Inheritance Flashcards
What is inheritance?
Inheritance is the transmission of genetic information from generation to generation
Where are chromosomes located?
Chromosomes are located in the nucleus of cells
What are chromosomes?
They are thread-like structures of DNA, carrying genetic information in the form of genes
What is a gene?
A gene is a short length of DNA found on a chromosome that codes for a specific protein (characteristic)
What is an example of a gene?
This could be a structural protein such as collagen found in skin cells, an enzyme or a hormone
What do genes control?
Genes control our characteristics
How do genes control our characteristics?
they code for proteins that play important roles in what our cells do
What is an allele?
Alleles are different versions of a particular gene
How many DIFFERENT chromosomes do humans have?
All humans have 23 different chromosomes in each cell
How many chromosomes do people have in general?
In most body cells, we have 2 copies of each chromosome, leading to a total of 46 chromosomes
What cells are an exception to having 46 chromosomes? How many do they have?
The gametes (egg and sperm cells) only have one copy of each chromosome, meaning they have a total of 23 chromosomes in each cell
What does it mean if a cell has a DIPLOID NUCLEI?
Nuclei with two sets of chromosomes are known as diploid nuclei
What does it mean if a cell has a HAPLOID NUCLEI?
Nuclei with one set of unpaired chromosomes are known as haploid nuclei
What is sex determined by?
Sex is determined by an entire chromosome pair (as opposed to most other characteristics that are just determined by one or a number of genes)
What sex chromosomes do females have?
XX
What sex chromosomes do males have?
XY
Who is responsible for determining the sex of the child?
As only a father can pass on a Y chromosome, he is responsible for determining the sex of the child
How can you show the inheritance of sex?
The inheritance of sex can be shown using a genetic diagram (known as a Punnett square)
What is different between a genetic diagram for sex inheritance and inheritance of other features?
the X and Y chromosomes taking the place of the alleles usually written in the boxes
What happens before a cell divides?
When cells divide their chromosomes double beforehand
Why do the chromosomes in a cell double before cell division?
This ensures that when the cell splits in two, each new cell still has two copies of each chromosome (is still diploid)
When is mitosis used?
This type of cell division is used for growth, repair of damaged tissues, replacement of cells and asexual reproduction
What is the definition of mitosis?
Mitosis is defined as nuclear division giving rise to genetically identical cells
- What is the first step to prepare for mitosis?
Just before mitosis, each chromosome in the nucleus copies itself exactly (forms x – shaped chromosomes)
- What happens after the chromosomes double before mitosis?
Chromosomes line up along the centre of the cell
- What happens to the chromosomes once they line up along the centre of the cell?
cell fibres pull them apart
- What happens once the cell fibres pull the chromosomes apart?
The cell divides into two; each new cell has a copy of each of the chromosomes
Why is mitosis so important?
All cells in the body (excluding gametes) are produced by mitosis of the zygote
What does mitosis replace?
Mitosis is important for replacing cells e.g, skin cells, red blood cells and for allowing growth (production of new cells e.g. when a zygote divides to form an embryo)
What 3 processes does mitosis occur in?
Growth
Repair
Asexual reproduction
How does mitosis help in growth?
mitosis produces new cells
How does mitosis help in repair?
to replace damaged or dead cells
How does mitosis help in asexual reproduction?
mitosis produces offspring that are genetically identical to the parent
What do all tissues in the human body contain?
Many tissues in the human body contain a small number of unspecialised cells
What are unspecialised cells called?
These are called stem cells
What is the function of stem cells?
their function is to divide by mitosis and produce new daughter cells that can become specialised within the tissue and be used for different functions
What type of cells does meiosis produce?
Meiosis is a type of nuclear division that gives rise to cells that are genetically different
What is meiosis used to produce?
It is used to produce the gametes (sex cells)
What must
meiosis result in?
The number of chromosomes must be halved when the gametes (sex cells) are formed
Why must the number of chromosomes be halved before meiosis?
Otherwise there would be double the number of chromosomes after they join at fertilisation in the zygote (fertilised egg)
What is meiosis described as?
This halving occurs during meiosis, and so it is described as a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells
- What happens at the beginning of meiosis?
It starts with chromosomes doubling themselves as in mitosis and lining up in the centre of the cell
- What happens after the chromosomes have lined up?
recombination occurs and then cell fibres will pull the pairs apart, each new cell will have one of each recombinant chromosome pair
- What happens in the second division?
chromosomes will line up along the centre of the cell, cell fibres will pull them apart (as with mitosis)
- How many cells does meiosis form and how can they be described?
Because of this double division, meiosis produces four haploid cells
What are examples of cells produced by meiosis?
Production of gametes e.g. sperm cells and egg cells, pollen grains and ovum
What does meiosis help with?
Increases genetic variation of offspring
How does meiosis increase genetic variation?
Meiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made, meaning that when gametes fuse randomly at fertilisation, each offspring will be different from any others
What is the difference between mitosis and meiosis in the number of cells produced?
MI - two cells produced (known as daughter cells)
ME - four cells produced (known as daughter cells)
What is the difference between mitosis and meiosis in the daughter cells?
MI - diploid
ME - haploid
What is the difference between mitosis and meiosis in the genetic differentiation of the daughter cells?
MI - genetically identical to each other and to the parent cell
ME - genetically different from each other and the parent cell
What is the difference between mitosis and meiosis in the number of divisions?
MI - one cell division occurs
ME - two cell division occurs
How many copies of each chromosome do we have?
2
How many copies do we have of each gene?
2 copies
How many copies do we have of each allele (for each gene)?
2
Why do we have 2 different alleles?
One of the alleles is inherited from the mother and the other from the father
What is a phenotype?
The observable characteristics of an organism (seen just by looking – like eye colour, or found – like blood type) is called the phenotype
What is a genotype?
The combination of alleles that control each characteristic is called the genotype
What 2 types of allele can there be?
Alleles can be dominant or recessive
What is the difference between recessive and dominant alleles in relation to the phenotype?
A dominant allele only needs to be inherited from one parent in order for the characteristic to show up in the phenotype
A recessive allele needs to be inherited from both parents in order for the characteristic to show up in the phenotype.
What happens if there is only one recessive allele?
If there is only one recessive allele, it will remain hidden and the dominant characteristic will show
What does it mean if a person is homozygous?
If the two alleles of a gene are the same, we describe the individual as being homozygous
What two different types of homozygous can a person be?
homozygous recessive
homozygous dominant
What does it mean if a person is homozygous recessive?
having two copies of the recessive allele
What does it mean if a person is homozygous dominant?
having two copies of the dominant allele
What does it mean if a person is heterozygous?
If the two alleles of a gene are different, we describe the individual as being heterozygous
What letter represents a dominant and recessive allele in a genetic diagram?
The dominant allele is given a capital letter and the recessive allele is given the same letter, but lower case
Can we tell the genotype from a person’s phenotype?
We cannot always tell the genotype of an individual for a particular characteristic just by looking at the phenotype
Why can’t we always tell a persons genotype from the phenotype?
a phenotype associated with a dominant allele will be seen in both a dominant homozygous and a dominant heterozygous genotype
What offspring will be produced if identically homozygous individual reproduce?
If two individuals who are both identically homozygous for a particular characteristic are bred together, they will produce offspring with exactly the same genotype and phenotype as the parents
Why is crossing two homozygous individuals called pure-breeding?
We describe them as being ‘pure breeding’ as they will always produce offspring with the same characteristics
Can heterozygous individuals participate in pure breeding?
A heterozygous individual can pass on different alleles for the same characteristic each time it is bred with any other individual and can therefore produce offspring with a different genotype and phenotype than the parents – as such, heterozygous individuals are NOT pure breeding
What is a monohybrid inheritance?
Monohybrid inheritance is the inheritance of characteristics controlled by a single gene (mono = one)
What does a punnett square show?
A Punnett square diagram shows the possible combinations of alleles that could be produced in the offspring
What is the ratio D to R when two heterozygous individuals are crossed?
There is more variation in this cross, with a 3:1 ratio of tall : short, meaning each offspring has a 75% chance of being dominant and a 25% chance of being recessive
What is the ratio D to R when one homozygous recessive individual and one heterozygous individual are crossed?
In this cross, there is a 1:1 ratio of dominant to recessive, meaning a 50% chance of the offspring being dominant and a 50% chance of the offspring being recessive
How can you find an unknown genotype?
Breeders can use a test cross to find out the genotype of an organism showing the dominant phenotype
What does a test cross involve?
This involves crossing the unknown individual with an individual showing the recessive phenotype – if the individual is showing the recessive phenotype, then its genotype must be homozygous recessive
How can we tell if the individual is heterozygous or homozygous from the test cross?
By looking at the ratio of phenotypes in the offspring, we can tell whether the unknown individual is homozygous dominant or heterozygous
How can we tell from the test cross that the organism is homozygous dominant?
tall and short plants
If the tall plant is homozygous dominant, all offspring produced will be tall
How can we tell from the test cross that the organism is heterozygous?
tall and short plants
If the tall plant is heterozygous, half the offspring will be tall and the other half will be short
What are family pedigree diagrams used to show?
Family pedigree diagrams are usually used to trace the pattern of inheritance of a specific characteristic (usually a disease) through generations of a family
What can we find out by a family pedigree?
This can be used to work out the probability that someone in the family will inherit the genetic disorder
What is an example of codominance?
Inheritance of blood group
What alleles are dominant and codominant in blood groups?
Alleles IA and IB are codominant, but both are dominant to IO
What does the letter I represent in blood group alleles?
I represents the gene and the superscript A, B and O represent the alleles
What does IA produce in the blood?
IA results in the production of antigen A in the blood
What does IB produce in the blood?
IB results in the production of antigen B in the blood
What does IO produce in the blood?
IO results in no antigens being produced in the blood
What does it mean if a characteristic is sex-linked?
When alleles that control a particular characteristic are found on the sex chromosomes, we describe the inheritance that results as ‘sex linked’
On which chromosome are the alleles usually on and why?
In almost all cases, there are only alleles on the X chromosome as the Y chromosome is much smaller
Why are males more likely to inherit sex-linked conditions?
Because males only have one X chromosome, they are much more likely to show sex-linked recessive conditions (such as red-green colour blindness and haemophilia)
Why are females less likely to inherit sex-linked conditions?
Females, having two copies of the X chromosome, are likely to inherit one dominant allele that masks the effect of the recessive allele
What is a woman known as if she has one recessive allele?
A female with one recessive allele masked in this way is known as a carrier; she doesn’t have the disease,
What is the chance that a female carriers offspring will have a sex-linked condition?
she has a 50% chance of passing it on to her offspring
If that offspring is a male, he will have the disease
What would be the result of a male who is colourblind a female carrier crossing?
In the cross above, there is a 25% chance of producing a male who is colourblind, a 25% chance of producing a female carrier, a 25% chance of producing a normal female and a 25% chance of producing a normal male
What are proteins made by?
Proteins are made by ribosomes
What forms proteins and what are they controlled by?
the sequence of amino acids controlled by the sequence of bases contained within DNA
Can DNA travel outside the nucleus? Why?
DNA cannot travel out of the nucleus to the ribosomes (it is far too big to pass through a nuclear pore)
How is the base code copied from the nucleus if it can’t leave the nucleus?
the base code of each gene is transcribed onto an RNA molecule called messenger RNA (mRNA).
What happens once the base code has been transcribed to mRNA?
mRNA then moves out of the nucleus and attaches to a ribosome
What does the ribosome do once it receives the mRNA?
The ribosome ‘reads’ the code on the mRNA in groups of three
What does each triplet of bases do?
Each triplet of bases codes for a specific amino acid
What does the ribosome do with the sequence of bases?
In this way the ribosome translates the sequence of bases into a sequence of amino acids that make up a protein
What happens once the amino acid has been assembled?
Once the amino acid chain has been assembled, it is released from the ribosome so it can fold and form the final structure of the protein
What does DNA control and how?
DNA controls cell function by controlling the production of proteins
What are examples of proteins?
The proteins may be enzymes, antibodies, or receptors for neurotransmitters
Why are some genes in certain cells not expressed?
Although all body cells in an organism contain the same genes, many genes in a particular cell are not expressed because the cell only makes the specific proteins it needs
