Inheritance: Autosomal Linkage | A-level Biology | OCR, AQA, Edexcel
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- Опубліковано 15 лип 2019
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The key points covered in this video include:
1. Introduction to Autosomal Linkage
2. Genetic Crosses and Autosomal Linkage
3. Example of Autosomal Linkage in the Drosophila Fruit Fly
Introduction to Autosomal Linkage
We know that there are 23 pairs of chromosomes in a human cell. One of these chromosome pairs are the sex chromosomes which determine whether an individual is male or female. The rest of the 22 pairs of chromosomes are homologous pairs and these 22 pairs are called autosomes. Autosomes are chromosomes which are not concerned with sex determination. Each chromosome can have many different genes located on it. Any genes which are located on the same chromosome are described as being linked. If two or more genes are located on the same autosome (non-sex chromosome) it is called autosomal linkage.
Genetic Crosses and Autosomal Linkage
Normally genes which are located on separate homologous chromosomes are independently separated into gametes during crossing over in meiosis. Consider a pea plant in which the gene for seed colour is on a different homologous pair to the gene for seed shape and which is heterozygous for both characteristics. When these homologous pairs cross over during meiosis, the alleles are independently separated from each other. This process is called the independent assortment of genes. This means that there are four possible combinations of alleles in the gametes. However, if two genes are closely located on the same chromosome then they are less likely to be separated during crossing over. This means that there will be just two possible combination of alleles in the gametes, because the alleles of the linked genes will be inherited together.
Example of Autosomal Linkage in the Drosophila Fruit Fly
In the Drosophila melanogaster fruit fly there is autosomal linkage between the gene for body colour and the gene for wing length. For the gene for body colour, the allele for a grey body is dominant to the allele for a black body. For the gene for wing length, the allele for normal wings is dominant over the allele for vestigial wings (tiny wings that don’t work). If two heterozygous Drosophila with grey bodies and normal wings are mated, we can work out the phenotype and genotype of the offspring using a Punnett Square. However, in this example we have to remember there is autosomal linkage between the gene for body colour and the gene for wing length. Step 1: Work out the parental genotypes. Step 2: Write out the parental gametes. Step 3: Work out the offspring genotypes. Step 4: Work out the offspring phenotypes. Step 5: Calculate the phenotype ratios. Therefore in autosomal linkage, a dihybrid cross of two heterozygotes results in ¾ of the offspring having both dominant characteristics and ¼ having both recessive characteristics.
this video made me question everything i've learned about chromosomes until i looked at the comments
this is why youtube never should've removed dislikes-
I was terribly confused until I read the comments and I understood the problem with this video. Thanks for sharing your knowledge for free, but please consider refining this video.
4:56 The blue chromosome shouldn't have both G and g because the sister chromatids should be genetically identical with the same alleles.
Instead the blue should have G and R on both chromatids and the yellow should have g and r on its chromatids.
At 6:20, therefore, it should have GG at the top and RR at the bottom.
Unfortunately there are mistakes in this video which need correcting. All alleles on sister chromatids are identical as they are copied during DNA replication before meiosis occurs (mistakes can occur during this process leading to mutations but so rare its not worth mentioning here). What is shown in the video are not homologous chromosomes but sister chromatids of one chromosome so the alleles should be the same. He should have drawn two blue chromosomes(a homologous pair) one with G on each sister chromatid and the other with g on each of the sister chromatids. Similarly he should have drawn two yellow homologous chromosomes one with RR and the other with rr.
He also worked out the gametes at 09:30 wrong. Because the genes are linked you can only get GN or gn gametes. You can get gametes that are Gn or gN if crossing over happens between homologous chromosomes. This would haowever be rare so there would only be very few if any offspring showing these new recombinant types.
@@bellagriffiths7751 thanks, I was actually getting really confused at that part.
i think he was calculating F2 generation
Why didn't you use all of the gametes for the punnet square? ended up having a 4x4 square- isn't it a dihybrid cross because there are two characteristics that are linked?
I think because GN are autosomally linked, so they tend to be inherited together vs. Gn or gN.
You seem to be representing homologous pairs of chromosomes as sister chromatids (at5:08). Surely the blue and yellow chromosomes are the homologous pair whilst the individual blue and yellow are sister chromatids joined via the red dot/centromere (and hence the alleles should be the same)?
You're right. I'm afraid they've made a mistake in the video.
Sooooo confusing. It looks like a single chromosome with two chromatically!!!
9:33 can someone explain why in the next punnett square, he only uses GN and gn for both? What if the dominant G allele occured with the n allele on the same chromosome? Doesn't that lead to Gn and consequently gN?
I asked my biology teacher about this and in fact, it could be Gn and gN autosomally linked. However, in the textbooks and most examples only use homozygous genotype parents i.e. GGNN and ggnn with gametes GN and gn respectively.
Exactly!!! I am a biology tutor. He skipped that part. That’s the case for non-linked cases. As this is assumed to be linked trait, the Grey Colour and normal wings are on the same chromosome so will always be inherited together. Therefore it’s 2 gametes.
Hi, good video but bit confusing at 9.44, I think you skipped the part where it’s autosomal linkage. There are 4 different gametes which would be the case in a non-linked inheritance. And here we are showing 2 because it’s linked.
At 9:48, why did you use only use those 2 combinations as the gametes in parents. If I wrote gN and Gn instead, it would change the phenotypic ratios. How would i know which combination to use?
I did the same as you Donna. also how would we know to use a 2 by 2 punnet square and not a a 4 by 4 punnet square?
@@Sara-vs1ww u would use the 4 by 4 square if they arent linked because the gametes would then have 4 different combinations. if they are linked then it would be a 2 by 2 punnet square
The main problem in this video is that he should include brackets when writing out the genotypes of the flies, which is (GN)(gn). This will result in getting parental gametes which are GN and gn only. If you don't include brackets, you will get GgNn which is like a normal dihybrid cross.. When we are talking about autosomal linkage, it is like compulsory to include brackets cuz it is not like a normal dihybrid cross.
And if you're using gametes which are GN and gn, you will get offsprings which have genotypes of (GN)(GN), (GN)(gn), (GN)(gn) and also (gn)(gn). The ratio is the same as the ratio in the video but the method of deriving the gametes is inappropriate. Could have explained it in a clearer way to avoid misunderstanding.. Need to include brackets as well. You can refer A Level Biology Textbook which also includes brackets...
I don't understand how he gets from having 4 parent gametes in step 2 (GN) (Gn) (gN) (gn) to just 2 gametes in step 3 (GN) to (gn)
you did not explain how to choose which genotype goes in the punnet square as we don't know which two characteristics are autosomally linked.. it could be Gn and gN that are autosomally linked how am i supposed to know which two to put in the punnet square
4:56 how can u say that both of them are homologous chromosomes? and genes are different in the same loci like how ever could R r in a chromosome will be in the same loci as G and g .This is not a definition of homologous pairs they should be carrying the same genes but different alleles as one from father and mother. Or they are just 2 different chromosomes not homologous???
The blue chromosomes is a homologous pair and the yellow chromosomes is the other homologous pair....i think
This is incorrect. They're both homologous chromosomes, which will indicate that they have the same loci. Thus placing both shape and colour at the same loci on the maternal and paternal chromosomes is a mistake in the video. You are in fact correct to point this out.
But the homologous pairs aren’t identical I swear.
@@davidmkwizu76 They have the same gene loci. However, can have different alleles. So no they're not.
This is only gonna confuse you. Don't follow the diagram. Follow them from the book.
Thank you for this
I dont understand at 8:50 u say that they can not be bred cuz they are hetrozygous cam someone plz explain why??
He isn't saying that they can't be bred together, when he puts the cross there he means that they *are* being bred.
@@negheg9270 thanks
Excellent explanations thank you very much
i wish you were my class teacher :(
This is why youtube shouldnt have removed dislikes, first video that comes up when you search for this and it is wrong apparently... nice
this video just confused me more great
there are lots of mistakes in this video
Yesss finally I get it
Thank youuu ❤️
good
Bruh your video is wrong
This video is terrible. Explanation all wrong. Homologous chromosomes should have alleles for the same gene. Not different genes as shown. Sister chromatids joint by centromere should have exactly the same alleles.
This is a very bad video. Why would you confuse people like that