, crosses between ww (white eye) individuals would yield a pure-breeding ww (white-eye) strain.

+ F 1 generation These two pure-breeding strains can be used to parent a new generation, the F1 generation. The F1 generation is the result of a cross between two pure- breeding strains—in this example WW and ww— and has the genotype Ww (heterozygous). Remember that by observing the phenotype of these heterozygous individuals, we know that the red-eye phenotype is dominant over the white-eye phenotype.

+ F 2 generation The heterozygous F 1 individuals can now be crossed together to produce the F 2 generation. This is a monohybrid cross. Heterozygotes (Ww) produce W and w gametes in equal frequency. Given that each parent produces two types of gametes, and gametes fuse randomly at fertilisation, there are four possible combinations of gametes. The eminent geneticist, R.C. Punnett, devised a simple method of showing the random combination of gametes and the genotypes of the resulting offspring: the Punnett square.

+ The diagram shows the gametes produced by the F 1 (Ww) individuals and the genotypes and phenotypes of the resulting F 2 generation. A 1 : 2 : 1 ratio of the genotypes WW : Ww : ww is observed in the F generation. The 1 : 2 : 1 ratio occurs because: in meiosis, heterozygous (Ww) individuals (both male and female) produce gametes in a 1 W : 1 w ratio. This is referred to as the Principle of Segregation. The W and w alleles are segregated (i.e., separated) into different gametes. Fertilisation occurs at random. E.g., a W sperm has equal chance of fertilising a W egg or a w egg given that these eggs are produced in equal frequency. The Punnett square takes both of these factors into account. In the case of blow fl y eye colour, the 1 WW : 2 Ww : 1 ww genotypic ratio yields a 3 : 1 red-eye (wild- type) : white-eye phenotypic ratio due to the dominance of the red-eye phenotype. (b)

+ In summary, a 3 : 1 phenotypic ratio will be observed in the F 2 generation for any trait whenever the following four conditions apply: the variation in the trait (in this example, white versus red eye) is controlled by a single gene the gene is on an autosome there are two alleles of the gene (in this example, W and w) one phenotype is dominant (in this example, red).

+ Pedigree charts Go through for drawing a pedigree chart on page 232 of your textbook.

+ How to read a pedigree chart? Can you see the trait in each generation of the family in which it occurs? No Yes Do the males mainly show the trait? Do daughters who show the trait have fathers with it also? Do the males mainly show the trait? Do daughters who show the trait have fathers with it also? Do the males mainly show the trait? Does the trait only pass from father to son? Do the males mainly show the trait? Does the trait only pass from father to son? No Yes Do all of the females and none of the sons show the trait when the father shows the trait and the mother does not? Do all of the females and none of the sons show the trait when the father shows the trait and the mother does not? On the Y chrom osome Sex linked dominant Autosomal dominant Autosomal recessive Sex linked recessive Yes No

+ Do the males mainly show the trait?

+ Do daughters who show the trait have fathers with it also?

+ Do the males mainly show the trait?

+ Does the trait only pass from father to son?

+ Do all of the females and none of the sons show the trait when the father shows the trait and the mother does not?