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Chapter 19 Heredity Objectives • define a gene as a unit of inheritance and distinguish clearly between the terms gene and allele • explain the terms dominant, recessive, codominant, homozygous, heterozygous, phenotype and genotype • predict the results of simple crosses with expected ratios of 3:1 and 1:1, using the terms homozygous, heterozygous, F1 generation and F2 generation • explain why observed ratios often differ from expected ratios, especially when there are small numbers of progeny • use genetic diagrams to solve problems involving monohybrid inheritance (Genetic diagrams involving autosomal linkage or epistasis are not required) Objectives • explain co-dominance and multiple alleles with reference to the inheritance of the ABO blood group phenotypes - A, B, AB, O, gene alleles IA, IB and Io • describe the determination of sex in humans - XX and XY chromosomes • describe mutation as a change in the structure of a gene such as in sickle cell anaemia, or in the chromosome number, such as the 47 chromosomes in a condition known as Down Syndrome • name radiation and chemicals as factors which may increase the rate of mutation Objectives • describe the difference between continuous and discontinuous variation and give examples of each • state that competition which arises from variation leads to differential survival of, and reproduction by, those organisms best fitted to the environment • give examples of environmental factors that act as forces of natural selection Objectives • assess the importance of natural selection as a possible mechanism for evolution • give examples of artificial selection such as in the production of economically important plants and animals Monohybrid Inheritance • Gregor Mendel (known as Father of Genetics) explained how qualities were inherited i.e the mechanism of heredity • Experimented on garden peas (Pisum sativum) Monohybrid Inheritance • Mendel selected several varieties of garden peas that has different characteristics. E.g : – Short and tall plants – Red or white flower plants – Seeds that were either yellow or green, or round and wrinkled • Inheritance involving only one pair of contrasting characters is called monohybrid inheritance •He crossed tall plants with dwarf plants •Pure bred (true breeding) varieties were used plants which when self fertilised produced offspring which resembled their parent •Seeds from the cross were then planted and he observed the hybrids. •These hybrids then selfpollinate and produce seeds that gave rise to F2 generation Some terms… Hybrid A hybrid is the offspring from two different varieties or species F1 generation Also known as first filial generation F2 generation Also known as second filial generation Results of the cross.. • All plants in the F1 generation were tall, resembling one of the parents • In the F2 generation, ratio of tall plants to that of short plants is in the ratio of 3:1 • He then performed the same experiment using other contrasting characters in peas Question • Why does the observed ratios differ from expected ratios, especially when there are small numbers of progeny? Results of all experiments • One trait or character remained unchanged in the F1 hybrids while the other trait seemed to disappear • This character then appear again in the F2 generation but only in about one-quarter of the total number of offsprings • Trait unchanged – dominant trait • Trait disappeared and appear again – recessive trait Mendel’s suggestion • Hereditary factors are responsible for the transmission of characteristics • Each characteristic controlled by a pair of factors in the cells of an organism • The two factors in each pair separate (segregate) during gamete formation and each gamete contain only one factor Mendel’s Law of Segregation • Fusion of gamete restores the diploid condition in the zygote • Gamete unite at random so that a predictable ratio of characteristics occurs among offspring (Fig 22.3) Heredity now Chromosomes • A structure where genetic material is found • Carry information for making new cells. This information carried in a molecule called DNA Gene • Small segment of DNA in a chromosome where a piece of genetic information is stored. • The place on the chromosome where the gene resides is called the gene locus • Each gene has a function (height, flower colour, seed colour) Allele • Each gene has a different forms and these alternative forms of the same gene are called alleles (tall and short, pink and white flowers, yellow and green seeds) Homologous chromosomes • In organisms, chromosomes come in pairs. One from father, one from mother • A pair of homologous chromosomes will have exactly the same sequence of gene loci • However, the alleles in those gene loci may not be the same (see example) • Homologous chromosomes are similar in shape and size (except sex chromosomes) – Fig 22.5 Alleles are alternative forms of a gene and occupy the same relative positions on a pair of homologous chromosomes For hair colour For shape of ear lobes For hair colour For shape of ear lobes Modelling genetic crosses Questions to bear in mind 1. Why did one of the characteristics disappear in the F1 generation ? 2. Why did this characteristic reappear in about one quarter of the F2 generation? 3. How do you know which allele is dominant and which is recessive? Rules for genetic crosses • Letters are used to represent alleles – Capital letters for dominant alleles – Corresponding small letters for recessive alleles – E.g. T – allele for tallness, t – allele for shortness • If organism is pure-bred, the two alleles are the same. Organism is said to be homozygous for that characteristic. – E.g. TT – homozygous dominant tt – homozygous recessive Tt – heterozygous Dominant and recessive • An allele is said to be dominant if it is always expressed in the appearance of an organism • E.g. the allele, T, for tall plants in pea is dominant to that for short plant, t. • Hence, with the pair of alleles, TT or Tt, the plants will always be tall. This shows the dominant allele is T • The effects of allele t which is recessive is seen when it is in tt form Table 19.2 Phenotype and Genotype Phenotype • Refers to expressed trait, that is the outward appearance or visible character of an organism (the characteristics of an organism which can be seen) Genotype • Genetic make-up of an organism, that is, the genes and their respective alleles (Genotype of TT or Tt, tt) • Hence, a dominant allele expresses itself and gives the same phenotype in both homozygous (TT) and heterozygous(Tt) condition • A recessive allele only expresses itself in the homozygous condition (tt), never in the heterozygous condition The test cross • Easy to tell genotype of an organism showing the recessive trait (tt) • Difficult to tell genotype of an organism showing the dominant trait (can be TT or Tt) • The genotype can only be identified by breeding experiments. How? • Cross the organism showing the dominant trait with an organism that is homozygous recessive – All offspring show dominant trait parent is homozygous dominant – Half the total number of offspring show dominant trait, the other half show recessive trait parent is heterozygous Refer to Pg 360 & 361 for worked example Try this: 1. In a breeding experiment a pure-bred black guinea pig was crossed with a pure-bred white one. All the F1 offspring were black. (a) Explain this information by means of a genetic diagram (b) If the F1 offspring were allowed to interbreed, what proportion of the F2 generation would be expected to be heterozygous? (c) If you were given a black guinea pig, how would you attempt to find out whether it is heterozygous or homozygous TYS 3.4 B MCQ Q1, 2, 3, 5, 7, 9, 10 Classwork Q11, 13, 15, 16, 17, 20, 21, 22, 25 Paper 2 Section A Q1, 3 Workbook Pg 151 MCQ Q2, 3, 4 Structured Questions Q1 Free Response Questions Q1 Incomplete Dominance or Co-dominance • In examples shown, one allele was dominant over the other • In many other cases, hybrid produced shows the effects of both alleles. Both alleles exert their effects so that the hybrid has a phenotype that is intermediate between that found in its parents • This is known as incomplete dominance / Codominance • Do example in the book (pg 364) Sex determination • In Man, the male has an X chromosome and a much shorter Y chromosome in each normal body cell • The female has 2 X chromosomes • In addition, each body cell has 22 pairs of autosomal chromosomes X Y