Inheritance Flashcards
What is a gene?
A sequence of DNA bases that codes for a specific polypeptide
What is a locus?
The particular region that the gene occupies on a chromosome
What is an allele?
Alternative forms of a particular gene, coding for an altered phenotype
Define genotype
The genetic make up of an individual - all of the alleles they possess
Define phenotype
The observable characteristics of an organism determined by the genotype
What is the gene pool?
All the alleles in a population at any one time
What is monohybrid inheritance?
The inheritance of a single gene eg that controlling a plant’s height
Why were pea plants a useful choice for Mendel’s inheritance experiments?
- They’re easy to grow
- They can self or cross fertilise
- They can produce flowers and fruits in the same year
How would Mendel ensure that offspring produced were only from the parents being investigated?
Pollen transferred by hand - receiving flower could be in a bag to prevent further cross pollination
Remove anthers before they mature so no self pollination
What is Mendel’s first ‘law of inheritance’?
The characteristics of an organism are determined by (factors) alleles, which occur in pairs. Only one allele of a pair is present in each gamete.
What is continuous variation?
Characteristic often has a range of values (bell shaped curve). Controlled by a number of genes. Eg height Environmental factors have an effect
What is discontinuous variation?
The characteristic has distinct categories controlled by a single gene. Environmental factors have no effect eg tongue rolling, blood type
What is the ‘backcross’ or test cross?
A method used in genetics to determine whether a particular dominant characteristic observed in an organism is caused by 1 or 2 dominant alleles (homozygous dominant or heterozygous). The unknown is always crossed with an individual displaying the recessive phenotype.
What is pure breeding?
Two organisms with the same phenotype that, when bred together, produce offspring with the same phenotype (homozygous)
What is codominance?
Both alleles for a gene are expressed when present in a heterozygote. Both alleles are written using a capital letter. Eg chickens with black feathers and chickens with white feathers produce speckled offspring
What is incomplete dominance?
The phenotype of the heterozygote is intermediate of the two homozygous parents eg red carnations crossed with white flowered carnations produce pink offspring
What is dihybrid inheritance?
Inheritance of 2 unlinked genes. Independent assortment of these genes produces recombinants (different allele combinations in the gametes)
What is Mendel’s second law (law of independent assortment)?
Either of a pair of alleles may combine randomly with either of another pair
What are the headings of the columns when working out chi squared?
Observed. Expected. O-E. (O-E)^2. (O-E)^2/E - Then work out the sum of (O-E)^2/E
What is the value for degrees of freedom?
One less than the number of categories
When do we accept/reject the null hypothesis?
- If the chi squared value is less than the value in the table, we accept the null hypothesis, any difference is down to chance
- If the chi squared value is greater than the value in the table, we reject the null hypothesis, therefore other factors are influencing
What 4 things to mention when concluding results of chi squared
- Chi squared value is less than/more than critical value at 5%
- Therefore there is/is no significant difference between observed and expected values
- Any difference is down to chance/other factors
- Null hypothesis is accepted/rejected
What are the 2 types of chromosome?
- Autosomes: any chromosome that is not a sex chromosome
- Sex chromosomes: a chromosome concerned in determining the sex of an organism, typically one of 2 kinds
What is sex linked inheritance?
Alleles that are carried on the X chromosome. Male Y chromosome is much smaller than the X so carries far fewer genes. For most genes carried on the X chromosome in the male there is only the one allele which must be expressed. Therefore in the male any recessive genes on the X chromosome will be expressed in the phenotype.
If a male with a sex-linked disease had a son, what are the chances of him having the disease?
No chance as the father will only pass on his Y chromosome to his son. Sons can only inherit the recessive allele from the mother
If a male with a sex-linked disease had a daughter, what are the chances of her having the disease?
She would become a carrier with no symptoms, unless the mother had also passed on a recessive allele and then she would have the disease.
Why might women with a carrier mother and an unaffected father want genetic counselling before having children?
To find out if they have inherited the recessive allele as this would make them carriers. If they passed this recessive allele onto their son, the son would have the disorder. They may decide to screen the embryo when they become pregnant.
Haemophilia is an example of a sex-linked condition in humans. The individual cannot produce enough of one particular blood clotting protein. What would be a symptom of this?
Blood doesn’t clot normally - slow, persistent bleeding
Duchenne muscular dystrophy is caused by a sex linked recessive allele of the dystrophin which stabilises cell membranes of muscle fibres. What are symptoms for individuals without this working allele?
Progressive weakening and wasting of the muscles
Why might there be a low number of children who have sex-linked diseases through affected parents?
Affected people might die before they can reproduce or decide they don’t wish to conceive so don’t pass the disease onto offspring
When do Mendel’s ratios apply (3:1 and 9:3:3:1)?
When genes are on different chromosomes - not linked
Genes on the same chromosome are linked and usually inherited together so Mendel’s ratios don’t apply
An organism with the genotype AaBb has 2 genes (A and B). That are found on different chromosomes - not linked. What gametes could be produced by this organism?
AB, Ab, aB, ab
In an organism, the genotype AaBb has 2 genes (A and B) that are found on the same chromosome - linkage. What gametes can be produced by this organism if crossing over doesn’t disrupt the linkage?
AB, ab
Summarise linkage
- Genes are inherited together
- Crossing over is very unlikely to separate them as the genes are too close together on the same chromosome
Summarise incomplete linkage
- Genes are on the same chromosome but are far apart
- Genes could be separated due to crossing over (at prophase I) and not be inherited together
- Rare (only 5-10% of the time)
What is a mutation?
An unpredictable change in the genetic material of an organism
What are the 2 types of mutation?
- Gene mutations: affect single genes
- Chromosome mutations: cause changes in the structure or number of whole chromosomes
Why are mutations important?
They provide an important source of genetic variation.
In which organisms are mutation rates more frequent?
Those with short life cycles and frequent cell division
When do chromosomal mutations most often occur?
During crossing over in prophase I and non-disjunction during anaphase I and II
What are mutagens? Give examples
Factors in the environment that increase the frequency of mutation in an organism
Ionising radiation eg X-rays
Mutagenic chemicals eg polycyclic hydrocarbons
Why is it incorrect to say that mutagens cause mutations?
Mutagens do not cause mutations but increased exposure to mutagens increases the rate of mutations occurring
What are carcinogens?
Mutagens that increase the rate of cancer
What regulates cell division in humans? What happens if these genes become mutated?
Proto-oncogenes and tumour-suppressor genes regulate cell division
They form oncogenes if mutated
What are oncogenes involved in?
Uncontrollable cell division and can lead to cancer
Tobacco smoke contains a number of harmful chemicals that affect human health. What do these include?
- Nicotine
- Polycyclic aromatic hydrocarbons
- Carbon monoxide
How may tar cause lung cancer?
Tar contains carcinogens which target DNA in the cells of the alveoli. If mutation occurs in a proto-oncogene or tumour suppressor gene, lung cancer may develop
What do gene mutations affect?
Single bases within a gene
What could a change in a single base of a gene result in?
- Change in DNA base sequence, resulting in a change in triplets/codons for amino acids
- Different amino acid sequence produced in translation
- Bonds form in different places and result in different shaped protein (may not function)
An enzyme is required for melanin to be produced. Explain how a mutation in the gene coding for this enzyme results in a lack of pigmentation in individuals with albinism
- Changes to the DNA base sequence results in a change in the sequence of amino acids during translation
- A mutation in the gene coding for the enzyme could result in the active site changing shape
- The enzyme cannot function and no melanin is produced
Give an example of a substitute gene mutation
Sickle cell anaemia
RBCs become sickle shaped, affecting the ability of the haemoglobin to bind to oxygen, resulting in anaemia
Mutant allele is co-dominant
Sickle cell trait confers some resistance to the malaria parasite. Why is this?
Plasmodium parasites cannot successfully reproduce within the RBCs because of their sickle shape. This is a selective advantage in countries where malaria is prevalent and so natural selection has meant this characteristic is not selected against
What are 3 ways chromosome mutations can occur?
- Changes in chromosome structure - errors occur when chromosomes exchange sections of DNA during crossing over at prophase I
- Changes in whole sets of chromosomes (polyploidy) - if a gamete receives 2 sets of chromosomes during a failed meiotic division, the gametes will be diploid rather than haploid
- Changes in chromosome number (nondisjunction) - process where faulty cell division results in one daughter cell getting two copies of a chromosome whilst one daughter cell gets none
Describe why someone may be born with Down’s syndrome (trisomy 21)
During meiosis in the female ovary, an oocyte with 2 copies of chromosome 21 is produced and survives. If this oocyte is fertilised, the zygote would have 3 copies of chromosome 21 - 2 from the mother and one from the father
State 3 features of Down’s syndrome
- Open facial features
- Low muscle tone
- Some degree of learning disability
Explain how a trisomy such as trisomy 16 in mice can occur in a zygote
Homologous chromosomes fail to separate/nondisjunction at anaphase I in oogenesis. Secondary oocyte has 2 copies of chromosome 16. Third added from sperm at fertilisation.
What is epigenetics?
The control of gene expression by modifying DNA or histone proteins, without affecting the DNA nucleotides sequence
How can the addition of a methyl group to nitrogenous bases lead to epigenetic changes?
It prevents the bases being recognised and therefore reduces the ability of the whole gene to be transcribed (expressed). If transcription doesn’t occur, translation will not occur and the polypeptide and eventual protein will not be produced
How does modification of histone proteins lead to epigenetic changes?
Histone proteins are used to organise the DNA in a chromosome. If the DNA becomes more tightly coiled around the histone proteins, this can prevent gene expression (transcription and translation). If coiled more loosely it can increase gene expression (transcription and translation)
