Determining Genotypes And Phenotypes Flashcards
Explain what homozygous means.
“Homo” means “the same”. The word homozygous means the two alleles the organism has for a given gene are the same. For example, if we think about the eye colour gene, there might be two alleles, b for blue eyes and B for brown eyes. An organism homozygous for that gene has two alleles which are the same, e.g. they have two B alleles or two b alleles, making their genotype BB or bb.
Explain what the word heterozygous means.
“Hetero” means “different”. The word heterozygous means the two alleles an organism has for a particular gene are different. For example, if we think about the eye colour gene, there might be two alleles, b for blue eyes and B for brown eyes. An organism which is heterozygous for that gene has two different alleles, e.g. they have one B allele and also a b allele, making their genotype Bb.
Define the terms dominant and recessive in regards to alleles.
Humans (as well as other animals and most plants) have two copies of every chromosome and therefore, two alleles for each gene (one on each of the chromosomes). The trait encoded by a dominant allele will be visible in the phenotype as long as there is at least one copy present. However, for a recessive allele, the trait encoded by the allele will only be present if there are two copies present - if both of the alleles the organism has are the recessive allele. Recessive alleles can be ‘masked’ or hidden by dominant alleles.
If we think about eye colour, with the b allele giving blue eyes and B giving brown eyes, the genotype BB gives brown eyes, but so does the genotype Bb. As long as there is at least one copy of the dominant allele, B, present, the organism will have brown eyes. However, the allele for blue eyes is recessive. To have blue eyes, the person must have two copies of the recessive allele, i.e. be bb. As long as a person has at least one brown eye allele, they will have brown eyes, regardless of what their other allele is for the eye colour gene.
Give an example of a homozygous recessive genotype.
bb
Give an example of a homozygous dominant genotype.
AA
Give an example of a heterozygous genotype.
Bb
What is meant by the phrase true-breeding or pure-breeding?
A true-breeding organism always passes down a particular trait, producing offspring with the same phenotype as itself. For example, if a black coloured rat is “true-breeding” or “pure-breeding” for coat colour, it will only ever produce black coloured offspring. The offspring might differ from their parents in other ways, but for this particular trait, they are the same.
Explain what a test-cross is and why a breeder would carry out a test-cross.
A test-cross is a way to investigate the phenotype of an organism. In a test cross, a breeder takes an organism whose genotype they want to investigate and crosses them with a homozygous recessive individual. They then examine the ratio of phenotypes in the offspring to determine the genotype of the individual they are interested in.
State the probability that a human mother will have a boy child bs a girl child, she has already had four sons. Explain how this will affect the probability that her next child is a daughter.
50% chance of a boy, and 50% chance of a girl. Even if the mother already has four sons, there is still a 50% chance that her next child will also be a son. This is because the biological sex of each child is independent.
