17. Inheritance

• Define chromosome as a thread-like structure of DNA, carrying genetic information in the form of genes

• Define gene as a length of DNA that codes for a protein

• Define allele as a version of a gene

• Describe the inheritance of sex in humans with reference to XX and XY chromosomes

• Explain that the sequence of bases in a gene is the genetic code for putting together amino acids in the correct order to make a specific protein (knowledge of the details of nucleotide structure is not required)

• Explain that DNA controls cell function by controlling the production of proteins (some of which are enzymes), antibodies and receptors for neurotransmitters

• Explain how a protein is made, limited to:

– the gene coding for the protein remains in the nucleus

– mRNA molecules carry a copy of the gene to the cytoplasm

– the mRNA passes through ribosomes

– the ribosome assembles amino acids into protein molecules

– the specific order of amino acids is determined by the sequence of bases in the mRNA (knowledge of the details of transcription or translation is not required)

• Explain that all body cells in an organism contain the same genes, but many genes in a particular cell are not expressed because the cell only makes the specific proteins it needs

• Define a haploid nucleus as a nucleus containing a single set of unpaired chromosomes, e.g. in gametes

• Define a diploid nucleus as a nucleus containing two sets of chromosomes, e.g. in body cells

• State that in a diploid cell, there is a pair of each type of chromosome and in a human diploid cell there are 23 pairs

• Define mitosis as nuclear division giving rise to genetically identical cells (details of stages are not required)

• State the role of mitosis in growth, repair of damaged tissues, replacement of cells and asexual reproduction

• State that the exact duplication of chromosomes occurs before mitosis

• State that during mitosis, the copies of chromosomes separate, maintaining the chromosome number (details of stages of mitosis are not required)

• Describe stem cells as unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions

• Define meiosis as nuclear division giving rise to cells that are genetically different (details of stages are not required)

• State that meiosis is involved in the production of gametes

• Define meiosis as reduction division in which the chromosome number is halved from diploid to haploid resulting in genetically different cells (details of stages are not required)

• Explain how meiosis produces variation by forming new combinations of maternal and paternal chromosomes (specific details are not required)

• Define genotype as the genetic make-up of an organism in terms of the alleles present

• Define phenotype as the observable features of an organism

• Define homozygous as having two identical alleles of a particular gene

• State that two identical homozygous individuals that breed together will be pure-breeding

• Define heterozygous as having two different alleles of a particular gene

• State that a heterozygous individual will not be pure-breeding

• Define dominant as an allele that is expressed if it is present

• Define recessive as an allele that is only expressed when there is no dominant allele of the gene present

• Interpret pedigree diagrams for the inheritance of a given characteristic

• Use genetic diagrams to predict the results of monohybrid crosses and calculate phenotypic

unknown genotype ratios, limited to 1:1 and 3:1 ratios

• Use Punnett squares in crosses which result in more than one genotype to work out and show the possible different genotypes

• Explain how to use a test cross to identify an unknown genotype

• Explain codominance by reference to the inheritance of ABO blood groups – phenotypes being A, B, AB and O blood groups and alleles being I^A, I^B and I^O

• Define a sex-linked characteristic as a characteristic in which the gene responsible is located on a sex chromosome and that this makes it more common in one sex than in the other

• Describe colour blindness as an example of sex linkage

• Use genetic diagrams to predict the results of monohybrid crosses involving codominance or sex linkage and calculate phenotypic ratios