Topic 7—A: Genetics- 4. Linkage Flashcards
What can linkage affect?
- The phenotypic ratios of monohybrid and dihybrid crosses
- You can use this variation from the expected ratios to identify that genes are linked
What is the genetic information for biological sex carried on?
- two sex chromosomes
In mammals, what chromosomes do females have?
Two X chromosomes (XX)
In mammals what chromosomes do males have?
One X chromosomes and one Y chromosome (XY)
What is the probability of having male offspring?
50%
What is the probability of having female offspring?
50%
Definition of sex-linked characteristics
When the allele that codes for the characteristic is located on a sex chromosome (X or Y)
Which chromosome is smaller?
- the Y chromosome is smaller than the X chromosome
- It carries fewer genes
Where are most genes on the sex chromosomes carried?
- On the X chromosome (called X-linked genes)
Why do males express the characteristic of the allele even if its recessive?
- As males only have one X chromosome they often only have one allele for sex-linked genes
- They only have one copy
- This makes males more likely than females to show recessive phenotypes for genes that are sex linked
What are the genetic disorders caused by faulty alleles located on sex chromosomes?
- colour blindness
- haemophilia
Where are the faulty alleles for both of these disorders carried?
- On the X chromosome and so are called X-linked disorders
- Y-linked disorders do exist but are less common
Colour blindness
- sex-linked disorder
- caused by a faulty allele carried on the X chromosome
- as its sex-linked both the chromosome and the allele are represented in the genetic diagram e.g. Xn where X represents the X chromosome and n the faulty allele for colour vision
- the Y chromosome doesn’t have an allele for colour vision so is just represented by Y
- Females would only need 2 copies of the recessive allele to be colour blind
- Males only need one copy
- This means colour blindness is much rarer in women than men
- females with one copy of the recessive allele are said to be carriers
What is a carrier?
- A person carrying an allele which is not expressed in the phenotype but that can be passed on to offspring
Punnet square for the sex-linked cross between a carrier female and an unaffected male
N- normal colour vision allele
n- faulty colour vision allele
Parents genotypes:
Carrier female: Unaffected male:
XN Xn. XNY
Gametes alleles: XN Xn. XN Y
XN. Xn XN XNXN. XNXn Y. XNY. XNY
Unaffected female
Carrier female
Unaffected male
Colour blind male
3:1 ratio of offspring without colour blindness:offspring with colour blindness
But when a female carrier (XN Xn) and a male without colour blindness have children only their males offspring are at risk of being colour blind
When will the ratio change?
If a female carrier (XNXn) and a male with colour blindness (XnY) have children
-Predicted ratio will then be 1:1- of offspring with colour blindness: offspring without colour blindness
Definition of autosome
- a chromosome that isn’t a sex chromosome
What are autosomal genes and where are they located?
- They are genes located on the autosome
What are the genes located on the same autosome said to be?
Linked
- this is because they will stay together during the independent segregation of chromosomes in meiosis I
- Their alleles will be passed on to the offspring together
- the only reason this wont happen is if crossing over splits them up first
The closer together the two genes are on the autosome the more likely they are said to be?
Linked
This is because crossing over is less likely to split them up
If two genes are autosomally linked what wont you get?
The phenotypic ratio you expect in the offspring of a cross
Example:
- in a dihybrid cross between two heterozygous parents you’d expect a 9:3:3:1 ratio in the offspring
- Instead, the phenotypic ratio is more likely to be expected for a monohybrid cross between two heterozygous parents (3:1) because the two autosomally-linked alleles are inherited together
- This means that a higher proportion of the offspring will have their parents (heterozygous) genotype and phenotype
What can you use the predicted phenotypic ratio to identify?
Autosomal linkage
Example
- a scientist was investigating autosomal linkage between the genes for eye colour and wing length in fruit flies
- the gene for normal wings (N) is dominant to the gene for vestigial wings (n) and the gene for red eyes (R) is dominant to the gene for purple eyes (r)
- The first cross the scientist carried out was between flies homozygous dominant for both normal wings and red eyes (NNRR) and flies homozygous recessive for both vestigial wings and purple eyes (nnrr)
- the resulting offspring were all heterozygous for normal wings and red eyes (NnRr)
- The second cross the scientist carried out was between these offspring (NnRr) and the flies homozygous recessive for vestigial wings and purple eyes (nnrr) he expected a 1:1:1:1 ratio
Parents genotypes- NnRr x nnrr
Gametes alleles- NR, Nr, nR, nr, nr, nr, nr, nr
Possible offspring:
nr. nr. nr. nr
Nr. NnRr NnRr NnRr NnRr
Nr Nnrr Nnrr Nnrr Nnrr
nR. nnRr nnRr nnRr nnRr
nr. nnrr nnrr nnrr nnrr
Normal wings, red eyes- 4
Normal wings, purple eyes- 4
Vestigial wings, red eyes- 4
Vestigial wings, purple eyes- 4
Phenotypic ration- 1:1:1:1
In order for the NnRr and nnrr genotypes to be common in the offspring what must have been linked?
the NR and nr alleles in the NnRr parent must have been linked
- this means that the NnRr parent produced mostly NR and nr gametes
- Some Nr and nR gametes were still made due to crossing over but there were fewer Nnrr and nnRr offspring overall
- As a result a higher proportion of the offspring have their parents phenotypes
