Monohybrid Inheritance Intermediate 2 Biology Unit 2: Environmental Biology and Genetics

Inherited Characteristics Inherited characteristics are passed on from one generation to the next through sexual reproduction Chromosomes are made up of genes Alleles are different forms of the same gene

Glossary Genotype Phenotype Homozygous Heterozygous genes possessed by an organism Phenotype physical appearance of an organism Homozygous 2 identical alleles for a particular characteristic Heterozygous 2 different alleles for a particular characteristic

Glossary Dominant Recessive allele which controls the development of a characteristic when it is present on only one chromosome. Recessive allele which control the development of a characteristic if present on both chromosomes.

Further definitions P – Parents F1 – first generation F2 – second generation

Genetics and symbols The dominant allele is given a capital letter The recessive allele is given a lower case letter In plants, tall is dominant to dwarf T = tall t = dwarf

True-breeding An individual that is true-breeding has two identical alleles for the characteristic True breeding plants are homozygous In pea plants TT is true-breeding tall plant tt is a true breeding dwarf plant

Monohybrid Inheritance This is a genetic cross involving only one gene (characteristic)

P phenotype P genotype Gametes F1 genotype F1 phenotype A cross between a true breeding tall pea plant with a true-breeding dwarf pea plant. P phenotype P genotype Gametes F1 genotype F1 phenotype

Members of the F1 generation are self-pollinated F1 phenotype F1 genotype Gametes Fertilisation F2 genotype ratio F2 phenotype ratio

Time to practice In pea plants, round is dominant to wrinkled seeds. A cross was carried out between a pea plant homozygous for round seeds and a pea plant with wrinkled seeds. Carry out the cross through to the F2 generation

P phenotype P genotype Gametes F1 genotype F1 phenotype In pea plants, round is dominant to wrinkled seeds. A cross was carried out between a pea plant homozygous for round seeds and a pea plant with wrinkled seeds. P phenotype P genotype Gametes F1 genotype F1 phenotype

Members of the F1 generation are self-pollinated F1 phenotype F1 genotype Gametes Fertilisation F2 genotype ratio F2 phenotype ratio

Monohybrid Inheritance practice questions Remember to state symbols used at start Lay out the cross using headings P phenotype P genotype Gametes F1 genotype F1 phenotype F1 self pollinated Fertilisation F2 genotype ratio F2 phenotype ratio

Polygenic inheritance This is inheritance when the characteristic is controlled by more than one gene eye colour in humans Seed mass in pea plants

Predicting number of offspring A monohybrid cross between two true-breeding parents will always produce a 3:1 phenotype ratio in the F2 generation. Often there is a difference between observed and expected results due to fertilisation being a random process which involves the element of chance.

Using a bead model to illustrate a monohybrid cross This model represents self-pollination of the F1 generation of pea plants following a cross between a true-breeding pea plant with lilac flowers and a white-flowered pea plant. You will be provided with two beakers containing 100 lilac beads and 100 white beads These are to represent heterozygous lilac-flowered pea plants

Using a bead model to illustrate a monohybrid cross Collect three beakers labelled LL, Ll and ll Use the “gamete” beads to form and classify “zygotes” Record your results as a tally mark for the three genotypes Express your results as a F2 phenotypic ratio.

Cross number LL Ll ll ratio 4 10 25 50 100

Human Inheritance A family tree can be used to trace a particular trait through several generations.

Working out genotypes using a family tree Let T = tongue rolling ability Let t = inability to roll tongue

Co-dominance Co-dominant alleles are both expressed in the phenotype of a heterozygous organism. E.g. Flower colour RR = red Rr = pink Rr = white This co-dominant cross would give a 1:2:1 phenotype ratio

Human Blood Groups There are three alleles for human blood groups IA IB Io IA and IB are co-dominant Io is recessive

Human Blood Groups genotype Blood Group Phenotype IA IA A IA Io IB IB IBIo IA IB AB Io Io o

Environmental impact on phenotype Phenotype is a result of interaction between the genotype the effect of the environment on growth and development. Some characters are unaffected by environmental factors Tongue rolling ability ABO blood groups

Same genotype, different phenotypes Identical twins in humans A twin fed a poorer diet will fail to reach full potential height Clones in plants (asexual reproduction) Arrowhead plant (pg193) Differences caused by environmental factors are not passed on to next generation

evolution Two scientists, two theories Acquired characteristics – Lamark Natural Selection - Darwin

Natural selection Variation Over-production Struggle for survival All species of organisms has a large reproductive potential Struggle for survival The environment can not support all offspring and this leads to competition

Survival of the fittest Offspring whose phenotypes are better adapted to their environment have a better chance of surviving Pass on genes to their offspring Offspring whose phenotype are less well-suited to environment die before reaching reproductive age Fail to pass on genes

Charles Darwin Observation 1: Observation 2: Deduction: Living things have the potential to produce more offspring than are needed to replace them. Observation 2: The number of organisms in the wild, however, usually remains the same. Deduction: Darwin deduced that some organisms must die before they reproduce.

Observation 3: Deduction: Observation 4: Most living things show variation. No two individuals are usually alike. Deduction: Darwin concluded that the organisms which survived were the best adapted or ‘fittest’ Observation 4: Some types of variation are passed on from parents to their offspring. Populations will slowly change or ‘evolve’ as successful parents pass on characteristics to their offspring.

Natural Selection in Action Peppered Moth Two forms Light brown with speckles Melanic (dark) form Both forms flay at night and rest on tree trunks during the day

Peppered moth “A modern example of evolution” Pale coloured, speckled peppered moth

Peppered moth “A modern example of evolution” Dark coloured peppered moth

Peppered moth “A modern example of evolution” Peppered moths on tree trunk

In non-polluted areas, tree trunks are covered with lichens, the light form is well camouflaged against the background. In polluted areas, toxic gases kill the lichens and soot particles darken the trunks. The melanic (dark) form is favoured by natural selection.

Natural Selection in Action Activity You will be given two different colours of punched paper Spread out 50 discs of each colour onto to the sheet of A3 paper Start the stopwatch/ stopclock and time one minute. During this time, use the forceps to pick out as many paper discs as possible Place the paper discs in a Petri dish. Count how many paper discs of each colour you have removed. If the total number is an odd number, pick out one more grain. Add more paper discs the paper to make it up to 100 again, by adding equal numbers of each colour. Make a note of how many grains there are of each colour now. The proportion of the two colours will now be different.

Time another minute and pick out as many paper discs as possible. Repeat this 4 more times Repeat the whole activity on a sheet of news paper

Trial one – on white paper Number of discs removed Number of discs replaced Percentage of discs in each colour white newspaper At start 1st pick 2nd pick 3rd pick 4th pick 5th pick

Trial one – on news paper Number of discs removed Number of discs replaced Percentage of discs in each colour white newspaper At start 1st pick 2nd pick 3rd pick 4th pick 5th pick