Variation

1. Variation

2. Explain what is meant by continuous and discontinuous variation, and give examples of each What does the term species mean State that variation can occur within a species

3. Species – group of organisms which can interbreed to produce fertile offspring. Variation – differences between members of the same species. There are 2 types of variation: 1. Discontinuous 2. Continuous

4. Discontinuous variation – can be observed, and separates members of a species into distinct groups, e.g. blood group, eye colour, etc. Discontinuous variation is usually presented in a bar graph.

5. Continuous variation – measured over a wide range, e.g., height, weight, pulse rate etc. 2 No. Pupils 1 0 100-110 111-120 121-130 Height (cm) Continuous variation is recorded as a histogram. See an example below.

6. Inheritance

7. Explain how the sex of a child is determined with reference to X and Y chromosomes From crosses identify examples of true-breeding, dominant and recessive characteristics Explain the differences between observed and predicted figures in monohybrid crosses Know the terms homozygous and heterozygous State the meaning of genotype In a cross identify generations as P, F1 and F2 State the meaning of phenotype Give examples of phenotypes of a chosen characteristic A fertilised egg has 2 sets of chromosomes State that every cell has 2 sets of chromosomes except for gametes which have 1 Each chromosome set from each parent contains 1 allele for each gene State that chromosomes contain genes Different forms of a gene are called alleles

8. Chromosomes carry genes, which control our inherited characteristics. Gene – unit of inheritance. Most human body cells have 46 chromosomes (or 23 pairs). We inherit one set of chromosomes from each parent, i.e., 23 from mum, and 23 from dad.

9. For one characteristic, you receive 2 genes – one from each parent. Different forms of a gene are called alleles. E.g., the gene for hair colour could carry information for brown or blonde hair etc.

10. If someone received a brown eye colour gene, and a blue eye colour gene, they would have brown eyes. This is because brown is dominant over blue. A dominant allele will always show in a person’s appearance. To have blue eyes, you would need 2 copies of the blue gene. Alleles such as this are called recessive. A recessive allele is always hidden by a dominant allele.

11. Phenotype – physical appearance of inherited characteristics. E.g., the possible phenotypes for eye colour – brown, blue, green, etc. For hair colour – brown, blonde, red, etc.

12. Genotype – combination of alleles for inherited characteristics. B for brown, capital as it is dominant. Lower case of the dominant letter. When writing a genotype, you assign letters. The letters are based on the dominant allele, which is given a capital letter. The recessive then becomes the lower case of the dominant. E.g. Brown eyes are dominant over blue, a person has a brown, and a blue gene, their genotype = Bb

13. Brown eyes are dominant over green eyes. A person has a brown gene, and a green gene. What is their genotype? Bb Even if the recessive allele has a different letter, it still takes the lower case of the dominant allele.

14. Genotypes can be: Homozygous – both alleles are the same Heterozygous – the alleles are different E.g., somebody with brown eyes could have either of the following genotypes: BB or Bb BB – homozygous Bb - heterozygous

15. The genotypes of individuals can be crossed to see what their offspring’s genotypes and phenotypes are. Brown eyes are dominant over blue. Cross between a homozygous dominant father, and a homozygous recessive mother.

16. P phenotypes Brown Blue P genotypes BB bb P gametes B b F1 genotypes Bb F1 phenotype ratio All brown eyes P = parents F1 = first generation of offspring. Number can be changed – F2 – second generation etc.

17. To work out genotypes of offspring, a punnettsquare can be used. Both parents have heterozygous genotypes for brown eyes. P genotypes = Bb, P gametes = B or b BB Bb Bb bb

18. BB Bb Bb bb F2 genotypes = BB, Bb, Bb, bb F2 phenotype ratio = 3 brown: 1 blue

19. 3:1 is the ratio of phenotypes when 2 heterozygous genotypes are crossed. This ratio is not always expressed as fertilisation is random.

20. A true-breeding organism has a homozygous genotype. If 2 true-breeding organisms for the same characteristic are crossed, the trait will always be present. When an organism exhibits a dominant trait, it is not obvious whether the genotype is homozygous or heterozygous. The genotype ca n be determined by a back cross, where possible genotypes are crossed by a recessive.

21. Mice: - Black fur = B, brown fur = b Black mouse produces brown offspring, what is the genotype? 2 possibilities: BB or Bb Do a punnett square testing both possibilities with a recessive genotype.

22. All offspring black fur Half black fur, half brown The black mouse must have genotype Bb

23. Determination of Sex • human body cells – 23 pairs of chromosomes • one pair = sex chromosomes • determine sex of person • X and Y chromosomes • Female = XX • Male = XY

24. all egg cells from mother – X • sperm cells from father – X or Y • => Father determines sex of child • 50% chance of child being male or female