1) Define Key words:
2) Inheritance of Gender in Humans
- There are 23 pairs of chromosomes in the nucleus of all diploid human cells.
- One of these pairs determine gender. These chromosomes are therefore called the sex chromosomes.
- For females these chromosomes are XX, for males the chromosomes are XY.
- All female egg cells contain only an X chromosome, male sperm cells may contain an X or a Y, thus the gender of the baby depends on which sperm cell fertilizes the egg cell.
- There is always a 50% chance of getting a boy and vice versa.
* 3) Explain that DNA controls cell function by controlling the Production of Proteins
- DNA controls the function of the cell by controlling the production of proteins (some which are enzyme), antibodies and receptors for neurotransmitters.
How Protein is made:
- the gene which codes for the protein is used to make a mRNA copy in the nucleus
- the mRNA copy (a molecules) leaves the nucleus, travels through the cytoplasm, to a ribosome
- the ribosome uses the mRNA copy to produce a chain of amino acids to make up the protein
- 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.
- the order of the amino acid chain is determined by the order of bases on the mRNA.
- this order is specific to each protein made.
* 4) Explain that all body cells in an organisms contain the Same Genes
- 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 protein it needs.
- These genes are therefore switched ‘on’ or ‘off’. When the gene is switched on, it is expressed, and the protein associated with the gene is synthesized. When the protein is not required, the gene is switched off.
5) Describe Mitosis
Mitosis is needed for:
6) Describe Meiosis
Meiosis is needed for:
7) Define Monohybrid Inheritance
- Is the inheritance of characteristics controlled by a single gene
- Can be determined using a Punnett Square, a genetic diagram that shows the possible combinations of alleles that could be produced in the offspring.
8) Define Key words
- Genotype: genetic makeup of an organism in terms of the alleles present (e.g. Tt or GG).
- Phenotype: observable features of an organism due to both its genotype and environment (e.g. tall plant or green seed).
- Homozygous: having two identical alleles of a particular gene. Two homozygous individuals that breed together will be pure-breeding.
- Heterozygous: having two different alleles of a particular gene. A heterozygous individual will not be pure-breeding.
- Dominant: is an allele that is expressed if it is present.
- Recessive: is an allele that is only expressed when there is no dominant allele of the gene present.
- Monohybrid cross: a genetic mix between two individuals who have homozygous genotypes, or genotypes that have completely dominant or completely recessive alleles. Used to observe how the offspring of homozygous individuals express the heterozygous genotypes they inherit from their parents.
9) Punnett Square
- Shows the possible allele combinations that can result in the offsping from parents with known genotypes.
Phenotypic ratio 3:1
10) Pedigree Diagram
- Used to trace the inheritance pattern of a genetic condition through a family
* 11) Explain how to use a Test Cross to Identify an Unknown Genotype
- A test cross involves mating an unknown genotype individual (eg. AA or Aa) with a known homozygous recessive (aa).
- The recessive alleles will always be masked by the presence of the dominant alleles.
- The phenotype of the offspring will reflect the genotype of the unknown parent.
- If unknown parent is homozygous dominant, all offspring will express dominant phenotype.
- If unknown parent is heterozygous, half the offspring should be dominant and half recessive.
* 12) Explain Co-Dominance
- Some alleles are co-dominant, meaning that neither is recessive, and they are both displayed in the phenotype.
- Example: blood groups. The three possible alleles for blood groups are A, B and O.
- A and B alleles are co-dominant, which leads to the AB blood group.
- O is recessive, and thus is only displayed in the phenotype if both parents have O blood groups.
* 13) Describe Colour Blindness as Example of Sex Linkage
- Sex-linked characteristic are 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.
- Example: any gene located on the Y chromosome can only be present in males as females do not have this chromosome. An example of a sex-linked characteristic is colour blindness, which is a recessive characteristic found on the X chromosome.
14) Define Variation
- Refers to the differences between individuals of the same species.
- Variation is beneficial to a species as it allows for natural selection to occurs and reduces the risk of extinction from disease.
Two types of variation:
- Genetic variation: each organism in a species has a different set of DNA. Meiosis increases genetic variation, each gamete has a different set of alleles. Fusion of two gametes can produce an entirely new set of genes.
- Phenotypic variation: refers to observable characteristics eg. height or hair color. Can be caused by genetic and environmental factors.
Variation can be
- Continuous: results in a range of phenotypes between two extremes, e.g. height in humans
- Discontinuous: is caused by genes alone, results in a limited number of distinct phenotypes (e.g. blood group limited to O, A, B or AB only in human)
15) Define Mutation
- Mutation is a genetic change. It is the way in which new alleles are formed.
- Gene mutation is a change in the base sequence of DNA.
- Can occur due to exposure to chemical and ionising radiation eg. X-rays. The greater the dose of radiation, the greater the chance of a mutation.
- Down’s syndrome, where a parent’s chromosomes are unevenly distributed in meiosis. In fertilisation, a zygote with a number of chromosomes that is not 46 is created (e.g. 23 + 24).
- Characteristics: broad forehead, short neck, downward-sloping eyes, short nose and mental retardation.
* 16) Explain effects of Change in Base Sequence of the Gene for Haemoglobin
- Another example is sickle-cell anemia, a condition where red blood cells become sickle shaped. Sickle cells carry less oxygen and can block blood vessels.
- This condition is caused by a change in the base sequence of the gene for haemoglobin, resulting in abnormal haemoglobin and sickle-shaped red blood cells.
- This allele is recessive, so it is only expressed if two copies of this allele are present (homozygous).
- The mutation can also have positive effects; people who are homozygous or heterozygous, i.e. have one sickle-cell allele and one unmutated allele, are immune to malaria, as the malaria parasite cannot infect the sickle-shaped cells.
- Sickle-cell anemia is commonly found in human population where malaria is common. This shows that natural selection for this gene is occurring in these areas, as those with the gene do not catch the disease and are more likely to survive.
17) Discuss the process of Adaptation
- A process, resulting from natural selection, by which populations become more suited to their environment over many generations.
- inherited feature that helps an organism to survive and reproduce in its environment.
- Plants that live in very dry climates, eg. cactus. Have to reduce water loss to increase survival.
- Fewer stomata: lesser diffusion of water vapour out of the plant via the stomata. Stomata are also sunken in pits in the leaf, which allows bubbles of moist air to be trapped around them, lowers the water potential gradient, so less water is lost from the leaf.
- Small, rolled leaves or spines: reduces the surface area of the leaf.
- Deep roots: allows plants to absorb water from the soil. Roots are also adapted to absorb lots of water when it rains for storage, e.g. in monsoon seasons.
- Thick waxy cuticle: provides a waterproof barrier around the leaf to prevent water loss.
- Plants that live in very wet conditions, eg. water lily and lotus. No need to minimize water loss.
- Leaf shape: leaves are usually large and flat to have a large surface area, promotes water loss.
- Stomata: positioned on the top of the leaf where the sun hits. There is also a large number of stomata, which are usually open to allow water vapour to diffuse out of the leaf.
- Thin/no waxy cuticle: no need to restrict water loss.
- Small root system: root systems can be shallow, water can diffuse directly into the stem.
18) Describe Natural Selection
- Through variation in the alleles of each species, each organism can develop positive and negative traits, resulting in variation within populations.
- Those with positive traits can adapt to the environment more effectively and thus more likely to compete better for resources and produce many offspring, and pass on their advantageous alleles to the next generation.
- Over time, those with negative traits will struggle for survival, fail to reproduce, get eaten up or die off.
- The change in adaptive features of a population over time as the result of natural selection.
The Process of Adaptation:
- The process, resulting from natural selection, by which population become more suited to their environment over many generations.
Example: Antibiotic resistance
- Mutation can occur in a bacterial cell allele, making it resistant to an antibiotic.
- When that antibiotic is administered, the non-resistant bacteria are killed, but the resistant bacterial is not killed.
- The resistant bacteria survive and reproduce, passing on the resistant allele to new resistant bacteria.
19) Describe Artificial Selection
- Selective Breeding is where human select animals or plants with desirable features are cross-breed together to make more offspring with these desirable features.
- Offspring with the most desirable features are chosen to continue the breeding programme and the process is repeated over a number of generations.
- As this is controlled by humans, it is known as artificial selection.
- The largest tomato is picked and selectively bred for their size.
- Crossing dogs with desirable qualities so that the alleles are passed on to their offspring.