Section 3: Reproduction and inheritance
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a) Reproduction
3.1: Understand the difference between sexual and asexual reproduction
Sexual reproduction involves male and female sex cells to fuse. It is best in a constantly changing environment to encourage variation. Asexual reproduction does not require sex cells; fertilization and variation do not take place. It is best to reproduce asexually in a stable environment.
Sexual reproduction involves male and female sex cells to fuse. It is best in a constantly changing environment to encourage variation. Asexual reproduction does not require sex cells; fertilization and variation do not take place. It is best to reproduce asexually in a stable environment.
3.2: Understand that fertilization involves the fusion of a male and female gamete to produce a zygote that undergoes cell division and develops into an embryo
At fertilization, male and female gametes fuse to produce a zygote that undergoes cell division. The random fertilization of gametes produces variation.
At fertilization, male and female gametes fuse to produce a zygote that undergoes cell division. The random fertilization of gametes produces variation.
3.3: Describe the structures of an insect-pollinated and a wind-pollinated flower and explain how each is adapted for pollination
Insect-pollinated flower
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Wind-pollinated flower
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3.4: Understand that the growth of the pollen tube followed by fertilization leads to seed and fruit formation
- The pollen grain lands on the stigma
- A pollen tube grows out of the pollen grain
- The pollen grain nucleus moves to the ovary of the flower through the pollen tube
- Fertilization takes place when the nucleus of the pollen grain fuses with the ovule
- An embryo forms by mitosis and the zygotes turn into seeds
- The ovary develops into a fruit around the seeds
3.5: Understand the conditions needed for seed germination
Water, oxygen and a suitable temperature are all required for seed germination. Water enters the seed through the micropyle (a tiny hole in the seed) and activates the enzymes in the seed. These enzymes break down the starch that is stored in the seed's food stores and this broken down starch is used by the seed as energy for growth. Oxygen is required for the seed to respire and a suitable temperature is required in order for the enzymes to work efficiently.
Water, oxygen and a suitable temperature are all required for seed germination. Water enters the seed through the micropyle (a tiny hole in the seed) and activates the enzymes in the seed. These enzymes break down the starch that is stored in the seed's food stores and this broken down starch is used by the seed as energy for growth. Oxygen is required for the seed to respire and a suitable temperature is required in order for the enzymes to work efficiently.
3.6: Understand how germinating seeds utilize food reserves until the seedling can carry out photosynthesis
- A developed seed contains an embryo and a store of food reserves
- When a seed starts to germinate, it obtains the glucose needed for respiration from its food reserves
- As the food reserves get used up, the mass of the seed decreases
- Once the plant develops leaves, its mass increases and the food reserves are no longer needed as the plant can photosynthesize
3.7: Understand that plants can reproduce asexually by natural methods (illustrated by runners) or by artificial methods (illustrated by cuttings)
Some plants reproduce asexually. This can happen naturally using runners:
Asexual reproduction can also be achieved using cuttings:
Some plants reproduce asexually. This can happen naturally using runners:
- The plant sends out runners - fast growing stems that grow sideways
- The runners take root and a new plant starts to grow a short distance away from the parent plant in order to reduce competition for resources
- The new plants are exact clones of the parent plant - no variation has taken place
Asexual reproduction can also be achieved using cuttings:
- Cuttings are taken from suitable parent plants and then planted
- The new plants will grow to be identical to the parent plant
3.8: Recall the structure and function of the male and female reproductive systems
The male reproductive system produces sperm. Sperm are male gametes. They are made in the testes, all the time after puberty. Sperm mix with a liquid to make semen, which is ejaculated form the penis into the vagina of the female during sexual intercourse. The female reproductive system produces ova. Ova are female gametes. An ovum is produced every 28 days form one of the two ovaries. It then passes into the Fallopian tube - this is where it might meet sperm that have entered the vagina during sexual intercourse. If it isn't fertilized by sperm, the ovum will break up and pass out of the vagina. If it is fertilized, the ovum starts to divide. The new cells will travel down the Fallopian tube to the uterus and attach to the endometrium (uterus lining). A fertilized ovum develops into an embryo. |
Male reproductive system
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Female reproductive system
- Ovary: produces eggs and sex hormones
- Endometrium: thickens during the menstrual cycle in preparation for a possible baby
- Cervix: the opening of the uterus
- Vagina: where sperm are deposited; unfertilized eggs exit through the vagina during the menstrual cycle
- Vulva: the opening of the vagina; sperm enter through here
- Uterus: the organ where an embryo grows
- Fallopian tube: a muscular tube that carries the ovum from the ovary to the uterus
3.9: Understand the roles of estrogen and progesterone in the menstrual cycle
Estrogen and progesterone both play a part in controlling the main events in the menstrual cycle. Estrogen causes the lining of the uterus to thicken and stimulates the release of LH (luteinizing hormone) which releases the ova. When the egg is released, progesterone is released from the corpus lutem, where it is produced, and maintains the lining of the womb. The lining comes away when progesterone levels fall.
Estrogen and progesterone both play a part in controlling the main events in the menstrual cycle. Estrogen causes the lining of the uterus to thicken and stimulates the release of LH (luteinizing hormone) which releases the ova. When the egg is released, progesterone is released from the corpus lutem, where it is produced, and maintains the lining of the womb. The lining comes away when progesterone levels fall.
3.10: Describe the role of the placenta in the nutrition of the developing embryo
Once an ovum has been fertilized, it develops into an embryo and this implants into the uterus. In the later stages of its development, the embryo is called a fetus. Once the embryo has been implanted, the placenta develops and lets the blood of the embryo and mother get very close and exchange food, oxygen and waste. Glucose, amino acids and fats travel through the mother’s bloodstream, into the uterus wall, and will cross into the child’s blood at the placenta. Having crossed into the child’s blood, they are then taken into the child. Substances from the child like carbon dioxide and urea move into the mother's bloodstream via the placenta as well. Arteries usually carry oxygenated blood, but the two umbilical arteries collect deoxygenated blood from the body of the fetus and carry it to the placenta. The blood in the umbilical arteries is pumped to the placenta by the fetal heart. Veins usually carry deoxygenated blood, but the single umbilical vein carries oxygenated and nutrient-rich blood from the placenta and delivers it to the fetal heart, which pumps it around the body of the fetus.
Once an ovum has been fertilized, it develops into an embryo and this implants into the uterus. In the later stages of its development, the embryo is called a fetus. Once the embryo has been implanted, the placenta develops and lets the blood of the embryo and mother get very close and exchange food, oxygen and waste. Glucose, amino acids and fats travel through the mother’s bloodstream, into the uterus wall, and will cross into the child’s blood at the placenta. Having crossed into the child’s blood, they are then taken into the child. Substances from the child like carbon dioxide and urea move into the mother's bloodstream via the placenta as well. Arteries usually carry oxygenated blood, but the two umbilical arteries collect deoxygenated blood from the body of the fetus and carry it to the placenta. The blood in the umbilical arteries is pumped to the placenta by the fetal heart. Veins usually carry deoxygenated blood, but the single umbilical vein carries oxygenated and nutrient-rich blood from the placenta and delivers it to the fetal heart, which pumps it around the body of the fetus.
3.11: Understand how the developing embryo is protected by amniotic fluid
The amniotic membrane forms and surrounds the embryo. This membrane is full of amniotic fluid which protects the embryo against knocks and bumps.
The amniotic membrane forms and surrounds the embryo. This membrane is full of amniotic fluid which protects the embryo against knocks and bumps.
3.12: Understand the roles of estrogen and testosterone in the development of secondary sexual characteristics
Sex hormones are released during puberty and they trigger secondary sexual characteristics:
Sex hormones are released during puberty and they trigger secondary sexual characteristics:
Estrogen
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Testosterone
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b) Inheritance
3.13: Understand that the nucleus of a cell contains chromosomes on which genes are located
Human body cells are diploid: they have two copies of each chromosome; one from each parent, arranged in pairs. A human cell nucleus contains 46 chromosomes - our diploid number is 46. Our sex cells, however, contain the haploid number of chromosomes - 23.
Human body cells are diploid: they have two copies of each chromosome; one from each parent, arranged in pairs. A human cell nucleus contains 46 chromosomes - our diploid number is 46. Our sex cells, however, contain the haploid number of chromosomes - 23.
3.14: Understand that a gene is a section of a molecule of DNA
DNA is a long list of instructions on how to put an organism together and make it work. Each separate gene in a DNA molecule is a chemical instruction that codes for a particular protein. Proteins are important as they control most processes in our bodies; they also determine inherited characteristics. By controlling the production of proteins, genes also control inherited characteristics.
DNA is a long list of instructions on how to put an organism together and make it work. Each separate gene in a DNA molecule is a chemical instruction that codes for a particular protein. Proteins are important as they control most processes in our bodies; they also determine inherited characteristics. By controlling the production of proteins, genes also control inherited characteristics.
3.15: Describe a DNA molecule as two strands coiled to form a double helix, the strands being linked by a series of paired bases: adenine (A) with thymine (T), and cytosine (C) with guanine (G)
A DNA molecule has two strands together in the shape of a double helix. The two strands are held together by chemicals called bases: adenine (A), thymine (T), cytosine (C) and guanine (G). There will always be equal amount of adenine and thymine, and equal amounts of cytosine and guanine. This is called complementary base pairing. The names of the four bases can be remembered by a simple acronym: All Tigers Come Growling.
A DNA molecule has two strands together in the shape of a double helix. The two strands are held together by chemicals called bases: adenine (A), thymine (T), cytosine (C) and guanine (G). There will always be equal amount of adenine and thymine, and equal amounts of cytosine and guanine. This is called complementary base pairing. The names of the four bases can be remembered by a simple acronym: All Tigers Come Growling.
3.16: Understand that genes exist in alternative forms called alleles which give rise to differences in inherited characteristics
Different versions of the same genes are called alleles. For example, there are several alleles that decide hair color: black, brown, blonde and ginger are all alleles of the same gene. Most of the time you hvev two copies of each gene - one from each parent.
Different versions of the same genes are called alleles. For example, there are several alleles that decide hair color: black, brown, blonde and ginger are all alleles of the same gene. Most of the time you hvev two copies of each gene - one from each parent.
3.17: Recall the meanings of the terms: dominant, recessive, homozygous, heterozygous, phenotype, genotype and codominance
- dominant - the allele that is expressed
- recessive - the allele that is not expressed, unless both alleles are recessive
- homozygous - when two of the same allele (dominant or recessive) are in the zygote (e.g. bb - homozygous recessive)
- heterozygous - when two different alleles are in the zygote (e.g. Bb, one dominant and one recessive)
- phenotype - the physical representation of the allele (e.g. brown eyes)
- genotype - the genes in the allele, expressed by letters (e.g. Bb)
- codominance - when there are two dominant alleles in the zygote (e.g. BB)
3.18: Describe patterns of monohybrid inheritance using a genetic diagram
Monohybrid inheritance is the inheritance of a specific gene. It can be shown by a type of genetic diagram called a Punnett square.
Monohybrid inheritance is the inheritance of a specific gene. It can be shown by a type of genetic diagram called a Punnett square.
3.19: Understand how to predict family pedigrees
Imagine you're cross-breeding hamsters, and that some have a normal, boring disposition while others have a leaning towards crazy acrobatics. And suppose you know that the behavior is due to one gene. Let's say that the allele which causes the crazy nature is recessive - so use a 'b'. And normal (boring) behavior is due to a dominant allele - call it 'B'.
Imagine you're cross-breeding hamsters, and that some have a normal, boring disposition while others have a leaning towards crazy acrobatics. And suppose you know that the behavior is due to one gene. Let's say that the allele which causes the crazy nature is recessive - so use a 'b'. And normal (boring) behavior is due to a dominant allele - call it 'B'.
- A crazy hamster must have the genotype bb (i.e. it must be homozygous for this trait)
- However, a normal hamster could have two possible genotypes - BB (homozygous) or Bb (heterozygous) because the dominant allele (B) overrules the recessive one (b).
- When you breed from two heterozygous hamsters, there's a 75% chance of having a normal, boring hamster, and a 25% chance of having a crazy one. You would expect a 3:1 ratio of normal : crazy hamsters. This ratio is called the phenotypic ratio.
- If you breed two homozygous hamsters there's only one possible offspring you can end up with - for example, breeding Bb and bb hamsters can only give offspring with a Bb genotype - and they'd all have a normal phenotype.
3.20: Predict probabilities of outcomes from monohybrid crosses
In genetic diagrams, letters are used to represent genes. Dominant alleles are always shown with a capital letter (e.g. 'B') and recessive alleles are always shown with a small letter (e.g. 'b'). The diagram on the left is called a Punnett square. You start by drawing a grid, then filling it in:
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show the possible combinations of the gametes. For example:
Genetic diagrams can be drawn in the same way to show codominant inheritance.
- Huntington's is a genetic disorder caused by a dominant allele, 'B', and so can be inherited if just one parent carries the defective gene
- The parent who carries the gene will be a sufferer too since the allele is dominant, but the symptoms don't start to appear until after the person is about 40
- As the Punnett square above shows, both parents are suffers of the disorder and each has a 50% chance of passing it onto their children
- There is a 3:1 phenotypic ratio in the children of carrier : unaffected child.
Genetic diagrams can be drawn in the same way to show codominant inheritance.
3.21: Recall that the sex of a person is controlled by one pair of chromosomes, XX in a female and XY in a male
There are 23 pairs of chromosomes in a human sex cell. The 23rd pair are sex chromosomes and they predict the gender of the baby. There is one pair of sex chromosomes from the mother and one pair from the father. If the two chromosomes are XX, the baby will be a female. If they are XY, the baby will be a male.
There are 23 pairs of chromosomes in a human sex cell. The 23rd pair are sex chromosomes and they predict the gender of the baby. There is one pair of sex chromosomes from the mother and one pair from the father. If the two chromosomes are XX, the baby will be a female. If they are XY, the baby will be a male.
3.22: Describe the determination of the sex of offspring at fertilization, using a genetic diagram
Genetic diagrams work in the same way as monohybrid crosses. They show the sex of the mother and father and the possible outcomes of the sex of a child. The only difference is that the outcomes will always be the same since one parent is always male and the other is always female:
Genetic diagrams work in the same way as monohybrid crosses. They show the sex of the mother and father and the possible outcomes of the sex of a child. The only difference is that the outcomes will always be the same since one parent is always male and the other is always female:
3.23: Understand that division of a diploid cell by mitosis produces two cells which contain identical sets of chromosomes
Mitosis results in two genetically identical, diploid, daughter cells. All of the cells in your body except for the sex cells are formed by mitosis from the zygote. The sex cells are formed by meiosis. In mitosis, a copy of each chromosome is made before the cell divides. During the division each daughter cell receives a copy of each chromosome. Mitosis is very important for the replacement of cells, for example, skin cells, gut cells and blood cells. In cancer the normal control of mitosis is lost, so the cells divide very rapidly and repeatedly. |
3.24: Understand that mitosis occurs during growth, repair, cloning and asexual reproduction
Cell division by mitosis occurs during:
Cell division by mitosis occurs during:
- growth - mitosis forms all the cells in the human body except the sex cells
- repair - the skin loses millions of cells every day; a layer of cells beneath the surface is constantly dividing to replace them
- cloning - mitosis is useful when cloning since the child will be genetically identical to the parent
- asexual reproduction - mitosis is useful during asexual reproduction since the child will be genetically identical to the parent
3.25: Understand that division of a cell by meiosis produces four cells, each with half the number of chromosomes, and that this results in the formation of genetically different haploid gametes
Meiosis is the special form of cell division that results in the sex cells or gametes. It involves two cell divisions and produces four haploid cells that are not genetically identical. In meiosis the chromosomes replicate to form two strands and then there are two cell divisions. The chromosomes are divided randomly between the daughter cells, so they are not all identical. During the first division of meiosis, one chromosome from each homologous pair goes into each daughter cell. During the second division this chromosome separates into two strands, and one part goes into each daughter cell. This makes it possible to have a lot of variety. |
3.26: Understand that random fertilization produces genetic variation in offspring
The offspring from sexual reproduction vary genetically. One reason for this is because of the huge variation in the sex cells, since they are produced by meiosis. Another reason is because of the random way in which fertilization takes place. In humans, any one of billions of sperm formed by a male during his life could, potentially, fertilize any one of the thousands of ova formed by a female.
The offspring from sexual reproduction vary genetically. One reason for this is because of the huge variation in the sex cells, since they are produced by meiosis. Another reason is because of the random way in which fertilization takes place. In humans, any one of billions of sperm formed by a male during his life could, potentially, fertilize any one of the thousands of ova formed by a female.
3.27: Recall that in human cells the diploid number is of chromosomes is 46 and the haploid number is 23
In a human, the diploid number of chromosomes is 46 and the haploid number is 23. However, not all cells in the human body contain the same number of chromosomes:
In a human, the diploid number of chromosomes is 46 and the haploid number is 23. However, not all cells in the human body contain the same number of chromosomes:
- gametes contain 23 chromosomes so they can fuse with another 23 during fertilization
- red blood cells don't contain chromosomes as they don't have nuclei
- skin cells contain a full set of 46 chromosomes
3.28: Understand that variation within a species can be genetic, environmental or a combination of both
Plants are either tall or short because of the genes they inherit. However, not all short ones or tall ones are exactly the same height. This is because of environmental factors that may influence their height:
Plants are either tall or short because of the genes they inherit. However, not all short ones or tall ones are exactly the same height. This is because of environmental factors that may influence their height:
- they may not all receive the same amount of sunlight and will not photosynthesize as well as others
- they may not receive the same amount of water and mineral ions form the soil
- they may not all receive the same amount of carbon dioxide
3.29: Understand that mutation is a rare, random change in genetic material that can be inherited
Mutations are changes in the genetic code of a person. The mutation of a gene is rare and completely random. A mutation changes the sequences of the DNA bases which could stop the production of a particular protein or produce a different kind of protein, which could lead to new characteristics and increase the chance of further mutation. Mutations can be inherited.
Mutations are changes in the genetic code of a person. The mutation of a gene is rare and completely random. A mutation changes the sequences of the DNA bases which could stop the production of a particular protein or produce a different kind of protein, which could lead to new characteristics and increase the chance of further mutation. Mutations can be inherited.
3.30: Describe the process of evolution by means of natural selection
Evolution takes place by means of natural selection. Natural selection can lead to change and adaptation within a population, which in time can lead to the formation of a new species. Living organisms always produce more offspring than are needed to replace them, but not all of these offspring grow up to become adults and breed themselves. Those organisms best suited to their local environment survive best and breed, passing on their genetic information to the next generation. This is natural selection. If conditions in an area change, the organisms that survive and breed will be those best suited to the new conditions. They may be different from the original organisms. In this way the different forms will become more and more different until eventually they are a new species. |
3.31: Understand that many mutations are harmful but some are neutral and a few are beneficial
Mutations are random changes in the genetic material that can be passed from one generation to the next. Some mutations are harmful, some are neutral, but a few are beneficial and give the organisms an advantage. All are rare.
Mutations are random changes in the genetic material that can be passed from one generation to the next. Some mutations are harmful, some are neutral, but a few are beneficial and give the organisms an advantage. All are rare.
3.32: Understand how resistance to antibiotics can increase in bacterial populations
Mutations can cause an increase in the resistance of bacterial populations to antibiotics. Bacteria reproduce frequently so mutations are not as rare as they are in other organisms. These mutations could mean that they are no longer susceptible to a certain type of antibiotic, and this makes it easier for them to survive and thrive, eventually possibly creating a new 'superbug' that is immune to different antibiotics due to evolution.
Mutations can cause an increase in the resistance of bacterial populations to antibiotics. Bacteria reproduce frequently so mutations are not as rare as they are in other organisms. These mutations could mean that they are no longer susceptible to a certain type of antibiotic, and this makes it easier for them to survive and thrive, eventually possibly creating a new 'superbug' that is immune to different antibiotics due to evolution.
3.33: Understand that the incidence of mutations can be increased by exposure to ionizing radiation and some chemical mutagens
The chance of mutation is increased by:
The chance of mutation is increased by:
- ionizing radiation: too much exposure to x-rays, gamma rays and ultraviolet light
- chemicals called mutagens which can be found in tobacco in cigarettes