Reproduction

• Define gamete

  Sex cells (sperm and egg cells) with half the usual number of chromosomes
• Define chromosome

  A long, coiled molecule of DNA that carries genetic information in the form of genes
• Define allele

  a version of a gene
• Define gene

  A section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein
• Define dominant

  describes an allele that is always expressed
• Define recessive

  Describes an allele that is only expressed in the absence of a dominant allele
• Define homozygous

  When someone has two identical alleles of a gene
• Define heterozygous

  When someone has two different alleles of a gene
• Define genotype

  An organism’s genetic composition. Describes all alleles
• Define genome

  The complete genetic material of an organism
• Define phenotype

  An organism’s observable characteristics. Due to interactions of the genotype and the environment
• Advantages of sexual reproduction

  • produces variation in the offspring
  • if the environment changes, variation gives a survival advantage by natural selection
  • natural selection can be speeded up by humans in selective breeding to increase food production
• Advantages of asexual reproduction

  • only one parent needed
  • more time and energy efficient as no need to find mate
  • faster than sexual reproduction
  • many identical offspring can be produced when conditions are favourable
• Potatoes can reproduce asexually

  Potatoes from one plant can be planted in the ground to produce new potato plants

  Why are all the new plants from a parent plant that is resistant to blight also resistant to blight?

  • no fusion of gametes/ asexual reproduction involves mitosis
  • so offspring are genetically identical to the parent plant
• Name the structures inside the cell nucleus that contain DNA

  chromosomes
• What type of substance does a gene code for?

  protein
• What is the type of cell division that produces sperm cells?

  meiosis
• How many sperm cells are formed when a cell divides my meiosis

  4
• How many chromosomes are in each human egg?

  23
• Ways embryo screening can help people with a family history of a genetic disorder

  • help them prepare
  • inform whether to consider having an abortion
  • find out if they have passed on the disorder
• Why is DNA described as a polymer?

  Made up of many repeating nucleotides
• Describe the structure of a nucleotide

  • phosphate is attached to a sugar
  • which has one of four bases attached to it
  • bases are A, C, G and T
• How do non-coding parts of DNA affect the expression of genes

  Switch genes off and on
• Differences between mitosis and meiosis

  • mitosis produces two daughter cells but meiosis produces four daughter cells
  • one cell division in mitosis but two cell divisions in meiosis
  • mitosis produces cells with two of each chromosome, but meiosis produces cells with one of each chromosome
  • mitosis produces genetically identical cells, but meiosis produces genetically different cells
• Similarities between the processes of mitosis and meiosis

  • DNA replicates once
  • increase in the number of sub-cellular structures
• Why are siblings likely to look similar but not the same?

  • gametes contain half the parents chromosomes/alleles present in each gamete cell are random
  • gametes fuse/join/ combine randomly
  • so each offspring will inherit different combinations of allele, so will look different overall
  • the alleles from which offspring are produced are the same so they look similar
• Benefits of understanding the human genome

  • diagnosis of inherited/genetic disorders
  • treatment of inherited disorders may be provided
  • understanding human evolution/ ethnic origins of a person/ ancestry
  • tracing human migration patterns
• Name of scientist whose work was the beginning of our modern understanding of genetics

  Gregor Mendel
• Explain how a change in one amino acid in an enxyme molecule could stop the enzyme working

  • different protein made
  • active site changed
  • so substrate does not fit
• Scientists want to produce a type of cattle that makes large volumes of low-fat milk

  How would a science selectively breed cattle to do so

  • find female with lowest fat in milk and highest milk yield
  • find male whose female offspring have highest milk yield and lowest fat in milk
  • cross the best for both features
  • select best offspring for both features from each generation and repeat for several generations
• Two parents have five children that all look different to each other. Why?

  • multiple genes determine appearance
  • different combinations of alleles
  • different environmental effects
  • from different egg/sperm
• What do A, T, C and G stand for?

  Adenine, thymine, cytosine and guanine
• Polydactyly is an inherited condition caused by a dominant allele.

  A man has polydactyly

  This mother also has it but his father does not.

  Why is the man heterozygous for polydactyly?

  • man has inherited polydactyly dominant allele from mother
  • man has inherited recessive allele from father
  • because if father had the dominant allele he would have polydactyly but he doesn't
• How is polydactyly inherited?

  • caused by dominant allele
  • so if either parent passes on the dominant allele, the child will have the condition
• Why the actual ratio of a certain allele is not necessarily the same as the predicted ratio from a Punnett square?

  • calculated value shows expected statistical outcome
  • combination of alleles is random so actual offspring will not necessarily follow statistical likelihood
• Why are less people born with cystic fibrosis than polydactyly


  • polydactyly is caused by a dominant allele

  • only one allele is required to inherit condition

  • higher likelihood of inheriting condition

  • polydactyly does not reduce life expectancy

  • so allele for polydactyly likely to be passed on to offspring

  • CF is caused by a recessive allele

  • two recessive alleles need to be inherited to inherit condition • so likelihood of inheriting condition relatively low

  • CF reduces life expectancy

  • so allele for CF less likely to be passed on to offspring
• symptoms of cystic fibrosis

  • regular lung infection
  • persistent coughing
  • shortness of breath
  • poor growth rate/weight gain
  • infertility
• Why would a carrier (heterozygote) of CF not suffer from the disorder?


  • cystic fibrosis caused by recessive allele
  • carrier will have the allele combination Cc
  • disorder only expressed if both alleles are recessive
• Main stages of cell division

  • DNA replicates/2 copies of the chromosomes are formed
  • one set of chromosomes is pulled to each end of the cell
  • the nucleus divides
  • cytoplasm and cell membrane divides
  • to form 2 identical cells
• Steps in production of haploid cell

  • copies of the DNA/genetic information are made
  • cell divides twice
  • to form four gametes
  • each with a single set of chromosomes
  • each daughter cell is genetically different