A2 OCR Biology Key Terms and Definitions Meiosis and Variation

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Asexual Reproduction
Reproduction in which genetically identical individuals are produced by mitosis; no gametes or fertilisation are involved.
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Sexual Reproduction
Reproduction in which two gametes (usually but not necessarily from two different parents) fuse to form a zygote; the offspring are genetically different from each other and their parent or parents.
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Gametes
Haploid cells specialised for reproduction; the nuclei of two gametes fuse together at fertilisation to form a diploid zygote.
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Fertilisation
The fusion of the nuclei of two gametes to form a zygote.
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Zygote
A diploid cell formed by the fusion of the nuclei of two haploid gametes.
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Meiosis
Reduction division; a type of nuclear division in which the nucleus divides twice to form four genetically different daughter cells from one parent cell, each containing half the number of chromosomes of the parent cell.
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Diploid
Containing two sets of chromosomes.
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Haploid
Containing one set of chromosomes.
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Genetic Variation
The events that take place during meiosis mix up the DNA in the different sets of chromosomes, causing genetic variation between offspring.
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Gene
A sequence of DNA nucleotides that codes for a polpeptide.
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Locus
The position on a chromosome at which a particular gene is found.
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Allele
One of two or more alternative forms of a gene.
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Independent Assortment
The result of the random orientation of each pair of homologous chromosomes on the equator of the spindle in metaphase of meiosis I, ensuring that either one of a pair of homologous chromosomes can be found with either one of another pair.
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Crossing Over
The exchange of alleles between chromatids of homologous chromosomes as a result of chiasma formation during prophase of meiosis I.
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Chiasma
A point at which a chromatid of one of a pair of homologous chromosomes breaks and rejoins to the chromatid of the other one of the pair, swapping genes between them; plural, chiasmata.
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Genetics
The study of inheritance.
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Cystic Fibrosis
A genetic disease caused by the recessive allele of a gene that codes for a membrane protein responsible for the passage of chloride ions out of cells.
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CFTR
A protein that acts as a transporter for chloride ions from inside a cell to the outside.
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Recessive Allele
An allele having an effect on the phenotype only when a dominant allele is not present.
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Dominant Allele
An allele having an effect on the phenotype even when a recessive allele is also present.
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Genotype
The alleles of a particular gene or genes possessed by an organism.
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Phenotype
The characteristics of an organism.
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Homozygous
A genotype in which both alleles of a gene are the same.
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Heterozygous
A genotype in which the alleles of a gene are different.
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Homologous
Having similar base sequences (in genes).
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Genetic Diagram
A conventional way of showing the genotypes of parents, their gametes and the genotypes and phenotypes of the offspring they would be expected to produce.
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Sex Chromosomes
The chromosomes that determine gender; in mammals, males are XY and females are XX.
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Autosome
Any chromosome other than a sex chromosome.
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Codominance
A situation in which both alleles have an effect on the phenotype in a heterozygous organism.
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Sex-Linked
A characteristic caused by a gene that is found on the non-homologous portion of the X chromosome.
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Factor VIII
A protein required for blood clotting, encoded by a gene found on the X chromosome and therefore sex-linked.
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Haemophilia
A sex-linked genetic disease caused by a recessive allele of the gene that encodes factor VIII.
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Dihybrid Inheritance
The study of the inheritance of two different genes.
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Linkage
The presence of two genes on the same chromosome, so that they tend to be inherited together and do not assort independently. (Genes on the same chromosomes are said to be linked).
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Nail Patella Syndrome (NPS)
An inherited condition caused by recessive alleles of two linked genes that affect the development of fingernails and the patella (kneecap).
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Chi-Squared (χ² test)
A statistical test that can be used to determine whether differences between observed and expected results are statistically significant or could simply be due to chance.
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Discontinuous Variation
Variation in which a feature of an individual fits into one of a few definite categories; it is likely to be caused by a small number of genes with a small number of alleles.
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Continuous Variation
Variation in which a feature of an individual does not fit into a definite category, but can have any value between two extremes; it is likely to be caused partly by the environment and/or by polygene or multiple alleles.
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Gene Pool
All the alleles of all the genes present in a population of interbreeding organisms.
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Stabilising Selection
Natural selection that tends to maintain the status quo.
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Directional (Evolutionary) Selection
Natural selection in which a change in environment or a change in the alleles present in the gene pool selects for a different feature than in the past, so that this feature becomes more common in successive generations.
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Evolution
A directional change in the characteristics of a population over time.
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Genetic Drift
A change in the characteristics of a population, or the proportions of different alleles in the population, as a result of chance; it is most likely to happen in small, isolated populations.
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Geographical Isolation
The separation of two populations by a geographical barrier.
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Reproductive Isolation
The inability of two populations to interbreed successfully.
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Biological Species Concept
The idea of a species as being a group of organisms with similar morphology and physiology, which are unable to breed successfully with other species.
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Phylogenetic (Evolutionary) Species Concept
The idea of a species as being a group of organisms that is geographically separated and morphologically or behaviourally distinct from other species, even if they may still be able to interbreed with individuals of another species.
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Artificial Selection
The choice by humans of individual animals or plants from which to breed over successive generations in order to improve the characteristics of the population to suit the requirements of humans.
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Artificial Insemination
The use of semen collected from a male animal to fertilise a female.
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The use of semen collected from a male animal to fertilise a female.
The determination of the characteristics of the offspring of an animal that is to be used in selective breeding but which, for some reason, is not able to show those characteristics itself, for example the milk yield of the daughters of a bull.
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Embryo Transplantion
The collection of embryos from one animal and their transfer for subsequent development into a surrogate mother.
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Other cards in this set

Card 2

Front

Reproduction in which two gametes (usually but not necessarily from two different parents) fuse to form a zygote; the offspring are genetically different from each other and their parent or parents.

Back

Sexual Reproduction

Card 3

Front

Haploid cells specialised for reproduction; the nuclei of two gametes fuse together at fertilisation to form a diploid zygote.

Back

Preview of the back of card 3

Card 4

Front

The fusion of the nuclei of two gametes to form a zygote.

Back

Preview of the back of card 4

Card 5

Front

A diploid cell formed by the fusion of the nuclei of two haploid gametes.

Back

Preview of the back of card 5
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