Inheritance Flashcards
What is a gene?
A length of DNA on a chromosome that codes for the production of one or more polypeptide chains and functional RNA
Define genotype.
The genetic constitution of an organism.
Define phenotype.
The expression of an organism’s genetic constitution, combined with its interaction
with the environment.
What is a locus?
The fixed position on a DNA molecule occupied by a gene.
What is an allele?
Different forms of a particular gene, found at the same locus on a chromosome. A single gene could have many alleles.
Define homozygous
Having two identical alleles of a gene e.g. FF or ff
Define heterozygous
Having two different alleles of a gene
e.g. Ff
What is a dominant allele?
An allele that is always expressed
Represented with a capital letter e.g. F
What is a recessive allele?
- An allele that is only expressed if 2 copies are present
- Represented with a small letter e.g. f
What is meant by codominant alleles?
Two dominant alleles that both contribute
to the phenotype, either by showing a blend of both characteristics, or the characteristics appearing together.
What is an autosome?
A chromosome that is not an X or Y
chromosome.
What is a sex chromosome?
A chromosome that determines the sex of an organism, e.g. X and Y chromosomes in humans and other mammals.
What is the F 1 generation?
The first generation of offspring resulting from the cross of two individuals in the
parental generation.
What is the F 2 generation?
The second generation of offspring resulting from the cross of two individuals in the F 1 generation.
Define monohybrid inheritance.
Where one phenotypic characteristic is controlled by a single gene.
What is Mendel’s first law of inheritance?
Law of segregation
Alleles separate randomly into gametes
Each parent passes one allele to their offspring
What is Mendel’s second law of inheritance?
Law of independent assortment
The alleles of genes assort independently of other genes during gamete formation
What is the purpose of a test cross?
Used to determine whether an individual
with a dominant trait is heterozygous or homozygous dominant.
Describe how a test cross is carried out.
An individual of unknown genotype is bred with a homozygous recessive individual
Offspring phenotypes are observed to determine
the genotype of the unknown individual
Define dihybrid inheritance.
The determination of a trait by the inheritance of two genes.
What does Mendelian inheritance assume?
It assumes that the genes involved are not linked.
What is meant by sex-linkage?
- An allele is located on one of the sex chromosomes
- Its expression depends on the sex of the individual
Give an example of a phenotype which shows sex-linkage.
- Haemophilia
- Duchenne muscular dystrophy
Describe haemophilia.
- X-linked recessive condition
- Results in excessive bleeding and blood that is slow to clot
Describe Duchenne muscular dystrophy.
- X-linked recessive condition
- Characterised by muscle degeneration and weakness
Why are haemophilia and Duchenne muscular dystrophy more common in males?
They are X-linked recessive conditions. Males only inherit one X chromosome so are more likely to express the gene in their phenotype.
What is linkage?
- Two or more genes positioned on the same autosome
- Unlikely to be separated by crossing over during meiosis so often inherited together
What is the chi-squared test?
A statistical test used to determine whether the difference between observed and expected data is due to chance or a real effect. Can be used to compare expected phenotypic ratios with observed ratios.
How is a chi-squared test performed?
Make a null hypothesis
Used Mendelian ratios to calculate the expected numbers
Calculate chi-squared value using chi-squared equation
Calculate the degrees of freedom
Select an appropriate significant level (normally 0.05)
Find the critical value
Compare the chi-squared value with the critical value
Accept or reject the null hypothesis
How are degrees of freedom calculated?
The number of categories minus 1.
If the chi-squared value is greater than or equal to the critical value, is the null hypothesis accepted or rejected?
Rejected.
What does it mean if the null hypothesis is rejected?
There is a significant difference between the observed and expected results.
If the chi-squared value is less than the critical value, is the null hypothesis accepted or rejected?
Accepted
What does it mean if the null hypothesis is accepted?
The difference between the observed and expected results is not significant. The results occured due to chance.
What is a mutation?
- An alteration to the volume, arrangement or structure of DNA
- May affect a single gene or a whole chromosome
When do most mutations occur?
- Crossing over in prophase !
- Non-disjunction in anaphase I and II
What is a gene mutation?
- A change to at least one nucleotide base in DNA or the arrangement of bases
- May occur spontaneously during DNA replication and can be beneficial, damaging, or neutral
Give an example of condition caused by a gene mutation.
Sickle cell anaemia
Describe sickle cell anaemia.
- Recessive genetic disorder caused by a substitution mutation on chromosome 11
- Results in abnormal haemoglobin which distorts red blood cells
What is a chromosome mutation?
A change in the structure or number of chromosomes, affecting many genes.
Give an example of condition caused by a chromosome mutation.
Down’s syndrome
Describe Down’s syndrome.
Genetic disorder characterised by delayed development and learning disabilities
* Due to non-disjunction, an affected individual possesses three copies of chromosome 21
What is chromosome nondisjunction?
Failure of homologous chromosomes to separate in meiosis I or sister chromatids to separate in meiosis II. Results in gametes with one extra or less chromosome than normal.
What is a mutagen?
A chemical, biological or physical agent that increases the rate of gene mutations above normal level.
Give some examples of mutagens.
lonising radiation (gamma radiation, UV, X-rays)
Chemicals (e.g. polycyclic hydrocarbons)
What is a carcinogen?
A type of mutagen that causes cancer.
What is an oncogene?
Mutations of proto-oncogenes that are activated continuously, resulting in uncontrolled cell division.
Define epigenetics,
The study of changes in gene expression that are not due to alterations in the nucleotide base sequence of DNA
How can histone modification affect gene expression?
- Alteration of histones by the addition of methyl, acetyl or phosphate groups
- Increases or decreases gene expression by causing the histone to coil more tightly or loosely
How can DNA methylation affect gene expression?
- Addition of a methyl (CH3) group to cytosine bases
- Prevents recognition of the bases, reducing gene expression
