Genetics Notes PDF - Chromosomes, Genes, and Alleles

Chromosomes Genes and DNA Inside every living cell there is a nucleus. The nucleus is the control centre of the cell and tells the cell what to do. Inside the nucleus are some thread like structures called Chromatin which condense to form chromosomes during cell division. Chromosomes are made of...

Chromosomes Genes and DNA Inside every living cell there is a nucleus. The nucleus is the control centre of the cell and tells the cell what to do. Inside the nucleus are some thread like structures called Chromatin which condense to form chromosomes during cell division. Chromosomes are made of one very long, condensed DNA molecule associated with proteins (in eukaryotic cells). The tightly coiled combination of DNA and proteins is called chromatin – this is what chromatids, and therefore chromosomes, are made of The DNA contained within chromosomes is essential for cell survival. Every chromosome consists of a long DNA molecule that codes for several different proteins. DNA (deoxyribose nucleic acid) molecules are large and complex. They carry the genetic code that tells the cell what proteins to make, and so determines the characteristics of a living thing. A length of DNA that codes for a single polypeptide or protein is called a gene. A gene is a section of DNA that carries the code for a particular protein. Each gene tells the cell to make a particular protein and so controls the development of a particular characteristic of an organism. Many genes are needed to carry all the genetic information for a whole organism. The position of a gene on a chromosome is its locus (plural: loci) Each gene can exist in two or more different forms called alleles. Different alleles of a gene have slightly different nucleotide sequences, but they still occupy the same position (locus) on the chromosome. If a person has inherited two of the SAME genes from their parents, then they are HOMOZYGOUS for that characteristic. e.g. If a person inherits two blue genes for eye colour their eye colour is homozygous blue. If a person inherits two DIFFERENT genes from their parents, then they are HETEROZYGOUS for that characteristic. e.g. IF a person inherits a blue gene and a brown gene for eye colour, their eye colour is heterozygous brown. Example of alleles One of the genes for coat colour in horses is Agouti. Hypothetically there are two different forms (alleles) of that gene found in horses: A and a Each allele can produce a different coat colour: o Allele A → black coat o Allele a → chestnut coat The genotype of an organism refers to the alleles of a gene possessed by that individual. E.g. An organism's genotype could be represented by the letters gg. The genotype of an individual affects their phenotype; a phenotype is the observable characteristics of an organism. E.g., every horse has two copies of a gene for coat colour in all of their cells, one on each homologous pair of chromosomes; the gene could be notated using the letters A and a o A horse that has two A alleles has the genotype AA and is homozygous. ▪ If the A allele codes for a black coat, then the phenotype of the horse would be black. o A horse that has two a alleles has the genotype aa and is also homozygous ▪ If the a allele codes for a chestnut coat then the phenotype of the horse would be chestnut Dominant & recessive Not all alleles affect the phenotype in the same way. Some alleles are dominant; they are always expressed in the phenotype no matter which other allele is present. o This means they are expressed in both heterozygous and homozygous individuals, e.g. ▪ A horse with the genotype AA would be said to be homozygous dominant and would have a black coat phenotype. ▪ A horse with the genotype Aa would be said to be heterozygous and would still have a black coat phenotype, as the allele for black coat colour is dominant over the lower- case allele. It is possible to refer to a heterozygous individual like this as a carrier of the recessive allele; the allele doesn't show in the phenotype but could still be passed on to offspring. Others are recessive; they are only expressed in the phenotype if no dominant allele is present. o This means that it is only expressed when present in a homozygous individual, e.g. A horse with the genotype aa would be said to be homozygous recessive and would have a chestnut coat phenotype due to the absence of the dominant black allele  Genetic Key Terms A gene is a short length of DNA found on a chromosome that can code for a particular characteristic (expressed by the formation of different proteins) Alleles are variations of the same gene. o As we have two copies of each chromosome, we have two copies of each gene and therefore two alleles for each gene. o One of the alleles is inherited from the mother and the other from the father. o This means that the alleles do not have to ‘say’ the same thing. o For example, two copies of a particular gene could contribute to eye colour, but one allele could code for brown eyes and one allele could code for blue eyes. The observable characteristics of an organism (seen just by looking - like eye colour or found – like blood type) is called the phenotype. The combination of alleles that control each characteristic is called the genotype. Alleles can be dominant or recessive. o A dominant allele only needs to be inherited from one parent in order for the characteristic to show up in the phenotype. o A recessive allele needs to be inherited from both parents in order for the characteristic to show up in the phenotype. o If there is only one recessive allele, it will remain hidden, and the dominant characteristic will show. If the two alleles of a gene are the same, we describe the individual as being homozygous (homo = same) o Homozygous dominant means having two copies of the dominant allele. o Homozygous recessive means having two copies of the recessive allele. If the two alleles of a gene are different, we describe the individual as being heterozygous (hetero = different) When completing genetic diagrams, alleles are abbreviated to single letters. o The dominant allele is given a capital letter, and the recessive allele is given the same letter, but lower case. Alleles are different forms of the same gene. You can only inherit two alleles for each gene, and they can be the same (homozygous) or different (heterozygous) Dominance Not all alleles affect the phenotype in the same way. Some alleles are dominant: they are always expressed in the phenotype. o This means they are expressed in both heterozygous and homozygous individuals. Others are recessive: they are only expressed in the phenotype if no dominant allele is present. o This means that it is only expressed when present in a homozygous individual. Example of dominance If for horses the allele A for a black coat is dominant and the allele a for a chestnut coat is recessive the following genotypes and phenotypes occur: o Genotype AA → black coat o Genotype Aa → black coat o Genotype aa → chestnut coat HOW TO WRITE THE ALLELES AND GENOTYPES? In each situation genotypes and alleles can be written based on the phenotype. For example: Tall is dominant over dwarf plants. Allele for tall is - T, allele for dwarf is – t (The first alphabet of the dominant feature to be written in capital for dominant allele, corresponding small alphabet to be written for recessive allele. NO OTHER ALPHABET SHOULD BE USED FOR WRITING THE GENOTYPES) For example Homozygous dominant: TT Homozygous recessive: tt Heterozygous: Tt / tT Heterozygous always show dominant phenotype. Key Terms & Definitions Table