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Sex-linked/genetic cross help!!urgent

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Could someone please help me -Why is this sex-linked and also why is the highlighted bit in the ms wrong? By the way I understand the first part of the question -it's just that I am confused with the second question. Please help me

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Reply 1

macpatgh-Sheldon

Hi,

All the red-eyed flies in the offspring are female (XX), all the white-eyed are male (XY), so the dominant allele for red eye must be on the X chromosome i.e. the gene is sex-linked. (this is supported by the fact that there are ZERO white-eyed females and ZERO red-eyed males; in general, almost all sex-linked genes/conditions (particularly diseases) are X-linked and the Y chromosome is a "sleeping partner". However, it has more recently been shown that there are also certain traits carried on the Y chromosome (these are transmitted strictly male to male i.e. father to son).

The print in bold is the reason for haemophilia occurring mostly in males e.g. only in kings of the royal family and not in quenns, being transmitted from an affected male via his carrier daughter to his grandson. Similarly, red-green colour blindness (protanopia) is a sex-linked recessive condition and is much more common in males because for a female to have it, she has to be homozygous for the abnormal allele, which is of course less likely.

M

Reply 2

h26

Original post by macpatelgh

Hi! Thanks for replying. But how do you know this is specifically sex linked -why not something else i.co-dominant etc.... Why are we drawing a genetic cross with x and y chromosomes?
Sorry if I am making you repeat anything

Reply 3

h26

Original post by macpatelgh

How do you know the dominant allele R is in the XX chromosome or in the X in XY?

Bump

Bump

it's sex-linked because this specific type of disease is carried by the 23rd chromosome (the one that determines your sex). to represent that it is carried by this chromosome it is done with x and y -- the signs for this chromosome will never change for the disease that it carries. the letter's change if it is another disease that is not sex-linked, but usually it can be any letter that has a significantly different upper case and lower case

Reply 7

h26

Original post by 8106

Thanks for replying! But the question isn't about a disease. So therefore it makes it hard to identify as sex linked. Also, the bit I've highlighted in yellow in the mark scheme is what I am really confused about. Could you please help with this?

Reply 8

LRxS

Original post by h26

How do you know the dominant allele R is in the XX chromosome or in the X in XY?

If it was not sex linked, and both parents are homozygous, the F1 generation would all display the dominant allele (as RR x rr only produces Rr offspring).

However, this is not the case, and based on the gender ratios we know that it is sex-linked.

The gene for eye colour is carried ONLY on the X-chromosome. This means that the recessive colour - white - is almost exclusively seen in males, as they do not have another X chromosome containing the dominant allele to “cancel” the recessive gene.

The cross carried out therefore was as follows:

XrXr (mother; white eyes, homozygous) || XRY (father, red eyes)

which gives the offspring XRXr and XrY in a 1:1 ratio - producing males that are all white-eyed and females that are all red-eyed.

I hope that helps!!

I will PM you the cross the student carried out.

Reply 9

h26

Original post by LRxS

Hi! Thanks a lot for replying. So I mostly understand this now. After thinking about it, the thing that is bothering me right now is:
How can you quickly identify that the parental gametes are---
XrXr (mother; white eyes, homozygous) || XRY (father, red eyes)

and not
XRXR (mother; white eyes, homozygous) || XrY (father, red eyes)
and not
XRXr (mother; white eyes, homozygous) || XrY (father, red eyes)

I am so sorry if I happen to repeat anything :frown:

I am just getting paranoid cause my exams are so near eek

Reply 10

OxFossil

Original post by h26

I am just getting paranoid cause my exams are so near eek

The biggest clue to you knowing that you have to give an answer using the sex chromosomes is in the way the question is presented.

First, it sorts the F1 generation by sex. Second, the F1 phenotypes are drastically different from a standard dominant/recessive autosomal segregation. Third - and this is the real giveaway - the question ii) is introduced by telling you about the sex chromosomes. There is absolutely no reason for the examiner to give you this information if it wasn't directly relevant to the answer! Together, this is enough to tell you that you are going to need to do a punnet square based on a sex linked gene.

I hope you can do the punnet square OK once you have got this.

You know that all the F1 females inherit one X chromosome from their mother and one from their father, and that all the F1 males inherit their X chromosome from their mother (only). The F1 females do not inherit anything on the Y chromosome from their father - as they are XX, and all the X chromosomes in the F1 males come from their mother.

All the F1 males are white-eyed, so neither of their mother's X chromosomes can contain the dominant R gene. She is therefore homozygous for white eyes. By deduction, therefore, the father must carry the R allele on his X chromosome (so all the F1 females are heterozygous XRXr)

Reply 11

LRxS

Original post by h26

Hi! Thanks a lot for replying. So I mostly understand this now. After thinking about it, the thing that is bothering me right now is:
How can you quickly identify that the parental gametes are---
XrXr (mother; white eyes, homozygous) || XRY (father, red eyes)
and not
XRXR (mother; white eyes, homozygous) || XrY (father, red eyes)
and not
XRXr (mother; white eyes, homozygous) || XrY (father, red eyes)

I am so sorry if I happen to repeat anything :frown:

I am just getting paranoid cause my exams are so near eek

Firstly, R codes for red, and r for white (but white is recessive).

To identify the parental gametes, start with the information you’ve got in the question - we know one parent has red eyes and one has white eyes, and that the Male is XY (look out for this as it isn’t always the case), so the options are:
1. XRXR / XrY (red female, white male)
2. XRXr / XrY (red female, white male)
3. XrXr / XRY (white female, red male)

The way to be sure if you’re not confident is to do the punnet square cross for ALL these possibilities, and see which one matches the results given in the question. However, for sex-linked traits, it can be a little quicker. We know that all the F1 males have white eyes, and that as they are Male they must have got a Y chromosome from their father. This means they can only get their X chromosome from their mother. Since none of them have red eyes, the mother has no R (red) alleles - meaning she is XrXr. We know there’s an R in their somewhere as the F1 females have red eyes - they get one X from each parent, and as we know hey get Xr from the mother, they must be getting XR from the father. So he is XRY. I hope that helps!!