chapter 20 p2 Flashcards
Codominance:
- occurs when two different alleles occur for a gene - both of which are equally dominant.
- As a result both alleles of the gene are expressed in the phenotype of the organism if present.
- One example of this condition is the colour of snapdragon flowers.
- Two equally dominant alleles exist, each of which codes for the colour of the flower:
1. An allele that codes for red flowers - the allele codes for the production of an enzyme which catalyses the production of red pigment from a colourless precursor.
2. An allele that codes for white flowers - the allele codes for an altered version of the enzyme which does not catalyse the production of the pigment, therefore the flowers are white.
In this example of codominance, three colours of flower can be produced:
Red flowers - the plant is homozygous for the allele coding for the production of red pigment.
White flowers - the plant is homozygous for the allele coding for no pigment production.
Pink flowers - the plant is heterozygous. The single allele present which codes for red pigmentation produces enough pigment to produce pink flowers.
figure 6
- The genetic cross in Figure 6 shows how pink flowers are produced.
- Two of the pink flowers produced in the F1 generation are then crossed in the second cross.
- When studying codominance, upper and lower case letters are not used to represent the alleles, as this would imply one allele is dominant and the other recessive.
- Instead a letter is chosen to represent the gene, in this example C for colour of flowers.
- The different alleles are then represented using a second letter which is shown as a superscript.
- In this example C^r is used to represent the allele coding for red flowers and C^w for the allele coding for white flowers.
Multiple alleles:
Some genes have more than two versions, they have multiple alleles.
However, as an organism carries only two versions of the gene (one on each of the homologous chromosomes) only two alleles can be present in an individual.
Your blood group is determined by a gene with multiple alleles.
The immunoglobulin gene (Gene I) codes for the production of different
antigens present on the surface of red blood cells. There are three alleles of this gene:
There are many different possible crosses. Figure 8 shows how parents of blood group A and B can reproduce to produce children who may display any of the four blood groups.
Determining sex:
- In humans and other mammals, as well as many other species, sex is genetically determined.
- Humans have 23 pairs of chromosomes of varying sizes and shapes.
- In 22 of the pairs, both members of the pair are the same but the 23rd pair, known as the sex chromosomes, are different.
- Human females have two X chromosomes, whereas a male has an X and a Y chromosome.
- The X chromosome is large and contains many genes not involved in sexual development.
- The Y chromosome is very small, containing almost no genetic information, but it does carry a gene that causes the embryo to develop as a male.
- Therefore the sex of the offspring will be determined by whether the sperm fertilising the egg contains a Y chromosome or an X.
Sex linkage:
- Some characteristics are determined by genes carried on the sex chromosomes - these genes are called sex linked.
- As the Y chromosome is much smaller than the X chromosome, there are a number of genes in the X chromosome that males have only one copy of.
- This means that any characteristic caused by a recessive allele on the section of the X chromosome, which is missing in the Y chromosome, occurs more frequently in males.
- This is because many females will also have a dominant allele present in their cells.
Haemophilia:
- Haemophilia is an example of a sex-linked genetic disorder.
- Patients with haemophilia have blood which clots extremely slowly due to the absence of a protein blood-clotting factor (in the majority of cases this is factor VIII).
- As a result injury can result in prolonged bleeding which, if left untreated, is potentially fatal.
- If a male inherits the recessive allele that codes for haemophilia (on their X chromosome) they cannot have a corresponding dominant allele on their Y chromosome, and so develop the condition.
- As a result the vast majority of haemophilia sufferers are males.
- Females who are heterozygous for the haemophilia coding gene are known as carriers.
- They do not suffer from the disorder, however they may pass on the allele to their children.
- This can result in the birth of a son who suffers from haemophilia.
- When showing the inheritance of a sex-linked condition the alleles are shown linked to the sex chromosome they are found on.
In this example, haemophilia is linked to the X chromosome, therefore:
- X^H is used to represent the dominant ‘healthy’ allele.
- X^h is used to represent the recessive allele coding for haemophilia (through the non-production of a blood-clotting protein).
- This is often known as the faulty allele.
- Y is used to represent the Y chromosome - it has no allele attached to it as it does not carry the gene which produces the specific blood-clotting protein (Figure 12).
- If a carrier female and a normal male have children, then in theory half of the male offspring produced will have the disorder (Figure 13).
- Half of the female offspring will be carriers. As male offspring only inherit an X chromosome from their mother, sons can only inherit the condition from their mother.
- However, an affected male can pass on the faulty allele to his daughters, resulting in them becoming carriers of the disorder (Figure 12).
Chapter 20.3 - Dihybrid inheritance
thousands of genes are inherited during fertilisation.
Dihybrid crosses are used to show the inheritance of two genes and this is known as dihybrid inheritance.
Dihybrid cross:
- A dihybrid cross is used to show the inheritance of two different characteristics, caused by two genes, which may be located on different pairs of homologous chromosomes.
- Each of these genes can have two or more alleles.
- A dihybrid cross is set out in a very similar format to the one used when studying a monohybrid cross - however, four alleles (two for each characteristic) are shown at each stage instead of two.
- A classic example is the inheritance of seed phenotype in pea plants.
- The seeds a pea plant produces can be produced in two different colours - yellow or green.
- They are also produced in two different shapes - round or wrinkled.
EXAMPLE P1
- In the dihybrid cross in Figure 2, a true breeding homozygous pea plant with yellow round seeds is crossed with a true breeding homozygous pea plant with green wrinkled seeds.
- All of the offspring produced in the F1 generation will have a heterozygous genotype - YyRr.
Therefore, they will all produce yellow round seeds as they will all have inherited one of each of the parental phenotype parental genotype dominant alleles from the yellow round seeded parent.
In the second example in Figure 2, two of the pea plants grown from the seeds of the F1 generation are now crossed.
EXAMPLE P2
When the phenotypes of the 16 possible combinations of alleles are identified (as shown in Figure 3), it is found that the expected ratio in the F2 generation is nine pea plants producing yellow round seeds, to three pea plants producing yellow wrinkled seeds to three pea plants producing green round seeds, to one pea plant producing green wrinkled seeds.
this is the expected ratio of the four different phenotypes.
As with all genetic crosses the actual ratio of offspring produced can differ from the expected. This may because:
sex linkage p2
Most often sex-linked genes are found on the longer X chromosome
Haemophilia is well known example of a sex-linked disease
Sex-linked genes are represented in the genotype by writing the alleles as superscript next to the sex chromosome. For example a particular gene that is found only on the X chromosome has two alleles G and g. The genotype of a heterozygous female would be written as X^GX^g. A males genotype would be written as X^G Y
