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monohybrid inheritance and genetic key terms

Page history last edited by tre (remote) 15 years, 2 months ago

 

image from flickr by mr happy under a creative commons license

 

Learning outcome

Explain the terms: gene, allele, genotype, phenotype, recessive, dominant, homozygote and heterozygote; and explain monohybrid inheritance, including the interpretation of genetic pedigree diagrams, in the context of traits such as cystic fibrosis, albinism, thalassaemia, garden pea height and seed morphology.

 

Monohybrid Inheritance

Mono means one. This means the characteristic is controlled by only one gene. Cystic Fibrosis is a good example of monohybrid inheritance because only the CFTR gene is implicated. Eye colour is thought to have several different genes involved.

 

Question: What do monosacharides and monohybrid inheritance have in common?

Highlight below for answer

 

mono means one, one sugar in the case of monosaccharides and one gene in the case of monohybrid inheritance

Pedigree diagrams

 

A pedigree diagram is useful for genetic counsellors to map the progression of a disease for in a family and calculate the chance that the offspring will be sufferers or carriers of that given disease.

 

Question: In the pedigree above showing cystic fibrosis inheritance. The individual labeled "4" in generation III is

  1. a healthy male.
  2. a healthy female.
  3. a male with cystic fibrosis.
  4. a female with cystic fibrosis.
  5. one of the parents.

 

Answer: A healthy female

Punnet Squares

These diagrams allow you to calculate the probability that parents will have offspring for any given traits. This is the same for plants and people and anything that passes on its genes! The parents characteristics are crossed with each other and this shows you all possible outcomes for the cross.

 

Question:  If one of the parents in the above punnet square is homozygous R, what is the chance of there being a yellow offspring?

Answer: No chance! 

 

Glossary of Terms

 

Gene

A piece of DNA which has a specific sequence of nucleotide bases. Each gene codes for a specific protein. An example of this in humans is the CF (cystic fibrosis) gene, which codes for the CFTR protein; this helps to transport chloride ions across cell membranes. An individual gene may have more than one form. These forms or alleles differ from each other in the sequences of their nucleotide bases and, as a result, produce slightly different proteins.

 

Allele

One of the different forms of a particular gene. In humans, a gene codes for the protein that transports chloride ions across the cell surface membrane. We all have this gene, the CF gene. The CF gene has two alternative forms or alleles. The normal allele, F, produces a functioning protein. The other allele, f, produces a protein which is very slightly different and does not function. If a person has two copies of the f allele, he or she will have cystic fibrosis.

An allele is a form of a particular gene. It is not part of a gene.

 

Phenotype

The characteristics of an organism, which result from the genes the organism possesses and the environment in which it lives. Himalayan rabbits have white fur but black feet, ears and tail. Cross-breeding pure-bred Himalayan rabbits produces young rabbits with these characteristics. The black pigment will only develop, however, in parts of the body which are at a low temperature. A Himalayan rabbit must have the gene for making black fur. The rabbit must also be in the right environment for this black pigment to develop.

 

Genotype

The genotype describes an organism in terms of the alleles it contains. In pea plants the allele for tall plants, T, is dominant to that for short plants, t. A short plant always has the genotype tt. This means each of its cells contains two copies of the t allele. The genotype of a tall plant, however, may be either TT or Tt.

 

Heterozygous

An organism which has two different alleles of a particular gene. In cystic fibrosis, F represents the normal allele and f represents the mutant allele. A person with genotype Ff is a heterozygote. This person is a carrier of cystic fibrosis, but does not have the disease. They could pass the disease on to their children.

 

Homozygous

An organism in which the alleles of a particular gene are identical to each other. For example, in cystic fibrosis, F represents the normal allele and f represents the mutant allele. There are two possible homozygotes. A person with the genotype FF does not have cystic fibrosis while a person with the genotype ff has cystic fibrosis.

Dominant

An allele is described as dominant if its effect is always shown. In peas, the allele for tall plants, T, is dominant to the allele for short plants, t. Because the allele for tall plants is dominant, plants with either the genotype TT or the genotype Tt will be tall.

 

Recessive

An allele is described as recessive if its effect is only shown when the other allele of the pair is identical. In peas, the allele for short plants, t, is recessive to the allele for tall plants, T. Only homozygous plants with the genotype tt will be short.

Where a gene has two alleles, one dominant to the other, use a capital letter for the dominant allele and a small letter for the recessive allele. Try not to use letters like C, M and P where the only difference between the capital and small letter is size.

 

genes and health

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