simple inheritance in animals and plants
In this section we will be looking at the ways in which organisms produce cells so that they can grow, repair themselves, and reproduce. In addition we will look at how inherited characteristics are passed on, and the genetics of a number of inherited diseases. In this topic there is a lot of opportunity to address ethical debates. You need to be aware of the ethics surrounding the acquisition of and use of stem cells, as well as issues related to whether people test for genetic conditions or not.
Mitosis
This is the form of cell division involved in growth, repair and replacement of cells |
Key terminology Gene - small packet of information that codes for a protein. Found on the chromosomes Chromosome - Thread-like structure that carries the genetic information in the nucleus of the cell. 46 pairs in normal human cells DNA - The material of inheritance. Makes up the chromosomes Nucleus - Contains all of the chromosomes. The control center of the cell Allele - a version of a gene Haploid - having one set of each chromosome (23 in the human gamete) Diploid - having two sets of each chromosome (23 pairs) Gamete - sex cell |
Cells produced by mitosis are genetically identical to the parent cells. This is because prior to division, the cell replicates its DNA, meaning daughter cells get exactly the same DNA in them. 2 daughter cells are produced per parent cell. Mitosis is used during asexual reproduction in, for example, yeast and bacteria.
Check your knowledge and find out about what happens when mitosis goes wrong here
Check your knowledge and find out about what happens when mitosis goes wrong here
Meiosis
This form of cell division is needed for sexual reproduction as it is used to produce the sex cells (gametes) - the sperm/pollen for the male, and the egg/ovule for the female. This form of cell division therefore only takes place within the reproductive organs of animals and plants. In humans this is the ovaries and testes.
Gametes only have half the number of chromosomes as the parent cell - 23 chromosomes in total in the human.
This form of cell division is needed for sexual reproduction as it is used to produce the sex cells (gametes) - the sperm/pollen for the male, and the egg/ovule for the female. This form of cell division therefore only takes place within the reproductive organs of animals and plants. In humans this is the ovaries and testes.
Gametes only have half the number of chromosomes as the parent cell - 23 chromosomes in total in the human.
In meiosis, there are 4 daughter cells for each parent cell.
Like in mitosis, the chromosomes copy themselves before any division takes place. However, in meiosis the cell divides not once but twice in quick succession. This gives 4 daughters, and also halves the chromosome number (here illustrated by just 4 chromosomes in the parent cell). Interestingly, due to mixing of DNA during meiosis, the daughter cells have genetic variation when compared to the parental cell. Upon fertilisation when the nucleus of a sperm and an egg cell fuse, there is a mixing of parental DNA creating a completely unique new individual. |
How to remember the difference:
Mitosis - making Identical two
Meiosis - making eggs (and sperm)
Differentiation and stem cells
Early embryos are made up of unspecialised cells. These cells rapidly become specialised during development so that by the time we are born they will have particular jobs to do. So, you will have muscle cells, nerve cells, blood cells, etc.
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Key terminology
Differentiation - the process by which cells become specialised Stem cells - unspecialised cells that have the ability to differentiate into many different forms of cell Multipotent - stem cells that are able to differentiate into only a subset of the types of cells in an organism Pluripotent - stem cells that are able to differentiate into all types of cells in an organism |
In a mature animal, cell division takes place by mitosis and, since differentiation has occurred, mitosis is restricted such that a given cell will normally only produce more of that type of cell. For example, a muscle cell will only divide to produce more muscle cells. It is slightly different in plants whereby growing points (known as meristems) maintain a population of undifferentiated cells which continue to divide throughout the life of the plant. The meristems are found in the roots and the growing tips, and their presence means plants continue to grow throughout their entire life.
Stem cells are unspecialised cells which have the capacity to differentiate into many different types of cells. They are found in embryos (embryonic stem cells - pluripotent) and in some adult tissues (adult stem cells - mulltipotent). You need to be aware of the ethical considerations of the use of embryonic stem cells, as well as the advantages and disadvantages of each.
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Look at this site for more information on what stems cells are and how they are used
The beauty of DNA
DNA (deoxyribonucleic acid) is the material of inheritance. It is found within the nucleus of the cell, packaged into chromosomes. Sections of chromosomes that control the expression of different proteins that the body makes are called 'genes'.
AMAZING RESOURCE |
DNA is made up of combinations of chemical bases (A, T, G and C). Genes contain tens of thousands of these bases. The order that they occur in a gene will determine the order of the amino acids that are used to construct the protein. Bases are 'read' in threes (triplets or codons), each set of three bases giving instructions for a particular amino acid. This is called the genetic code
Mendel - the grandaddy of genetics!
Gregor Mendel was an Austrian monk who worked in the mid-19th Century. He conduced thousands of experiments on pea plants and eventually sussed out that certain characteristics were passed on from parents to their offspring (what we now refer to as inheritance). Sadly the reasoning behind inheritance wasn't understood at the time (indeed, not until after Mendel's death) so his work was widely rejected by the scientific community. To find out more about Mendel and the significance of his work, watch this interesting video.
Gregor Mendel was an Austrian monk who worked in the mid-19th Century. He conduced thousands of experiments on pea plants and eventually sussed out that certain characteristics were passed on from parents to their offspring (what we now refer to as inheritance). Sadly the reasoning behind inheritance wasn't understood at the time (indeed, not until after Mendel's death) so his work was widely rejected by the scientific community. To find out more about Mendel and the significance of his work, watch this interesting video.
Using DNA
DNA fingerprinting is a forensic technique that exploits the fact that everybody's DNA is unique (with the exception of identical twins who come from the same fertilised egg). Here, small amounts of DNA are compared in order to determine a match. It is often used to solve a crime (where a sample of DNA from the scene will be compared to samples taken from suspects), and in paternity testing (where a child's DNA is compared to that from potential dads). Go Jeremy Kyle! |
To find out more about how tiny amounts of DNA can be used by scientists for a variety of reasons, go to this excellent website
How inheritance actually works
You inherit your genes from your parents. Your father gives you one set of chromosomes (half of your DNA) and your mother gives you your second set of chromosomes (the other half of your DNA). 23 chromosomes from mum and 23 chromosomes from dad, making a total of 46 chromosomes.
As you learnt in the section on meiosis above, sperm and egg only contain 23 chromosomes meaning upon fertilisation, the resulting embryo has 46 chromosomes.
Sex determination
Females have the sex chromosomes XX and males have the sex chromosomes XY. The Y chromosome determines whether the child is a boy or a girl. There is a 50% chance of having either a boy or a girl.
As you learnt in the section on meiosis above, sperm and egg only contain 23 chromosomes meaning upon fertilisation, the resulting embryo has 46 chromosomes.
Sex determination
Females have the sex chromosomes XX and males have the sex chromosomes XY. The Y chromosome determines whether the child is a boy or a girl. There is a 50% chance of having either a boy or a girl.
Punnet squares
These are a useful way of working out what the result of a genetic cross is. A genetic cross is what we call a combination of genes from both parents. You can use them for all inherited characteristics, from the colour of your eyes or hair, to whether you are likely to suffer from an inherited disease - more about this below. Genes are always given a letter (it is arbitrary - it doesn't matter what letter a gene has been given). The two alleles for that gene are written with the same letter but the dominant allele will be given a capital letter, whereas the recessive allele will have a lowercase letter. In the example here, let's imagine we are looking at a gene for Huntington's disease (a dominant condition), and it has been given the letter 'H'. Dad has got the genotype Hh (he is heterozygous), as has mum. There will both have Huntington's disease as they both have the dominant allele, H. Remember only one chromosome (and therefore one allele) will go into any gamete so 50% of the gametes will have an 'H' and 50% will have an 'h' for each parent. The Punnet square shows the result of this cross. |
Genetics terminology
There are a lot of very big complicated words in this topic that you need to know the meaning of. Make sure you are clear about all of the following words Homozygous - an individual that has two identical alleles for a given gene Heterozygous - an individual that has two different alleles for a given gene Phenotype - the physical appearance of an individual as a result of gene expression Genotype - the genetic makeup of an individual Dominant - an allele that is expressed even if only present on one chromosome Recessive - an allele that will only be expressed if it is present on both chromosomes Carrier - someone who has the recessive allele for a condition but don't express the condition themselves |
Make sure you practice Punnet squares and are comfortable using them to work out the result of genetic crosses. In an exam they might not even tell you what the genotype is of the parents, just that they have had a child with a certain condition. They will expect you to use Punnet squares to work out how that happened.
Inherited conditions in humans
You need to know about two inherited conditions:
You need to know about two inherited conditions:
Polydactyly
This is a dominant condition It causes multiple digits Polydactyly is not life-threatening but people often have extra digits removed!
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Cystic fibrosis
This is a recessive condition It causes thick mucus which clogs up their lungs and digestive system, resulting in repeated infections. It also affects their reproductive system, meaning CF patients are often infertile. Sufferers have a reduced life expectancy |
There are procedures in place that can be used to screen for genetic conditions. As for the use of embryonic stem cells, you need to be aware of the advantages, disadvantages and ethical considerations of these procedures.
Have a look at this activity on BBC Bitesize for some more information and examples on inheritance