WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.501 Unit C B 3: Genetics. 00:00:09.167 --> 00:00:12.639 Gametes are sex cells: sperm and egg cells in humans. 00:00:12.789 --> 00:00:18.601 During fertilisation, their nuclei join (fuse) to form a fertilised egg called a zygote. 00:00:21.267 --> 00:00:25.174 Body cells are usually diploid, with two sets of chromosomes. 00:00:25.324 --> 00:00:30.581 Human diploid body cells have 23 pairs: 46 chromosomes in total. 00:00:33.233 --> 00:00:36.217 Gametes are haploid, with one set of chromosomes. 00:00:36.367 --> 00:00:39.428 Human gametes contain 23 chromosomes. 00:00:42.100 --> 00:00:51.261 Meiosis starts with a diploid cell, copies its D N A, and involves two divisions to produce four haploid cells with half the original chromosome number. 00:00:53.933 --> 00:00:57.176 The cells produced by meiosis are genetically different. 00:00:57.326 --> 00:01:03.522 Meiosis and the random combination of gametes at fertilisation contribute to genetic variation. 00:01:06.200 --> 00:01:17.483 Fertilisation restores the diploid chromosome number: a human sperm nucleus with 23 chromosomes combines with an egg nucleus with 23 to give a zygote with 46. 00:01:20.133 --> 00:01:28.686 Unlike meiosis, mitosis normally produces two genetically identical daughter cells with the same chromosome number as the parent cell. 00:01:28.836 --> 00:01:33.191 The detailed stages of meiosis are not required for Combined Science. 00:01:35.867 --> 00:01:41.529 D N A is a long molecule (a polymer) made from repeating units called nucleotides. 00:01:41.679 --> 00:01:45.913 Each nucleotide contains a sugar, a phosphate group and one base. 00:01:48.567 --> 00:01:54.188 The four D N A bases are adenine (A), thymine (T), guanine (G) and cytosine (C). 00:01:54.338 --> 00:01:57.005 Their sequence carries genetic information. 00:01:59.667 --> 00:02:03.410 D N A consists of two strands coiled into a double helix. 00:02:03.560 --> 00:02:06.361 Each strand has a sugar, phosphate backbone. 00:02:09.033 --> 00:02:14.712 Bases form complementary pairs between the strands: A pairs with T, and C pairs with G. 00:02:14.862 --> 00:02:17.721 Weak hydrogen bonds join the paired bases. 00:02:17.871 --> 00:02:20.068 A pairs with T and C with G. 00:02:20.218 --> 00:02:23.479 DNA is drawn untwisted to make the pairing clear. 00:02:28.133 --> 00:02:33.732 If one strand has the sequence ATGC, the complementary sequence on the other strand is TACG. 00:02:36.400 --> 00:02:39.384 Chromosomes contain long D N A molecules. 00:02:39.534 --> 00:02:43.749 A gene is a section of D N A that codes for a specific protein. 00:02:46.400 --> 00:02:48.640 An allele is a version of a gene. 00:02:48.790 --> 00:02:53.547 Differences between alleles can lead to differences in inherited characteristics. 00:02:56.200 --> 00:03:02.982 The genome is the entire D N A of an organism, including D N A that does not form protein-coding genes. 00:03:03.132 --> 00:03:05.180 It is not just a list of genes. 00:03:07.833 --> 00:03:13.967 Crush soft fruit such as strawberry to break up its tissues and increase contact with the extraction solution. 00:03:16.633 --> 00:03:19.947 Mix the crushed fruit with detergent and salt solution. 00:03:20.097 --> 00:03:27.158 Detergent breaks down cell and nuclear membranes, releasing D N A; salt helps the D N A strands collect together. 00:03:29.833 --> 00:03:33.866 Filter the mixture to remove solid pieces of cells (cell debris). 00:03:34.016 --> 00:03:40.468 The liquid that passes through the filter is the filtrate; it contains dissolved D N A and other substances. 00:03:43.133 --> 00:03:46.685 Carefully add cold ethanol as a layer above the filtrate. 00:03:46.835 --> 00:03:56.365 D N A cannot dissolve in cold ethanol (it is insoluble), so it comes out of solution (precipitates) as pale, stringy material where the two layers meet. 00:03:59.033 --> 00:04:03.982 The visible strands are many D N A molecules together, not a single double helix. 00:04:04.132 --> 00:04:10.752 Follow practical safety instructions: ethanol is flammable, avoid ignition sources, and use eye protection. 00:04:13.433 --> 00:04:18.446 Genotype is the combination of alleles an organism has for a gene or genes. 00:04:18.596 --> 00:04:24.759 Phenotype is its observable characteristics, influenced by genotype and often the environment. 00:04:27.433 --> 00:04:36.414 For a simple inherited characteristic, use a capital letter for the dominant allele and the same lower-case letter for the recessive allele, such as A and a. 00:04:39.067 --> 00:04:46.223 An organism is homozygous for a gene when its two alleles are the same: two capital A alleles or two lowercase a alleles. 00:04:46.373 --> 00:04:50.321 It is heterozygous when they differ: capital A with lowercase a. 00:04:53.000 --> 00:04:55.411 In a simple dominant–recessive model, 00:04:55.561 --> 00:05:04.871 the dominant allele is expressed in the phenotype when one or two copies are present: two capital A alleles and capital A with lowercase a show the dominant phenotype. 00:05:07.533 --> 00:05:13.526 A recessive allele is expressed in this model only when two copies are present: two lowercase a alleles. 00:05:13.676 --> 00:05:19.371 A heterozygous individual can carry and pass on a recessive allele without showing its phenotype. 00:05:22.033 --> 00:05:25.195 Dominant does not mean more common, stronger or better. 00:05:25.345 --> 00:05:33.419 Most human characteristics involve several genes; eye colour is not accurately explained by a single simple dominant, recessive pair. 00:05:36.100 --> 00:05:38.723 A monohybrid cross follows one gene. 00:05:38.873 --> 00:05:46.562 Write the parents’ genotypes, determine the alleles in their possible gametes, then combine one allele from each parent in a Punnett square. 00:05:49.233 --> 00:05:57.381 For capital A, lowercase a crossed with capital A, lowercase a, each parent can produce gametes containing capital A or lowercase a. 00:05:57.531 --> 00:06:06.702 The four equally likely combinations are two capital A alleles, capital A with lowercase a, capital A with lowercase a, and two lowercase a alleles. 00:06:06.852 --> 00:06:11.684 Each square is an equally likely allele combination, not a guaranteed child. 00:06:16.367 --> 00:06:24.207 For two heterozygous parents, the genotype ratio is one homozygous dominant to two heterozygous to one homozygous recessive. 00:06:24.357 --> 00:06:29.187 With complete dominance, the phenotype ratio is three dominant to one recessive. 00:06:31.867 --> 00:06:41.263 For two heterozygous parents, the probability of two lowercase a alleles is one out of four, which equals zero point two five, or twenty five percent. 00:06:41.413 --> 00:06:46.617 The probability of the dominant phenotype is three out of four, or seventy five percent. 00:06:49.300 --> 00:06:50.815 For capital A, 00:06:50.965 --> 00:06:53.823 lowercase a crossed with two lowercase a alleles, 00:06:53.973 --> 00:07:02.633 half the expected offspring are heterozygous and half are homozygous recessive: a one to one genotype and phenotype ratio in this simple model. 00:07:05.300 --> 00:07:13.885 For two capital A alleles crossed with two lowercase a alleles, all expected offspring are capital A with lowercase a and show the dominant phenotype. 00:07:16.567 --> 00:07:19.148 Each offspring is a separate chance event. 00:07:19.298 --> 00:07:26.293 Four offspring do not have to include exactly one recessive individual even when its probability is 25 percent. 00:07:28.967 --> 00:07:32.793 Expected number equals probability times total offspring. 00:07:32.943 --> 00:07:42.815 If a recessive phenotype has probability 0.25, the expected number among 80 offspring is 20; this is an expectation, not a guarantee. 00:07:45.467 --> 00:07:48.783 A pedigree chart tracks a characteristic through a family. 00:07:48.933 --> 00:07:55.620 Squares usually represent males, circles females; shading indicates who has the characteristic according to the key. 00:07:58.300 --> 00:08:02.994 A horizontal line joins parents and a branching line connects them to offspring. 00:08:03.144 --> 00:08:07.149 Use the key and relationships before assigning possible genotypes. 00:08:09.800 --> 00:08:18.193 For an autosomal recessive condition (one carried on a chromosome other than X or Y), affected individuals are two lowercase a alleles. 00:08:18.343 --> 00:08:23.815 An unaffected individual may be two capital A alleles or capital A with lowercase a. 00:08:23.965 --> 00:08:29.756 Someone with capital A with lowercase a is a carrier: they can pass on the allele without having the condition. 00:08:32.433 --> 00:08:41.637 If two unaffected parents have an affected child with an autosomal recessive condition, both parents must be carriers: each has capital A with lowercase a. 00:08:41.787 --> 00:08:47.141 Two unaffected carriers can have an affected child with an autosomal recessive condition. 00:08:51.800 --> 00:08:55.881 An affected two lowercase a alleles parent passes a to every child. 00:08:56.031 --> 00:09:02.504 An unaffected child of that parent must therefore be capital A with lowercase a in the simple recessive model. 00:09:05.167 --> 00:09:08.927 Some pedigree evidence leaves more than one possible genotype. 00:09:09.077 --> 00:09:15.818 Give all possible genotypes when appropriate; do not assign two capital A alleles to every unaffected individual. 00:09:18.500 --> 00:09:24.089 In the usual human X X/X Y model, females have X X and males have X Y. 00:09:24.239 --> 00:09:28.193 Sex chromosomes are one pair within the 23 chromosome pairs. 00:09:30.867 --> 00:09:35.539 Egg cells carry an X chromosome; sperm cells carry either X or Y. 00:09:35.689 --> 00:09:41.779 Fertilisation with an X-bearing sperm gives X X, and with a Y-bearing sperm gives X Y. 00:09:44.433 --> 00:09:51.494 An X X crossed with X Y Punnett square gives an approximately one to one chance of X X or X Y offspring. 00:09:51.644 --> 00:09:55.557 The sperm supplies the chromosome determining the outcome in this model. 00:09:55.707 --> 00:10:02.191 The standard X X, X Y model gives equal expected probabilities of X X and X Y offspring. 00:10:06.867 --> 00:10:11.011 The outcome of one fertilisation does not change the chance for the next. 00:10:11.161 --> 00:10:18.519 This model describes the standard GCSE inheritance calculation; biological sex development can be more complex. 00:10:21.200 --> 00:10:26.494 Variation means differences in characteristics between individuals of the same species. 00:10:26.644 --> 00:10:30.620 It can have genetic causes, environmental causes, or both. 00:10:33.300 --> 00:10:36.497 Genetic variation arises from different alleles. 00:10:36.647 --> 00:10:43.953 Mutations can create new alleles, while meiosis and fertilisation produce new combinations of existing alleles. 00:10:46.633 --> 00:10:49.149 A mutation is a change in D N A. 00:10:49.299 --> 00:10:56.252 Copying errors can occur when D N A is replicated, and some radiation or chemicals can increase mutation rates. 00:10:58.933 --> 00:11:04.906 Most mutations have no effect on phenotype, some have a small effect, and a few have a large effect. 00:11:05.056 --> 00:11:08.551 A mutation is not automatically harmful or beneficial. 00:11:11.233 --> 00:11:18.709 Environmental variation produces acquired characteristics, such as a scar or a change in body mass from diet. 00:11:18.859 --> 00:11:22.899 Acquired changes do not usually change the alleles passed to offspring. 00:11:25.567 --> 00:11:32.400 Many characteristics, such as height, are influenced by multiple genes and the environment, including nutrition. 00:11:35.067 --> 00:11:38.775 Continuous variation has a range of values, such as height. 00:11:38.925 --> 00:11:43.926 Discontinuous variation has distinct categories, such as ABO blood group. 00:11:46.600 --> 00:11:54.247 Continuous measurements are often grouped into intervals and displayed in a histogram; distinct categories can be shown in a bar chart. 00:11:54.397 --> 00:11:56.807 Select a suitable graph for the data. 00:11:59.467 --> 00:12:05.681 Some continuous traits show an approximately normal, bell-shaped distribution, with most values near the mean. 00:12:05.831 --> 00:12:10.006 Not every characteristic or data set follows a normal distribution. 00:12:12.667 --> 00:12:18.766 The Human Genome Project determined a reference sequence of the human genome and helped identify genes. 00:12:18.916 --> 00:12:24.472 It involved D N A from multiple people, rather than mapping only one individual’s genes. 00:12:27.133 --> 00:12:35.089 Comparing genomes can help identify gene variants associated with inherited conditions and improve understanding of disease. 00:12:37.767 --> 00:12:43.600 Genetic information can help predict some disease risks and guide which medicines may be effective. 00:12:43.750 --> 00:12:49.519 A risk prediction is not a certainty, especially where many genes and the environment are involved. 00:12:52.200 --> 00:12:57.053 Potential benefits include more targeted treatment and improved diagnosis. 00:12:57.203 --> 00:13:03.901 Decisions about using genetic information also involve privacy, consent and possible discrimination. 00:13:06.567 --> 00:13:08.401 That completes Genetics. 00:13:08.551 --> 00:13:11.897 Revisit the notes and test yourself on the revision website.