ASB 325 Animal breeding_Week 2_Mendelian Genetics.pdf

ASB 325: ANIMAL BREEDING Week two Dr K. Thutwa Learning outcomes At the end of the lesson the students will be able to; State the Mendelian laws in details Explain the different forms of dominance Calculate the phenotypic and genotypic ratio when animals with dominant...

ASB 325: ANIMAL BREEDING Week two Dr K. Thutwa Learning outcomes At the end of the lesson the students will be able to; State the Mendelian laws in details Explain the different forms of dominance Calculate the phenotypic and genotypic ratio when animals with dominant alleles for a particular trait are mated with those with recessive alleles. Differentiate between Mendelian traits and quantitative traits. 1. MENDELIAN GENETICS In 1866 Gregor Mendel published results of his experiments in which he had investigated inheritance in garden peas (Pisum sativum). From these findings Mendel discovered the existence of discrete hereditary elements (now called genes) and the rules determining their transmission from parents to offspring during sexual reproduction.  Mendel also discovered that the genes exist in pairs in the cells of individuals. Germ cells i.e. egg and sperm carry one or the other of these paired genes in equal proportion. These findings led Mendel to postulate two rules, laws or principles: Rule 1: Segregation - this states that during the formation of gametes the paired elements (genes) separate or segregate randomly such that each gamete receives one or the other of the elements (genes). These genes which are present in duplicate (2N) in parents segregate in the formation of gametes (N) and recombine as discrete units at fertilisation (2N). Rule 2: Independent assortment - this states that segregation of the members of a pair is independent of the segregation of other pairs during the processes leading to formation of the gametes.  These rules led to a branch of animal breeding called Mendelian genetics. Mendelian genetics refers to the inheritance of chromosomal genes following the laws governing the transmission of chromosomes to subsequent generations. Mendelian genetics is concerned with traits controlled by a single gene or few genes. Dominance Mendel discovered that the expression of a gene at a locus depends on the other alternative gene (allele) present at that locus. This interaction between genes at a single locus such that in heterozygotes one allele has more effect than the other is called Dominance. The allele with the greater effect is dominant over its recessive counterpart.  There are four (4) forms of dominance; i. Complete dominance – a form of dominance in which the expression of the heterozygote is identical to the expression of the homozygous dominant genotype. ii. Partial dominance – a form of dominance in which the expression of the heterozygote is intermediate to the expressions of the homozygous genotypes and more closely resembles the expression of the homozygous dominant genotype. iii. No dominance – a form of dominance in which the expression of the heterozygote is exactly midway between the expressions of the homozygous genotypes. iv.Overdominance – a form of dominance in which the expression of the heterozygote is outside the range defined by the expressions of the homozygous genotypes and most closely resembles the expression of the homozygous dominant genotype  Traits that display Mendelian ratios are called qualitative traits. Example of complete dominance: polled homozygous bulls are mated to horned homozygous cows. The gene that codes for polledness is dominant to the gene that codes for horns. Calculate the genotype and phenotype ratios in: (i) First generation (ii)Intra-mating F1 Answer The assumptions are that the allele P is the dominant gene and this codes for polledness while the allele p is recessive and codes for horns. (i) In the first generations all progeny will be polled and all progeny will be heterozygous (since they are all Pp) (ii) In the second generation resulting from mating F1 heterozygous animals. Here we will have some animals polled, while others are horned. Please work out the genotypic and phenotypic ratios. (Answer Phenotypic ratio 3 polled : 1 horned; Genotypic ratio 1PP: 2Pp: 1pp). These principles or rules of inheritance discovered by Mendel ultimately became the foundation of genetics and a major factor in the development of modern biology. 2. QUANTITATIVE GENETICS Quantitative genetics is the extension of Mendelian genetics, which is concerned with traits controlled by many genes. Each gene has a small effect on the trait and the environment modifies the genetic effect. Quantitative traits are based on the premise that genes are the basis of inheritance and that genes are subject to the same laws of transmission and have the same general properties as the genes of qualitative traits.  Once there are more than one gene controlling a trait (polygenic) there is more variation and it is impossible to determine genotype and Mendelian ratios. Differences between Mendelian and quantitative genetics are: (i) Quantitative traits are controlled by many genes, each with a small effect and genetic effects are modified by the environment, comparatively, qualitative traits are controlled by one or few genes each gene with a large effect and the environment has no effect in most instances. (ii) Quantitative traits do not display Mendelian ratios, which are readily displayed in qualitative traits. (iii) Quantitative traits are based on objective measurement such as kg, l , and mm. These are expressed in terms of mean and standard deviation whereas qualitative traits are based on counts and ratios. (iv) In quantitative traits, the population (large groups of individuals) are the basis of study whereas in qualitative traits few individuals are informative. Non-Mendelian concepts There are concepts of quantitative genetics, which are not Mendelian in origin. These are epistasis, pleiotropy and linkage. Epistasis – An interaction among genes at different loci such that the expression of genes at one locus depends on the alleles present at one or more other loci. Where there is epistasis one gene at a locus masks another gene at a different locus.  Genes are always together hence traits are also always displayed together. For example colour in Labrador retrievers (dogs) is determined by two genes at different loci. This leads to three (3) colours - black, chocolate and yellow.  Pleiotropy is when one gene affects two different traits. This is good when the two traits are desirable because traits are always together. This is bad when one of the trait is undesirable because they cannot be broken-up. For example there is a gene in Drysdale breed of sheep that codes for medullated fibres and also horns in males and scours in female.  Linkage is when genes close together on a chromosome tend to stick together. The further the genes are on a chromosome the higher the chance that they may separate through crossing over i.e. traits can be broken up by crossing over. Most economically important traits are not simple in terms of inheritance but quantitative in nature. Therefore, Mendelian genetics is of little use in practical animal breeding, quantitative genetics is the major interest in animal breeding.