AQA GCSE Science BIOLOGY 2 Unit B2.7 Cell division and inheritance

• Know and understand that characteristics are passed on from one generation to the next in both plants and animals.

• Know and understand that simple genetic diagrams can be used to show this.

• Know and understand that there are ethical considerations in treating genetic disorders.

• You should be able to use your skills, knowledge and understanding to:

  • Be able to explain why Mendel proposed the idea of separately inherited factors and why the importance of this discovery was not recognised until after his death.

  • Be familiar with principles used by Mendel in investigating monohybrid inheritance in peas.

    • His worked involved (as far as he could tell) crossing different pure bred pea plants of a particular characteristic eg a particular colour or tall or short plants and then cross-breeding the offspring.

    • He showed that the

  • Understand that Mendel’s work preceded the work by other scientists which linked Mendel’s ‘inherited factors’ with chromosomes.

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  • Genetic table for crossing tall pea with dwarf pea
    Parent genotypes: TT x tt
    Gametes: T and T
    Genotypes of children	T	T
    t	Tt	Tt
    t	Tt	Tt

  • This gives 100% tall plants (genotype Tt), but they all carry the allele t for dwarf pea plants

  • The diagrams above and below give a modern genetic interpretation of Mendel's results from initially crossing a pure line of tall pea plants with a pure line of dwarf pea plants (F1) and then cross-breeding their offspring to give F2.

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  • Genetic table for crossing tall pea plants
    Parent genotypes: Tt x Tt
    Gametes: T and T
    Genotypes of children	T	t
    T	TT	Tt
    t	Tt	tt

  • The first resulting offspring (F1) were all tall pea plants, and these were then crossed with each other, to give the second set of offspring (F2) shown above.

  • This gave approximately 75% tall plants (genotype TT or Tt) and 25% dwarf pea plants (genotype tt)

  • Mendel found that the second cross produced tall : dwarf pea plants in the approximate ratio of 3 : 1.

  • Be able to interpret genetic diagrams, including family trees.

  • HT only: construct genetic diagrams of monohybrid crosses and predict the outcomes of monohybrid crosses and be able to use the terms homozygous (same alleles eg XX or TT) genes, heterozygous (different alleles eg XY or Tt), phenotype (gene expression - the outcome!) and genotype (gene type),

    • If you are a Foundation Tier candidate, you should be able to interpret genetic diagrams of monohybrid inheritance and sex inheritance but will not be expected to construct genetic diagrams or use the terms homozygous, heterozygous, phenotype or genotype.

  • Be able to predict and/or explain the outcome of crosses between individuals for each possible combination of dominant and recessive alleles of the same gene

  • Be able to make informed judgements about the social and ethical issues concerning the use of stem cells from embryos in medical research and treatments

  • Be able to make informed judgements about the economic, social and ethical issues concerning embryo screening.

•  Data in examination questions may be given for unfamiliar contexts.

AQA GCSE Science BIOLOGY Unit B2.7.1 Cell division

• a) Know and understand that in body cells the chromosomes are normally found in pairs.

  • Know that body cells divide by mitosis.

  • Knowledge and understanding of the stages in mitosis and meiosis is not required.

• b) Know and understand that the chromosomes contain the genetic information.

• c) Know and understand that when a body cell divides by mitosis:

  • Cell division my mitosis (diagram above, notes below)

  • (i) Copies of the genetic material are made ie copies of the DNA, which form into identical chromosomes.

  • (ii) Then the cell divides once to form two genetically identical body cells, in the process both full sets of chromosomes become enclosed in a cell membrane complete with the necessary cytoplasm, so the separation into two cells is complete.

  • Throughout section 2.7 you should develop an understanding of the relationship from the molecular level upwards between genes, chromosomes, nuclei and cells and to relate these to tissues, organs and systems (see sections B2.2 and B2.3).

    • DNA is the acronym for deoxyribonucleic acid and these giant molecules have all the coded instructions for reproduction and developing an organism and keeping the organism alive!

    • In the nucleus of a cell the DNA is collected together in huge sections called chromosomes.

    • Shorter sections of chromosomal DNA are called genes contain the code instructions to make specific proteins or differentiate the functions of specific cells etc. (etc. meaning everything!).

• d) Know and understand that mitosis occurs during growth or to produce replacement cells to replace damaged ones in both plants and animals.

• e) Body cells have two sets of chromosomes but sex cells (gametes) have only one set.

• f) Know the cells in reproductive organs: testes and ovaries in humans, divide to form gametes (see diagram below).

  • For Foundation Tier students, knowledge of meiosis is restricted to where the process occurs and that gametes are produced by meiosis.

• g) Know and understand the type of cell division in which a cell divides to form gametes is called meiosis (see below).

• h) HT only: Know and understand that when a cell divides to form gametes in sexual reproduction:

  • Cell division my meiosis (see diagram above, notes below)
  • copies of the genetic information are made, each gamete has 23 chromosomes

  • then the cell divides twice to form four gametes, each with a single set of chromosomes.

    • This double cell division process is called meiosis.

• i) Know and understand that when gametes join at fertilisation, a single body cell with new pairs of chromosomes is formed.

  • Know and understand that a new individual then develops by this cell repeatedly dividing by mitosis.

  • You should understand that genetic diagrams are biological models which can be used to predict the outcomes of crosses.

  • Egg cells carry the female DNA and food reserves for the embryo, the sperm carries the male DNA, and, enzymes in its head to break down the cell membrane of the egg cell so it can enter and combine with the egg cell.

  • The fertilised cell has 23 + 23 = 46 chromosomes and so inherits characteristics from both parents (male + female).

• j) Know and understand that most types of animal cells differentiate at an early stage whereas many plant cells retain the ability to differentiate throughout life.

  • Know that in mature animals, cell division is mainly restricted to repair and replacement.

• k) Know and understand that cells from human embryos and adult bone marrow, called stem cells, can be made to differentiate into many different types of cells, eg nerve cells.

  • Knowledge and understanding of stem cell techniques is not required.

  • Stem cells are found in early human embryos and have the potential to be converted into any type of cell found in the human body.

  • Adults have stem cells in their marrow but these can only be converted into a few specific type of cells - you may have heard the phrase 'bone marrow transplant'. A bone marrow transplant is a gene therapy procedure that involves replacing damaged bone marrow with healthy bone marrow stem cells. Stem cells in bone marrow produce three important types of blood cells : red blood cells – which carry oxygen around the body, white blood cells – which help fight infection and platelets – which help stop bleeding. It is used to treat sufferers of leukaemia, non-Hodgkin's lymphoma and sickle cell anaemia.

• l) Know and understand that human stem cells have the ability to develop into any kind of human cell.

• m) Know and understand that treatment with stem cells may be able to help conditions such as paralysis.

  • It is hoped to be able to grow nerve cells for people disabled by a spinal injury.

  • Stem cell research is a very controversial area.

    • The ethical issue of using embryos for medical purposes is abhorrent to some people.

    • This is the argument of 'potential life' versus help for seriously ill 'living people'.

    • It is possible to use unwanted embryos from fertility clinics because there is no other source of universal stem cells.

    • Stem cell research is allowed in some countries like the UK, but there are very strict guideline as to how it can be carried out.

• n) Know and understand that the cells of the offspring produced by asexual reproduction are produced by mitosis from the parental cells.

  • Know and understand that in asexual reproduction the offspring contain the same alleles as the parents.

  • Some plants reproduce by mitosis, so all new plants have identical genes and so are identical plants.

AQA GCSE Science BIOLOGY Unit B2.7.2 Genetic variation

• a) Know and understand that sexual reproduction gives rise to variation because, when gametes fuse, one of each pair of alleles comes from each parent.

• b) Know and understand that in human body cells, one of the 23 pairs of chromosomes carries the genes that determine sex.

  • In females the sex chromosomes are the same (XX); in males the sex chromosomes are different (XY).

  • All human cells have 22 matched pairs of chromosomes but the 23rd chromosome is different.

  • Men have an X and Y chromosome and women have two X chromosomes.

  • In the first stage of the meiosis of sperm cells, there is a 50% chance of having an X or Y chromosome in the new sperm cell. Egg cells only have one X chromosome.

  • Therefore on egg fertilisation there is a 50% chance of an XX or XY combination ie a 50% chance of being male or female (see table and diagram below).

    • Note use of the word 'chance'. These 'chances' are the probable outcome of many sexual reproductions.

    • In any data set, because of the random combinations of the gametes (from available possibilities), the outcome is unlikely to be perfectly 1:1, but more likely 48% : 50% (0.48 : 0.52,  0.92 : 1.0) or 51% to 49% (0.51 : 0.49, 1.00 : 0.96)

    • So bear this idea in mind when ratios like 1 : 3 etc. are quoted i.e. in reality as well as the possibility of 1.00 : 3.00, for other data sets it might be 0.97 : 3.00 or 1.00 to 3.02).

      • My good Irish wife Molly, has a cousin who has seven sons and no daughters!

      • So much for statistical probability and the dominance of the XY genotype!

  • Genetic table for human sex determination
    Parent genotypes: XX (female) x XY (male)
    Gametes: X and Y
    Genotypes of children	X	Y
    X	XX	XY
    X	XX	XY

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• c) Know and understand that some characteristics are controlled by a single gene.

  • Each gene may have different forms called alleles.

• d) Know and understand that an allele that controls the development of a characteristic when it is present on only one of the chromosomes is a dominant allele.

  • This is important when interpreting genetic diagrams (see later with the genetic disorder polydactyly).

• e) Know and understand that an allele that controls the development of characteristics only if the dominant allele is not present is a recessive allele.

  • This is important when interpreting genetic diagrams (see later with the genetic disorder cystic fibrosis).

• f) Know and understand that chromosomes are made up of large molecules of DNA (deoxyribonucleic acid) which has a double helix structure.

  • You are not expected to know the names of the four bases or how complementary pairs of bases enable DNA replication to take place.

  • DNA in the nucleus contains all the coded instructions for an organism to grow and develop ie everything it needs to know to function and reproduce!

• g) Know and understand that a gene is a small section of DNA.

  • Genes code for specific proteins and the type of cell they form part of.

• h) HT only: Know and understand that each gene codes for a particular combination of amino acids which make a specific protein.

• i) Know and understand that each person (apart from identical twins) has unique DNA.

  • Know that this can be used to identify individuals in a process known as DNA fingerprinting in forensic science and your DNA can be checked against a database of previous suspects or convicted criminals!

    • All you need is a sample of blood, hair, semen or skin.

    • It can also be used to identify if an individual is a relative of another.

    • As I was working on this page in 2013, the bones of King Richard III have been found by archaeologists in the City of Leicester, England. Chromosomal DNA was extracted from the bones and compared with one of the few known descendents of his family (a man in Canada, I think?) and a family match established. The bones showed that Richard III had a deformed back ('hunchback'), but you didn't need DNA to confirm that!

    • Since writing the above paragraph. on re-visiting Leicester, I took a photograph of the DNA evidence for confirming the bones found were those of Richard III (image below from the exhibition in the medieval Guildhall in Leicester from the work done by Leicester University).

      • They compared the mitochondrial DNA of Michael Ibsen and a 2nd matrilineal (lineage 2), with that of DNA extracted from the bones of Richard III.

      • You can see the matching base peaks (colour coded) for the specific and characteristic sequence based on the four bases G (guanine), A (adenine), C (cytosine) and T (thymine) found in the structure of the compared DNA molecules.

      • The sequence reads in sections such as ...GAACAAGCTATGTA.... etc.

  • Knowledge and understanding of genetic fingerprinting techniques is not required.

AQA GCSE Science BIOLOGY Unit B2.7.3 Genetic disorders

• a) Know and understand that some disorders are inherited.

• b) Know and understand that polydactyly/polydactyl – having extra fingers or toes – is caused by a dominant allele of a gene and can therefore be passed on by only one parent who has the disorder.

  • Genetic table for polydactyly
    Genotypes of parents: pp x Pp affected and normal
    Gametes: P,p plus p,p
    Genotypes of children	P	p
    p	Pp	pp
    p	Pp	pp

  • 

  • Polydactyly is caused by the dominant allele P (se doesn't need genotype PP).

  • The genetic diagram shows that there is a 50% chance of a child suffering from polydactyly if just one of the parents is a carrier.

    • Polydactyly is a physical condition in which a person has more than five fingers per hand or five toes per foot. Having an abnormal number of digits (6 or more) can occur on its own, without any other symptoms or disease. Polydactyly may be passed down (inherited) in families and this trait involves only one gene that can cause several variations.

• c) Cystic fibrosis (a disorder of cell membranes) must be inherited from both parents.

  • The parents may be carriers of the disorder without actually having the disorder themselves.

  • It is caused by a recessive allele of a gene and can therefore be passed on by parents, neither of whom has the disorder.

  • Genetic table for cystic fibrosis
    Genotypes of parents: Ff x Ff normal but both carriers
    Gametes: F,f plus F,f
    Genotypes of children	F	f
    F	FF	Ff
    f	Ff	ff

  • 

  • Cystic fibrosis is caused by a recessive allele f (so it needs genotype ff).

  • The genetic diagram shows that both parents must be carriers of the recessive allele and there is a 3/4 chance of having a normal child (FF non-carrier or Ff carrier) and a 1/4 chance of having a child with cystic fibrosis (ff sufferer and carrier).

    • Cystic fibrosis is a genetic disorder disease passed down through families. Cystic fibrosis causes thick, sticky mucus to build up in the lungs, digestive tract, and other areas of the body and is one of the most common chronic lung diseases in children and young adults. Sadly, it is a life-threatening disorder caused by a defective gene which causes the body to produce abnormally thick and sticky fluid, called mucus. The thick mucus builds up in the breathing passages of the lungs (causing lung infections) and in the pancreas, the organ that helps to break down and absorb food (causing digestion problems).

• d) Embryos can be screened for the alleles that cause these and other genetic disorders.

  • Knowledge and understanding of embryo screening techniques is not required.

• Your practical work to develop your skills and understanding may have included the following:

  • observation or preparation and observation of root tip squashes to illustrate chromosomes and mitosis

  • using genetic beads to model mitosis and meiosis and genetic crosses

  • making models of DNA

  • extracting DNA from kiwi fruit.