Genetics Flashcards
Mitosis
This results in the formation of two genetically identical daughter cells.
Mitosis is used in asexual reproduction.
Meiosis
This results in the formation of four genetically non-identical daughter cells.
Meiosis is used in sexual reproduction.
Mitosis and Meiosis
Meiosis: Four genetically non-identical daughter cells
Mitosis: Two genetically identical daughter cells
Meiosis and Mitosis
Sexual reproduction is the fusion (joining together) of male and female gametes. Cells in reproductive organs divide by meiosis to create gametes. Gametes contain half of the number of chromosomes (containing DNA) found in all body cells.
Cells split
Each cell has a pair of each chromosome (diploid cell).
During meiosis each pair of chromosomes replicate and the cell splits in two.
Further cell splitting
There are now two identical cells.
The diploid cell divides again.
Haploid cells created
This creates four genetically different gametes that each have half the number of chromosomes of the parent cell. Cells that have only one copy of each chromosome (such as gametes) are called haploid cells.
Gametes fertilise
During sexual reproduction, the male gamete fertilises the female gamete and the fertilised cell now has the normal number of chromosomes (46 in humans).
Mitosis
Once the gametes have combined, the new cell divides by mitosis (the cell grows asexually).
As soon as the embryo reaches a certain size, cells begin to differentiate (specialise).
After the gametes have combined, the new cell divides by
mitosis which increases the number of cells. As soon as the embryo reaches a threshold size, cells begin to differentiate.
Sexual Reproduction
The process of reproduction where the nuclei of two gametes (sex cells) fuse to form a zygote (a process known as fertilisation), producing offspring that are genetically different to one another.
Male and female gametes fuse
Sperm and egg cells are the gametes in animals.
Pollen and ovum cells contain the gametes in flowering plants.
Sexual Reproduction
In sexual reproduction, male and female gametes fuse. Sperm and egg cells are the gametes in animals. Pollen and ovum cells contain the gametes in flowering plants.
Advantages of Sexual Reproduction
Some organisms can reproduce sexually or asexually. Fungi, malaria parasites and strawberry plants can all do this.
Variation in offspring
Variation in the offspring increases the chances of a population being able to survive environmental change by natural selection. This is because some individuals are likely to be adapted to the new conditions.
Artificial selection
Humans can speed up artificial selection through selective breeding of plants and animals.
This depends on genetic variation. This process has many benefits, including boosting food production and breeding fast horses.
An advantage of sexual reproduction is
variation in the offspring, increasing the chances of a population being able to survive environmental change by natural selection.
Asexual Reproduction
Asexual reproduction is the process of reproduction that forms genetically identical offspring from only one parent. Asexual reproduction is common in single-celled organisms and some plants.
Advantages of Asexual Reproduction
If an organism can reproduce sexually or asexually, it often reproduces asexually when conditions are good
Lots of identical offspring
If conditions are favourable, producing lots of identical offspring is positive.
Faster
Asexual reproduction is faster than sexual reproduction.
No mate
No mate is required. Therefore, asexual reproduction is more efficient with regards to time and energy.
The Genome
The genome is the entire (all) genetic material of an organism.
DNA
DNA is double helix polymer, which means it is a polymer (a large molecule made up of many subunits) made up of two strands forming a twisted, ladder shape.
Genes
A gene is a small section of DNA. Genes code for a sequence of amino acids, which combine to give a specific protein.
Chromosomes
Long strands of DNA are coiled up to form chromosomes.
Chromosomes contain many genes.
Human body cells contain 23 pairs of chromosomes, one of each pair coming from each parent.
Organisation of Genetic Material
The nucleus of eukaryotic cells contains chromosomes made of DNA molecules.
Each chromosome contains a large number of genes. Each gene tells how a specific protein should be made.
Is a Gene Bigger than DNA?
A gene is a segment of DNA. But there can be fragments of DNA that are smaller than a gene. This question is like asking whether a city or a community is bigger. They are kind of the same depending on how you define them.
DNA Extraction
DNA can easily be extracted from fruit:
Grind: Grind a sample of fruit (such as strawberry) in soapy water.
This breaks open cells, releasing the DNA from within cells. Filter: Filter the sample to produce a filtrate. Ethanol: Very slowly, pour ice-cold ethanol into the filtrate. The DNA moves into the ethanol by precipitation, and can then be removed with a wire loop.
Genome Sequencing
In 2003, the Human Genome Project was completed. Researchers had successfully studied the entire human genome. Since then, extensive research has been carried out on it. This has vastly increased our understanding.
Personalised medicine
Knowing the details of a patient’s genome might help doctors chose the medications that will help them the most.
Gene therapy
The Human genome project has been useful for gene therapy (inserting new or modified genes into a cell’s genome to treat a disease).
Gene identification
Identifying genes linked to different disorders. This would allow those at risk to make informed lifestyle decisions based on the known risk factors.
How could the completion of the Human Genome Project, and subsequent research on the human genome, be beneficial in tackling Cystic Fibrosis?
The knowledge we have gained from the Human Genome Project and subsequent research can help us in identifying Cystic Fibrosis genes, allowing those carrying it to be made aware.
It could also further our understanding of the disorder and how we should be treating it, and whether treatment should be different for people with key genome differences.
It could lead to a treatment involving gene therapy.
Nucleotide
DNA is a polymer made up of two long strands of small units that repeat throughout the structure, called nucleotides. Each of these is made up of a sugar, a phosphate and a base attached to the sugar.
DNA
DNA is a polymer consisting of two long strands of small units that repeat throughout the structure, called nucleotides. Each of these is made up of a sugar, a phosphate and a base attached to the sugar.
DNA Structure
The two long strands within each DNA molecule are held together by weak hydrogen bonds between opposite bases.
Each base has one other specific base with which it pairs:
T pairs with A.
G pairs with C.
The sugar and phosphates of nucleotides form the long strands.
What is known as a permanent change in the nucleotide sequence of DNA?
A mutation
If an individual possesses the genotype BB, we would say they were:
Homozygous, dominant
What is the pair of chromosomes responsible for determining male characteristics?
XY
Variation
Within a population, variation describes differences in the characteristics of individuals.
Genetics
Genetics (inherited genes).
All genetic variation is the result of mutations, some of which are then inherited and passed onto the next generation.
Genetics and environment
A combination of genetic and environmental causes.
The environment
The conditions in which the organism developed.
Variation
In sexually reproducing populations, there are many different combinations of alleles. This means that genetic variation is high. Variation can be the result of inherited genes or the environment or both.
Alleles
Whilst sexual reproduction is capable of shuffling pre-existing alleles, only mutations can generate new alleles.
Mutations
Most mutations (permanent changes in nucleotide sequences of DNA) do not affect the phenotype. Of the small number that do affect the phenotype, it is rare that a phenotypic change gives a significant survival advantage
Phenotype
A phenotype refers to the set of observable characteristics of an individual.
Survival advantage
If a mutation creates a new phenotype that is better adapted to environmental changes (than the rest of the population), the mutation is likely to spread throughout the population over a small number of generations.
How can mutations lead to human evolution?
Most mutations (permanent changes in nucleotide sequences of DNA) do not affect the phenotype.
But if a mutation creates a new phenotype that is better adapted to environmental changes (than the rest of the population), the mutation is likely to spread throughout the population over a small number of generations. For example, over time the size of the human brain has increased, as those born with a bigger brain were found to be at a survival advantage.
Mutations
A mutation is a permanent change in the nucleotide sequence of DNA. Mutations happen continuously and normally only slightly affect proteins or don’t affect them at all. Occasionally, a mutation may change the structure or shape of a protein.
Undesirable change
The outcome of a mutation is almost always detrimental to protein function.
For example, in enzymes, the substrate may no longer be able to bind to the active site. In structural proteins, their strength may be reduced.
Survival advantage
More rarely, a mutation may give a survival advantage, such as resistance to an antibiotic in bacteria.
These mutations can be beneficial and represent the foundation of evolution by natural selection.
Humanity’s Understanding of Genetics
Before the mid-19th century, the consensus was that sexual reproduction produced offspring that exhibited (had) a blend of characteristics.
Mid-19th century
Gregor Mendel, an Austrian monk, performed breeding experiments on pea plants.
This work showed that characteristics were determined by inherited “units” passed from parents.
Late 19th century
The first observation of how chromosomes behave during cell division.
Early 20th century
The similarity between the behaviour of chromosomes and Mendel’s ‘units’ was recognised.
Consequently, it was decided that the ‘units’ were located on chromosomes.
Additionally, the ‘units’ were renamed genes.
Mid-20th century
Technological advancements allowed scientists to work out the structure of DNA.
The mechanism by which genes operate was also unearthed at this time.
Alleles
Alleles are different forms of the same gene. Humans have pairs of every gene and in one gene, each half of the pair may have different alleles. People’s characteristics are determined by the alleles that they have.
Dominant Alleles
A dominant allele is always expressed, regardless of the identity of the other allele.
It only needs one copy present to be expressed (BB or Bb).
It is represented by a capital letter, e.g. B.
If B is the allele for brown eyes: When a person has a copy of the B allele, they will have brown eyes, no matter what other allele is present
Recessive Alleles
A recessive allele is only expressed if the other allele is also recessive.
It is represented by a lowercase letter e.g. b. It needs two copies to be present to be expressed (bb).
If b is the allele for blue eyes: A person can only have blue eyes if both of their alleles are b.
In most cases, a characteristic results
from multiple genes interacting. However, sometimes, a single gene is responsible for a characteristic. Red/green colour blindness is an example of a characteristic determined by a single gene.
A dominant allele is
always expressed, regardless of what the other allele is. It only needs one copy to be expressed (to be in) a person.
Genotype vs Phenotype
When talking about the inheritance of characteristics, we use the words genotype and phenotype.
Genotype
Genotype refers to the combination of alleles an organism has.
If the two alleles are different, we say that the person is heterozygous (Bb).
If the two alleles are the same, we say that the person is homozygous (BB or bb).
Phenotype
A phenotype is an observed characteristic of an individual.
The phenotype is determined by the interaction between the genotype and environment.
Earlobes being attached or free is an example of a phenotype, where the alleles present will determine a characteristic, unless the environment interferes.
A phenotype is the term given to
the observed characteristics of an individual. This is the result of an interaction between the genotype and environment
Punnett Squares
Most characteristics are the result of many genes interacting, but monohybrid inheritance refers to the inheritance of traits determined by a single gene.
Punnett squares are diagrams that help us to visualise the outcome of a monohybrid cross. In these diagrams, a capital letter shows a dominant allele.
The alleles of the parents are drawn along the top and side of a grid.
The pairs of alleles that the offspring could have are then filled in on the grid.
Pea Plant Punnett Square
Pea plants can be either green or yellow. The green colour allele is dominant over the yellow allele.
We represent the dominant green allele with G, and the recessive yellow allele with g on a Punnett square.
With one homozygote recessive parent and one heterozygote parent (shown above):
There is a 50% chance of offspring being yellow.
Ratio of green:yellow offspring will be 1:1.
What name do we give to diagrams that help us to visualise the outcome of a monohybrid cross?
Punnett squares
Punnett Squares
Punnett squares are diagrams that help us to visualise the outcome of a monohybrid cross. In these diagrams, a capital letter shows a dominant allele.
Pea Colour
An example of monohybrid inheritance is pea plant colour. Pea plants can be either green or yellow.
Green
The allele for green colour is dominant over the yellow. We represent the dominant green allele with G.
Yellow
The allele for yellow colour is recessive. We represent the recessive yellow allele with g.
If two heterozygote parents breed together, what percentage of their offspring would be expected to have a homozygous recessive genotype?
Homozygotes have one copy of each allele of a gene.
Sex Determination
Healthy human body cells contain 23 pairs of chromosomes, only one of which is responsible for determining sex (or gender). This pair of chromosomes are called the sex chromosomes and can either be X or Y.
Females
Females are XX, so an egg contains one X chromosome
Males
Males are XY, so a sperm contains either one X or one Y chromosome.
Gender Determination
The Punnett square shows that half of the embryo’s will be female, XX, and half will be male, XY.
Gender Punnett Square
The Punnett square for determining gender is shown above:
The mother is always XX. The father is always XY. 50% of the children are XX (girls) and 50% are XY (boys).
Codominance
Human blood group can be split into four categories: A, B, AB and O.
Multiple alleles
There are multiple alleles for the gene that controls blood group: IA, IB, and IO.
IA and IB are expressed equally (neither allele is dominant or recessive). This is known as codominance.
IO is recessive.
Cell markers
Red blood cells have ‘markers’ on their cell membrane. People with the A blood group have ‘A-markers’ on their cells; those in blood group B have ‘B-markers’; those with AB blood group have both ‘A- and B-markers’; those in blood group O have no markers.
Genotypes
The genotypes IAIA and IAIO result in blood group A.
The genotypes IBIB and IBIO result in blood group B.
The genotype IAIB causes blood group AB.
The O blood group is caused by genotype IOIO.
Inherited Disorders
People can develop disorders if they inherit certain alleles.
Example - cystic fibrosis
Cystic fibrosis is a disorder of cell membranes. It causes thick, sticky mucus to build-up in the lungs and digestive system.
Cystic Fibrosis Punnett Square
For cystic fibrosis, f is the recessive suffering allele, and F is the dominant non-suffering allele.
On the Punnett square above, both parents are non-sufferers, but carry the recessive suffering allele.
Their children will have a 25% chance of having cystic fibrosis.
This can also be described as a ratio:
Non-sufferers:sufferers = 3:1
Cystic Fibrosis Family Tree
We can use the knowledge that cystic fibrosis is caused by a recessive allele to work out the allele combinations of each individual in a family tree. If one of their offspring is a sufferer, and neither of the parents is a sufferer: The offspring must have two recessive alleles in order to suffer.
The parents are non-sufferers, so must have at least one of the dominant, non-recessive alleles each.
If either parent were homozygote dominant, the child could not be a sufferer.
Therefore, the parents must both be heterozygotes.
How many cystic fibrosis alleles does an individual need to have to suffer from the disease?
Two
The inheritance of certain alleles can result in the development of disorders, such as
cystic fibrosis. Cystic fibrosis is a disorder of cell membranes, which can result in the build-up of thick, sticky mucus in the lungs and digestive system. It is caused by a recessive allele.
Polydactyly
Polydactyly is a disorder where the sufferer has extra fingers or toes.
It is caused by a dominant allele. With one suffering parent (heterozygote), and one non-suffering parent (homozygote recessive): 50% of the offspring will have extra toes or fingers
Polydactyly
Polydactyly is a disorder where the sufferer has extra fingers or toes. It is caused by a dominant allele.
