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Genetic constitution of a population A Review of Mendelian Genetics Lecture 2 Aim • To review with students terms and principles of population genetics Lecture objectives • By the end of the topic, students should be able to • Define terms related to genetic constitution of a population • Define genes and the link between genes and population • Explain gene discovery • Explain the process of cell division and inheritance Definition of some genetic terms Slide taken from: Julius van der Welf, GEN 251- Applied Animal Breeding Notes A Population A breed, strain or variety or herd that breed together and exchange genetic material more than the rest Genetically it is a breeding group, unlike individuals Breed, Variety or Strain Breed is a homogenous sub-specific group of domestic livestock with definable and identifiable external characteristics These characteristics enable it to be separated by visual appraisal from other similarly defined groups within the species Variety or strain is a subdivision of a breed that has been developed with special characteristics such as size, early maturity, coat type etc Breed, Variety or Strain A population of chickens A Ross broiler strain Breed, Variety or Strain Different breeds of goats A population of chickens A Population Must always be defined and decisions should be made where one population ends and another one begins Shows continuity in time Under similar conditions, remains practically unchanging in both quality and number over an indefinite time A Population Practically real population changes over time After some generations a large breeding herd may lose its identity through dispersal and cannot be identified as a population any more This may also occur in a similar way to a breed in the course of a changing economic environment A Population In a population, generations overlap A population is a collection of individuals Individual is a collection of genes In quantitative genetics, the centre of interest is the population and its qualities or level of performance Defining a Gene Genes are genetic factors These are hereditary units of animals They are maintained in a population And are passed from one generation to another Defining a Gene When goats are mated, one expects a goat and not sheep Same for cattle, poultry And these resemble each other This shows that they share a common property that is inherited And this is a gene Where are genes located? An animal body contains cells, that contain nucleus In the nucleus, are chromosomes – rod like structures On the chromosomes are DNA – long double strand It is this DNA that contains encodes, genetic information that is inherited Chromosome A structure that is seen as rod – like during cell division under microscope that is in the nucleus of cells, on which genes are located There are several genes but few chromosomes This gives evidence that genes are on chromosomes Chromosome Chromosomes occur in pairs, a member being called homologous chromosomes The number of chromosomes differ (diploid – pairs) between species Species Haploid (n) Diploid (2n) Cattle 30 60 Sheep 27 54 Goats 30 60 Pigs 19 38 Chickens 39 78 Humans 23 42 Chromosome pair It is these genes on the chromosome that are responsible for colour, shape, position, height and numerous other characteristics, generally called traits Position on a chromosome where a particular gene is located is called a locus (pl, loci) Chromosome pair Types of chromosomes Autosomes Are homologus – two copies of each carrying same genes. But these genes may be of different forms - alleles Sex chromosomes Are responsible for sex characteristics In most species, they carry different genes and are of different sizes Mammals have XX in females, XY in males Birds have ZW in females, ZZ in males Cell division Life begins with a single cell which is a fertilised egg (zygote) This has diploid number of chromosomes that divides into daughter cell Each cell receives a full complement of chromosomes This process is called mitosis The process repeats and cells continue to divide to form different parts and tissues of the body Cell division What happens is that the chromosomes double in number Duplicate pairs then separate, one pair to each of the daughter cells This maintains the number of chromosomes in all parts of the body This process starts very early in embryonic stage Gamete formation The zygote is formed by the fusion of two special cells, the gametes One gamete comes from each of the parents A sperm from the father An unfertilised egg from the mother Only half the number of chromosomes appear in each gamete i.e. only one of each member of homologous chromosome pairs go into each gamete Gamete formation This process maintains number of chromosomes from generation to generation This process of cell division is called meiosis The number of chromosomes that appear in the gamete is called haploid number The choice of which chromosome goes into each gamete is random Gamete formation The diploid number is restored during fertilisation The sperm and the egg each contribute haploid number to the fused zygote The zygote now has normal diploid number of chromosomes according to the species The zygote then starts the process of mitosis throughout its life Inheritance It is chromosomes that are passed from parents to offspring through gametes This means those genes on the chromosomes will be the ones transmitted to the offspring If homologous chromosomes contained genes that influence a trait differently (allele) then two offspring will look different, hence the observed variation Inheritance Since chromosomes occur in pairs, genes also function in pairs However, on a single locus, there may be one or more alternative forms of a gene, and this is called an allele These alleles have different influence on a trait Gene discovery It all started by Gregor Mendel Who was working on peas in the 19th Century And published his work in 1886 That there are certain characteristics that are inherited Based on certain rules and hypotheses Gregor Mendel • Studied basic genetic principles • Used pea plants • Theory was not well understood during his life • Mendel, G. 1866. Experiments on Plant Hybridization. Transactions of the Brünn Natural History Society. • http://www.esp.org/foundations/genetics/classical/gm-65.pdf Slide taken from: Dr. David Buchanan, ANSI 3433 - Animal Breeding Notes Gregor Mendel https://www.google.com/#q=Mendel%2C+G.+1866.+ Mandatory reading • Chapter 2, pages 7 – 19 of Geoff Simm ~ Genetic Improvement of Cattle and Sheep