17.5) Monohybrid inheritance
Allele: is a version of a gene.
Genotype: is the genetic makeup of an organism in terms of the alleles present.
Phenotype: is the observable features of an organism.
Homozygous: is having two identical alleles of a particular gene. Two identical homozygous individuals that breed together will be pure-breeding.
Heterozygous: is having two different alleles of a particular gene. A heterozygous individual will not be pure-breeding.
Dominant: is an allele that is expressed if it is present.
Recessive: is an allele that is only expressed when there is no dominant allele of the gene present.
Pedigree diagrams and inheritance:
Pedigree diagrams are similar to family trees and can be used to demonstrate how genetic diseases can be inherited.
They include symbols to indicate whether individuals are male or female and what their genotype is for a particular genetic characteristic.
- the organism with the dominant trait is always crossed with an organism with the recessive trait
- if ANY offspring show the recessive trait, the unknown genotype is heterozygous
- if ALL the offspring have the dominant trait, the unknown genotype is homozygous dominant
- large numbers of offspring are needed for reliable results
If both genes of an allelomorphic pair produce their effects in an individual (ie. neither allele is dominant to the other) the alleles are said to be co-dominant.
The inheritance of the human ABO blood groups provides an example of codominance.
The gene controlling human ABO blood groups has three alleles, not just two:
- I^A and I^B are not dominant over one another
- both are dominant over I^O
The table shows the possible genotypes (alleles present) and phenotypes (blood group).
Since the alleles for groups A and B are dominant to that for group O, a group A person could have the genotype I^AI^A or I^AI^O. Similarly for group B. There are no alternative genotypes for groups AB and O.
Sex-linked characteristic is one in which the gene responsible is located on a sex chromosome, which makes it more common in one sex than the other.
Colour blindness is an example:
- In the following case, the mother is a carrier of colorblindness (X^CX^c). This means she shows no symptoms of colour blindness, but the recessive allele causing color blindness is present on one of her X chromosomes.
- The father has normal colour vision (X^CY).
- If the gene responsible for a particular condition is present only on the Y chromosome, only males can suffer from the condition because females do not possess the Y chromosome.
- F1 genotypes: X^CX^C X^CX^c X^CY X^cY
- F1 phenotypes: 2 females with normal vision; 2 males, one with normal vision, one with colour blindness.