A4. Linkage Flashcards
Inheritance of sex-linked characteristics
The genetic information for biological sex is carried on two sex chromosomes.
In mammals, females have two _ chromosomes (__) and males have one X chromosome and one _ chromosome (__). Figure 1 is a genetic diagram that shows how sex is inherited. From this you can see that the probability of having male offspring is __% and the probability of having female offspring is __%.
Figure 1: Genetic diagram showing the inheritance of sex.
The genetic information for biological sex is carried on two sex chromosomes.
In mammals, females have two X chromosomes (XX) and males have one X chromosome and one Y chromosome (XY). Figure 1 is a genetic diagram that shows how sex is inherited. From this you can see that the probability of having male offspring is 50% and the probability of having female offspring is 50%.
Inheritance of sex-linked characteristics
Some characteristics are sex-linked. That means the _________ that _____for them are located on a ___ ______________. The _ chromosome is _________than the X chromosome and carries ______genes. So most genes on the sex chromosomes are only carried on the X chromosome (called X-linked genes).
As males only have one X chromosome they often only have one allele for sex-linked genes. So because they only have one copy, they express the characteristic of this allele even if it’s ____________. This makes males more likely than females to show __________phenotypes for genes that are sex-linked.
Genetic disorders caused by faulty alleles located on sex chromosomes include colour blindness and haemophilia. The faulty alleles for both of these disorders are carried on the X chromosome and so are called X-linked disorders. Y-linked disorders do exist but are less common.
Some characteristics are sex-linked. That means the alleles that code for them are located on a sex chromosome. The Y chromosome is smaller than the X chromosome and carries fewer genes. So most genes on the sex chromosomes are only carried on the X chromosome (called X-linked genes).
As males only have one X chromosome they often only have one allele for sex-linked genes. So because they only have one copy, they express the characteristic of this allele even if it’s recessive. This makes males more likely than females to show recessive phenotypes for genes that are sex-linked.
Genetic disorders caused by faulty alleles located on sex chromosomes include colour blindness and haemophilia. The faulty alleles for both of these disorders are carried on the X chromosome and so are called X-linked disorders. Y-linked disorders do exist but are less common.
Inheritance of sex-linked characteristics - example
Figure 2 on the next page shows a genetic diagram for colour blindness. Colour blindness is a sex-linked disorder caused by a faulty allele carried on the X chromosome. As it’s 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 need two copies of the recessive allele to be colour blind, while 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. A carrier is a person carrying an allele which is not expressed in the phenotype but that can be passed on to offspring.
Here’s how to draw a Punnett square for the sex-linked cross between a carrier female and an unaffected male:
(3 steps)
Step 1: Make sure you’re clear what the letters mean.
Step 2: Work out the alleles the gametes would have.
Step 3: Cross the parents’ gametes to show the possible offspring.
Inheritance of sex-linked characteristics - figure 2 explained
In the figure 2, there’s a :ratio of offspring without colour blindness: offspring with colour-blindness. But when a female carrier and a male without colour-blindness have children (as in this example), only their male offspring are at risk of being colour-blind. So you can also say that there’s a predicted _:: ratio of female offspring without colour- blindness: male offspring without colour-blindness: male offspring with colour-blindness. This ratio will change if a female carrier (XᴺXᴺ) and a male with colour-blindness (XᴺY) have children. The predicted ratio will then be :- of offspring with colour-blindness: offspring without colour-blindness. The ratio will be the same for offspring of each sex. You only end up with this predicted ratio for a monohybrid F2 cross with a sex-linked characteristic.
In the figure 2, there’s a 3:1 ratio of offspring without colour blindness: offspring with colour-blindness. But when a female carrier and a male without colour-blindness have children (as in this example), only their male offspring are at risk of being colour-blind. So you can also say that there’s a predicted 2:1:1 ratio of female offspring without colour- blindness: male offspring without colour-blindness: male offspring with colour-blindness. This ratio will change if a female carrier (XᴺXᴺ) and a male with colour-blindness (XᴺY) have children. The predicted ratio will then be 1:1- of offspring with colour-blindness: offspring without colour-blindness. The ratio will be the same for offspring of each sex. You only end up with this predicted ratio for a monohybrid F2 cross with a sex-linked characteristic.
Tip: Males can’t be _________of X-linked disorders because they only have one copy of each chromosome, so if they have the allele they have the disease-whether it’s recessive or not.
Tip: Males can’t be carriers of X-linked disorders because they only have one copy of each chromosome, so if they have the allele they have the disease-whether it’s recessive or not.
Tip: Here’s a diagram to show the predicted phenotypic ratio of offspring if a female carrier and male with colour-blindness have children:
____ offspring are predicted to have colour-blindness and ____aren’t. This simplifies to a : ratio.
Two offspring are predicted to have colour-blindness and two aren’t. This simplifies to a 1:1 ratio.
Linkage of autosomal genes
Autosome is the fancy name for any chromosome that isn’t a sex chromosome. Autosomal genes are the genes located on the autosomes. Genes on the same autosome are said to be linked that’s because they’ll stay together during the _____________ ___________of chromosomes in _________ _, and their _______will be _________ __ to the offspring together. The only reason this won’t happen is if crossing over splits them up first. The closer together two genes are on the ________, the more closely they are said to be ________. This is because __________ _____is less likely to split them up.
Autosome is the fancy name for any chromosome that isn’t a sex chromosome. Autosomal genes are the genes located on the autosomes. Genes on the same autosome are said to be linked that’s because they’ll stay together during the independent segregation of chromosomes in meiosis I, and their alleles will be passed on to the offspring together. The only reason this won’t happen is if crossing over splits them up first. The closer together two genes are on the autosome, the more closely they are said to be linked. This is because crossing over is less likely to split them up.
Figure 3a: Autosomal genes being split up during crossing over.
Tip: Independent segregation is the…
Crossing over is when…
It happens in meiosis I before ________________ _____________.
Tip: Independent segregation is the random division of homologous (paired) chromosomes into separate daughter cells during meiosis. Crossing over is when two homologous chromosomes ‘swap bits’. It happens in meiosis I before independent segregation.
If two genes are ____________ ________, you won’t get the phenotypic ratio
you expect in the offspring of a cross.
If two genes are autosomally linked, you won’t get the phenotypic ratio
you expect in the offspring of a cross.
Figure 3b: Independent segregation of autosomes during meiosis I
Linkage of autosomal genes - example
In a dihybrid cross between two heterozygous parents you’d expect a ::_:_ratio in the offspring. Instead, the phenotypic ratio is more likely to be that expected for a monohybrid cross between two ____________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.
So you can use the predicted phenotypic ratio to identify autosomal _______.
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 that 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.
So you can use the predicted phenotypic ratio to identify autosomal linkage.
Linkage of autosomal genes - 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 as shown in Figure 4:
Figure 4: Genetic diagram showing the expected phenotypic ratio for a dihybrid cross between one heterozygous parent and one homozygous parent.
However, the results the scientist got for the NnRr x nnrr cross showed an 8:1:1:8 ratio
In order for the NnRr and nnrr genotypes to be so common in the offspring, the NR alleles and the 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.
In order for the NnRr and nnrr genotypes to be so common in the offspring, the NR alleles and the 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.
Tip: Crossing the offspring with one of the parents is known as a ______ _______.
Tip: Crossing the offspring with one of the parents is known as a back cross.
Tip: Watch out :::ratio is expected here because the cross is between a homozygous parent and a heterozygous parent not two heterozygous parents (which would be a _:::_ratio).
Tip: Watch out 1:1:1:1 ratio is expected here because the cross is between a homozygous parent and a heterozygous parent not two heterozygous parents (which would be a 9:3:3:1 ratio).
