Flashcards in Variation and Artificial Selection (Chapter 17) Deck (50)
What are the 5 things that cause genetic variation?
1) independent assortment of chromosomes (and ∴ alleles) during meiosis
2) crossing over between chromatids of homologous chromosomes during meiosis
3) random mating between organisms within a species
4) random fertilisation of gametes
How does independent assortment, crossing over, random mating and random fertilisation cause genetic variation?
- They reshuffle existing alleles in the population
- Offspring have combinations of alleles which differ from those of their parents and from each other
- This genetic variation produces phenotypic variation
How does mutation cause genetic variation?
- Mutation does not reshuffle alleles already present
- It can produce completely new alleles e.g. by a new base sequence occurring in a gene as a result of a mistake during DNA replication
What is a gene mutation?
An unpredictable change in a gene
What is a new allele caused by mutation normally like and what does this mean?
The new allele is very often recessive ∴ it often does not show up in the population for some generations until by chance two descendants of organisms in which the mutation happened mate and produce offspring
What is the effect of mutations in somatic cells?
They often have no effect on the organism and cannot be passed to offspring by sexual reproduction
What is the effect of mutations in cells that divide to form gamers in ovaries or testes?
They may be inherited by offspring as the gametes may contain the mutated gene and if the gamete fuses to form a zygote, all the cells in the organism produced by the single cell dividing will contain the mutated gene
What does genetic variation passed on by parents to offspring give?
Differences in phenotype, providing the raw material on which natural selection can act
What does variation within a population mean?
That some individuals have features that give them an advantage over other members of that population
How else is variation in phenotype caused?
- By the environment in which organisms live e.g. if an organisms has better food when growing, they will be larger
- Variation caused by the environment is not passed on by parents to their offspring
What is discontinuous variation?
Qualitative differences which fall into clearly distinguishable categories, with no intermediates
What is an example of discontinuous variation?
The four possible ABO blood groups: A, B, AB, O
What is continuous variation?
Quantitative difference which may be small and difficult to distinguish
What is an example of continuous variation?
Height - there are no distinguishable height classes, instead there is a range of heights between two extremes
What are two characteristics that discontinuous and continuous variation share?
Both qualitative and quantitative difference in phenotype may be inherited and both may involve several different genes
What are the characteristics of discontinuous variation that are different?
1) different alleles at a single gene locus have large effects on the phenotype
2) different genes have quite different effects on the phenotypes
What are the characteristics on continuous variation that are different?
1) different alleles at a single gene locus have small effects on the phenotype
2) different genes have the same, often additive effect on the phenotype
3) a large numbers of genes may have a combined effect on a particular phenotypic effect (polygenes)
Describe the example of continuous variation in height
- Height is controlled by two unlinked genes (i.e. genes on different chromosomes): A/a and B/b
- a/b contribute x cm in height, A/B contrite 2x cm in height
- Since the effect of such genes is additive, aabb = 4xcm and AABB = 8x cm - the other genotypes will fall between these two extremes
- ∴ a cross between two heterozygous will produce phenotypes in a normal distribution with 6x cm being the most common height
What happens if more genes for height with an additive effect (polygenes), all on different chromosomes and possibly with more than two alleles are involved?
The number of discrete height classes would increase as more genes are involved and the difference between these classes gets less
What do environmental effects smooth out?
Differences between different classes
What happens if two or more of the genes are linked on the same chromosome?
- This could reduce the number of classes of offspring and increase the differences between them
- However, crossing over in meiosis restores variation
What do environmental effects do?
- They may allow the full genetic potential height to be reached or may shunt it in some way
- They can influence which genes are expressed
How could environmental effects on height prevent the full genetic potential being reached?
1) someone might have less food or less nutritious food than someone else with the same genetic contribution
2) a plant may be in a lower light intensity or in soil with fewer nutrients than another plant with the same genetic potential height
What is the example of environmental effects on the colouring of Himalayan rabbits, Siamese and Burmese cats?
- The development of dark tips to ears, nose, paws and tail
- This colouring is caused by an allele which allows the formation of the dark pigment only at low temperatures
- The extremities are the coldest part of the animal, so the colour is produced there
What is the experiment to show environmental effects?
1) cross two pure-bred varieties of maize (Black Mexican and Tom Thumb), which differ in cob length
2) F1 genetically different from parents but genetically the same as each other
3) variation in parents and F1 shows environmental effects
4) F2 shows much wider variation in cob length (both genetic and environmental)
What does pure-bred mean?
Homozygous at many loci
What is it important to know in selective breeding about variation and why?
- It is important to know how much phenotypic variation is genetic and how much is environmental in origin
- This is bc there is no point in selecting parents for a breeding programme on the basis of environmental variation
What is artificial selection?
When humans purposefully apply selection pressures to populations
What are desired features of cattle?
Fast growth rates, high milk yield and docility (making the animal easier to control)