Genetic Variation Flashcards
(42 cards)
What is variation?
Variation is variety and differences in the gene pool.
Variation in the gene pool increases the likelihood of survival.
What is variation caused by?
Variation is caused by mutation, environmental factors, sexual reproduction, random fertilisation, meiosis - independent assortment, segregation, crossing over. (MRS SICE)
What are the types of cell division?
There are two types of cell division.
Mitosis is for growth, repair, and asexual reproduction,
Meiosis is for sexual reproduction and ensures variation.
Describe the difference between mitosis and meiosis
Mitosis is the splitting of chromosomes into two chromatids, while meiosis is halving of homologous pairs, into separate chromosomes.
Describe the process of meiosis. (10 steps)
- Prophase I: Chromosomes shorten and thicken, chromatids become visible. Parts of adjacent chromatids can cross over and combine.
- Metaphase I: Chromosomes arrange themselves along cell equator in their homologous pairs. Spindle fibres pull the homologous chromosomes apart.
- Anaphase I: homologous pairs seperate and move toward cell poles. Cell membrane closes up between them.
- Telophase I: Two new cells form.
- Metaphase II: chromosomes line up again on equator of cell.
- Anaphase II: Chromatids seperate and move towards cell poles. Cell membranes close between them.
- Telophase II: Four new haploid cells form.
How does sexual reproduction ensure variation?
Sexual reproduction ensures variation in two different ways.
Crossing over - most crucial
Independent assortment - limited variation
Segregation - limited variation
How does crossing over ensure variation?
When homologous pair chromosomes align themselves across the equator (during meiotic division), small sections of chromosomes (containing genes) break off from their chromosomes and attach themselves onto the other one at the same locations.
The sister chromatids are no longer the same after crossing over with another chromosome. But, the chromosomes are still homologous pairs because the centromeres and genes have the same locus still and genes contained in each have not been changed, only the alleles in each.
This means that when meiosis occurs the gametes created have different genetic makeup.
How does independent assortment ensure variation?
When homologous pairs line up on the equator, they are in random positioning in terms of maternal and paternal being ‘on the left or the right’. This means that when the pairs are split up, each of the daughter cells has a random combination of paternal and maternal chromosomes from the parent cell.
How does segregation ensure variation?
When chromosomes in homologous pairs are split up, each daughter cell has different alleles. This creates variation because each daughter cell has a totally unique set of alleles
How does random fertilisation ensure variation?
Any one of the all genetically different sperm will fertilize the egg. This means there are many combinations of alleles that could be created at fertilisation. Different alleles in each gamete cause variation in the zygote. Different combinations/mixtures of the parent’s maternal and paternal alleles in each gamete cause variation.
What is the genotype?
this is the combination of genes you carry e.g. BB, Bb, bb
What is the phenotype?
the physical expression of the gene, e.g. blue eyes
Define ‘heterozygous’
having both a dominant and a recessive allele; Bb
Define ‘homozygous’
a pair of alleles that are the same; BB, bb
Define ‘dominant’
the allele which is expressed in the phenotype
Define ‘recessive’
the allele that is only expressed if there are two of that allele present (bb)
What does the term ‘pure breeding’ mean?
If an organism is ‘pure breeding’ it means that it shows no variation in the particular character being studied, i.e. all of the offspring produces by either self-fertilisation (in plants) or by breeding with others who are identical for this characteristic, show the same phenotype as the parent.
Describe the process of a ‘test cross’
A test cross involves mating the organism in question with an individual that is homozygous recessive for that trait. If the recessive phenotype appears in the offspring then the organism in question must be heterozygous.
(Remember with any test cross you can never be 100% certain that a parent is homozygous dominant. Test crosses have an element of uncertainty and need multiple breeding attempts in order to increase the certainty of the
unknown parent’s genotype.)
Describe dominant and recessive interactions
In dominant/recessive interactions, there are heterozygous, homozygous dominant, and homozygous recessive. The heterozygous individuals have the dominant phenotype.
What is incomplete dominance?
In incomplete dominance, the heterozygous individuals have an intermediate phenotype. Their phenotype is a combination of both the dominant and recessive conditions/alleles.
What is co-dominance?
In codominance, the heterozygous individuals display both phenotypes simultaneously.
Why can a gene have two or more different alleles? (Multiple alleles)
A gene can have two or more different alleles. The genes responsible for the human blood groups have multiple alleles. To further complicate matters, some of these alleles show complete dominance, while others show co-dominance. Human blood groups (known as ABO blood groups) are controlled by three alleles.
Describe what is meant by the term ‘lethal alleles’
Lethal alleles are alleles that produce a phenotypic effect that causes the death of the organism. Lethal alleles usually arise due to a mutation in an essential gene. The mutation means the gene cannot produce a functioning version of an essential protein. The lethal allele may be completely dominant, incompletely dominant, or recessive.
What are dihybrid crosses?
Dihybrid crosses show possible combinations involving two genes and the possible alleles of those genes.
