WJEC Biology for A2: Student Bk

YOU SHOULD KNOW › › › ››› The difference between genes and alleles ››› How to set out a genetic cross and predict ratios of offspring ››› Explain the derivation of the ratios 3:1, 1:1, 9:3:3:1 and 1:1:1:1 ››› How to use the chi 2 test to compare observed and expected results of a genetic cross St u dy point A gene controls a characteristic and alleles are different versions of the gene in the same way that a car (‘gene’) could be a BMW or a Porsche (‘alleles’). St u dy point For a heterozygous gene, the dominant allele is always written before the recessive allele e.g. Tt. Key Terms Gene : A sequence of DNA that codes for a polypeptide and which occupies a specific locus on a chromosome. Allele : A variant nucleotide sequence for a particular gene at a given locus, which codes for an altered phenotype. Examtip Learn the definitions of genetic terms. Genetic terms A gene is the physical unit of heredity. It is a sequence of DNA that occupies a specific site, or locus , on a chromosome, and codes for a polypeptide. Polypeptides and proteins determine structures and functions in living organisms, and so the gene is also thought of as the sequence of DNA that codes for a characteristic. There are several genes that control coat colour in dogs. In one, the nucleotide sequence codes for an enzyme that makes the fur black, but a small difference in the sequence gives a slightly altered pigment molecule that makes the fur brown. Small changes in nucleotide sequences of the same gene make small changes in the polypeptides they encode and produce different characteristics. The different sequences are called alleles . A single gene may have one, two or many alleles. Alleles always occupy the same locus because they are versions of the same gene. Organism Gene Alleles Human Number of fingers 5; 6 Freckles Present; absent Rhesus blood group Positive; negative Sweet pea Height Tall; dwarf Seed colour Yellow; green Flower position Terminal; axial A diploid individual has one of each chromosome from each parent and, therefore, one copy of each gene from each parent. If the alleles of a particular gene are the same from both parents, the individual is homozygous for that gene. If they are different, the individual is heterozygous for that gene. The genotype of an individual is all the alleles that they contain. Their phenotype can be described as their appearance, but it is much more than that, because it includes characteristics that cannot be seen, such as blood group. Both the genotype and the environment determine the characteristics of an individual so the phenotype is better thought of as the expression of the genotype in a specific environment. There is a gene that codes for stem height in sweet pea and some other plants. An allele of this gene, given the symbol T, codes for the production of gibberellin, a growth regulator that elongates the stem, so plants with the T allele are tall. An alternative allele, given the symbol t, does not code for the production of gibberellin so the plants that do not have a T allele are dwarf. Each plant has two alleles, which gives three possible combinations: Allele combination Effect Phenotype Description of genotype tt No gibberellin produced Dwarf Homozygous recessive TT Gibberellin produced Tall Homozygous dominant Tt Gibberellin produced Tall Heterozygous The T allele is always expressed when it is present, so it is described as dominant . The t allele is not expressed in the presence of the T allele, and is only expressed when it is homozygous. It is described as recessive . One-year old female labradors 195 Inheritance