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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