inheritance Flashcards
Inheritance is
the transmission of genetic information from generation to generation
Chromosomes are located in
the nucleus of cells
chromosomes are
thread-like structures of DNA, carrying genetic information in the form of genes
A gene is
a short length of DNA found on a chromosome that codes for a specific protein
Genes control
our characteristics as they code for proteins that play important roles in our cells
Alleles are
different versions of the same gene; for example, the gene for eye colour has alleles for different colours like brown and blue:
Diploid & Haploid Nuclei
All humans have 23 different chromosomes in each cell
In most body cells, not including the gametes (sex cells), we have 2 copies of each chromosome, leading to a total of 46 chromosomes
Nuclei with two sets of chromosomes are known as diploid nuclei
The gametes (egg and sperm) only have one copy of each chromosome, meaning they have a total of 23 chromosomes in each cell
Nuclei with one set of unpaired chromosomes are known as haploid nuclei
Sex, or gender, is determined by
an entire chromosome pair (as opposed to most other characteristics that are just determined by one or a few pairs of genes)
Females have the sex chromosomes
XX
Males have the sex chromosomes
XY
As only the father can… they are responsible for
- pass on either an X or Y chromosome,
- pass on either an X or Y chromosome,
XX & XY Chromosomes shown using s genetic diagram
the sequence of bases in a gene is
the genetic code for putting together amino acids in the correct order to make a specific protein
Explain how a protein is made
– the gene coding for the protein remains in the nucleus – mRNA molecules carry a copy of the gene to the cytoplasm – 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
DNA controls cell function by
controlling the production of proteins (some of which are enzymes), antibodies and receptors for neurotransmitters
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
Mitosis
Stem Cells
Many tissues in the human body contain a small number of unspecialised cells
These are called stem cells and 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
mitosis is used for
growth, repair of damaged tissues, replacement of cells and asexual reproduction
Mitosis is defined as
nuclear division giving rise to genetically identical cells
cells divide their chromosomes double beforehand in mitosis because
it ensures that when the cell splits in two, each new cell still has two copies of each chromosome (is still diploid)
Meiosis is a
a type of nuclear division that gives rise to cells that are genetically different
meiosis is used to
produce the gametes (sex cells)
Meiosis
The number of chromosomes must be halved when the gametes (sex cells) are formed
Otherwise there would be double the number of chromosomes after they join at fertilisation in the zygote (fertilized egg)
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
It starts with chromosomes doubling themselves as in mitosis and lining up in the centre of the cell
After this has happened the cells divide twice so that only one copy of each chromosome passes to each gamete
We describe gametes as being haploid – having half the normal number of chromosomes
Because of this double division, meiosis produces four haploid cells
eiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made
This means that when gametes fuse randomly at fertilisation, each offspring will be different from any others
Differences between Mitosis & Meiosis
A gene is
a short length of DNA found on a chromosome that codes for a particular characteristic (expressed by the formation of different proteins)
Alleles are
variations of the same gene
phenotype
The observable characteristics of an organism
genotype
The combination of alleles that control each characteristic
Alleles can be dominat or recessive
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
If there is only one recessive allele, it will remain hidden and the dominant characteristic will show
If the two alleles of a gene are the same, we describe the individual as being homozygous (homo = same)
An individual could be homozygous dominant (having two copies of the dominant allele), or homozygous recessive (having two copies of the recessive allele)
If the two alleles of a gene are different, we describe the individual as being heterozygous (hetero = different)
When completing genetic diagrams, alleles are abbreviated to single letters
The dominant allele is given a capital letter and the recessive allele is given the same letter, but lower case
homozygous
If the two alleles of a gene are the same
heterozygous
f the two alleles of a gene are different
When completing genetic diagrams, alleles are
abbreviated to single letters
The dominant allele is given a capital letter and the recessive allele is given the same letter, but lower case
In the example above, the phenotypes and genotypes from left to right would be:
Phenotype – brown eyes; genotype – BB (homozygous dominant)
Phenotype – blue eyes; genotype – bb (homozygous recessive)
Phenotype – brown eyes; genotype – Bb (heterozygous)
we cannot always tell the genotype of an individual for a particular characteristic just by looking at the
phenotype
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 – we describe them as being ‘pure breeding’ as they will always produce offspring with the same characteristic
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
Monohybrid inheritance is
the inheritance of characteristics controlled by a single gene (mono = one)
Monohybrid inheritance an be determined using a
genetic diagram known as a Punnett square
A Punnett square diagram shows
s the possible combinations of alleles that could be produced in the offspring
From this the ratio of these combinations can be worked out
Remember the dominant allele is shown using a capital letter and the recessive allele is shown using the same letter but lower case
You should always write the dominant allele first, followed by the recessive allele
The height of pea plants is controlled by a single gene that has two alleles: tall and short
The tall allele is dominant and is shown as T
The small allele is recessive and is shown as t
‘Show the possible allele combinations of the offspring produced when a pure breeding short plant is bred with a pure breeding tall plant’
The term ‘pure breeding’ indicates that the individual is homozygous for that characteristic
The tall plant’s genotype is TT
The short plant’s genotype is tt
The possible combination of alleles in the offspring, shown in a Punnett square, is:
‘Show the possible allele combinations of the offspring produced when two of the offspring from the first cross are bred together’
All of the offspring of the first cross have the same genotype, Tt (heterozygous), so the possible combinations of offspring bred from these are:
‘Show the results of crossing a heterozygous plant with a short plant’
The height of pea plants is controlled by a single gene that has two alleles: tall and short
The tall allele is dominant and is shown as T
The small allele is recessive and is shown as t
The heterozygous plant will be tall with the genotype Tt
The short plant is showing the recessive phenotype and so must be homozygous recessive – tt
The results of this cross are as follows:
How to construct Punnett squares
Determine the parental genotypes
Select a letter that has a clearly different lower case, for example: Aa, Bb, Dd
Split the alleles for each parent and add them to the Punnett square around the outside
Fill in the middle four squares of the Punnett square to work out the possible genetic combinations in the offspring
You may be asked to comment on the ratio of different allele combinations in the offspring, calculate a percentage chances of offspring showing a specific characteristic or just determine the phenotypes of the offspring
Completing a Punnett square allows you to predict the probability of different outcomes from monohybrid crosses
Identifying an Unknown Genotype
Breeders can use a test cross to find out the genotype of an organism showing the dominant phenotype
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
By looking at the ratio of phenotypes in the offspring, we can tell whether the unknown individual is homozygous dominant or heterozygous
‘A plant breeder has a tall plant of unknown genotype. How can they find out whether it is homozygous dominant or heterozygous?’
The short plant is showing the recessive phenotype and so must be homozygous recessive – tt
If the tall plant is homozygous dominant, all offspring produced will be tall:
Family Pedigrees
Family pedigree diagrams are usually used to trace the pattern of inheritance of a specific characteristic (usually a disease) through generations of a family
This can be used to work out the probability that someone in the family will inherit the genetic disorder
Males are indicated by the square shape and females are represented by circles
Affected individuals are red and unaffected are blue
Horizontal lines between males and females show that they have produced children (which are shown underneath each couple)
The family pedigree above shows:
both males and females are affected
every generation has affected individuals
That there is one family group that has no affected parents or children
the other two families have one affected parent and affected children as well
Codominance
Some genes have alleles that are equally dominant and so are both expressed equally in the phenotype
This is known as codominance
Both codominant alleles are shown with upper case letters in genetic diagrams, but the letters used are different
For example, feather colour in hens may be white, black or speckled (it has both white feathers and black feathers)
The alleles can be shown as…
There are three possible genotypes:…
There are also three possible phenotypes:
- W for white and B for black
- WW, BB and BW
- WW = white, BB = black, and BW = speckled
Inheritance of Blood Group
Inheritance of blood group is an example of codominance
There are three alleles of the gene governing this instead of the usual two
Alleles IA and IB are codominant, but both are dominant to IO
I represents the gene and the superscript A, B and O represent the alleles
IA results in the production of antigen A in the blood
IB results in the production of antigen B in the blood
IO results in no antigens being produced in the blood
These three possible alleles can give us the following genotypes and phenotypes:
image is wrong
‘Show how a parent with blood group A and a parent with blood group B can produce offspring with blood group O’
The parent with blood group A has the genotype IAIO.
The parent with the blood group B has the genotype IBIO.
We know these are their genotypes (as opposed to both being homozygous) as they are able to produce a child with blood group O and so the child must have inherited an allele for group O from each parent
Parents with these blood types have a 25% chance of producing a child with blood type O
Sex-Linked Characteristics
When alleles that control a particular characteristic are found on the sex chromosomes, we describe the inheritance that results as ‘sex linked’
In almost all cases, there are only alleles on the X chromosome as the Y chromosome is much smaller
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)
Females, having two copies of the X chromosome, are likely to inherit one dominant allele that masks the effect of the recessive allele
A female with one recessive allele masked in this way is known as a carrier; she doesn’t have the disease, but she has a 50% chance of passing it on to her offspring
If that offspring is a male, he will have the disease
The results of a cross between a normal male and a female who is a carrier for colourblindness is as follows:
Sex-Linked Characteristics ad diseases
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)
Females, having two copies of the X chromosome, are likely to inherit one dominant allele that masks the effect of the recessive allele
A female with one recessive allele masked in this way is known as a carrier; she doesn’t have the disease, but she has a 50% chance of passing it on to her offspring
If that offspring is a male, he will have the disease
The results of a cross between a normal male and a female who is a carrier for colourblindness is as follows:
