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17. Inheritance

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Topic 17.1

17.1 Chromosomes, genes and proteins

Objectives in this topic

State that chromosomes are made of DNA, which contains genetic information in the form of genes

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

Define an allele as an alternative form of a gene

Describe the inheritance of sex in humans with reference to X and Y chromosomes

State that the sequence of bases in a gene determines the sequence of amino acids used to make a specific protein (knowledge of the details of nucleotide structure is not required)

Explain that different sequences of amino acids give different shapes to protein molecules

Explain that DNA controls cell function by controlling the production of proteins, including enzymes, membrane carriers and receptors for neurotransmitters

Explain how a protein is made, limited to: • the gene coding for the protein remains in the nucleus • messenger RNA (mRNA) is a copy of a gene • mRNA molecules are made in the nucleus and move to the cytoplasm • the mRNA passes through ribosomes • the ribosome assembles amino acids into protein molecules • the specific sequence 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 most 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

Describe a haploid nucleus as a nucleus containing a single set of chromosomes

Describe a diploid nucleus as a nucleus containing two sets of chromosomes

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

Topic 17.2

17.2 Mitosis

Objectives in this topic

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

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

State that the exact replication of chromosomes occurs before mitosis

State that during mitosis, the copies of chromosomes separate, maintaining the chromosome number in each daughter cell

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

Topic 17.3

17.3 Meiosis

Objectives in this topic

State that meiosis is involved in the production of gametes

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

Topic 17.4

17.4 Monohybrid inheritance

Objectives in this topic

Describe inheritance as the transmission of genetic information from generation to generation

Describe genotype as the genetic make-up of an organism and in terms of the alleles present

Describe phenotype as the observable features of an organism

Describe homozygous as having two identical alleles of a particular gene

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

Describe heterozygous as having two different alleles of a particular gene

State that a heterozygous individual will not be pure-breeding

Describe a dominant allele as an allele that is expressed if it is present in the genotype

Describe a recessive allele as an allele that is only expressed when there is no dominant allele of the gene present in the genotype

Interpret pedigree diagrams for the inheritance of a given characteristic

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

Describe codominance as a situation in which both alleles in heterozygous organisms contribute to the phenotype

Explain the inheritance of ABO blood groups: phenotypes are A, B, AB and O blood groups and alleles are IA, IB and Io

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

Describe red-green 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

ConceptIGCSE Biology