Inheritance means the passing down of genetic information from generation to generation.
To get that information, DNA needs to get from parent to offspring.
The way this happens is meiosis, then fertilization.
In other words, sperm and egg are made (these are called 'gametes') by the process of meiosis and then the sperm must fertilize (fuse with) the egg.
The way this happens is meiosis, then fertilization.
In other words, sperm and egg are made (these are called 'gametes') by the process of meiosis and then the sperm must fertilize (fuse with) the egg.
Meiosis
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Meiosis basics
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When meiosis occurs
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How meiosis occurs
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Importance of meiosis
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Meiosis is the process that forms gametes, or sex cells. Sex cells are what combine during fertilization to produce offspring. Meiosis occurs in the ovaries/testes. In humans, gametes are eggs and sperm.
All of the cells in our body are diploid (2n). This means that each chromosome has a homologous pair. We have 46 chromosomes in our cells, and they all pair up, making 23 homologous pairs. Gametes are the exception to this - they are haploid (n). This means that there is only one of each chromosome, no homologous pairs. There are only 23 chromosomes in our haploid cells - in our gametes - in an egg or sperm.
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When two haploid (n) cells come together during fertilization (egg+sperm), a zygote is produced (a fertilized egg is called a zygote). Two cells, each with 23 chromosomes (one of each) come together causing their DNA to be in one nucleus. As the chromosomes are in the same place, they can now pair up. So once a sperm (n) and an egg (n) become one, you end up with a diploid (2n) cell which is the offspring.
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Gametes are not identical. There are 23 homologous pairs of chromosomes and either chromosome from each pair can end up in the gamete - this means there are so many different possible combinations of chromosomes that could end up in the sperm/egg. This is important to know.
When does meiosis occur?
In a human life cycle, meiosis occurs to produce the gametes. In males, this is once puberty has started and occurs continuously. In females, meiosis begins while they are in the womb, but hormones produced during puberty allows the process to finish. This is the only time meiosis occurs in the human life cycle. To produce gametes. Mitosis on the other hand occurs once a zygote is produced (the result of fertilization) in order to produce an embryo, fetus, and so on. It will occur at a higher rate during growth stages of our life cycle and is also used for cell repair/replacement.
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How does meiosis occur?
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You don't need to know the names of all the steps of meiosis. It does help to remember them but knowing the names will not be assessed. You do need to know how meiosis occurs and the benefits of it.
Independent assortment:
Crossing over:
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Meiosis helps to create genetic variation within a population.
Meiosis is beneficial as it produces gametes that are genetically different to the parent cell, and when the gametes combine the offspring produced is made from combination of genes from both mother and father. The offspring is genetically unique, there is no other organism on the Earth with the exact same DNA sequences on their chromosomes. Among humans and all living organisms, this creates genetic variation.
Genetic variation is very important to all species. Lets make this about plants. Say you have a flower garden in your backyard and in the garden are 10 flowering plants that are genetically identical - the sequence of bases on all of their chromosomes is the same. Now lets say one night the temperature drops unusually low and there's a frost. You get up the next morning and find all your flowers have been destroyed by the frost - not one surviving plant. Terrible! Now, if you had 10 genetically different flowering plants in the same garden and experienced a frost just like this one, things may turn out differently. This is because there is variation in the DNA sequences (and therefore proteins and characteristics) of these plants. It could just be that one or two flowering plants survive the frost because of a combination of alleles they have, that the others don't. It could just be that because of these small differences in your flowering plant population, the whole population isn't wiped out. The one or two plants that survived can now live longer and reproduce, passing on the favourable alleles that allowed them to survive the frost. Because of meiosis, and the resulting genetic differences, the whole population can survive environmental change. This is very important to the survival and evolution of species as diversity allows them to survive droughts, floods, frosts, diseases, fires, all sorts of changes to the environment. Some individuals will die from these changes but due to small genetic differences, some will be able to survive and ultimately keep the species alive.
Here's the link to a website that explains genetic variation really well. |
Fertilization
This must happen for an offspring to be produced. Sperm must fuse with egg and form new homologous pairs of chromosomes. Each offspring gets 23 chromosomes from their mother and 23 from their father. Maternal and paternal chromosomes pair up to make new homologous pairs. As each egg ever made is different, and same with sperm, this means that each time fertilization happens it is with a different egg and sperm. This makes each offspring unique!
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Sex determination
How and why did you end up a girl and not a boy?
A sperm is haploid (has 23 chromosomes, not 46) - so is an egg. So when a sperm and egg fuse during fertilization, the zygote ends up with a full set - 46. The chromosomes match up with their homologous pairs. Sex is determined by the chromosomes you get in the 23rd pair. Because mum's female, she has 2 X chromosomes for her 23rd pair, but males have a different combination. To be male, you must have 1 X and 1 Y chromosome. So dad has the genotype XY.
When fertilization happens, the zygote will definitely receive an X from mum (as mum can only give X's). It completely depends on the sperm that fertilizes the egg to which gender the offspring will be - if a sperm carrying an X fertilizes the egg, the offspring will be XX and be female. However, if the sperm is carrying the Y chromosome and fertilizes the egg then the offspring will have the genotype XY and be male. Each time fertilization happens there is a 50% chance of the offspring being male of female!
Monohybrid inheritance
This is all about how you get your hair colour or eye colour.
Monohybrid means when you cross two individuals (i.e. parents) and look at one gene in their offspring.
Monohybrid means when you cross two individuals (i.e. parents) and look at one gene in their offspring.
From here on, alleles are given letters. The letter is always the same (as it represents the gene) but it can be upper case or lower case (e.g. the letter E, but written as E or e). Upper case signifies dominance whereas a lower case letter is recessive. For the same gene, there is always one dominant allele and one recessive allele, at least!
New terms used to describe someone's genotype:
Homozygous - homo meaning two of the same allele. That person's homologous chromosome pair for the gene [whatever the gene is] both contain the same allele (ie. EE or ee, not Ee). |
Heterozygous - hetero meaning two different alleles. That person's homologous chromosome pair for that gene [whatever the gene is] contain one of each allele (ie. Ee, not EE or ee). |
Dominant - When present in the genotype this allele will always show up in the phenotype. Only one is needed to 'win' and to be seen. |
Recessive - Can only show up in phenotype if there are two in the genotype. If there is ever a dominant allele and a recessive allele in a genotype, the dominant is shown in the phenotype. 2 recessive alleles are always needed for that trait to be seen. |
Monohybrid cross
A monohybrid cross is when you take two individuals and cross them (mate them), and look at the resulting offspring for one gene. We use monohybrid crosses to predict the genotype, and thus the phenotype, of the offspring that would result from a cross between those two individuals. For example, you are looking at the hair colour gene (we'll give that gene the letter B. You could give it a different letter though!).
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Example:
Black hair is dominant over brown hair. Therefore we can give black hair the B and brown hair the b. B/b represents hair colour, the different cases represent dominant/recessive.
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We are going to cross a homozygous dominant woman with a homozygous recessive man.
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Possible genotype and phenotype ratios
(there are only a few you can get)
We'll do H for hairy (think back to guinea pigs) and h for hairless
(there are only a few you can get)
We'll do H for hairy (think back to guinea pigs) and h for hairless
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Pedigree charts
You need to be able to analyse these to work out some genotypes!
Let's use a 2016 exam question to practice...
How to work out the four individuals:
1 - number one is unaffected (use the key to work this out). In the question it says photic sneezing is dominant to unaffected. This means we know unaffected must be recessive. For a recessive trait to show up in the phenotype, you need two copies of that allele in the genotype. This makes 1's genotype aa.
2 - number two is affected (use the key to work this out). If number two was homozygous dominant (AA) then each offspring will get an A (dominant allele) and, as only one is needed in the genotype to show up in the phenotype, all offspring would be affected. However this is not the case - 4 out of 5 offspring are unaffected (making them aa). This means number two must have a recessive allele hidden in his genotype. This makes her Aa.
11 and 12 - same reasoning as for two. Both must have one A allele (because they're affected) and A always masks a. However one of the offspring of 11 and 12 (number 14) is unaffected (aa) - remember offspring get one allele from each parent - so one a must have come from the mother and the other from the father. This means the mother and father of 14 (11 and 12) must both carry an a. This makes them Aa.
1 - number one is unaffected (use the key to work this out). In the question it says photic sneezing is dominant to unaffected. This means we know unaffected must be recessive. For a recessive trait to show up in the phenotype, you need two copies of that allele in the genotype. This makes 1's genotype aa.
2 - number two is affected (use the key to work this out). If number two was homozygous dominant (AA) then each offspring will get an A (dominant allele) and, as only one is needed in the genotype to show up in the phenotype, all offspring would be affected. However this is not the case - 4 out of 5 offspring are unaffected (making them aa). This means number two must have a recessive allele hidden in his genotype. This makes her Aa.
11 and 12 - same reasoning as for two. Both must have one A allele (because they're affected) and A always masks a. However one of the offspring of 11 and 12 (number 14) is unaffected (aa) - remember offspring get one allele from each parent - so one a must have come from the mother and the other from the father. This means the mother and father of 14 (11 and 12) must both carry an a. This makes them Aa.
Here's a question from the 2014 exam:
Test crosses
A test cross is a method used to find out the genotype of an individual who possesses a dominant phenotype.
- This is of relevance if you are wanting to create a pure-breeding pedigree (a family lineage where no alleles are hidden)
- For example, HH and Hh both produce Hairy guinea pigs, but how do we know it's genotype just from looking at it? All we can see is that it is hairy!
By looking at this guinea pig, we know its phenotype for the 'hair' allele is hairy (H). Hairy is dominant over hairless. If there was a hairless guinea pig here instead, we would know its genotype. It would be hh. Remember any time a dominant allele is present it is expressed in the phenotype, so if the phenotype is not the dominant one, we know the genotype must contain two recessive alleles. |
All we know from looking at the guinea pig is that it is hairy, but we don't know if it is HH or Hh. To create a pure-breeding line, we want the guinea pig to be HH. To find out its genotype we must cross (mate) it with a guinea pig that is homozygous recessive for that gene (hairless - hh) and look at the offspring from that cross.
This is one of those things you can just memorize because it will always be the same!