Patterns of inheritance Flashcards
What is chlorosis
when plants leaves look pale or yellow because the cells aren’t producing the normal amount of chlorophyll reducing a plants ability to photosynthesise
How do environmental factors affect plants phenotype?
o Mineral deficiencies: lack of iron or magnesium. Iron is needed as a cofactor by some enzymes that make chlorophyll and Magnesium is found at the heart of the chlorophyll molecule
o Virus infections: When viruses infect plants, interfering with the metabolism of cells so they can no longer support the synthesis of chlorophyll
o Lack of light: in absence of light, plants turn off chlorophyll production to conserve resources
How is body mass affected by environmental factors and genetic factors
- Environmental factors include: amount of food eaten, amount of exercise or presence of disease.
- Occasionally, obesity is a result of the genetic makeup of an organise e.g. the obese mouse has a mutation on chromosome 7 causing the pattern of fat deposition in its body to be altered
Define genotype and phenotype
- Genotype: combination of alleles an organism inherits for a characteristic
- Phenotype: observable characteristics of an organism
Define the dominant allele
• Dominant allele: Version of the gene that will always be expressed if present in an organism
o Means an individual showing the dominant characteristic could have one or two copies of the dominant gene
Define the recessive allele
• Recessive allele: only expressed if two copies of this allele are present meaning if an individual has a recessive phenotype, you also know the genotype
Define homozygous and heterozygous
- Homozygous: two identical alleles for a characteristic
* Heterozygous: they have two different alleles for a characteristic
What is continuous variation
a characteristic that can take any value within a range, caused by genetic and environmental factors and controlled by a number of genes
- e.g. leaf surface area, animal mass, skin colour
What is discontinuous variation
a characteristic that can only appear in specific (discrete) values, caused by mostly genetic factors and controlled by one or two genes
- e.g. blood group, albinism and round and wrinkled pea shape
What is true breeding
• organisms that contain homozygous alleles for a particular gene
What is codominance
• Occurs when two different alleles occur for a gene – both of which are equally dominant so both alleles of the gene are expressed in the phenotype
When are F1 offspring formed and when are F2 offspring formed
F1 = homozygous genetic cross F2 = heterozygous genetic cross
How is sex determined
- Sex is genetically determined
- 23rd pair of chromosomes are known as sex chromosomes – females have two X chromosomes whereas males have an X and a Y
- X chromosome: large and contains many genes involved in sexual development
- Y chromosome: smaller, containing almost no genetic information but does carry gene causing an embryo to develop as a male
What is sex linkage
• Characteristics determined by genes carried on sex hormones are genes that are sex linked
• Y chromosome is much smaller than X so there are a number of genes in the X chromosome that males have only one copy of – means that any characteristic caused by a recessive allele on the section of the X chromosome which is missing in the Y chromosome
o Many females also have a dominant allele present in their cells
Describe how haemophilia is an example of a sex-linked genetic disorder
- If a male inherits the recessive allele that codes for haemophilia they cannot have a corresponding dominant allele on their Y chromosome and so develop the condition
- Females who are heterozygous for the haemophilia coding gene are carriers
How is a dihybrid cross used and how is it different to a monohybrid cross
• Used to show the inheritance of two different characteristics caused by two genes which may be located on different pairs of homologous chromosomes – each of these genes can have two or more alleles
• Same as monohybrid cross but 4 alleles instead of two
• You can produce an expected ratio of the four different phenotypes but as with all genetic crosses the actual ratio of offspring produced can differ from the expected as:
o Fertilisation of gametes is a random process so in a small sample a few chance events can lead to a screwed ratio
o Genes being studied are both on the same chromosome, linked genes. If no crossing over occurs the alleles for the two characteristics will always be inherited together
Define autosomal linkage
when genes that are linked are found on one of the other pairs of chromosomes
Describe how linked genes differ from normal genes
- Linked genes are inherited as one unit – no independent assortment during meiosis unless the alleles are separated by chiasmata – tend to be inherited together
- Linked genes cannot undergo normal random shuffling of alleles during meiosis and expected ratios will not be produced in the offspring
Define recombinant offspring
• have different combinations of alleles than either parent
Define recombinant frequency and describe how it is calculated
• The closer the genes are on a chromosome the less likely they are to be separated during crossing over and the fewer recombinant offspring produced
• Recombinant frequency: measure of the amount of crossing over that has happened in meiosis
o Number of recombinant offspring total number of offspring
o RC of 50% indicates that there is no linkage and the genes are on separate chromosomes
o 50% indicates there is gene linkage and the random process of independent assortment has been hindered
o As the degree of crossing over is determined by how close the genes are on the chromosome – closer they are the less likely they will be separated during crossing over
What is epistasis
• The interaction of genes at different loci e.g. gene regulation (regulatory genes controlling the activity of structural genes – lac operon)
Define hypostatic and epistatic
- Hypostatic: gene that is affected by another gene
* Epistatic: gene that affects the expression of another gene
When does dominant apostasies occur and what influence does this have
• Dominant epistasis occurs if a dominant allele results in a gene having an effect on another gene
o happens if an epistatic gene coded for an enzyme that modified one of the precursor molecules in the pathway and the next enzyme in the pathway would lack a suitable substrate molecule and so the pigment would again not be produced
o all of the genes in the sequence would be masked
How is allele frequency calculated
p^2 + 2pq + q^2 = 1 p+q=1 p^2 is frequency of homozygous dominant q^2 is frequency of homozygous recessive 2pq is frequency of heterozygous