Genetics- Patterns of Inheritance Flashcards
What are the causes of genetic variation?
- Genetic differences
- Environmental influences
- Usually a combination of both
Features of continuous variation:
- Normal distribution curve
- Polygenic inheritance
- Environmental influences
Features of discontinuous variation:
- Mostly genetic
- Controlled by alleles of one gene or a very small number of genes
- Qualitative
- Small number of distinct classes or categories
What are the causes of genetic variation?
-Independent assortment
-Crossing over
-Mutations
-Random Fertilisation
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What is the Mendelian ratio for a monohybrid cross?
3:1
What is the Mendelian ratio for a dihybrid cross?
9:3:3:1
What is Epistasis?
Occurs when there are multiple alleles of two or more genes interacting with one another to control one phenotypic trait. The presence of one genes suppresses the expression of another gene
What is a hypostatic gene?
One that is suppressed
What is an epistatic gene?
Gene that suppresses the expression of another
How does crossing over cause genetic variation?
In Prophase I, sections of DNA on sister chromatids become entangled and break off and rejoin, resulting in exchange of DNA. This means that new combinations of alleles are created.
How does independent assortment create variation?
During metaphase I, the chromosomes line up on the metaphase plate in random order, independently of each other. This can result in many different combinations of alleles facing the poles of the cell so when it divides, each cell contains varied genetic information
How does random fertilisation create variation?
Gametes contain different alleles for genes therefore when gametes fuse, new combinations of alleles are formed. Also, multiple sperm compete to fuse with the egg so any combination can be created this way.
How do mutations create variation?
Changes in base sequence means that bonds are broken and reformed changing the specific 3D tertiary shape. This means that proteins may not be able to carry out their function therefore affecting the phenotype.
What does the Chi-squared test?
Measures the size of difference between the results you get and those expected, and whether that difference is significant or not.
How to work out expected number for Chi Squared test?
Total number x ratio of each phenotype
What is polygenic inheritance?
A single phenotype is controlled by two or more genes at different loci
What does the recombination frequency measure?
The amount of crossing over that has happened in meiosis
Recombination frequency equation:
Number of recombiant offspring/Total number of offspring x100
What can the recombination frequency be used for?
Recombination frequency for a number of characteristics coded for by genes on the same chromosome can be used to map the genes on the chromosome. RF of 1% relates to a distance of one map unit on a chromosome
What is co-dominance?
Both alleles are dominant and expressed in the phenotype. Alleles are independent and the phenotype results from full expression of both alleles
Why is blood group an example of co-dominance?
Both A and B are co-dominant over O.
What is the Rhesus factor?
People who are rhesus positive have antigen D on their RBC. R+ is dominant over R-
How is sex determined?
23rd pair of chromosomes in humans are the sex chromosomes, which are both different. Females have two X chromosomes while males have one X and one Y chromosome. The Y chromosome only contains a gene which will cause the embryo to develop as a male. Therefore, the sex of the offspring is determined by whether the sperm fertilising the egg contains an X or Y chromosome
What is sex-linkage?
Some characteristics are determined by genes carried on sex chromosomes. As the Y chromosome is much smaller than the X chromosome, there are a number of genes that males do not have a copy of. Therefore, any characteristic caused by a recessive allele on the section of the X chromosome, which is missing in the Y chromosome, will occur more frequently in males. Females, however, will often have a dominant allele present in their cells
What is an example of a sex-linked genetic disorder?
Haemophilia is a condition where patients have blood which clots incredibly slowly due to the absence of a protein blood-clotting factor. This condition affect males more as if they have a recessive allele on their X chromosome, they cannot have a corresponding dominant allele on the Y chromosome and so will develop the condition
What is recessive epistasis?
If the presence of two recessive alleles at a gene loci leads affects the expression of a gene
What is dominant epistasis?
If a dominant allele has an effect on expression of another gene
What is population genetics?
Investigates how allele frequencies within populations change over time
What is a gene pool?
Total of all genes in a population at any given time
What is genetic drift?
Change in allele frequency due to the random nature of alleles being passed on from parents to their offspring. In some cases, the existence of a particular allele can disappear from a population all together. Occurs more frequently in small populations
What is the founder effect?
When a small population is separated from the main population and geographically isolated. This leads to a much smaller gene pool so any alleles that were rare in the original population will suddenly have a much higher frequency in the offspring
What is stabilising selection?
An example of natural selection where alleles that are in the extremes (e.g. very high or low birth weight) reduce in frequency as they are less likely to survive. Therefore, there is an increase in the frequency of ‘average’ alleles.
What is directional selection?
Occurs when there is a change in the environment and the ‘normal’ phenotype is no longer advantageous. Organisms with rare alleles and extreme phenotypes are positively selected and therefore the allele frequency shifts towards the extreme phenotype and evolution eventually occurs.
What is a genetic bottleneck?
Where few individuals within a population survive an event or change, thus reducing the gene pool. Only the alleles of the surviving members of the population are able to be passed on to offspring
What is selective breeding?
When animals or plants with favourable characteristics are chosen to be bred
Why may crops be selectively bred?
- Increased yield
- Resistance (drought, disease, wind)
- Improve taste
- Desired nutrients
- Longer shelf life
What are the pros of aritificial selection?
- Produce higher yields to meet demands of growing population
- Friendlier and prettier pets?
- Produce animals that produce more milk, meat, eggs, etc.
What are the cons of artificial selection?
- Loss of genes in a population
- Animal discomfort (cows with large udders struggle to walk and some chicken are too heavy to stand and are forced to lay down)
- Can lead to inbreeding, which causes health problem in offspring
