- Created by: maddieecarr
- Created on: 30-11-19 15:32
B12.1: Types of Reproduction
- in asexual reproduction, there is no fusion of gametes and only one parent. there is no mixing of genetic information, leading to genetically identical offspring
- only mitosis is involved in asexual reproduction
- sexual reproduction involves the fusion of gametes formed by meiosis. meiosis leads to the formation of non-identical cells, sperm and egg cells in animals and pollen and egg cells in flowering plants.
- in sexual reproduction there is a mixing of genetic information that leads to variation in the offspring.
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B12.2: Cell Division in Sexual Reproduction
- cells in the reproductive organs divide by meiosis to form the gametes
- body cells have two sets of chromosomes, gametes only have one set.
- in meiosis, the genetic material is copied and then divided twice to form four gametes. each gamete has a single set of chromosomes.
- all gametes are genetically different from each other.
- gametes join at fertilisation to restore the normal number of chromosomes. the new cell divides by mitosis. the number of cells increase and as the embryo develops, the cells differentiate
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B12.3: DNA and the Genome
- the genome of an organism is the entire genetic material of that organism.
- the whole human genome has now been studied and this will have great importance for medicine in the future.
- the genetic material in the nucleus of a cell is composed of DNA. DNA is a polymer made up of two strands forming a double helix.
- a gene is a small section of DNA on a chromosome. each gene codes for a particular sequence of amino acids to make a specific protein.
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B12.4: Inheritance in Action
- some characteristics are controlled by a single gene. each gene may have different forms called alleles.
- the alleles present, or genotype, operate at a molecular level to develop characteristics that can be expressed as the phenotype.
- if two alleles are the same, the individual is homozygous for that trait. if the alleles are different they are heterozygous.
- a dominant allele is always expressed in the phenotype, even if only one copy is present. a recessive allele is only expressed if two copies are present.
- most characteristics are the result of multiple genes interacting rather than a single gene.
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B12.5: More About Genetics
- direct proportion and ratio are used to express the outcome of a genetic cross.
- punnett squares and family trees are used to understand genetic inheritance.
- punnett squares predict the outcome of a monohybrid cross.
- ordinary body cells contain 23 pairs of chromosomes: 22 control body characteristics and the sex chromosome pair controls the gender.
- in humans, the female sex chromosome is **, in males it is XY.
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B12.6: Inherited Disorders
- some disorders are inherited.
- polydactyly is a dominant phenotype which can be inherited from either or both parents.
- cystic fibrosis is a recessive phenotype and is caused by recessive alleles which must be inherited from both parents.
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B12.7: Screening for Genetic Disorders
- cells from embryos and fetuses can be screened for alleles that may cause many genetic disorders.
- embryo and fetal cells are used to identify genetic disorders but screening raises economic, social and ethical issues.
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